Bss cell planning

BSS cell planning Planning requirementsWhen planning a mobile telephone system, the aim is to create a communications network that fulfils the following requirements:

Provides the desired capacity. Offers good frequency efficiency. Implemented at low cost. High grade of service.

These requirements, when analyzed, actually conflict with one another. Therefore the operating network is always a solution achieved through compromise. The cost of different network configurations can vary considerably. From an engineering point of view it would be worth while using the most frequency efficient solutions despite their high cost, but a mobile telephone network is so huge an investment that the financial factors are always going to limit the possibilities. The effect of limited funds is particularly obvious when the first stage of the network is being built. Consequently, economical planning is a condition for giving the best possible service from the start. The use of the GSM900, EGSM, and DCS1800 frequency bands, create many propagation based problems. Because the channel characteristics are not fixed, they present design challenges and impairments that must be dealt with to protect MS telephone users from experiencing excessively varying signal level and lack of voice quality. It is important to be able to predict the RF path loss between the BTS and the MS within the coverage area in different types of environment. To do this it is necessary to have knowledge of the transmitter and receiver antenna heights, the nature of the environment and the terrain variations. Planning factors When planning the network there are a number of major factors which must be considered to enable the overall system requirements to be met.

1. click here Planning tools. 1. click here GSM frequency spectrum:

Modulation techniques and channel spacing.1. click here Traffic capacity:

Unit of measure and grade of service.1. click here Capacity calculations:

Typical call parameters.1. click here Control channel calculations:

Number of CCCH per BTS cell.Number of SDCCH per BTS cell.Control channel configurations.

1. GPRS effective load. 1. click here Propagation effects on GSM frequencies:

Introduction to decibels.Fresnel zoneRadio refractive index.Environmental effects on propagation.Multipath propagation.Free space loss.Plane earth loss.Antenna gain.Clutter factor.Power budget and system balance.

1. click here Frequency re-use: Re-use patterns.Carrier to interference ratio.Co-channel interference.Adjacent channel interference.Sectorization of sites.

1. click here Overcoming adverse propagation effects:

Frequency/baseband/synthesizer hopping.Block and diagonal interleaving.Directional antennas, sectorization.Uplink and downlink power control.Discontinued transmissions.Receive diversity.Equalization.

1. click here Subscriber environment: Environment.Future planning.

1. click here The microcellular solution. Planning tools

IntroductionIn order to predict the signal strength in a cell area it would be necessary to make many calculations, at regular intervals, from the BTS. The smaller the interval the more accurate the propagation model. Also the calculations would need to be performed at regular distances along each radial arm from the BTS, to map the signal strength as a function of distance from the BTS. The result, is the necessity to perform hundreds of calculations for each cell. This would be time consuming in practice, but for the intervention of the software planning tool. This can be fed with all the details of the cell, such as:

Type of terrain. Environment. Heights of antennas.

It can perform the necessary number of calculations needed to give an accurate picture of the propagation paths of the cell. Several planning tools are available on the market, such as Netplan or planet, and it is up to the users to choose the tool(s) which suit them best. After calculation and implementation of the cell, the figures should then be checked by practical measurements. This is because, with all the variable factors in propagation modelling, an accuracy of 80% would be considered excellent. GSM frequency spectrum The GSM900 frequency spectrumThe original GSM frequency spectrum was allocated in 1979. This consisted of two sub-bands 25 MHz wide. The frequency range is:

Uplink range 890 MHz - 915 MHz. Downlink range 935 MHz - 960 MHz.

It is usual for the uplink frequencies - mobiles transmit to the BTS - to be on the lowest frequency band . This is because there is a lower free space path loss for lower frequencies. This is more advantageous to the mobile as it has a reduced transmit output power capability compared to the BTS. The two bands are divided into channels, a channel from each band is then paired with one of the pair allocated for uplink and one for the downlink. Each sub-band is divided into 124 channels, these are then given a number known as the Absolute Radio Frequency Channel Number (ARFCN). So a mobile allocated an ARFCN will have one frequency to transmit on and one to receive on. The frequency spacing between the pair is always 45 MHz for GSM. The spacing between individual channels is 200 kHz and at the beginning of each range is a guard band. It can be calculated that this will leave 124 ARFCNs for allocation to the various network operators. These ARFCNs are numbered 1 to 124 inclusive To provide for future network expansion more frequencies were allocated to GSM as they became available. An extra 10 MHz was added on to the two GSM bands and this became known as Extended GSM (EGSM). The EGSM frequency range is:-


This allows another 50 ARFCNs to be used bringing the total to 174. These additional ARFCNs are numbered 975 to 1023 inclusive.

One thing to note is that original Phase 1 MSs can only work with the original GSM frequency range and it requires a Phase 2 MS to take advantage of the extra ARFCNs. As the operator cannot guarantee that his network will have a significant number of Phase 2 MS, care must be taken when using EGSM frequencies not to make holes in the network for Phase 1 MSs. The DCS1800 frequency spectrumAs GSM evolved it was decided to apply the technology to the Personal Communications Networks. This required changes to the air interface to modify the frequency range over which it operates. The modified frequency range is:


This provides 374 ARFCNs with a frequency separation of 95 MHz between uplink and downlink frequencies. In the UK these ARFCNs have been shared out between the four network operators, refer to Figure 3-1. Two of these, Orange and One to One operate exclusively in the DCS1800 range while the other two, Vodafone and Cellnet have been allocated DCS1800 channels on top of their GSM900 networks. ARFCNs are numbered from 512 to 885 inclusive The portion at the top of the band is used by Digital enhanced Cordless telephony (DECT).

Figure 3-1 UK network operatorsThe PCS1900 frequency spectrumThis is another adaptation of GSM into the 1900 MHz band. It is used in the United States where the Federal Communications Commission has divided the band into 300 ARFCNs and issued licences to various operators to implement GSM networks. The frequency separation is 80 MHz. The frequency range is :


Absolute radio frequency channel capacityEach RF carrier supports eight time division multiplexed physical channels and each of these is capable of supporting speech or signalling information. The maximum number of RF carriers at any one BTS site is 24 for M-Cell6 and 25 for BTS6. Therefore the maximum number of physical channels available at a BTS site is 24 x 8 = 192, for M-Cell6 and 25 x 8 = 200, for BTS6.

Figure 3-2 Eight TDMA timeslots per RF carrierModulation techniques and channel spacingThe modulation technique used in GSM is Gaussian minimum shift keying. This works by shaping the data to be modulated with a Gaussian filter. The filter removes some of the harmonics from the data square wave producing a more rounded shape. When this is applied to a phase modulator the result is a modified envelope shape at the output of the modulator. The bandwidth of this envelope is narrower than that of a comparable one produced from non-filtered data. With each modulating carrier occupying a narrower bandwidth, more efficient use can be made of the overall bandwidth available. The bandwidth allocated to each carrier frequency in GSM is 200 kHz. The actual bandwidth occupied by a transmitted GSM carrier is far greater than 200 kHz, even with Gaussian filtering. The signal therefore overlaps into surrounding frequencies, as illustrated in Figure 3-3. If two carriers from the same or adjacent cells are allocated adjacent frequencies or channel numbers they will interfere with each other because of the described overlapping. This interference is unwanted signal noise. All noise is cumulative, so starting with a large amount by using adjacent channels our wanted signal will soon deteriorate below the required quality standard. For this reason adjacent frequencies should never be allocated to carriers in the same or adjacent cells. Figure 3-3 illustrates the fact that the actual bandwidth of a GMSK modulated signal is considerably wider than the 200 kHz channel spacing specified by GSM. At the channel overlap point the signal strength of the adjacent channel is only -10 dB below that of the wanted signal. While this just falls within the minimum carrier to interference ratio of 9 dB, it is not insignificant and must be planned around so that allocation of adjacent frequencies in adjacent cells never occurs. One other consideration about channel spacing that must be considered is when using combiners. If a cavity combining block is used the frequencies for combining must be separated by at least three ARFCNs otherwise it could cause intermodulation products and spurious frequency generation. These could interfere with other carriers further away in the radio spectrum, possibly in adjacent cells, so they would not necessarily be a problem to the home cell so the source of interference becomes more difficult to locate.

Figure 3-3 Modulation techniques and channel spacing

Traffic capacityDimensioningOne of the most important steps in cellular planning is system dimensioning. To dimension a system correctly and hence all the supporting infrastructure, some idea of the projected usage of the system must be obtained (for example; the number of people wishing to simultaneously use the system). This means traffic engineering. Consider a cell with N voice channels, the cell is therefore capable of carrying N individual simultaneous calls. The traffic flow can be defined as the average number of concurrent calls carried in the cell. The unit of traffic intensity is the Erlang, traffic defined in this way can be thought of as a measure of the voice load carried by the cell. The maximum carried traffic in a cell is N Erlangs, which occurs when there is a call on each voice channel all of the time. If during a time period T (seconds), a channel carries traffic is busy for t (seconds), then the average carried traffic, in Erlangs, is t/T. The total traffic carried by the cell is the sum of the traffic carried by each channel. The mean call holding time is the average time a channel is serving a call. Channel blockingThe standard model used to dimension a system is the Erlang B model, which models the number of traffic channels or trunks required or a given grade of service and given offered traffic. There will be times when a call request is made and all channels or trunks are in use, this call is then blocked. The probability of this happening is the grade of service of the cell. If blocking occurs then the carried traffic will be less than the offered traffic. If a call is blocked, the caller may try again within a short interval. Repeated call attempts of this type increase the offered traffic above the level if there had been an absence of blocking. Because of this effect the notion of offered traffic is somewhat confused, however, if the blocking probability is small, it is reasonable to ignore the effect of repeated call attempts and assume that blocked calls are abandoned. The number of calls handled during a 24 hour period varies considerably with time. The figure opposite shows the type of traffic load that might be expected on a typical call. There are usually two peaks during week days, although the pattern can change from day to day. Across the typical day the variation is such that a one-hour period shows greater usage than any other. From the hour with the least traffic to the hour with the greatest traffic, the variation can exceed 100:1. To add to these fairly regular variations, there can also be unpredictable peaks caused by a wide variety of events (for example; the weather, natural disasters, conventions, sports events). In addition to this, system growth must also be taken into account. There are a set of common definitions to describe this busy hour traffic loading. Busy Hour: The busy hour is a continuous period during which traffic volume or number of call attempts is the greatest. Peak Busy Hour: The busy hour each day it is not usually the same over a number of days. Time Constant Busy Hour: The one-hour period starting at the same time each day for which the average traffic volume or call attempts count is greatest over the days under consideration. Busy Season Busy Hour: The engineering period where the grade of service criteria is applied for the busiest clock hour of the busiest weeks of the year. Average Busy Season Busy Hour: The average busy season busy hour is used for trunk groups and always has a grade of service criteria applied. For example, for the Average Busy Season Busy Hour load, a call requiring a circuit in a trunk group should not encounter All Trunks Busy (ATB) no more than 1% of the time. Peak loads are of more concern than average loads when engineering traffic routes and switching equipment. Traffic flowIf mobile traffic is defined as the aggregate number of MS calls (C) in a cell with regard to the duration of the calls (T) as well as their number, then traffic flow (A) can be defined as: Traffic Flow (A) = C x T

Where: C is: the calling rate per hour.

T the average holding time per call. Suppose an average hold time of 1.5 minutes is assumed and the calling rate in the Busy Hour is 120, then the traffic flow would be 120 x 1.5 = 180 call-minutes or 3 call hours. One Erlang of traffic intensity on one traffic channel means a continuous occupancy of that particular traffic channel.

Considering a group of traffic channels, the traffic intensity in Erlangs is the number of call-seconds per second or the number of call-hours per hour. As an example; if there were a group of 10 traffic channels which had a call intensity of 5 Erlangs, then half of the circuits would be busy at the time of measurement. Grade of serviceOne measure of the quality of service is how many times a subscriber is unsuccessful in setting up a call (blocking). Blocking data states what grade of service is required and is given as a percentage of the time that the subscriber is unable to make a call. Typical blocking for the MS-BSC link is 2% with 1% being acceptable on the BSC-MSC link. There is a direct relationship between the grade of service required and the number of channels. The customers desired grade of service has a direct affect on the number of channels needed in the network. Capacity calculations IntroductionThis section provides information on how to determine the number of control channels required at a BTS. This information is required for the sizing of the links to the BSC, and is required when calculating the exact configuration of the BSC required to support a given BSS. Typical call parameters The number of control channels and GPROC2s required at a BTS depend on a set of call parameters; typical call parameters for BTS planning are given in Table 3-1.

Table 3-1 Typical parameters for BTS call planning

Parameter Assumed Value

Call duration T = 120 seconds

Ratio of SMSs per call S = 0.1

Ratio of location updates to calls: non-border location area l = 2

Ratio of location updates to calls: border location area l = 7

Ratio of IMSI detaches to calls Id = 0

Location update factor: non-border location area (see below) L = 2

Location update factor: border location area (see below) L = 7

Number of handovers per call H = 2.5

Paging Rate in pages per second P = 3

Time duration for location update TL = 4 seconds

Time duration for SMSs TSMS = 6 seconds

Time duration for call set-ups TC = 5 seconds

Guard time for SDCCHs Tg = 4 seconds

Probability of blocking for TCHs PB-TCH < 2%

Probability of blocking for SDCCHs PB-SDCCH < 1%

The location update factor (L) is a function of the ratio of location updates to calls (I), the ratio of IMSI detaches to calls (Id) and whether the short message sequence (type 1) or long message sequence (type 2) is used for IMSI detach; typically Id = 0 (that is IMSI detach is disabled) as in the first formula given below. When IMSI detach is enabled, the second or third of the formulas given below should be used. The type of IMSI detach used is a function of the MSC. If IMSI detach is disabled: If IMSI detach type 1 is enabled: If IMSI detach type 2 is enabled: Control channel calculations Introduction

There are four types of air interface control channels, they are: Broadcast control channel (BCCH). Common control channel (CCCH). Standalone dedicated control channel (SDCCH). Cell broadcast channel (CBCH), which uses one SDCCH.

There are three configurations of control channels, each occupies one radio timeslot: A combined control channel.

One BCCH plus three CCCH plus four SDCCH.or

A non-combined control channel. One BCCH plus nine CCCH (no SDCCH).

plus An SDCCH control channel.

Eight SDCCH.Each sector/cell requires a BCCH, so one of the configurations is always required. The number of air interface control channels required for a site, is dependent on the:

Number of pages. Location updates. Short message services. Call loading. Setup time.

Only the number of pages and access grants affects the CCCH. The other information uses the SDCCH. GPRS control channel RF provisioningControl channels can be equipped to a GPRS carrier or to a circuit switched GSM carrier to support GPRS traffic channels. If the control channel timeslot(s) are assigned to a GPRS carrier, this reduces the number of available GPRS timeslots from eight to a smaller number in direct proportion to the number used as control channels. Alternatively, by equipping the control channels to the circuit switched GSM carrier, all eight timeslots on the GPRS carrier remain available for use as GPRS timeslots. The network planner needs to combine the GSM circuit switched signalling requirements with the GPRS signalling requirements in order to plan the appropriate level of control channel support. This planning guide provides the planning rules that enable the network planner to evaluate whether a combined BCCH can be used, or if a non-combined BCCH is required. The decision to use a non-combined BCCH is a function of the combined GPRS and GSM signalling load on the PAGCH ,and on the number of SDCCH channels required to support the GSM circuit switched traffic. The use of a combined BCCH is desirable because it may permit the use of only one timeslot on a carrier that is used for signalling. A combined BCCH can offer 4 more SDCCH blocks for use by the GSM circuit switched signalling traffic. If more than an average of three CCCH blocks, or more than four SDCCH blocks, is required to handle the signalling load, more control channel timeslots are required. The planning approach for GPRS/GSM control channel provisioning is to determine whether a combined BCCH is possible, given the combined GPRS and GSM load on the CCCH control channel. When more than three CCCH blocks and less than nine CCCH blocks are required to handle the combined load, the use of a combined BCCH is not possible. When more than nine CCCH blocks are needed, one or more timeslots are required to handle the CCCH signalling. In this case, it may be advantageous to use a combined BCCH again, depending on the CCCH and SDCCH load. The determination of how many CCCH and SDCCH blocks are required to support the circuit switched GSM traffic is deferred to the network planning that is performed with the aid of the relevant planning information for GSM. The network planning that is performed using the planning information determines how many CCCH and SDCCH blocks are required, and subsequently how many timeslots in total are required, to support the CCCH and SDCCH signalling load. The downlink control channels are: FCCH, SCH, BCCH, PAGCH. The Paging Access Grant CHannel (PAGCH) consists of paging messages and access grant messages. The downlink control channel load is determined by evaluating the combined GSM circuit switched signalling traffic load and the GPRS signalling traffic load on the PAGCH.

The uplink control channel is the Random Access CHannel (RACH). It is assumed that by adequate provisioning of the downlink portion of the Common Control CHannel (CCCH), the uplink portion is implicitly provisioned with sufficient capacity. The provisioning of the Paging Access Grant CHannel (PAGCH) is estimated by calculating the combined load from the GPRS pages, GSM pages, GPRS access grant messages, and GSM access grant messages. The calculation is performed by adding the estimated GPRS and GSM paging blocks for the BTS cell to the estimated number of GPRS and GSM access grant blocks for the BTS cell, and dividing that sum by the CCCH utilization factor. Equation 19 should be evaluated to determine whether the number of PAGCHs is greater than three. If the evaluation is greater than three, three CCCH blocks are not sufficient: a non-combined BCCH must be used, independent of the number of SDCCH channels that are calculated as part of the BSS GSM circuit switched planning. If more than nine CCCH blocks are needed, more non-combined timeslots may be required. Example control channel configurations are shown in Table 3-2.

Table 3-2 Control channel configurations

Timeslot 0 Other timeslots Comment

1 BCCH + 3 CCCH + 4 SDCCH

N x 8 SDCCH One combined BCCH. The other timeslot may or may not be required depending on the support of circuit switched traffic, where the value of N can be >=0.

1 BCCH + 9 CCCH N x 8 SDCCH Non-combined BCCH. The value of N is >=1.

1 BCCH + 9 CCCH

N x 8 SDCCH, 9 CCCH

Non-combined BCCH. This is an example of one extra timeslot of CCCHs added in support of GPRS traffic. The value of N is >= 1.

The number of GPRS and GSM paging blocks are summed together in Equation 20. Equation 19Each term in the above equation is determined as per Equation 21 and Equation 22. Where: NPAGCH is: The average number of paging / Access Grant blocks rounded up to an integer.

NPCH The average number of paging blocks required at a cell.

NAGCH The average number of Access Grant blocks required at a cell.

UCCCH This is a utilization factor based on the percentage of the CCCH bandwidth that can be reliably used. A typical value for UCCCH is 30%.

The number of GPRS and GSM paging blocks are summed together in Equation 20. Equation 20Each term in the above equation is determined as per Equation 21 and Equation 22.

Where: NPCH is: The average number of paging blocks in support of GPRS and GSM traffic required at a cell.

NPCH_GPRS The average number of paging blocks in support of GPRS traffic.

NPCH_GSM The average number of paging blocks in support of GSM traffic.

Equation 21

Where: NPCH_GPRS is: The average number of paging blocks in support of GPRS traffic required at a cell.

GPRS_Page_Rate The number of GPRS pages transmitted to a BTS cell per second.

Equation 22

Where: NPCH_GSM is: The average number of paging blocks in support of GSM traffic required at a cell.

GSM_Page_Rate The number of GSM pages transmitted to a BTS cell per second. Where the denominator factor of 1.5 in Equation 21 and Equation 22 reflects that one page can be used for an average of 1.5 mobiles. The factor of 4.25 is the number of paging messages per second supported by one CCCH block.

The factors of 1.5 in Equation 21 and in Equation 22 take into account the paging message packing efficiency experienced at the cell. The number of GPRS and GSM access grant channel blocks is summed in Equation 23. Equation 23

Where: NAGCH is: The average number of access grant blocks required at a cell.

NAGCH_GPRS The average number of GPRS access grant blocks required at a cell.

NAGCH_GSM The average number of GSM access grant blocks required at a cell. Each term in Equation 23 above is determined by Equation 24 and Equation 25 respectively. Equation 24Where: NAGCH_GPRS is: The number of GPRS access grant blocks required at a cell.

lBURST_GPRS

This number includes all downlink bursts per second in support of all uplink and downlink GPRS temporary data flow (TBF) originations. GPRS data traffic includes all SMS traffic carried by the GPRS infrastructure. Additionally, this factor includes routeing area updates and cell updates.

Before the GPRS network is operational, the above values in Equation 24 must be determined by the operator. Once the network is operational, these values can be obtained by inspecting the BSS busy hour statistics. Equation 25The factors in the above Equation 25 are defined as follows.

Where: NAGCH_GSM is: The average number of GSM access grant blocks required at a cell.

(lambda)CALL_GSM The call arrival rate per second.

(lambda)L_GSM The location update rate per second.

(lambda)S_GSM The number of SMS messages per second. Number of CCCH per BTS cell The following factors should be considered when calculating the number of CCCH per BTS cell:

The CCCH channels comprise the paging and access grant channel (PAGCH) in the downlink, and the random access channel (RACH) in the uplink. The PAGCH is subdivided into access grant channel (AGCH) and paging channel (PCH).

If the CCCH has a low traffic requirement, the CCCH can share its timeslot with SDCCHs (combined BCCH). If the CCCH carries a high traffic a non-combined BCCH must be used:

o Combined BCCH (with four SDCCH). Number of CCCH blocks = 3.Number of CCCH blocks reserved for AGCH ag_blks_res is 0 to 2.Number of CCCH blocks available for PCH/AGCH is 3 to 1.

Non combined BCCH. Number of CCCH blocks = 9.Number of CCCH blocks reserved for AGCH ag_blks_res is 0 to 7.Number of CCCH blocks available for PCH is 9 to 2.

When a non-combined BCCH is used, it is possible to add additional CCCH control channels (in addition to the mandatory BCCH on timeslot 0). These additional CCCH control channels are added, in order, on timeslots 2, 4, and 6 of the BCCH carrier. Thus creating cells with 18, 27, and 36 CCCH blocks. These configurations would only be required for very high capacity cells or in large location areas with a large number of pages.

Each CCCH block can carry one message. The message capacity of each CCCH block is 4.25 messages/second.

The AGCH is used to send immediate assignment and immediate assignment reject messages. Each AGCH immediate assignment message can convey channel assignments for up to two MSs. Each AGCH immediate assignment reject message can reject channel requests from up to four MSs.

The PCH is used to send paging messages. Each PCH paging message can contain pages for up to four MSs using TMSI or two MSs using IMSI. If no paging messages are to be sent in a particular CCCH block, then an immediate assignment/immediate assignment reject message can be sent instead.

The current Motorola BSS implementation applies the following priority (highest to lowest) for downlink CCCH messages:

Paging message (if not reserved for AGCH). Immediate assignment message. Immediate assignment reject message.

Thus, for example, if for a particular PAGCH sub-channel there are always paging messages (that is high paging load) waiting to be sent, no immediate assignment or immediate assignment reject messages will be sent on that PAGCH sub-channel. Hence the option to reserve CCCH channels for AGCH.

It can normally be assumed that sufficient capacity exists on the uplink CCCH (RACH) once the downlink CCCH (PAGCH) is correctly dimensioned.

A number of other parameters may be used to configure the CCCH channels. Some of these are:

o Number of paging groups. Each MS is a member of only one paging group and only needs to listen to the PCH sub-channel corresponding to that group. Paging group size is a trade off between MS idle-mode battery life and speed of access (for example, a lot of paging groups, means the MS need only listen very occasionally to the PCH but as a consequence it takes longer to Page that MS resulting in slower call setup as perceived by a PSTN calling party).

o Number of repetitions for MSs attempting to access the network on the RACH. o Time MS must wait between repetitions on the RACH.

Precise determination of the CCCH requirements may be difficult; however, a number of statistics can be collected (for example ACCESS_PER_PCH, ACCESS_PER_AGCH by the BSS and these may be used to determine the CCCH loading and hence perform adjustments.

Calculate the number of CCCHs per BTS cell The following planning actions are required:

Determine the number of CCCHs per BTS. The average number of blocks required to support AGCH and PCH is given by:

The average number of blocks required to support AGCH only is given by:The average number of blocks required to support TMSI paging only is given by:The average number of blocks required to support IMSI paging only is given by:The access grant rate is given by:

Where: UCCCH is: the CCCH utilization.

lAGCH the access grant rate (per second).

P the paging rate per second.

lcall the call arrival rate per second.

lL the location update rate per second.

lS the number of SMSs per second. Number of SDCCH per BTS cell Determining the SDCCH requirement is an important part of the planning process. The SDCCH is where a large portion of call setup messaging takes place. As the number of calls taking place in a BTS increases, greater demand is placed on the control channel for call setup. The following factors should be considered when calculating the number of SDCCH per BTS cell:

To determine the required number of SDCCHs for a given number of TCHs per sector, the call, location update, and SMS (point to point) rates must be determined. Refer to the equations below for information on calculating these rates. Once these rates are determined, the required number of SDCCHs for the given number of TCHs can be determined. Refer to the equations below for information on calculating the required number of SDCCHs.

The rates for SMS are for the SMSs taking place over an SDCCH. For MSs involved in a call, the SMS may take place over the TCH, and may not require the use of an SDCCH.

Calculating the number of SDCCHs required is necessary for each cell at a BTS site. The equation below for NSDCCH is used to determine the average number of SDCCHs. The

number of Erlangs, e, is the number of Erlangs supported by a given sector based on the

number of TCHs in that sector. To determine the number Erlangs support by a sector use Erlangs B. Use Erlang B to determine the required number of SDCCHs necessary to support the desired grade of service.

The number of location updates will be higher for sites located on the borders of location areas, as compared to inner sites of a location area. See Figure 3-4.

Figure 3-4 Location area diagramCalculate the number of SDCCHs per BTS cell The following planning actions are required:

Determine the number of SDCCHs per BTS. The average number of SDCCHs is given by:

The call rate (calls per hour) is given by:The location update rate (LU per hour) is given by:The SMS rate (SMS per hour) is given by:

Where: NSDCCH is: the average number of SDCCHs.


Tc the time duration for call setup.

lLU the location update rate.

TL the time duration of location updates.

Tg the guard time for SDCCH.

lS the number of SMSs per second.

TS the time duration of SMS (short message service setup).

e the number of Erlangs per cell.

T the average call length in seconds.

L the ratio of location updates to calls.

S the ratio of SMSs to calls.

Control channel configurations Table 3-3 and Table 3-4 give typical control channel configurations based on the typical BTS planning parameters given in Table 3-1. Control channel configurations for non-border location areaTable 3-3 is for the non-border location area cell, where the ratio of location updates to calls is 2.

Table 3-3 SDCCH planning for typical parameters (non-border location area)

Number of

RTFs

Number of

TCHsNumber of

ErlangsNumber of SDCCHs

Timeslot utilization

Timeslot 0 Other timeslots

1 7 2.94 4 1 BCCH + 3 CCCH + 4 SDCCH

2 14 8.20 8 1 BCCH + 9 CCCH 8 SDCCH

3 22 14.9 8 1 BCCH + 9 CCCH 8 SDCCH

4 30 21.9 12 1 BCCH + 3 CCCH + 4 SDCCH 8 SDCCH


6 45 35.6 16 1 BCCH + 9 CCCH 2 x 8 SDCCH

7 53 43.1 16 1 BCCH + 9 CCCH 2 x 8 SDCCH

8 61 50.6 20 1 BCCH + 3 CCCH + 4 2 x 8 SDCCH

SDCCH

9 69 58.2 20 1 BCCH + 3 CCCH + 4 SDCCH 2 x 8 SDCCH


11

12

Control channel configurations for border location areaTable 3-4 is for the border location area cell, where the ratio of location updates to calls is 7.

Table 3-4 SDCCH planning for typical parameters (border location area)

Number of

RTFs

Number of

TCHsNumber of




1 6 2.28 8 1 BCCH + 9 CCCH 8 SDCCH


3 21 14.0 16 1 BCCH + 9 CCCH 2 x 8 SDCCH


5 36 27.3 24 1 BCCH + 9 CCCH 3 x 8 SDCCH




9 66 55.3 40 1 BCCH + 9 CCCH 5 x 8 SDCCH


The GPRS planning process

Overview of the GPRS planning processThe GPRS planning process documentation has the following structure:

Introduction to the planning process. GPRS network traffic estimation and key concepts. Air interface planning process.

Introduction to the GPRS planning process

Overview the GPRS planning process introductionThe Introduction to the GPRS planning process has the following structure:

Determination of expected load or overload. Network planning flow.

Determination of expected loadThe planning process begins by determining the expected GPRS load (applied load) to the system. The next step is to determine the effective load to the system by weighting the applied load by network operating parameters. These parameters consist of the expected BLock Error Rate (BLER) based on the cell RF plan, the protocol overhead (GPRS protocol stack, that is TCP/IP, LLC, SNDCP, RLC/MAC), the expected advantage from V.42bis compression and TCP/IP header compression, and the multislot operation of the mobiles and infrastructure. The effective load at a cell is used to determine the number of GPRS timeslots required to provision a cell. The provisioning process can be performed for a uniform load distribution across all cells in the network or on an individual cell basis for varying GPRS cell loads. The number of GPRS timeslots is the key piece of information that drives the BSS provisioning process in support of GPRS. The planning process also uses network generated statistics, available after initial deployment, for replanning a network. The statistics fall into two categories: PCU specific statistics, and GSN (SGSN + GGSN) statistics. In a later section of this document, all of the statistics collected from the GPRS infrastructure are listed. The statistics that are expected to be useful for network replanning are identified. In this planning document, the statistics used for planning purposes are grouped into four categories: Stats_A, Stats_B, Stats_C, and Stats_D, as indicated in Figure 3-5.

Figure 3-5 GPRS network planning flowchartNetwork planning flowThe remaining chapters of the planning guide are presented in support of the GPRS network planning flowchart (Figure 3-5). The network planning flow is as follows:

GPRS network traffic estimation and key concepts: This text is intended to introduce the key concepts involved in planning a network. Because GPRS introduces the concept of a switchable timeslot that can be shared by both the GSM circuit switched infrastructure and by the GPRS infrastructure, much of the following text is dedicated to the discussion of this topic.

Customer inputs to the planning process: This chapter provides a table of inputs that can serve as a guide in the planning process. In subsequent planning sections, references are made to parameters in this table of inputs. A key piece of information that is needed for the planning process is the RF cell plan. This subsection discusses the impact of different cell plans on the GPRS provisioning process, and how to use this information in order to determine the number of GPRS timeslots that are required on a per cell basis.

BSS planning: The hardware and communication link provisioning rules are detailed in this section based on the number of timeslots required. The number of timeslots is determined from the applied cell load requirements (cell throughput) that are provided by the network planner.

GSN complex planning: The hardware and communication links are determined in this section.

GPRS network statistics for network replanning: The statistics collected by the BSS and GSN are listed in tabular form, and the statistics that could be valuable for network replanning are identified.

Planning examples: A planning example is provided for both the BSS and GSN portions of the GPRS infrastructure.

Recommended planning guidelines: Based on the network planning rules, a few recommended planning guidelines are provided in this section.

GPRS network traffic estimation and key concepts

Overview of the GPRS network traffic estimation and key conceptsThe GPRS network traffic estimation and key concepts section has the following structure:

Introduction to the GPRS network traffic estimation and key concepts. Dynamic timeslot mode switching. Carrier timeslot allocation examples. BSS timeslot allocation methods. Provisioning the network with switchable timeslots. Recommendation.

Introduction to the GPRS network traffic estimation and key concepts

The GPRS network planning is fundamentally different from the planning of circuit switched networks. One of the fundamental reasons for the difference is that a GPRS network allows the queuing of data traffic instead of blocking a call when a circuit is unavailable. Consequently, the use of Erlang B tables for estimating the number of trunks or timeslots required is not a valid planning approach for the GPRS packet data provisioning process. The GPRS traffic estimation process starts by looking at the per cell GPRS data traffic profile such as fleet management communications, email communications, web browsing, and large file transfers. Once a typical data traffic profile mix is determined, the required network throughput per cell can be calculated as measured in kbits per second. The desired network throughput per cell is used to calculate the number of GPRS timeslots required to support this throughput on a per-cell basis. The estimated GPRS network delay is derived based on computer modeling of the delay between the Um interface and the Gi interface. The results are provided in the planning guide. The network delay can be used to determine the mean or average time it takes to transfer a file of arbitrary length. In order to simulate the delay, the following factors are considered: traffic load per cell, mean packet size, number of available GPRS carrier timeslots, distribution of CS-1 and CS-2 rate utilization, distribution of Mobile Station (MS) multislot operation (1,2,3, or 4), and BLER. Use of timeslotsThe use of timeslots on a GPRS carrier is different from how they are used in the GSM circuit switched case. In circuit switched mode, an MS is either in the idle mode or dedicated mode. In the dedicated mode, a circuit is assigned through the infrastructure whether or not a subscriber is transporting voice or data. In the Idle mode, the network knows where the MS is, but there is no circuit assigned. In the GPRS mode, a subscriber uses the infrastructure timeslots for carrying data only when there is data to be sent. However, the GPRS subscriber can be attached and not sending data and this still presents a load to the GSN portion of the GRPS system, and must be accounted for when provisioning the GPRS infrastructure, that is, in state 2 as explained below. The GPRS mobile states and conditions for transferring between states are provided in Table 3-5 and shown in Figure 3-6 in order to specify when infrastructure resources are being used to transfer data. The comment column specifies what the load on the infrastructure equipment is for that state and only in state 3 does the infrastructure equipment actually carry user data. The infrastructure equipment is planned such that many more MSs can be attached to the GPRS network, that is in state 2, than there is bandwidth available to simultaneously transfer data. One of the more significant input decisions for the network planning process is to determine and specify how many of the attached MSs are actively transmitting data in the Ready state 3. In the Standby state 2, no data is being transferred but the MS is using network resources to notify the network of its location. The infrastructure has equipment limits as to how many MSs can be in state 2. When the MS is in state 1, the only required infrastructure equipment support is the storage of MS records in the HLR. Network provisioning requires planning for traffic channels and for signalling channels also referred to as control channels. The BSS GSR 4.1 release combines the circuit switched and GPRS control channels together as BCCH/CCCH. This planning guide provides a planning procedure in a later section for determining the BCCH/CCCH control channel capacity needed.

Table 3-5 MM State Model of MS

Present state #

Present state Next state Condition for state

transfer Comments (Present state)

1 IDLE READY(3) GPRS Attach

Subscriber is not monitored by the infrastructure, that is not attached to GPRS MM, and therefore does not load the system other than the HLR records.

2 STANDBY READY(3) PDU Transmission

Subscriber is attached to GPRS MM and is being actively monitored by the infrastructure, that is MS and SGSN establish MM context for subscriber IMSI, but no data transmission occurs in this state.

3 READY IDLE(1) GPRS Detach Data transmission through the infrastructure occurs in the Ready state

3 READY STANDBY(2) Ready timer expiry The ready timer (T3314) default time is

or force to Standby (The network or the MS can send a GMM signalling message to invoke force to Standby.)

32 seconds. The timer value can be modified during the signalling process by MS request. 2-60 sec. in 2 sec. increments or 61-1800 sec. in 60 sec. increments.

Figure 3-6 MM state models for MS and SGSNDynamic timeslot mode switchingThis section proposes a network planning approach when utilizing dynamic timeslot mode switching of timeslots on a GPRS carrier. The radio interface resources can be shared dynamically between the GSM circuit switched services and GPRS data services as a function of service load and operator preference. The timeslots on a GPRS carrier can be reserved for GPRS use, for circuit switched use only, or allocated as switchable. Motorola uses the term switchable to describe a timeslot that can be dynamically allocated for GPRS Data service or for circuit switched service. The timeslot allocation is performed such that the GPRS reserved timeslots are allocated for GPRS use before switchable timeslots. GSM circuit switched timeslots are allocated to the circuit switched calls before switchable timeslots. The switchable timeslots are allocated with priority given to circuit switched calls. Motorola has a BSS feature called Concentration at BTS. This feature enables the terrestrial backhaul resources to be dynamically assigned over the E1 links between the BSC and BTS. The terrestrial backhaul resources are managed and allocated in increments of 16 kbit/s. When the concentration-at-BTS feature is enabled, it is important to have a sufficient level of terrestrial backhaul resources provisioned. This feature has the concept of reserved and switchable BSC-to-BTS resources. This concentration-at-BTS feature allows the network planner to allocate dedicated or reserved backing pools to reserved GPRS timeslots so that there is a guaranteed level of terrestrial backing available to GPRS traffic. It is recommended that the reserved backing pool is made large enough to serve the expected busy hour GPRS traffic demands on a per BTS site basis. It is possible for the circuit switched portion of the network to be assigned all of the switchable terrestrial backing under high-load conditions and, in effect, block GPRS access to the switchable timeslots at the BTS. In addition, the reserved GPRS pool of backing resources can be taken by the circuit switched portion of the network when BSC-to-BTS E1 outages occur, and when emergency pre-emption type of calls occur and cannot be served with the pool of non-reserved resources. The concentration-at-BTS feature does not take the last switchable backhaul timeslot until all of the GPRS traffic has be transmitted, in the case when there are no provisioned reserved GPRS timeslots at the cell site. Provisioning rules for the concentration-at-BTS feature are described in the planning information. Background and discussionThe initial Motorola BSS GPRS infrastructure product permits up to one carrier per cell to be provisioned as a GPRS carrier. The GPRS carrier can also be the BCCH/CCCH carrier. Alternatively, the GPRS carrier can be specified to use all eight timeslots for GPRS traffic and one of the GSM circuit switched carriers in the cell can be designated as the BCCH/CCCH carrier. The GPRS carrier can be provisioned to carry a mix of circuit switched traffic and GPRS traffic. There are three provisioning choices:

Reserved GPRS timeslots allocated only for GPRS use. Switchable timeslots dynamically allocated for either GSM circuit switched traffic or GPRS

traffic (designated as switchable timeslots by Motorola). Remaining GPRS carrier timeslots, if any, only for circuit switched use.

The BSS supports a user definable number of GPRS timeslots and reserved GPRS timeslots. The BSS calculates the number of switchable timeslots by taking the number of operator allocated GPRS timeslots minus the number of operator allocated reserved GPRS timeslots. The number of circuit switched timeslots on a non-BCCH GPRS carrier is equal to eight timeslots minus the number of GPRS timeslots, that is GPRS timeslots include reserved plus switchable timeslots. The network planner may have some of the following network planning goals in mind when trying to determine when to use reserved timeslots versus and when to use switchable:

Use reserved timeslots to guarantee a minimum GPRS quality of service.

Use switchable timeslots to provide low circuit mode blocking and high GPRS throughput when the voice busy hour and the GPRS busy hour do not coincide.

Use switchable timeslots to provide higher GPRS throughput without increasing the circuit switched blocking rate. If all the GPRS carrier timeslots are provisioned as switchable, the last available timeslot is not given to a circuit switched call until transmission of all the GPRS traffic on that last timeslot is completed. Therefore, there is a circuit switched blocking on that last timeslot until the timeslot becomes free.

Use switchable timeslots to provide some GPRS service coverage in low GPRS traffic volume areas.

Use switchable timeslots to provide extra circuit switched capacity in spectrum limited areas. In order to make the decision on how to best allocate reserved and switchable timeslots, the network planner needs to have a good idea of the traffic level for both services. The proposal in this planning guide is to drive the allocation of switchable timeslots and reserved GPRS timeslots from a circuit switched point of view. Start by looking at the circuit switched grade of service objectives and the busy hour traffic level, as measured in Erlangs. Once the circuit switched information is known, the potential impact on switchable timeslots can be analysed. The GPRS quality of service can be planned by counting the number of available reserved GPRS timeslots, and by evaluating the expected utilization of the switchable timeslots by the circuit switched portion of the network during the GPRS busy hour. Carrier timeslot allocation examplesThe following two-carrier configuration examples explore different ways a two-carrier system may provision switchable and reserved GPRS timeslots. All blank timeslots in the following figures are available only for circuit switched traffic use. The BSS starts the reserved GPRS timeslot allocation at the top of the carrier (timeslot 7), and then allocates the switchable timeslots, followed by circuit-switched-use-only timeslots. When GPRS and GSM signalling requirements are added together to be served by a two-carrier cell, it is highly likely that one timeslot will be used for BCCH and another timeslot allocated as an SDCCH timeslot. Therefore, the following examples A to example E assume that there is an extra timeslot allocated as an SDCCH timeslot (SD) for GSM signalling purposes. In Example A, Figure 3-7, only four timeslots are used for GPRS on carrier 1; two are reserved GPRS timeslots (R), and two are switchable timeslots (S). One timeslot is used for BCCH (B) and another timeslot for SDCCH (SD), and two timeslots for circuit- switched-only use (blank). In Example B, Figure 3-8, the GPRS signalling information is carried on the BCCH (B) of carrier 1 and SDCCH GSM signalling on a separate timeslot (SD). A separate carrier (carrier 2) is used to carry the GPRS data traffic. In this example, three timeslots are reserved GPRS timeslots and two are switchable. The remaining three timeslots on the second carrier are for circuit-switched-only use(blank). In Example C, Figure 3-9, all GPRS timeslots are configured as switchable timeslots on the BCCH carrier 1 and no reserved GPRS timeslots are configured. Again, one timeslot is assigned for SDCCH signalling use. In Example D, Figure 3-10, all GPRS timeslots are configured as switchable timeslots on the non-BCCH carrier, carrier 2. In Example E, Figure 3-11, all eight GPRS timeslots are configured as reserved timeslots on the non-BCCH carrier, carrier 2. Timeslot allocation for examples A and BB: BCCH/CCCH timeslot for GPRS/GSM signalling SD: SDCCH timeslot for GSM signalling R: Reserved GPRS timeslot S: Switchable timeslot Blank: Circuit-switched-use-only timeslots Figure 3-7 provides a timeslot allocation example A.

Figure 3-7 Example AFigure 3-8 provides a timeslot allocation example B.

Figure 3-8 Example BTimeslot allocation for examples C, D, and EB: BCCH/CCCH for GPRS/GSM signalling

SD: SDCCH for GSM signalling R: Reserved PDCH S: Switchable PDCH Blank: Circuit-switched-use-only timeslots Figure 3-9 provides a timeslot allocation example C.

Figure 3-9 Example CFigure 3-10 provides a timeslot allocation example D.

Figure 3-10 Example DFigure 3-11 provides a timeslot allocation example E.

Figure 3-11 Example EBSS timeslot allocation methodsThe BSS algorithm that is used in order to determine allocation of switchable timeslots gives priority to circuit switched calls. Consequently, if a switchable timeslot is being used by a GPRS mobile and a circuit switched call is requested after all other circuit switched timeslots are used, the BSS takes the timeslot away from the GPRS mobile and gives it to the circuit switched mobile. The switchable timeslot can be re-allocated back to the GPRS mobile when the circuit switched call ends. The number of reserved GPRS timeslots can be changed by the operator in order to guarantee a minimum number of dedicated GPRS timeslots at all times. The operator provisions the GPRS carrier by selecting the number of timeslots that are allocated as reserved and switchable, and not by specifically assigning timeslots on the GPRS carrier. Motorola has implemented an idle circuit switched parameter that enables the operator to strongly favour circuit switched calls from a network provisioning perspective. By setting the idle parameter to 0, this capability is essentially turned off. The use of the idle circuit switched parameter is as follows. When a circuit switched call ends on a switchable GPRS timeslot and the number of idle circuit switched timeslots is greater than an operator settable threshold, the BSS re-allocates the borrowed timeslot for GPRS service. When the number of idle timeslots is less than or equal to a programmable threshold, the BSS does not allocate the timeslot back for GPRS service, even if it is the last available timeslot for GPRS traffic. If the BSS needs to use the last switchable timeslot in a cell for a circuit switched call when all of the timeslots are allocated as switchable, re-allocation of the timeslot to circuit switched must wait until there is no GPRS traffic in the cell. There is no GPRS traffic in the cell when all of the GPRS uplink and downlink BSS infrastructure queues are empty. At this point, the BSS can re-allocate the last switchable timeslot back as a circuit switched timeslot. If one or more timeslots in a cell are allocated as reserved, the last switchable timeslot is allocated immediately on demand for a circuit switched call. Multislot mobile operation requires that contiguous timeslots are available. The BSSl takes the lowest numbered switchable timeslot in such a manner as to maintain contiguous GPRS timeslots for multislot GPRS operation. The BSS attempts to allocate as many timeslots as requested in multislot mode, and then backoff from that number as timeslots are not available. For example, suppose that timeslots 3 and 4 are switchable, and timeslots 5,6, and 7 are GPRS reserved (see Figure 3-12). When the BSS needs to re-allocate a switchable timeslot from GPRS mode to circuit switched mode, the BSS assigns timeslot 3 before it assigns timeslot 4 for circuit switched mode. Timeslot allocation for Figure 3-12B: BCCH/CCCH for GPRS/GSM signalling SD: SDCCH for GSM signalling R: Reserved PDCH S: Switchable PDCH Blank: Circuit-switched-use-only timeslots Figure 3-12 provides a timeslot allocation with reserved and switchable timeslots.

Figure 3-12 GPRS carrier with reserved and switchable timeslotsIf the Emergency Call Pre-emption feature is enabled, the BSS selects the air timeslot that carries the emergency call from the following list: (most preferable listed first)

1. Idle circuit switched. 1. Idle or in-service switchable GPRS timeslot (from lowest to highest). 1. In-service circuit switched. 1. Idle or in-service reserved GPRS timeslot (from lowest to highest).

Provisioning the network with switchable timeslots

Provisioning the network with switchable timeslots can offer flexibility in the provisioning process for combining circuit switched and GPRS service. This flexibility is in the form of additional available network capacity to both the circuit switched and GPRS subscribers, but not simultaneously. Because the BSS favours circuit switched use of the switchable timeslots, the network planner should examine the demand for switchable timeslots during the circuit switched busy hour and during the GPRS busy hour. Normally the operator provisions the circuit switched radio resource for a particular Grade Of Service (GOS) such as 2%. This means that 2 out of 100 circuit switched calls are blocked during the busy hour. If the operator chooses to use the new switchable timeslot capability, it is now possible to share some GPRS carrier timeslots between the circuit switched calls and the GPRS calls. During the circuit switched busy hour, the circuit switched use of these switchable timeslots may dominate their use. The circuit switched side of the network has priority use of the switchable timeslots, and attempts to provide a better grade of service as a result of the switchable timeslots being available. The example in Table 3-6 assumes that the planning is being performed for a cell that has two carriers. The first carrier is for circuit-switched-only use as shown in Table 3-6. The second carrier is a GPRS carrier; all eight timeslots are configured as switchable as shown in Figure 3-13. The table was created using the Erlang B formula in order to determine how many circuit switched timeslots are required for a given grade of service. The table covers the range of 2 Erlangs to 9 Erlangs of circuit switched traffic in order to show the full utilization of two carriers for circuit switched calls. The purpose of the table is to show how the circuit switched side of the network allocates switchable timeslots during the circuit switched busy hour in an attempt to provide the best possible GOS, assumed to be 0.1% for the purposes of this example. The comments column in the table is used to discuss what is happening to the availability of switchable timeslots for GPRS data use as the circuit switched traffic increases, as measured in Erlangs. This example does show some Erlang traffic levels that cannot be adequately served by two carriers at the stated grade of service listed in the tables. This occurs at the 7 and 8 Erlang levels for 0.1% GOS. In these cases, all of the switchable timeslots are used up on the second carrier in an attempt to reach a 0.1% GOS. For the 9 Erlang traffic level, 2 carriers is not enough to serve the circuit switched traffic at a 2% GOS. This would indicate a need for a second circuit switched carrier, in addition to the first circuit switched carrier and the GPRS carrier. Timeslot allocation for B: BCCH/CCCH for GPRS/GSM signalling SD: SDCCH for GSM signalling R: Reserved PDCH S: Switchable TCH Blank: Circuit-switched-use-only timeslots Assumptions: 2 Carrier site. Figure 3-13 shows one circuit switched carrier with one BCCH/CCCH timeslot, one SDCCH timeslot, and six TCH timeslots.

Figure 3-13 1 circuit switched carrier with 1 BCCH/CCCH timeslot, 1 SDCCH timeslot and 6 TCH timeslots

Figure 3-14 shows one GPRS carrier with all timeslots (eight TCHs) designated as switchable.

Figure 3-14 One GPRS carrier with all timeslots (eight TCHs) designated as switchableTable 3-6 shows part of the switchable timeslot utilization.

Table 3-6 Switchable timeslot utilization (part A)

GOSPlanned circuit

switched Erlangs/cell

Total number of circuit switched

timeslots required including

switchable

Number of switchable timeslots

necessary to provide GOS

Comments

2% 2 6 0 During off busy hour time periods, the GPRS carrier most likely carries only

GPRS traffic. Therefore, GPRS network planning should be performed assuming there are 8 timeslots available for GPRS traffic.

0.1% 2 8 2

During circuit switched busy hour at least 2 of the switchable timeslots are occasionally used by the circuit switch side of the network in an attempt to provide the best possible GOS - assumed to be on the order of 0.1%.

2% 3 8 2

During the circuit switched busy hour, 2 of the switchable timeslots are occasionally used by the circuit switch side of the network in an attempt to provide the 2% GOS.

0.1% 3 10 4

During the circuit switched busy hour, 4 of the switchable timeslots are occasionally used by the circuit switch side of the network in an attempt to provide the best possible GOS - assumed to be on the order of 0.1%.

2% 4 9 3

0.1% 4 12 6

2% 5 10 4

0.1% 5 14 8 All of the switchable timeslots are occasionally used to satisfy the 0.1% GOS.

Table 3-7 shows more switchable timeslot utilization.

Table 3-7 Switchable timeslot utilization (part B)

GOSPlanned circuit


Total number of circuit switched timeslots required including

switchable

Number of switchable timeslots necessary to provide

GOSComments

2% 6 12 6

0.1% 6 15 9 There are not enough switchable timeslots to reach 0.1% GOS.

2% 7 13 7


2% 8 14 8

All of the switchable timeslots are occasionally used to satisfy the 2% GOS.


2% 9 15 9 There are not enough

switchable timeslots to reach 2% GOS


RecommendationThe following recommendation is offered when using switchable timeslots. It is important to determine the GOS objectives for circuit switched traffic and QoS objectives for GPRS traffic prior to selecting the number of switchable timeslots to deploy. During the circuit-switched-busy-hour, potentially all switchable timeslots are occasionally used by the circuit switched calls. The circuit switched timeslot allocation mechanism continues to assign switchable timeslots as circuit switched timeslots as the circuit switched traffic continues to increase. Therefore, if there is a minimum capacity requirement for GPRS services, the network planner should plan the GPRS carrier with enough reserved timeslots in order to handle the expected GPRS data traffic. This ensures that there is a minimum guaranteed network capacity for the GPRS data traffic during the circuit switched busy hour. During the circuit-switched-off-busy-hours, the switchable timeslots could be considered as available for use by the GPRS network. Therefore, in the circuit switched off busy hours potentially all switchable timeslots could be available for the GPRS network traffic. The BSS call statistics should be inspected to determine the actual use of the switchable timeslots by the circuit switched services. The circuit-switched-busy-hour and the GPRS-busy-hour should be monitored to see if they overlap when switchable timeslots are in use. If the busy hours overlap, an adjustment may be needed to the number of reserved timeslots allocated to the GPRS portion of the network in order to guarantee a minimum GPRS quality of service as measured by GPRS throughput and delay. Furthermore, one or more circuit switched carriers may need to be added to the cell being planned or replanned so that the switchable timeslots are not required in order to offer the desired circuit switched grade of service. In conclusion, assume switchable timeslots are occasionally unavailable for GPRS traffic during the circuit switched portion of the network busy hour. Provision enough reserved timeslots for GPRS traffic during the circuit switched busy hour to meet the desired minimum GPRS quality of service objectives, as measured by GPRS data throughput. The following step-wise process is proposed when determining how best to allocate GPRS carrier timeslots. AssumptionsThe process assumptions are:

A GPRS carrier can be added to a cell in addition to circuit switched carriers. A circuit switched carrier can be used to provide the control channels (BCCH/CCCH/SDCCH)

on one or more timeslots as needed. The number of circuit switched timeslots are determined as part of the BSS planning effort

prior to the GPRS planning effort. When the concentration-at-BTS feature is enabled, a sufficient pool of reserved backing

resources is provisioned in support of the number of reserved GPRS timeslots in order to meet the GPRS QoS objectives.

Step 1Determine how many reserved GPRS timeslots are needed on a per-cell basis in order to satisfy a GPRS throughput QoS. The GPRS reserved timeslots should equal the sum of the active and standby timeslots that are allocated to a carrier. Step 2If there are any timeslots left on the GPRS carrier after step 1, consider using them as switchable timeslots. The use of switchable timeslots can potentially offer increased capacity to both the GPRS and circuit switched traffic if the traffic is staggered in time. Step 3If there is a need to use some timeslots on the GPRS carrier to satisfy the circuit switched GOS objectives and the timeslot requirement overlaps with the number of reserved GPRS timeslots, consider adding another circuit switched carrier to the cell. Step 4

After deploying the GPRS carrier, review the network statistics listed in the Network statistics section on a continuous basis in order to determine whether the reserved GPRS timeslots, switchable GPRS timeslots, and circuit switched timeslots are truly serving the GOS and QoS objectives. As previously discussed, the use of switchable timeslots can offer network capacity advantages to both circuit switched traffic and GPRS traffic as long as the demand for these timeslots is staggered in time. GPRS Air interface planning processOverview of the GPRS air interface planning process structureThe air interface planning process is documented as follows:

Introduction to the air interface planning process. Air interface interface throughput. Throughput estimation process: step 1. Throughput estimation process: step 2. Throughput estimation process: step 3. Throughput estimation process: step 4.

Introduction to the GPRS air interface planning processThe air interface planning process uses the range of values listed in Table 3-8 to Table 3-13. If network values are not available at the time a network is planned, typical or recommended values are provided where appropriate. The minimum values are given for the maximum capacity of a minimum system, and the typical values are used as standard model parameters.

Table 3-8 Air interface planning inputs (part A)

Variable Minimum value

Typical value

Maximum value Assumptions/ variable use

CS rate ratio, CS-1/CS-2

Approx. 0 % 10% 100 %

CS rate ratio is determined by the Cell plan, mean TBF size and use of Acknowledge mode. Refer to cell plan tables: Table 3-14, Table 3-15 and Table 3-16.

V.42 bis compression ratio

1 2.5 4

A ratio of 1 means there is no compression and a ratio of 4 is the theoretical maximum, which is most likely never realized. Most users see a compression advantage in the range of 2-to-3 over the air interface between the MS and the SGSN. The compression ratio is used in Equation 3.

The air interface planning process uses the range of values listed in Table 3-8 to Table 3-13.

Table 3-9 Air interface planning inputs (part B)

Variable Minimum value Typical value Maximum

value Assumptions/ variable use

BLER 0 10% 100%

The BLock Error Rate (BLER) is largely determined by the cell RF plan. The typical value is an average rate. There are separate BLERs for CS-1 and CS-2 rates that are RF plan specific.

FTD 0.7 second

3 seconds for a 3 kbyte file, subject to network load and multislot operation.

File size dependent

This is the File Transit Delay (FTD) objective measured in seconds from the Um interface to the Gi interface. The minimum delay is the approximate delay for a RLC block of 23 bytes or less, which is the minimum system limit with only one user on the system. The FTD value is determined by Equation 4.

The number of 0 Network 8 This number can represent reserved

GPRS timeslots per cell dependent and/or switchable timeslots as explained

from Figure 3-7 to Figure 3-14.

Number of active GPRS timeslots per PCU with redundancy

30 Network dependent 240

This is the number of timeslots simultaneously in use with N+1 redundancy. This number is used to calculate the number of PRP and PICP boards to equip at the PCU using the PCU planning rules tabled in Chapter 5.


Table 3-10 Air interface planning inputs (part C)


Typical value


Number of GPRS users monitored at the PCU with redundancy


This is the number of mobiles that can be monitored in addition to the mobiles actually using timeslots. This value reflects N+1 redundancy. This number reflects the coverage capability of the PCU.

Number of active GPRS timeslots per PCU without redundancy


This is the number of timeslots simultaneously in use without N+1 redundancy. This number is used to calculate the number of PRP and PICP boards to equip at the PCU using the PCU planning rules tabled in Chapter 5.

Number of GPRS users monitored at the PCU without redundancy


This is the number of mobiles that can be monitored in addition to the mobiles actually using timeslots without N+1 redundancy. This number reflects the coverage capability of the PCU.


Table 3-11 Air interface planning inputs (part D)


Typical value Maximum value Assumptions/ variable use

Mean LLC PDU packet size (bytes)

20 435 1,580 This parameter is used in determining the cell and subscriber throughput capacities.

Data traffic /subscriber (peak)

0 98 kbytes/hour

No maximum limit other than what the network is provisioned to support.

This parameter is the expected GPRS load of a subscriber. This figure should include the SMS traffic carried as GPRS data.


Table 3-12 Air interface planning inputs (part E)



Total number of GPRS pages per attached subscriber

0 0.6 No maximum limit other than what the network is provisioned to support.

This effects the signalling traffic load over the SGSN-to-PCU (Gb) interface, the PCU-to-BSC interface(GSL), and the BSC-to-BTS interface(RSL). The GPRS paging traffic must be added to the circuit switched signalling traffic at the BSC in order to determine the total signalling traffic between

the BSC and reporting BTSs. This parameter is also used to determine the GPRS load on the CCCH.

Number of data transfers per hour per subscriber

0

No maximum limit other than what the network is provisioned to support

This number is used to determine the provisioning of the control channels (CCCH provisioning).

Number of BSCs supporting GPRS per OMC-R serving area


This establishes how many PCUs are required per OMC-R serving area. The size of the PCU is determined from the GPRS subscriber profile. (Provision 1 PCU per BSC.)


Table 3-13 Air interface planning inputs (part F)


Typical value


Equipment redundancy (BSS PCU & GSN)

No Yes More equipment can be deployed when redundancy is desired.

E1 redundancy No Yes

Extra E1 lines are deployed for GSL, GDS, GBL, and Gi links when redundancy is desired. The extra E1 lines provide logical redundancy because the traffic is load shared over the redundant links.

Air interface throughputThe GPRS data throughput estimation process given in this planning guide is based upon the Poisson process for determining the GPRS mobile packet transfer arrivals to the network and for determining the size of GPRS data packets generated or received by the GPRS mobiles. A number of wired LAN/WAN traffic studies have shown that packet interarrival rates are not exponentially distributed. Recent work argues that LAN traffic is much better modelled using statistically self-similar processes instead of Poisson or Markovian processes. Self-similar traffic pattern means the interarrival rates appear the same regardless of the timescale at which it is viewed (in contrast to Poisson process, which tends to be smoothed around the mean in a larger time scale). The exact nature of wireless GPRS traffic pattern is not known due to lack of field data. In order to minimize the negative impact of underestimating the nature of the GPRS traffic, it is proposed in this planning guide to limit the mean GPRS cell loading value to 50% of the system capacity. Using this cell loading factor has the following advantages:

Cell overloading due to the bursty nature of GPRS traffic is minimized. The variance in file transit delay over the Um-to-Gi interface is minimized such that the delay

can be considered a constant value for the purposes of calculating the time to transfer a file of arbitrary size.

LAN/WAN wireline studies have also shown that even when statistically valid studies are performed, the results come out very different in follow-up studies. It turns out that web traffic patterns are very difficult to predict accurately and, therefore, it is highly recommended that the network planner makes routine use of the GPRS network statistics. About the stepsThe following steps 1 and 2 are used for dimensioning the system. Step 1 needs to be performed prior to step 2 in order to calculate the number of GPRS timeslots that should be provisioned on a per cell basis.

Steps 3 and 4 are optional. These steps are included in this section so that an over-the-air file transfer time can be calculated for any size file. The results from steps 3 and 4 are dependent upon the choices made in steps 1 and 2. Step 1: throughput estimation processChoose a cell plan in order to determine the expected BLER and percentage of time data is transferred at the CS-1 rate and at the CS-2 rate. The cell plan that is chosen for GPRS may be determined by the plan currently in use for the GSM circuit switched portion of the network. However, it may be necessary to change an existing cell plan used for GSM circuit switched in order to get better BLER performance for the GPRS portion of the network. After the cell plan is chosen, the network planner can move on to step 2. The PCU dynamically selects the best CS-1 or CS-2 rate in order to maximize the GPRS data throughput on a per mobile basis. The CS-1 and CS-2 rate selection is performed periodically during the TBF. Simulations were performed (see Impact of the Radio Interface on GPRS System Dimensioning - a Simulation Study, Draft 0.1 of June 1999) for two typical frequency hopping cell configurations; results for a 1x3 cell reuse pattern with 2/6 hopping are shown in Table 3-14 (which is hopping on 2 carriers over 6 frequencies) and results for a 1x1 cell reuse pattern with 2/18 hopping are shown in Table 3-15 (which is hopping on 2 carriers over 18 frequencies). Results for a non-hopping cell configuration with a TU-3 model is shown in Table 3-17 provide a chart of the cell coverage area and cell C/I performance for the non-hopping case. The following tables were created, based on the simulations, in order to indicate the percentage of the time the CS-2 rate would be chosen over the CS-1 rate and at what mean BLER. The simulation results indicate that the higher data rate of the CS-2 more than offsets its higher BLER rate in the majority of the cell coverage area, resulting in the CS-2 rate being chosen most of the time. Reviewing the following tables it can be seen that under good cell C/I conditions, better throughput may be obtained by provisioning the GPRS timeslots on the BCCH carrier as indicated by Table 3-16.

Table 3-14 1 x 3 2/6 hopping

Parameter CS-1 rate CS-2 rate

% Rate chosen 10 90

% Mean BLER 50 20



% Rate chosen 10 90

% Mean BLER 56 14

Table 3-16 Non-hopping TU-3 model


% Rate chosen 0 100

% Mean BLER 10 3

Table 3-17 provides the cell C/I performance, as measured in dBs, as a function of cell area coverage for the TU-3 model.

Table 3-17 Cell coverage versus carrier-to-interface (C/I)

% cell coverage 90 80 70 60 50 40

C/I 12 16 18 20 22 24

The cell plans assume a regular cell reuse pattern for the geographic layout and for the allocation of frequencies. The computer simulation generated the above cell plan using a typical urban 3 kph model, a propagation law with a Radius (R) exponent of -3.7 and a shadowing function standard deviation of 5 db.

If non-regular patterns are used, a specific simulation study may be required to match the particular cell characteristics. The simulation process is outside the scope of this planning guide and the network planner should contact Motorola for additional simulation results. Step 2: throughput estimation processStep 2 determines the number of GPRS timeslots that need to be provisioned on a per cell basis. Timeslot provisioning is based on the expected per-cell mean GPRS traffic load, as measured in kbit/s. The GPRS traffic load includes all SMS traffic routed through the GSN. The SMS traffic is handled by the GPRS infrastructure in the same manner as all other GPRS traffic originating from the PDN. The cell BLER and CS rate characteristics chosen in step 1 provide the needed information for evaluating the following Equation 1. Equation 1Equation 2

Where: is:

Mean_trf_ld The mean traffic load, as measured in kbit/s, is defined at the LLC layer therefore all the higher layer protocol overheads (for example, TCP, UDP, IP, SNDCP, LLC) are encapsulated in this load figure.

Denom_1 Denominator 1 is used in Equation 1.

PDCH The number of timeslots per cell, maximum 8.

%CS1 The percent of time data transmission occurs using the CS-1 coding scheme.

CS1BLER The mean BLER rate for CS-1.

%C2S The percent of time data transmission occurs using the CS-2 coding scheme.


3/23 The CS-1 RLC/MAC overhead percentage, that is 20 bytes payload.


Mean_ld_f The mean load factor for the number of active timeslots to provision at a cell. The recommended value is 50% of the number GPRS timeslots provisioned at a cell.

TBF SETUP REL factor

TBF SETUP and Release Factor. The recommended value 0.45. This factor is an interim solution whilst the Overlapping TPF feature is being completed.

The number of PDCH timeslots calculated in Equation 1 includes the number of active timeslots and the number of standby timeslots. The Mean_load_factor of 50% determines the ratio of active timeslots to standby timeslots. For example, if Equation 1 evaluated to 8 timeslots, 4 timeslots would be counted as active timeslots and 4 timeslots as standby timeslots. It is important to differentiate between the required number of active timeslots and the required number of standby timeslots because it directly effects the provisioning of the communication links and the PCU hardware. The active timeslots are timeslots that are simultaneously carrying data. The standby timeslots are timeslots that are being monitored by the PCU for an uplink or downlink timeslot request. A request on a standby timeslot for an active timeslot is granted for an active timeslot as soon as one becomes available at the PCU. For example, when the PCU is provisioned to handle 30 active timeslots and all of them are in use, at least one of these 30 active timeslots must become available in order to move a standby timeslot to active state. The use of active timeslots and standby timeslots enables several cells to share the PCU resource. While one cell is experiencing a high load condition, using all eight GPRS timeslots for instance, another cell operating below its mean load averages out the GPRS traffic load at the PCU. The E1s between the BTS and BSC must be provisioned to handle the number of timeslots calculated in Equation 1 because all of the timeslots can become active under high load conditions. Throughput estimation process: step 3 (optional)Step 3 is optional, and the results can be used in optional step 4. Step 3 is intended to be used as an aid in determining the size of a file that is to be transferred as an LLC PDU from the mobile to the SGSN, by using Equation 3. The file size consists of the application file to be transferred, which includes any application-related overhead. In addition to the application file, there is transport and network layer protocol overhead, TCP and IP. Finally, there is GPRS Link Layer Control (LLC) and SubNetwork Convergence (SNDCP) protocol overhead. The application file plus all of the protocol overhead summed together makes up the one or more LLC_PDU frames that constitute the file to be transferred.

The percentage of protocol overhead depends on the transport layer used, such as TCP or UDP. For example, the TCP/IP protocol overhead is 40 bytes when TCP/IP header compression is not used. When TCP/IP header compression is used, the TCP/IP header can be reduced to 5 bytes from 40 bytes after the first LLC frame is transferred. The use of header compression continues for as long as the IP address remains the same. Figure 3-15 illustrates the typical LLC_PDU frame with the user application payload and all of the protocol overhead combined for the case of no TCP/IP header compression.

Figure 3-15 LLC PDU layoutIf V.42bis application data compression is used, the effective file size for transmission is reduced by the data compression factor which can range from 1 to 4. Typically, V.42bis yields a 2.5 compression advantage on a text file, and close to no compression advantage (factor=1) on image files and very short files. Equation 3Where: File_size_LLC is: The files size in bytes to be transferred measured at the LLC layer.

Appln The user application data file size measured in bytes.

LLC_payload The maximum LLC PDU payload of 1527 bytes.

protocol_overhead The protocol overhead for TCP/IP/SNDCP/LLC/CRC is 53 bytes without header compression and 18 bytes with header compression.

V.42bis_factor Application data compression is over the range of 1 to 4, a typical value is equal to 2.5.

ExampleA 3 kbytes application file transfer requires the following number of bytes to be transferred at the LLC_PDU layer: Application= 3 kbytes Assume V.42bis_factor = 1, that is no application data compression No header compression: File_size_LLC = 3000 + roundup (3000/1527) x 53 = 3106 bytes With header compression: The first LLC_PDU the header is not compressed, and all subsequent LLC_PDUs are compressed. For this size file of 3000 bytes, only 2 LLC_PDU transmissions are required so the File_size_LLC is: File_size_LLC = 3000 + 53+18 = 3071 bytes

Throughput estimation process: step 4 (optional)The network planner can use step 4 to determine how long it takes to transfer a file of an arbitrary size over the Um-to-Gi interface. The application file is segmented into LLC PDU frames as illustrated previously. The File Transit Delay (FTD) is calculated in Equation 4 by using the following information: total number of RLC blocks of the file, BLER, number of timeslots used during the transfer, and mean RLC Transit Delay (RTD) value. Equation 4 does not include the effects of acknowledgement messages. The reason is that the largest effect is in the uplink direction, and it is expected that the downlink direction will dominate the cell traffic. The DL sends an acknowledgement message on an as-needed basis, whereas the uplink generates an acknowledgement message every 2 out of 12 RLC_Blocks. It is expected that the downlink acknowledgement messages will not significantly effect the file transit delay in the downlink direction. Equation 4Where: FTD is: The file transit delay measured in seconds.

RTD

This is the transit delay time from the Um interface to the Gi interface for a file size of only 1 RLC/MAC block of data. RTD is estimated to be 0.9 seconds when system running at 50% capacity. This parameter will be updated when field test data is available.

RLC_Blocks This is the total number of RLC blocks of the file. This can be calculated by dividing file_size_LLC by the corresponding RLC data size of 20 bytes for CS-1 and 30 bytes for CS-2.

mslot This is the mobile multislot operating mode; the value can be from 1 to 4.

CSBLER This is the BLER for the specific CS rate. The value is specified in decimal form. Typical values range form 0.1 to 0.2.

The RTD parameter is directly correlated to the system utilization and the mean packet size. When the cell approaches its throughput capacity limit, the RTD value increases dramatically, and the infrastructure starts to drop packets. Simulation data indicates that when traffic load is minimal, the RTD value is at a minimum limit of 0.7 seconds. At a cell throughput capacity of 50%, the RTD increases to 0.9 seconds. It is recommended that cell throughput provisioning be performed at the mean cell capacity level of 50%. Provisioning for a mean cell throughput greater than 50% greatly increases the likelihood of dropped packets, and RTD values of over 2.6 seconds can occur. The assumptions used in the simulation to determine the RTD value at a mean cell throughput level of 50% are: 25% of the cell traffic at the CS-1 rate and 75% of the cell traffic at the CS-2 rate, BLER 10%, mobiles multislot distribution 1:2:3:4 = 20:50:20:10, 8 PDCH, DL, mean LLC_PDU packet size of 435 bytes. For example, a 3 kbyte application file transit time at the CS-2 rate, using one timeslot, BLER = 10%, and no header or V.42 bis compression is: 3 Kbyte file transit time over Um-to-Gi interface = 0.9 + Roundup (3106/30) x 0.02 x 1.1 / 1 = 3.2 seconds

Where: File_size_LLC is: = 3106 bytes (as calculated in the previous example)

CS-2 payload = 30 bytes

Air time for one RLC/MAC block = 0.02 seconds

(1+CSBLER) = 1.1

Multislot operation = 1 Propagation effects on GSM frequencies Propagation productionMost of the methods used to predict propagation over irregular terrain are actually terrain based, since they are designed to compute the diffraction loss and free space loss based upon the path profile between the transmitter and the receiver. A widely used technique in the United Kingdom is the prediction method used by the Joint Radio Committee of the Nationalized Power Industries (JRC). This method utilizes a computerized topographical map in a data base, providing some 800,000 height reference points at 0.5 km intervals covering the whole of the UK. The computer predicts the received signal level by constructing the ground path profile between the transmitter and receiver using the data base. The computer then tests the path profile for a line of sight path and whether Fresnel-zone clearance is obtained over the path. The free space and plane earth propagation losses are calculated and the higher value is chosen. If the line of sight and Fresnel-zone clearance test fails, then the programme evaluates the loss caused by any obstructions and grades them into single or multiple diffraction edges. However, this method fails to take any buildings into account when performing its calculation, the calculations are totally based upon the terrain features. Although the use of topographical based calculations are useful when designing mobile communication systems, most mobile systems are centred around urban environments. In these urban environments, the path between transmitter and the receiver maybe blocked by a number of obstacles (for example; buildings), so it is necessary to resort to approximate methods of calculating diffraction losses since exact calculations for each obstacle then become extremely difficult.

Introduction to decibels Decibels are used to express power output levels, receiver input levels and path losses. The reason they are used is to simplify the calculations used when planning radio systems. Any number maybe expressed as a decibel (dB). The only requirement is that the original description and scale of unity is appended to the dB, so indicating a value which can be used when adding , subtracting, or converting dBs. For example for a given power of 1 mW it may be expressed as 0 dBmW, the mW refers to the fact that the original scale of measurement was in thousandths of a watt. For a power of 1 W the equivalent in dBs is 0 dBW. The decibel scale is logarithmic and this allows very large or very small numbers to be more easily expressed and calculated. For example take a power of 20 watts transmitted from a BTS which was .000000001 W at the receiver. It is very difficult to accurately express the total power loss in a simple way.

By converting both figures to decibels referenced to 1 mW, 20 W becomes 32 dBmW and .000000001 W is -60 dBmW. The path loss can now be expressed as 92 dBmW. Multiplication and division also become easier when using dBs. For figures expressed as dBs to multiply them together simply add the db figures together. This is the equivalent in decimal of multiplying. For division simply take one dB figure from the other. Another example is for every doubling of power figures the increase in dBs is 3 dB and for every halving of power the decrease is 3 dB. Table 3-18 gives examples of dB conversions.

Table 3-18 dBmW and dBW to Power conversion

dBmW dBW Power dBmW dBW Power

+ 59 29 800 W + 7 - 23 5 mW

+ 56 26 400 W + 4 - 26 2.5 mW

+ 53 23 200 W + 1 - 29 1.25 mW

+ 50 20 100 W 0 - 30 1 mW

+ 49 19 80 W - 3 - 33 0.5 mW

+ 46 16 40 W - 6 - 36 0.25 mW

+ 43 13 20 W - 9 - 39 0.125 mW

+ 40 10 10 W - 10 - 40 0.1 mW

+ 39 9 8 W - 20 - 50 0.01 mW

+ 36 6 4 W - 30 - 60 1 mW

+ 33 3 2 W - 40 - 70 0.1 mW

+ 30 0 1 W - 50 - 80 0.01 mW

+ 27 - 3 500 mW - 60 - 90 1 nW

+ 24 - 6 250 mW -70 -100 0.1 nW

+ 21 - 9 125 mW - 80 - 110 0.01 nW

+ 20 - 10 100 mW - 90 - 120 1 pW

+ 17 - 13 50 mW - 100 - 130 0.1 pW

+ 14 - 16 25 mW -103 - 133 0.01 pW

+ 11 - 19 12.5 mW - 106 - 136 0.001 pW

+ 10 - 20 10 mW

Fresnel zone The Fresnel (pronounced Fresnel) actually consists of several different zones, each one forming an ellipsoid around the major axis of the direct propagation path. Each zone describes a specific area depending on the wavelength of the signal frequency. If a signal from that zone is reflected of an obstacle which protrudes into the zone, it means that a reflected signal as well as the direct path signal will arrive at the receiver. Radio waves reflected in the first Fresnel zone will arrive at the receiver out of phase with those taking the direct path and so combine destructively. This results in a very low received signal strength. It is important when planning a cell to consider all the radio paths for obstacles which may produce reflections from the first Fresnel zone because if they exist it is like planning permanent areas of no coverage in certain parts of the cell. In order to calculate wether or not this condition exists the radius of the first Fresnel zone at the point where the object is suspected of intruding into the zone must be calculated. The formula, illustrated in Figure 3-16, is as follows:

Where: F1 is: the first Fresnel zone.

d1 distance from Tx antenna to the obstacle.

d2 distance from Rx antenna to the obstacle.

l wavelength of the carrier wave.

d total path length. Once the cell coverage has been calculated the radio path can be checked for any objects intruding into the first Fresnel zone. Ideally the link should be planned for no intrusions but in some cases they are unavoidable. If that is the case then the next best clearance for the first Fresnel zone is 0.6 of the radius. When siting a BTS on top of a building care must be taken with the positioning and height of the antenna to ensure that the roof edge of the building does not intrude into the first Fresnel zone.

Figure 3-16 First Fresnel zone radius calculationRadio refractive index It is important when planning a cell or microwave radio link to have an understanding of the effects changes in the Radio Refractive Index (RRI) can have on microwave communications, also what causes these changes. RRI measurements provide planners with information on how much a radio wave will be refracted by the atmosphere at various heights above sea level. Refraction, Figure 3-17, is the changing of direction of propagation of the radio wave as it passes from a more dense layer of the atmosphere to a less dense layer, which is usual as one increases in height above sea level. It also occurs when passing from a less dense layer to a dense layer. This may also occur under certain conditions even at higher altitudes.

Figure 3-17 RefractionThe main effect to cell planners is that changes in the RRI can increase or decrease the cell radius depending on conditions prevailing at the time. The RRI is normally referenced to a value n at sea level. The value will vary with seasons and location but for the UK the mean value is 1.00034. This figure is very cumbersome to work with so convention has converted n to N.

Where: N is: (n-1) x 10 to the power of 6. The value of N now becomes 340 units for the UK. The actual seasonal and global variations are only a few 10 s of units at sea level. The value of N is influenced by the following :

The proportion of principle gasses in the atmosphere such as nitrogen, oxygen, carbon dioxide, and rare gasses. These maintain a near constant relationship as height is increased so although they affect the RRI the affect does not vary.

The quantity of water vapour in the atmosphere. This is extremely variable and has significant effects on the RRI.

Finally the temperature, pressure, and water vapour pressure have major effects on the RRI. All the above will either increase or decrease the RRI depending on local conditions, resulting in more or less refraction of a radio wave. Typically though for a well mixed atmosphere the RRI will fall by 40 N units per 1 km increase in height above sea level. Measurement of the RRIThere are two main ways of measuring the RRI at any moment in time. Firstly by use of Radio Sonds. This is an instrument which is released into the atmosphere, normally attached to a balloon. As it rises it measures the temperature, pressure, and humidity. These are transmitted back to the ground station with a suitable reference value. The measurements of pressure are made every 35 m, humidity every 25 m, and temperature every 10 m. These together provide a relatively crude picture of what the value of the RRI is over a range of heights. The second method is a more serious means of measuring the RRI. It uses fast response devises called refractometers. These maybe carried by a balloon , aircraft, or be spaced apart on a high tower. These instruments are based upon the change in resonant frequency of a cavity with partially open ends caused by the change in RRI of air passing through the cavity. This gives a finer measurement showing variations in the RRI over height differences of a little over one meter. This is illustrated by the graph in Figure 3-18. The aircraft mounted refractometer can give a detailed study over several paths and heights.

Figure 3-18 Measurement of the RRIEffects of deviations from the normal lapse rate

The lapse rate of 40 N per km is based on clear sky readings with good atmosphere mixing. Normally a radio system is calibrated during these conditions and the height alignment in the case of a microwave point to point link is determined. It is easier to see the effects on a microwave point to point system when examining the effects of uneven variations of the RRI. Figure 3-19A shows an exaggerated curved radio path between two antennas under normal conditions. The signal is refracted by the atmosphere and arrives at the receiving antenna. Figure 3-19B illustrates the condition known as super refraction where the radio waves are not diffracted enough. This occurs when the lapse rate is less than 40 N per km. Under these conditions the main signal path will miss the receive antenna. Similar effects on a cell would increase the cell size as the radio waves would be propagated further resulting in co-channel and adjacent channel interference. The second effect is where the RRI increases greater than 40 N per km. This results in the path being refracted too much and not arriving at the receive antenna. This condition is known as sub-refraction. While this will not cause any interference as with super refraction, it could result in areas of no coverage. See Figure 3-19C. The last effect is known as ducting and occurs when the refraction of the radio wave produces a path which matches the curvature of the Earth. If this happens radio waves are propagated over far greater distances than normal and can produce interference in places not normally subjected to any.

Figure 3-19 Effects on a microwave systemEvents which can modify the clear sky lapse rateThere are four main events which can modify the clear sky lapse rate and they are as follows: Radiation nightsThis is the result of a very sunny day followed by clear skies overnight. The Earth absorbs heat during the day and the air temperature rises. After sunset the Earth radiates heat into the atmosphere and its surface temperature drops. This heat loss is not replaced resulting in air closer to the surface cooling faster than air higher up. This condition causes a temperature inversion and the RRI profile no longer has a uniform lapse rate. This effect will only occur overland and not water as water temperature variations are over a longer period of time. Advection effectsThis effect is caused by high pressure weather fronts moving from land to the sea or other large expanses of water. The result is warm air from the high pressure front covering the relatively cool air of the water. When this combination is then blown back over land a temperature inversion is caused by the trapped cool air. It will persist until the air mass strikes high ground where the increase in height will mix and dissipate the inversion. SubsidenceThis occurs again in a high pressure system this time overland when air descending from high altitude is heated by compression as it descends. This heated air then spreads over the cooler air below. This type of temperature inversion normally occurs at an altitude of 1 km but may occasionally drop to 100 m where it cause severe disruption to radio signals. Frontal systemsThis happens when a cold front approaching an area forces a wedge of cold air under the warmer air causing a temperature inversion. These disturbances tend to be short lived as the cold front usually dissipate quickly. Although those described above are the four main causes of RRI deviations, local pressure, humidity and temperature conditions could well give rise to events which will affect the RRI.

Environmental effects on propagation At the frequency range used for GSM it is important to consider the effects that objects in the path of the radio wave will have on it. As the wave length is approximately 30 cm for GSM900 and 15 cm for GSM1800, most objects in the path will have some effect on the signal. Such things as vehicles, buildings, office fittings even people and animals will all affect the radio wave in one way or another. The main effects can be summarized as follows:

Attenuation. Reflection. Scattering. Diffraction. Polarization changes.

Attenuation

This will be caused by any object obstructing the wave path causing absorption of the signal. The effects are quite significant at GSM frequencies but still depend on the type of materials and dimensions of the object in relation to the wavelength used. Buildings, trees and people will all cause the signal to be attenuated by varying degrees.

Figure 3-20 AttenuationReflectionThis is caused when the radio wave strikes a relatively smooth conducting surface. The wave is reflected at the same angle at which it arrived. The strength of the reflected signal depends on how well the reflector conducts. The greater the conductivity the stronger the reflected wave. This explains why sea water is a better reflector than sand.

Figure 3-21 ReflectionScatteringThis occurs when a wave reflects of a rough surface. The rougher the surface and the relationship between the size of the objects and the wave length will determine the amount of scattering that occurs.

Figure 3-22 ScatteringDiffractionDiffraction is where a radio wave is bent off its normal path. This happens when the radio wave passes over an edge, such as that of a building roof or at street level that of a corner of a building. The amount of diffraction that takes place increases as the frequency used is increased. Diffraction can be a good thing as it allows radio signals to reach areas where they would not normally be propagated.

Figure 3-23 DiffractionPolarization changesThis can happen any time with any of the above effects of due to atmospheric conditions and geomagnetic effects such as the solar wind striking the earths atmosphere. These polarisation changes mean that a signal may arrive at the receiver with a different polarisation than that which the antenna has been designed to accept. If this occurs the received signal will be greatly attenuated by the antenna.

Figure 3-24 PolarizationMultipath propagation Rayleigh and Rician fadingAs a result of the propagation effects on the transmitted signal the receiver will pick up the same signal which has been reflected from many different objects resulting in what is known as multipath reception. The signals arriving from the different paths will all have travelled different distances and will therefore arrive at the receiver at different times with different signal strengths. Because of the reception time difference the signals may or may not be in phase with each other. The result is that some will combine constructively resulting in a gain of signal strength while others will combine destructively resulting in a loss of signal strength. The receiving antenna does not have to be moved very far for the signal strength to vary by many tens of dBs. For GSM900 a move of just 15 cm or half a wavelength will suffice to observe a change in signal strength. This effect is known as multipath fading. It is typically experienced in urban areas where there are lots of buildings and the only signals received are from reflections and refractions of the original signal. Rayleigh environmentThis type of environment has been described by Rayleigh. He analysed the signal strength along a path with a moving receiver and plotted a graph of the typical signal strength measured due to multipath fading. The plot is specifically for non line of sight, Figure 3-25, and is known as Rayleigh distribution, Figure 3-26.

Figure 3-25 Propagation effect - Rayleigh fading environmentFigure 3-26 Rayleigh distribution

Rician environmentWhere the signal path is predominantly line of sight, Figure 3-27, with insignificant reflections of refractions arriving at the receiver, this is know as Rician distribution, Figure 3-28. There are still fades in signal strength but they rarely dip below the threshold below which they will not be processed by the receiver.

Figure 3-27 Propagation effect - Rician environmentFigure 3-28 Rician distribution

Comparison of DCS1800 and GSM900: From a pure frequency point of view it would be true to say that DCS1800 generally has more fades than GSM900. However, they are usually less pronounced. Receive signal strengthA moving vehicle in an urban environment seldom has a direct line of sight path to the base station. The propagation path contains many obstacles in the form of buildings, other structures and even other vehicles. Because there is no unique propagation path between transmitter and receiver, the instantaneous field strength at the MS and BTS exhibits a highly variable structure. The received signal at the mobile is the net result of many waves that arrive via multiple paths formed by diffraction and scattering. The amplitudes, phase and angle of arrival of the waves are random and the short term statistics of the resultant signal envelope approximate a Rayleigh distribution. Should a microcell be employed, where part of a cell coverage area be predominantly line of sight then Rician distribution will be exhibited. Free space lossThis is the lose of signal strength that occurs as the radio waves are propagated through free space. Free space is defined as the condition where there are no sources of reflection in the signal path. This is impossible to achieve in reality but it does give a good starting point for all propagation loss calculations. Equally important in establishing path losses is the effect that the devices radiating the signal have on the signal itself. As a basis for the calculation it is assumed the device is an isotopic radiator. This is a theoretical pin point antenna which radiates equally in every direction. If the device was placed in the middle of a sphere it would illuminated the entire inner surface with an equal field strength. In order to find out what the power is covering the sphere, the following formula used:

Where: Pt is: the input power to the isotopic antenna.

d the distance from the radiator to the surface of the sphere. This formula illustrates the inverse square law that the power decreases with the square of the distance. In order to work out the power received at a normal antenna the affective aperture (Ae) of the receiving antenna must be calculated. The actual received power can be calculated as follows: Now if P is substituted with the formula for the power received over the inner surface of a sphere and Ae with its formula the result is: Free space path lossThis is the ratio of the actual received power to the transmitted power from an isotopic radiator and can be calculated by the formula: Logs are used to to make the figures more manageable. Note that the formula is dependant on distance and frequency. The higher the frequency the shorter the wavelength and therefore the greater the path loss. The formula above is based on units measured in metres. To make the formula more convenient it can be modified to use kilometre and megahertz for the distance and frequency. It becomes:

Where: d is: the distance in km.

f the frequency in MHz. Plane earth lossThe free space loss as stated was based solely on a theoretical model and is of no use by itself when calculating the path loss in a multipath environment. To provide a more realistic model, the earth in its role as a reflector of signals must be taken into account. When calculating the plane earth loss the model assumes that the signal arriving at the receiver consists of a direct path component and a reflective path component. Together these are often called the space wave. The formula for calculating the plane earth loss is:

This takes into account the different antenna heights at the transmitter and receiver. Although this is still a simple representation of path loss. When this formula is used is implies the inverse fourth law as opposed to the inverse square law. So for every doubling of distance there is a 12 dB loss instead of 6 db with the free space loss calculation.

The final factors in path loss are the ground characteristics. These will increase the path loss even further depending on the type of terrain, refer to Figure 3-29. The ground characteristics can be divided into three groups:

1. Excellent ground. For example sea water, this provides the least attenuation so a lower path loss.

1. Good ground. For example rich agricultural land, moist loamy lowland and forests. 1. Poor ground. For example Industrial or Urban areas, rocky land. These give the highest

losses and are typically found when planning Urban cells. Figure 3-29 Plane earth loss

Clutter factorThe propagation of the RF signal in an urban area is influenced by the nature of the surrounding urban environment. An urban area can then be placed into two sub categories; the built up area and the suburban area. The built up area contains tall buildings, office blocks, and high-rise residential tower blocks, whilst a suburban area contains residential houses, playing fields and parks as the main features. Problems may arise in placing areas into one of these two categories, so two parameters are utilised, a land usage factor describing the percentage of the area covered by buildings and a 'degree of urbanization' factor describing the percentage of buildings above storeys in the area.

Where: B is: the clutter factor in dBs.

F the frequency of RF signal.

L the percentage of land within 500m square occupied by buildings.

H the difference in height between the squares containing the transmitter and receiver.

K 0.094U - 5.9

U the percentage of L occupied by buildings above 4 storeys. A good base station site should be high enough to clear all the surrounding obstacles in the immediate vicinity. However, it should be pointed out that employing high antennas increases the coverage area of the base station. However, it will also have adverse effects on channel re-use distances because of the increased possibility of co-channel interference. Antenna gainThe additional gain provided by an antenna can be used to enhance the distance that the radio wave is transmitted. Antenna gain is measured against an isotopic radiator. Any antenna has a gain over an isotopic radiator because in practice it is impossible to radiate the power equally in all directions. This means that in some directions the radiated power will be concentrated. This concentration or focusing of power is what enables the radio waves to travel further than those that if it were possible were radiated from an isotopic radiator. See Figure 3-30.

Figure 3-30 Focusing of powerThe gain of a directional antenna is measured by comparing the signal strength of a carrier emitted from an isotopic antenna and the directional antenna. First the power of the isotopic radiator is increased so that both receive levels are the same. The emitted powers required to achieve that are then compared for both antennas. The difference is a measure of gain experienced by the directional antenna. It will always have some gain when compared to an isotopic radiator. See example in Figure 3-31.

Figure 3-31 Measurement of gainIn this example to achieve a balanced receive level the isotopic radiator must have an input power of 1000 W as opposed to the directional antenna which only requires 10 W. The gain of the directional antenna is 100 or 20 dBi.

Where: i is: for isotopic. The more directional the antenna is made the more gain it will experience. This is apparent when sectorizing cells . Each sectored cell will require less transmit power than the equivalent range omni cell due to the gain of its directional antenna, typically 14 to 17 dBi. The gain is also present in the receive path though in all cases the gain decreases as the frequency increases. That is why the uplink mobile to BTS frequency is usually the lowest part of the frequency range. This gives a slight gain advantage to the lower power mobile transmitter. Propagation in buildings

With the increased use of hand portable equipment in mobile cellular systems combined with the increased availability of cordless telephones, it has become essential to study RF propagation into and within buildings. When calculating the propagation loss inside a building, Figure 3-32, a building loss factor is added to the RF path loss. This building loss factor is included in the model to account for the increase in attenuation of the received signal when the mobile is moved from outside to inside a building. This is fine if all users stood next to the walls of the building when making calls, but this does not happen, so the internal distance through which the signal must pass which has to be considered. Due to the internal construction of a building, the signal may suffer form spatial variations caused by the design of the interior of the building. The building loss tends to be defined as the difference in the median field intensity at the adjacent area just outside the building and the field intensity at a location on the main floor of the building. This location can be anywhere on the main floor. This produces a building median field intensity figure which is then used for plotting cell coverage areas and grade of service. When considering coverage in tall buildings, coverage is being considered throughout the building, if any floors of that building are above the height of the transmitting antenna a path gain will be experienced.

Figure 3-32 In building propagationThe Okumura methodIn the early 1960's a Japanese engineer named Okumura carried out a series of detailed propagation tests for land mobile radio services at various different frequencies. The frequencies were 200 MHz in the VHF band and 453, 922, 1310, 1430, and 1920 MHz in the UHF band. The results were statistically analyzed and described for distance and frequency dependencies of median field strength, location variabilities and antenna height gain factors for the base and mobile stations in urban, suburban, and open areas over quasi-smooth terrain. The correction factors corresponding to various terrain parameters for irregular terrain, such as rolling hills, isolated mountain areas, general sloped terrain, and mixed land/sea path were defined by Okumura. As a result of these tests carried out primarily in the Tokyo area, a method for predicting field strength and service area for a given terrain of a land mobile radio system was defined. The Okumura method is valid for the frequency range of 150 to 2000 MHz, for distances between the base station and the mobile stations of 1 to 100 km, with base station effective antenna heights of 30 to 100m. The results of the median field strength at the stated frequencies were displayed graphically. Different graphs were drawn for each of the test frequencies in each of the terrain environments (for example; urban, suburban, hilly terrain) Also shown on these graphs were the various antenna heights used at the test transmitter base stations. The graphs show the median field strength in relation to the distance in km from the site. As this is a graphical representation of results it does not transfer easily into a computer environment. However, the results provided by Okumura are the basis on which path loss prediction equations have been formulated. The most important work has been carried out by another Japanese engineer named Hata. Hata has taken Okumura's graphical results and derived an equation to calculate the path loss in various environments. These equations have been modified to take into account the differences between the Japanese terrain and the type of terrain experienced in Western Europe.

Figure 3-33 Okumura propagation graphsHata's propagation formulaHata used the information contained in Okumura's propagation loss report of the early 1960's, which presented its results graphically, to define a series of empirical formulas to allow propagation prediction to be done on computers. The propagation loss in an urban area can be presented as a simple formula of:

Where: A is: the frequency.

B the antenna height function.

R the distance from the transmitter.

Hata using this basic formula which is applicable to radio systems is the UHF and VHF frequency ranges, added an error factor to the basic formula to produce a series of equations to predict path loss. To facilitate this action Hata has set a series of limitations which must be observed when using this empirical calculation method:

Where: Frequency range (fc) is: 100 - 1500 MHz

Distance (R) 1 - 20 km

Base station antenna height (hb) 30 - 200 M

Vehicular antenna height (hm) 1 - 10 M Hata defined three basic formulas based upon three defined types of coverage area; urban, suburban and open. It should be noted that Hata's formula predicts the actual path loss, not the final signal strength at the receiver. Urban Area: Lp = 69.55 + 26.16 log10fc - 13.82.log10hb - a (hm)# + (44.9 - 6.66. log10hb).log10R dB

Where: # is: Correction factor for vehicular station antenna height. Medium - Small City:

a(hm) = (1.1 . log10fc - 0.7).hm - (1.56.log10fc - 0.8) Large City:

a(hm) = 3.2 (log10 11.75 hm)2 - 4.97 Where: fc is: >400 MHz.

Suburban Area: Lps = Lp [Urban Area] - 2.[log10 (f/28)]2 - 5.4 dB Rural Area: Lpr = Lp [Urban Area] - 4.78.(log10fc)2 + 18.33.log10fc - 40.94 dB Power budget and system balanceIn any two-way radio system the radio path losses and equipment output powers must be taken into account for both directions. This is especially true in a mobile network where there are different characteristics for the uplink and downlink paths. These include receive path diversity gain in the uplink only, the possibility of mast head amplifiers in the uplink path, the output power capability of the mobile is a lot less than that of the BTS and the sensitivity of the BTS receiver is usually better than the mobiles. If these differences are not considered it is possible that the BTS will have a service area far greater than that which the mobile will be able to use due to its limited output power. Therefore the path losses and output powers in the uplink and downlink must be carefully calculated to achieve a system balance. One where the power required of the mobile to achieve a given range is equitable to the range offered by the power transmitted by the BTS. The output powers of the BTS and mobile are unlikely to be the same for any given distances due to the differences in uplink and downlink path losses and gains as described above. Once the area of coverage for a site has been decided the calculations for the power budget can be made. The system balance is then calculated which will decide the output powers of the BTS and mobile to provide acceptable quality calls in the area of coverage of the BTS. The BTS power level must never be increased above the calculated level for system balance. Although this seems a simple way to increase coverage, the system balance will be different and the mobile may not be able to make a call in the new coverage area. To increase the cell coverage, an acceptable way is to increase the gain of the antenna. This will affect both the uplink and downlink therefore maintaining system balance. Where separate antennas are used for transmit and receive they must be of similar gain. If the cell size is to be reduced then this is not a problem as the BTS power can be altered and the mobiles output power is adaptive all the time. There is a statistic in the BTS that checks the path balance every 480 ms for each call in progress. The latest uplink and downlink figures reported along with the actual mobile and BTS transmit powers are used in a formula to give an indication of the path balance.

GSM900 path lossFigure 3-34 and Figure 3-35 compare the path losses at different heights for the BTS antenna and different locations of the mobile subscriber between one and 100 km cell radius.

Figure 3-34 BTS antenna height of 50 m, MS height of 1.5 m (GSM900)Figure 3-35 BTS antenna height of 100 m, MS height of 1.5 m (GSM900)

Path loss GSM900 vs DCS1800

Figure 3-36 illustrates the greater path loss experienced by the higher DCS1800 frequency range compared to the GSM900 band. The cell size is typical of that found in urban or suburban locations. The difference in path loss for the GSM900 band at 0.2 km compared with 3 km is 40 dB, a resultant loss factor of 10,000 compared to the measurement at 0.2 km.

Figure 3-36 Path loss vs cell radius for small cellsFrequency re-use Introduction to re-use patternsThe network planner designs the cellular network around the available carriers or frequency channels. The frequency channels will be allocated to the network provider from the GSM, EGSM, or DCS1800 band as shown below: Within this range of frequencies only a finite number of channels may be allocated to the planner. The number of channels will not necessarily cover the full frequency spectrum and there has to be great care taken when selecting/allocating the channels. Installing a greater number of cells will provide greater spectral efficiency with more frequency re-use of available frequencies. However, a balance must be struck between spectral efficiency and all the costs of the cell. The size of cells will also indicate how the frequency spectrum is used. Maximum cell radius is determined in part by the output power of the mobile subscriber (MS) (and therefore, its range) and interference caused by adjacent cells. Remember that the output power of the MS is limited in both the GSM900 and DCS1800 systems. Therefore to plan a balanced transmit and receive radio path the planner must make use of the path loss and thus the link budget. The effective range of a cell will vary according to location, and can be as much as 35 km in rural areas and as little as 1 km in a dense urban environment.

Figure 3-37 Frequency re-useRe-use patternThe total number of radio frequencies allocated is split into a number of channel groups or sets. These channel groups are assigned on a per cell basis in a regular pattern which repeats across all of the cells. Thus, each channel set may be re-used many times throughout the coverage area, giving rise to a particular re-use pattern (for example; 7 cell re-use pattern, Figure 3-38).

Figure 3-38 7 cell re-use patternClearly, as the number of channel sets increases, the number of available channels per cell reduces and therefore the system capacity falls. However, as the number of channel sets increases, the distance between co-channel cells also increases, thus the interference reduces. Selecting the optimum number of channel sets is therefore a compromise between quality and capacity. 4 site - 3 cell re-use patternDue to this increase in frequency robustness within GSM, different re-use frequency patterns can be adopted, which gives an overall greater frequency efficiency. The most common re-use pattern is 4 site with 3 cells. With the available frequency allocation divided into 12 channels sets numbered a1-3, b1-3, c1-3, and d1-3. The re-use pattern is arranged so that the minimum re-use distance between cells is at least 2 to 1. The other main advantage of this re-use pattern is if a new cell is required to be inserted in the network, then there is always a frequency channel set available which will not cause any adjacent channel interference.

Figure 3-39 4 site - 3 cell re-use pattern2 site - 6 cell re-use patternAnother solution to possible network operators capacity problems may be an even higher frequency re-use pattern. The re-use pattern, shown in Figure 3-40, uses a 2 site - 6 cell re-use. Therefore: 2 sites repeated each with 6 cells = 2 x 6 = 12 groups. If the operator has only 24 carriers allocated for their use, they are still in a position to use 2 carriers per cell. However this may be extremely difficult and may not be possible to implement. It also may not be possible due to the current network configuration. However, the subscribers per km ratio would be improved.

Figure 3-40 2 site - 6 cell re-use patternCarrier/ Interference (C/I) ratioWhen a channel is re-used there is a risk of co-channel interference which is where other base stations are transmitting on the same frequency.

As the number of channel sets increases the number of available channels per cell reduces and therefore capacity reduces. But the interference level will also reduce, increasing the quality of service. The capacity of any one cell is limited by the interference that can be tolerated for a given grade of service. A number of other factors, apart from the capacity, effect the interference level:

Power control (both BTS and MS). Hardware techniques. Frequency hopping (if applied). Sectorization. Discontinuous transmission (DTX).

Carrier/Interference measurements taken at different locations within the coverage of a cell can be compared to a previously defined acceptable criterion. For instance, the criterion for the C/I ratio maybe set at 8 dB with the expectation that the C/I measurements will be better than that figure, for 90% of cases (C/I90). For a given re-use pattern, the predicted C/I ratio related to the D/R ratio can be determined, to give overall system comparison. For example:

Figure 3-41 Carrier interference measurementsOther sources of interferenceAdjacent Channel Interference: This type of interference is characterised by unwanted signals from other frequency channels `spilling over' or injecting energy into the channel of interest. With this type of interference being influenced by the spacing of RF channels, its effect can be reduced by increasing the frequency spacing of the channels. However, this will have the adverse effect of reducing the number of channels available for use within the system. The base station and the mobile stations receiver selectivity can also be designed to reduce the adjacent channel interference. Environmental Noise: This type of interference can also provide another source of potential interference. The intensity of this environmental noise is related to local conditions and can vary from insignificance to levels which can completely dominate all other sources of noise and interference. There are also several other factors which have to be taken into consideration. The interfering co-channel signals in given cell would normally arise from a number of surrounding cells not just one. What effect will directional antennas have when employed? Finally, if receiver diversity is to be used, what type and how is implementation to be achieved? Sectorization of sitesAs cell sizes are reduced, the propagation laws indicate that the levels of carrier interference tend to increase. In a omni cell, co-channel interference will be received from six surrounding cells all using the same channel sets. Therefore, one way of significantly cutting the level of interference is to use several directional antennas at the base stations, with each antenna radiating a sector of the cell, with a separate channel set. Sectorization increases the number of traffic channels available at a cell site which means more traffic channels available for subscribers to use. Also by installing more capacity at the same site there is a significant reduction in the overall implementation and operating costs experienced by the network operator. By using sectorized antennas, sectorization allows the use of geographically smaller cells and a tighter more economic re-use of the available frequency spectrum. This results in better network performance to the subscriber and a greater spectrum efficiency. The use of sectorized antennas allows better control of any RF interference which results in a higher call quality and an improved call reliability. More importantly for the network designer sectorization extends and enhances the cells ability to provide the in-building coverage that is assumed by the hand portable subscriber. Sectorization provides the flexibility to meet uneven subscriber distribution by allowing if required an uneven distribution of traffic resources across the cells on a particular site. This allows a more efficient use of both the infrastructure hardware and the available channel resources. Finally, with the addition of diversity techniques an improved sensitivity and increased interference immunity are experienced in a dense urban environment. Overcoming adverse propagation effects Hardware techniques

Multipath fading is responsible for more than just deep fades in the signal strength. The multipath signals are all arriving at different times and the demodulator will attempt to recover all of the time dispersed signals. This leads to an overlapping situation where each signal path influences the other, making the original data very hard to distinguish. The example opposite shows three component paths of the original signal which after demodulation should give three examples of the original data. This is not the case in reality as the output will be the result of the combination of the three inputs. As is shown in the diagram the output is very different making it difficult to decide wether the data should represent a 1 or a 0. This problem is known as inter symbol interference (ISI) and is made worse by the fact that the output from the demodulator is rarely a square wave. The sharp edges are normally rounded off so that when time dispersed signals are combined it makes it difficult to distinguish the original signal state. Another factor which makes things even more difficult is that the modulation technique Gaussian minimum shift keying, itself introduces a certain amount of ISI. Although this is a known distortion and can under normal conditions be filtered out, when it is added to the ISI distortion caused by the time delayed multipath signals it makes recovery of the original data that much harder. Frequency hoppingFrequency hopping is a feature that can be implemented on the air interface, (for example; the radio path to the MS), to help overcome the effects of multipath fading. GSM recommends only one type of frequency hopping, baseband hopping; but the Motorola BSS will support an additional type of frequency hopping called synthesizer hopping. Baseband hoppingBaseband Hopping is used when a base station has several DRCU/TCUs available. The data flow is simply routed in the baseband to various DRCU/TCUs, each of which operates on a fixed frequency, in accordance with the assigned hopping sequence. The different DRCU/TCUs will receive a specific individual timeslot in each TDMA frame containing information destined for different MSs. There are important points to note when using this method of providing frequency hopping.

There is a need to provide as many DRCU/TCUs as the number of allocated frequencies. The use of remote tuning combiners, cavity combining blocks or hybrid combiners is

acceptable in BTS6 applications. Within M-Cell equipment applications the use of either combining bandpass filter/hybrid or

cavity combining block is acceptable. Synthesizer hoppingSynthesizer hopping uses the frequency agility of the DRCU/TCU to change frequencies on a timeslot basis for both transmit and receive. The SCB in the DRCU and the digital processing and control board in the TCU calculates the next frequency and programmes one of the pair of Tx and Rx synthesizers to go to the calculated frequency. As the DRCU/TCU uses a pair of synthesizers for both transmit and receive, as one pair of synthesizers is being used the other pair are returning. There are important points to note when using synthesizer hopping:

Instead of providing as many DRCU/TCUs as the number of allocated frequencies, there is only a need to provide as many DRCU/TCUs as determined by traffic plus one for the BCCH carrier.

The output power available with the use of hybrid combiners must be consistent with coverage requirements.

Therefore as a general rule, cells with a small number of carriers will make good candidates for synthesizer hopping, whilst cells with many carriers will be good candidates for baseband hopping. There is also the other rule. There can only be one type of hopping on a BTS site, not a combination of the two. Error protection and detection To protect the logical channels from transmission errors introduced by the radio path, many different coding schemes are used. Figure 3-43 illustrates the coding process for speech, control and data channels; the sequence is very complex. The coding and interleaving schemes depend on the type of logical channel to be encoded. All logical channels require some form of convolutional encoding, but since protection needs are different, the code rates may also differ. The coding protection schemes,shown in Figure 3-42, are: Speech channel encodingThe speech information for one 20 ms speech block is divided over eight GSM bursts. This ensures that if bursts are lost due to interference over the air interface the speech can still be reproduced.

Common control channel encoding20 ms of information over the air will carry four bursts of control information, for example BCCH. This enables the bursts to be inserted into one TDMA multiframe. Data channel encodingThe data information is spread over 22 bursts. This is because every bit of data information is very important. Therefore, when the data is reconstructed at the receiver, if a burst is lost, only a very small proportion of the 20 ms block of data will be lost. The error encoding mechanisms should then enable the missing data to be reconstructed.

Figure 3-42 The coding processFigure 3-43 Error protection and detection

Speech channel encoding The BTS receives transcoded speech over the Abis interface from the BSC. At this point the speech is organized into its individual logical channels by the BTS. These logical channels of information are then channel coded before being transmitted over the air interface. The transcoded speech information is received in frames, each containing 260 bits. The speech bits are grouped into three classes of sensitivity to errors, depending on their importance to the intelligibility of speech. Class 1aThree parity bits are derived from the 50 Class 1a bits. Transmission errors within these bits are catastrophic to speech intelligibility, therefore, the speech decoder is able to detect uncorrectable errors within the Class 1a bits. If there are Class 1a bit errors, the whole block is usually ignored. Class 1bThe 132 Class 1b bits are not parity checked, but are fed together with the Class 1a and parity bits to a convolutional encoder. Four tail bits are added which set the registers in the receiver to a known state for decoding purposes. Class 2The 78 least sensitive bits are not protected at all. The resulting 456 bit block is then interleaved before being sent over the air interface. The encoded speech now occupies 456 bits, but is still transmitted in 20 ms thus raising the transmission rate to 22.8 kbit/s.

Figure 3-44 Speech channel encodingChannel coding for enhanced full rate The transcoding for enhanced full rate produces 20 ms speech frames of 244 bits for channel coding on the air interface. After passing through a preliminary stage which adds 16 bits to make the frame up to 260 bits the EFR speech frame is treated to the same channel coding as full rate. The additional 16 bits correspond to an 8 bit CRC which is generated from the 50 Class 1a bits plus the 15 most important Class 1b bits and 8 repetition bits corresponding to 4 selected bits in the original EFR frame of 244 bits. Preliminary channel coding for EFREFR Speech Frame

50 Class 1a + 124 Class 1b + 70 Class 2 = 244 bits Preliminary Coding

8 bit CRC generated from 50 Class 1a + 15 Class 1b added to Class 1b bits 8 repetition bits added to Class 2 bits

Output from preliminary coding 50 Class 1a + 132 Class 1b + 78 Class 2 = 260 bits

EFR frame of 260 bits passed on for similar channel coding as Full Rate. Figure 3-45 Preliminary coding for enhanced full rate speech

Control channel encoding Figure 3-46 shows the principle of the error protection for the control channels. This scheme is used for all the logical signalling channels, the synchronization channel (SCH) and the random access burst (RACH). The diagram applies to SCH and RACH, but with different numbers. When control information is received by the BTS it is received as a block of 184 bits. These bits are first protected with a cyclic block code of a class known as a Fire Code. This is particularly suitable for the detection and correction of burst errors, as it uses 40 parity bits. Before the convolutional

encoding, four tail bits are added which set the registers in the receiver to a known state for decoding purposes. The output from the encoding process for each block of 184 bits of signalling data is 456 bits, exactly the same as for speech. The resulting 456 bit block is then interleaved before being sent over the air interface.

Figure 3-46 Control channel codingData channel encoding Figure 3-47 shows the principle of the error protection for the 9.6 kbit/s data channel. The other data channels at rates of 4.8 kbit/s and 2.4 kbit/s are encoded slightly differently, but the principle is the same. Data channels are encoded using a convolutional code only. With the 9.6 kbit/s data some coded bits need to be removed (punctuated) before interleaving, so that like the speech and control channels they contain 456 bits every 20 ms. The data traffic channels require a higher net rate (`net rate' means the bit rate before coding bits have been added) than their actual transmission rate. For example, the 9.6 kbit/s service will require 12 kbit/s, because status signals (such as the RS-232 DTR (data terminal ready)) have to be transmitted as well. The output from the encoding process for each block of 240 bits of data traffic is 456 bits, exactly the same as for speech and control. The resulting 456 bit block is then interleaved before being sent over the air interface. The encoded control information now occupies 456 bits but is still transmitted in 20 ms thus raising the transmission rate to 22.8 kbit/s.

Figure 3-47 Data channel encodingMapping logical channels onto the TDMA frame structure InterleavingHaving encoded, or error protected the logical channel, the next step is to build its bitstream into bursts that can then be transmitted within the TDMA frame structure. It is at this stage that the process of interleaving is carried out. Interleaving spreads the content of one traffic block across several TDMA timeslots. The following interleaving depths are used:

Speech - 8 blocks Control - 4 blocks Data - 22 blocks

This process is an important one, for it safeguards the data in the harsh air interface radio environment. Because of interference, noise, or physical interruption of the radio path, bursts may be destroyed or corrupted as they travel between MS and BTS, a figure of 10-20% is quite normal. The purpose of interleaving is to ensure that only some of the data from each traffic block is contained within each burst. By this means, when a burst is not correctly received, the loss does not affect overall transmission quality because the error correction techniques are able to interpolate for the missing data. If the system worked by simply having one traffic block per burst, then it would be unable to do this and transmission quality would suffer. It is interleaving that is largely responsible for the robustness of the GSM air interface, enabling it to withstand significant noise and interference and maintain the quality of service presented to the subscriber.

Table 3-19 Interleaving

TRAU Frame Type Number of GSM Bursts spread over

Speech 8

Control 4

Data 22

Diagonal interleaving - speechFigure 3-48 illustrates, in a simplified form, the principle of the interleaving process applied to a full-rate speech channel. The diagram shows a sequence of `speech blocks' after the encoding process previously described, all from the same subscriber conversation. Each block contains 456 bits, these blocks are then

divided into eight blocks each containing 57 bits. Each block will only contain bits from even bit positions or bits from odd bit positions. The GSM burst will now be produced using these blocks of speech bits. The first four blocks will be placed in the even bit positions of the first four bursts. The last four blocks will be placed in the odd bit positions of the next four bursts. As each burst contains 114 traffic carrying bits, it is in fact shared by two speech blocks. Each block will share four bursts with the block preceding it, and four with the block that succeeds it, as shown. In the diagram block 5 shares the first four bursts with block 4 and the second four bursts with block 6.

Figure 3-48 Diagonal interleafing - speechTransmission - speechEach burst will be transmitted in the designated timeslot of eight consecutive TDMA frames, providing the interleaving depth of eight. Table 3-20 shows how the 456 bits resulting from a 20 ms speech sample are distributed over eight normal bursts. It is important to remember that each timeslot on this carrier may be occupied by a different channel combination: traffic, broadcast, dedicated or combined. The FACCH will steal a 456 bit block and be interleaved with the speech. Each burst containing a FACCH block of information will have the appropriate stealing flag set.

Table 3-20 Distribution of 456 bits from one 20 ms speech sample

Distribution Burst

0 8 16 24 32 40 ..........................448 even bits of burst N

1 9 17 25 33 41 ..........................449 even bits of burst N + 1



4 12 20 28 36 44 ..........................452 odd bits of burst N + 4




Rectangular interleaving - controlFigure 3-49 illustrates, in a simplified form, the principle of rectangular interleaving. This is applied to most control channels. The diagram shows a sequence of `control blocks' after the encoding process previously described. Each block contains 456 bits, these blocks are then divided into four blocks each containing 114 bits. Each block will only contain bits for even or odd bit positions. The GSM burst will be produced using these blocks of control. Transmission - controlEach burst will be transmitted in the designated timeslot of four consecutive TDMA frames, providing the interleaving depth of four. The control information is not diagonally interleaved as are speech and data. This is because only a limited amount of control information is sent every multiframe. If the control information was diagonally interleaved, the receiver would not be capable of decoding a control message until at least two multiframes were received. This would be too long a delay.

Figure 3-49 Rectangular interleaving - controlDiagonal interleaving - dataFigure 3-50 illustrates, in a simplified form, diagonal interleaving applied to a 9.6 kbit/s data channel. The diagram shows a sequence of `data blocks' after the encoding process previously described, all from the same subscriber. Each block contains 456 bits, these blocks are divided into four blocks each containing 114 bits. These blocks are then interleaved together. The first 6 bits from the first block are placed in the first burst. The first 6 bits from the second block will be placed in the second burst and so on. Each 114 bit block is spread across 19 bursts and the total 456 block will be spread across 22 bursts.

Data channels are said to have an interleaving depth of 22, although this is sometimes also referred to as an interleaving depth of 19. Transmission - dataThe data bits are spread over a large number of bursts, to ensure that the data is protected. Therefore, if a burst is lost, only a very small amount of data from one data block will actually be lost. Due to the error protection mechanisms used, the lost data has a higher chance of being reproduced at the receiver. This wide interleaving depth, although providing a high resilience to error, does introduce a time delay in the transmission of the data. If data transmission is slightly delayed, it will not effect the reception quality, whereas with speech, if a delay were introduced this could be detected by the subscriber. This is why speech uses a shorter interleaving depth.

Figure 3-50 Diagonal interleaving - dataVoice Activity Detection - VAD VAD is a mechanism whereby the source transmitter equipment identifies the presence or absence of speech. VAD implementation is effected in speech mode by encoding the speech pattern silences at a rate of 500 bit/s rather than the full 13 kbit/s. This results in a data transmission rate for background noise, known as comfort noise, which is regenerated in the receiver. Without comfort noise the total silence between the speech would be considered to be disturbing by the listener. Discontinuous Transmission - DTX DTX increases the efficiency of the system through a decrease in the possible radio transmission interference level. It does this by ensuring that the MS does not transmit unnecessary message data. DTX can be implemented, as necessary, on a call by call basis. The effects will be most noticeable in communications between two MS. DTX in its most extreme form, when implemented at the MS can also result in considerable power saving. If the MS does not transmit during silences there is a reduction in the overall power output requirement. The implementation of DTX is very much at the discretion of the network provider and there are different specifications applied for different types of channel usage. DTX is implemented over a SACCH multiframe (480 ms). During this time, of the possible 104 frames, only the 4 SACCH frames and 8 Silence Descriptor (SID) frames are transmitted.

Figure 3-51 SACCH Multiframe (480 ms)Receive diversityIn its simplest case, multipath fading arises from destructive interference between two transmission paths. The deepest instantaneous fade occurring at the frequency for which the effective path length difference is an odd multiple of half wavelengths. If two receive antennas are mounted a defined distance apart, then it follows that the probability of them simultaneously experiencing maximum fade depth at a given frequency is very much less than for the single antenna situation. There are three ways of utilizing this concept:

The receiver can be switched between the two RF receive paths provided two antennas. The RF signals from two receive paths can be phase aligned and summed. The phasing can be made so as to minimize the distortion arising from the multipath

transmission. Each of the methods has advantages and disadvantages. In the case of the switched configuration, its simply chooses the better of the two RF signals which is switched through to the receiver circuitry. Phase alignment has the advantage of being a continuously optimized arrangement in terms of signal level, but phase alignment diversity does not minimize distortion. The Motorola DRCU/TCU uses this diversity concept. The distortion minimizing approach, whilst being an attractive concept, has not yet been implemented in a form that works over the full fading range capabilities of the receivers and therefore has to switch back to phase alignment at low signal levels. This means a rather complex control system is required. It must be emphasized that diversity will not usually have any significant effect on the mean depression component of fading, but the use of phase alignment diversity can help increase the mean signal level received.

Figure 3-52 Receive diversityEqualizationAs mentioned in multipath fading, in most urban areas the only signals received are multipath. If nothing was done to try and counter the effects of (Inter Symbol Interference) ISI caused by the time dispersed signals, the Bit Error Rate (BER) of the demodulated signal would be far too high, giving a very poor quality signal, unacceptable to the subscriber. To counter this a circuit called an equalizer is built into the receiver. The equalizer uses a known bit pattern inserted into every normal burst transmitted, called the training sequence code. This allows the equaliser to assess and modify the effects of the multipath component, resulting in a far cleaner less distorted signal. Without this equalizer the quality of the circuit would be unacceptable for the majority of time. Training sequence codeThe training sequence code, Figure 3-53, is used so that the demodulator can estimate the most probable sequence of modulated data. As the training sequence is a known pattern, this enables the receiver to estimate the distortion ISI on the signal due to propagation effects, especially multipath reception. The receiver must be able to cope with two multipaths of equal power received at an interval of up to 16 microseconds. If the two multipaths are 16 micro seconds delayed then this would be approximately equivalent to 5-bit periods. There are 32 combinations possible when two 5-bit binary signals are combined. As the transmitted training sequence is known at the receiver, it is possible to compare the actual multipath signal received with all 32 possible combinations reproduced in the receiver. From this comparison the most likely combination can be chosen and the filters set to remove the multipath element from the received signal. The multipath element can be of benefit, once it has been identified, as it can then be recombined with the wanted signal in a constructive way to give a greater received signal strength. Once the filters have been set, they can be used to filter the random speech data as it is assumed they will have suffered from the same multipath interference as the training sequence code. The multipath delay is calculated on a burst by burst basis, as it is constantly changing.

Figure 3-53 Training sequence codeSubscriber environment Subscriber hardwareSystem quality, (for example; voice quality) system access and grade of service, as perceived by the customer, are the most significant factors in the success of a cellular network. The everyday subscriber neither knows or really cares about the high level of technology incorporated into a cellular network. However, they do care about the quality of their calls. What the network designer must remember is that it is the subscriber who chooses the type of equipment they wish to use on the network. It is up to the network provider to satisfy the subscriber whatever they choose. The output power of the mobile subscriber is limited in a GSM system to a maximum of 8 W for a mobile and a minimum of 0.8 W for a hand portable. For a DCS1800 system, the mobile subscriber is restricted to a maximum of 1 W and a minimum of 250 mW hand portable. EnvironmentNot only does the network designer have to plan for the subscribers choice of phone, the designer has to plan for the subscribers choice as to where they wish to use that phone. Initially when only the mobile unit was available, system coverage and hence subscriber use was limited to on street, high density urban or low capacity rural coverage areas. During the early stages of cellular system implementation the major concern was trying to provide system coverage inside tunnels. However, with the advances in technology the hand portable subscriber unit is now firmly established. With this introduction came new problems for the network designer. The portable subscriber unit provides the user far more freedom of use but the subscriber still expected exactly the same service. The subscriber now wants quality service from the system at any location. This location can be on a street, or any floor of a building whether it be the basement or the penthouse and even in lifts, refer to Figure 3-54. Thus greater freedom of use for the subscriber gives the network designer even greater problems when designing and implementing a cellular system.

Figure 3-54 The subscriber environmentDistribution

Not only do network designers have to identify the types of subscriber that use the cellular network now and in the future, but at what location these subscribers are attempting to use their phones. Dense urban environments require an entirely different design approach, due to considerations mentioned earlier in this chapter, than the approach being used to design coverage for a sparsely populated rural environment. Road and rail networks have subscribers moving at high speed, so this must be accounted for when planning the interaction between network entities whilst the subscriber is using the network. Even in urban areas, the network designer must be aware that traffic is not necessarily evenly distributed. An urban area may contain sub-areas of uneven distribution such as a business or industrial district, and may have to plan for a seasonal increase of traffic due to, say, a convention centre. It is vitally important that the traffic distribution is known and understood prior to network design, to ensure that a successful quality network is implemented.

Figure 3-55 Subscriber distributionMost demandingThe network designer must ensure that the network is designed to ensure a quality service for the most demanding subscriber. This is the hand portable subscriber. The hand portable now represents the vast majority of all new subscriber units introduced into cellular networks. So clearly the network operators, and hence the network designers, must recognise this. Before commencing network design based around hand portable coverage, the network designer must first understand the limitations of the hand portable unit and secondly, what the hand portable actually requires from the network. The hand portable phone is a small lightweight unit which is easy to carry and has the ability to be used from any location. The ability of the unit to be used at any location means that the network must be designed with the provision of good in-building coverage as an essential element. To further complicate the network designers job, these hand portable units have a low output power:

0.8 W to 8 W for GSM900. 0.25 W to 1 W for DCS1800.

So the distance at which these units can be used from a cell is constrained by RF propagation limitations. For practical purposes, the actual transmit power of the hand portable should be kept as low as possible during operation. This helps not only from an interference point of view, but this also helps to extend the available talk time of the subscriber unit, which is limited by battery life.

Future planningNormal practice in network planning is to choose one point of a well know re-use model as a starting point. Even at this early stage the model must be improved because any true traffic density does not follow the homogeneous pattern assumed in any theoretical models. Small-sized heavy traffic concentrations are characteristic of the real traffic distributions. Another well known traffic characteristic feature is the fast descent in the density of traffic when leaving city areas. It is uneconomical to build the whole network using a standard cell size, it becomes necessary to use cells of varying sizes. Connecting areas with different cell sizes brings about new problems. In principle it is possible to use cells of different size side by side, but without careful consideration this may lead to a wasteful frequency plan. This is due to the fact that the re-use distance of larger cells is greater than that of smaller cells. The situation is often that the borders are so close to the high density areas that the longer re-use distances mean decreased capacity. Another solution, offering better frequency efficiency, is to enlarge the cell size gradually from small cells into larger cells. In most cases the traffic concentrations are so close to each other that the expansion cannot be completed before it is time to start approaching the next concentration, by gradually decreasing the cell size. This is why the practical network is not a regular cluster composition, but a group of directional cells of varying size. Besides this need for cells of different size, the unevenness of the traffic distribution also cause problems in frequency planning. Theoretical frequency division methods applicable to homogenous clusters cannot be used. It is quite rare that two or more neighbouring cells need the same amount of channels. It must always be kept in mind that the values calculated for future traffic distribution are only crude estimates and that the real traffic distribution always deviates from these estimates. In consequence, the network plan should be flexible enough to allow for rearrangement of the network to meet the real traffic needs.

ConclusionIn conclusion there are no general rules for radio network planning. It is a work of experimenting and reiterating. By comparing different alternatives, the network designers should find a plan that both fulfils the given requirements and keeps within practical limitations. When making network plans, the designers should always remember that every location in a network has its own conditions, and all local problems must be tackled and solved on a individual basis. The microcellular solutionLayered ArchitectureThe basic term layered architecture is used in the microcellular context to explain how macrocells overlay microcells. It is worth noting that when talking of the traffic capacity of a microcell it is additional capacity to that of the macrocell in the areas of microcellular coverage. The traditional cell architecture design, Figure 3-56, ensures that, as far as possible, the cell gives almost total coverage for all the MSs within its area.

Figure 3-56 Layered architectureCombined cell architectureA combined cell architecture system, Figure 3-57, is a multi-layer system of macrocells and microcells. The simplest implementation contains two layers. The bulk of the capacity in a combined cell architecture is provided by the microcells. Combined cell systems can be implemented into other vendors networks. Macrocells: Implemented specifically to cater for the fast-moving MSs and to provide a fallback service in the case of coverage holes and pockets of interference in the microcell layer. Macrocells form an umbrella over the smaller microcells. Microcells: Microcells handle the traffic from slow-moving MSs. The microcells can give contiguous coverage over the required areas of heavy subscriber traffic. Picocells: Low cost installation by using in-building fibre optics or telephone wiring with a HDSL modem, easily expanded to meet capacity requirements. Efficient use of the frequency spectrum due to low power radios causing low interference to external networks. Higher quality speech compared with external illumination of the building due to improved uplink quality.

Figure 3-57 Combined cell architectureCombined cell architecture structureA combined cell architecture employs cells of different sizes overlaid to provide contiguous coverage. This structure is shown in Figure 3-58. Some points to note:

Macrocell and microcell networks may be operated as individual systems. The macrocell network is more dominant as it handles the greater amount of traffic. Microcells can be underlayed into existing networks. Picocells can be introduced as a third layer or as part of the second layer.

Figure 3-58 Combined cell architecture structureExpansion solution As the GSM network evolves and matures its traffic loading will increase as the number of subscribers grow. Eventually a network will reach a point of traffic saturation. The use of microcells can provide high traffic capacity in localised areas. The expansion of a BTS site past its original designed capacity can be a costly exercise and the frequency re-use implications need to be planned carefully (co-channel and adjacent channel interference). The use of microcells can alleviate the increase in congestion, the microcells could be stand-alone cells to cover traffic hotpots or a contiguous cover of cells in a combined architecture. The increased coverage will give greater customer satisfaction. Previous Chapter · First Page · Previous Page · Next Page · Last Page · Next Chapter

Introduction to Cell StatisticsDescription of Cell Statistics

Quality of NetThis chapter includes descriptions of the cell statistics. Cell statistics are organized into the following groups:

Call clearing. Concentric cell.

Connection establishment. Directed retry. Emergency call access. Extended range cell. General handover. Inter-BSS handover. Intra-BSS handover. Intra-cell handover. Multiband. TCH assignment. Usage congestion.

Call clearingCall clearing statistics are used to track the number calls lost due to RF failures, cipher mode failures, or procedure timeouts. The following are call clearing statistics:

click here CIPHER_MODE_FAIL. click here CLR_REQ_TO_MSC. click here RF_LOSSES_SD. click here RF_LOSSES_TCH.

Concentric cellConcentric cell statistics are used to track concentric cell feature processes. The following are concentric cell statistics:

click here TCH_CONG_INNER_ZONE. click here TCH_USAGE_INNER_ZONE. click here ZONE_CHANGE_ATMPT. click here ZONE_CHANGE_SUC.

Connection establishmentConnection establishment statistics are used to track access, allocation, channel request, and MSC page request functions. The following are connection establishment statistics:

click here ACCESS_PER_AGCH. click here ACCESS_PER_PCH. click here ACCESS_PER_RACH. click here ALLOC_SDCCH. click here ALLOC_SDCCH_FAIL. click here ALLOC_TCH. click here ALLOC_TCH_FAIL. click here CHAN_REQ_CAUSE_ATMPT. click here CHAN_REQ_MS_BLK. CHAN_REQ_MS_FAIL. click here CONN_REFUSED. click here CONN_REQ_TO_MSC. click here INV_EST_CAUSE_ON_RACH. click here OK_ACC_PROC. click here OK_ACC_PROC_SUC_RACH. click here PAGE_REQ_FROM_MSC.

Directed retryDirected retry statistics are used to track congestion processes associated with the directed retry feature. The following are directed retry statistics:

click here CONGEST_EXIST_HO_ATMPT. click here CONGEST_STAND_HO_ATMPT.

Emergency call accessEmergency call access statistics are used to track emergency call processes. The following are emergency call access statistics:

click here NUM_EMERG_ACCESS. click here NUM_EMERG_REJECTED.

click here NUM_EMERG_TCH_KILL. click here NUM_EMERG_TERM_SDCCH.

Extended range cellExtended range statistics are used to track extended range feature processes. The following are extended range cell statistics:

click here ER_INTRA_CELL_HO_ATMPT. click here ER_INTRA_CELL_HO_SUC. click here TCH_USAGE_EXT_RANGE.

General handoverGeneral handover statistics are used to track processes common to all types of handovers. The following are handover statistics:

click here BAD_HO_REFNUM_MS. click here HO_FAIL_NO_RESOURCES.

Inter-BSS handoverInter-BSS handover statistics are used to track inter-BSS handover processes. The following are inter-BSS handover statistics:

click here HO_REQ_MSC_FAIL. click here HO_REQ_MSC_OK. click here IN_INTER_BSS_HO. click here OUT_INTER_BSS_HO.

Intra-BSS handoverIntra-BSS handover statistics are used to track intra-BSS handover processes. The following are intra-BSS handover statistics:

click here IN_INTRA_BSS_HO. click here OUT_HO_CAUSE_ATMPT click here OUT_INTRA_BSS_HO.

Intra-cell handoverIntra-cell handover statistics are used to track intra-cell handover processes. The following is the only intra-cell handover statistic:

click here INTRA_CELL_HO. MultibandMultiband statistics are used to track multiband feature processes. The following are multiband statistics:

click here INTERBAND_ACTIVITY. click here MS_ACCESS_BY_TYPE. click here MS_TCH_USAGE_BY_TYPE. click here OUT_HO_CAUSE_ATMPT.

TCH assignmentTCH assignment statistics are used to track successful, unsuccessful, and total traffic channel assignment processes. The following are TCH assignment statistics:

click here MA_CMD_TO_MS. click here MA_CMD_TO_MS_BLKD. click here MA_FAIL_FROM_MS. click here MA_REQ_FROM_MSC. click here TOTAL_CALLS.

Usage congestionUsage congestion statistics are used to track congestion processes not related to the directed retry feature. The following are usage congestion statistics:

click here AVAILABLE_SDCCH. click here AVAILABLE_TCH. click here BUSY_SDCCH. click here BUSY_TCH. click here CALLS_QUEUED. click here CLASSMK_UPDATE_FAIL.

click here FLOW_CONTROL_BARRED. click here MA_COMPLETE_TO_MSC. click here PCH_AGCH_Q_LENGTH. click here PCH_Q_PAGE_DISCARD. click here SDCCH_CONGESTION. click here SECOND_ASSIGN_ATMPT. click here SECOND_ASSIGN_SUC. click here SMS_INIT_ON_SDCCH. click here SMS_INIT_ON_TCH. click here SMS_NO_BCAST_MSG. TCH_CONGESTION. click here TCH_DELAY. click here TCH_Q_LENGTH. click here TCH_USAGE.

DYNET failures click here DYNET_ASSIGN_FAIL. click here DYNET_CALL_REJECTS.

ACCESS_PER_AGCHDescriptionThis statistic counts Immediate Assignment messages sent on the Access Grant Channel (AGCH) of a cell. Access Grants for more than one MS may be contained in one Access Grant message. An Access Grant for more than one MS is only pegged once. This count includes Immediate Assignment, Immediate Extended, and Immediate Assignment Reject messages sent on the AGCH of a cell. PeggingThis statistic is pegged when an Access Grant message is sent on the AGCH to signal the availability of an allocated channel. AnalysisThis statistic can be used for trend analysis of Intermediate Assignment messages sent on the AGCH of a cell.

Reference GSM 4.03, 3.0.3, 4.3.2.1. GSM 4.08, 3.8.0, 3.3.1.2.1, 7.1.2, 9.1.17-19. GSM 12.04, 3.1.0, B.1.1.10.

Usage Radio resource allocation statistics. Network planning.

Basis Cell.

Type Counter.

ACCESS_PER_PCHDescriptionThis statistic counts the number of Paging Request messages sent on the air interface. A paging message could contain up to four pages if paging by TMSI and up to two pages if paging by IMSI. PeggingThis statistic is pegged when a Paging Request message is sent on the Paging CHannel (PCH) of a cell. A page for more than one MS is only pegged once. AnalysisThis statistic can be used for trend analysis of Paging Request messages sent on the PCH of a cell.

Reference GSM 4.03, 3.0.3, 4.3.2.1. GSM 4.08, 3.8.0, 3.3.2.1, 7.1.1 and 9.1.21-23. GSM 12.04, 3.1.0, B.1.1.10.

Usage Radio resource allocation. Network planning.

Basis Cell.

Type Counter.

ACCESS_PER_RACHDescriptionThis statistic counts Channel Request messages received by the BSS on the Random Access CHannel (RACH) of a cell. A Channel Request message is used by the MS to request allocation of a dedicated channel (to be used as an SDCCH) by the network, in response to a paging message (incoming call) from the network or as a result of an outgoing call/supplementary short message service dialled from the MS. It is also used as part of the call re-establishment procedure. PeggingThis statistic pegs received requests that:

Succeeded (resulting in channel assignment). Failed (did not result in channel assignment).

AnalysisThis statistic can be used for trend analysis of Channel Request messages sent on the RACH of a cell.

Reference GSM 4.08, 3.8.0, 3.3.1.1, 3.3.2.2, 4.5.1.6, 5.5.4, 7.1.2, 9.1.8 and 9.2.4. GSM 4.03, 3.03 and 4.3.2.1. GSM 12.04, 3.2.0, B.1.1.10.

Usage

RF loss. Congestion. Quality of service monitoring. Network planning.

Basis Cell.

Type Counter.

ALLOC_SDCCHDescriptionThis statistic is the sum of the number of times a SDCCH is successfully seized. If a TCH is reconfigured as an SDCCH, only the SDCCH statistics will be configured. A Channel Request message is used by the MS to request allocation of a dedicated channel (to be used as an SDCCH) by the network, in response to a paging message (incoming call) from the network or as a result of an outgoing call/supplementary short message service dialled from the MS. It is also used as part of the call re-establishment procedure. SDCCH seizure is caused by immediate assignment, handover, and channel assignment procedures. Congestion is signalled by the Immediate Assignment Reject or the Assignment/Handover Failure message. PeggingThis statistic is pegged each time an Immediate Assignment message is sent by the BSS. An Immediate Assignment message will be sent upon:

Successful handover on an SDCCH channel. Successful immediate assignment on an SDCCH channel.

This statistic may be pegged for reasons other than calls requiring a traffic channel, such as location updates. AnalysisThis statistic can be used for trend analysis of SDCCH seizures.

Reference GSM 12.04, 3.1.0, B.1.3.3.

GSM 4.08, 3.8.0, 3.3.12, 3.4.3.1, 3.4.4.1, 9.1.2 to 9.1.4, 9.1.14 to 9.1.19.

Usage

RF loss. Congestion. Quality of service monitoring: Call set-up. Network planning. Ladder: Immediate assignment successful, Intra-cell handover, Inter-BSS handover, Connection establishment, Intra-BSS handover. Key statistic: SDCCH_CONGESTION.

Basis Cell.

Type Counter.

ALLOC_SDCCH_FAILDescriptionThis statistic counts the number of times that an attempt at SDCCH seizure was rejected because of SDCCH congestion. If a TCH is reconfigured as an SDCCH, only the SDCCH statistics will be incremented. Congestion is signalled by the Immediate Assignment Reject or the Assignment/Handover Failure message. PeggingThis statistic is pegged when an Immediate Assignment Reject message is sent to the MS in response to a channel request because no SDCCH channels were available to be allocated. It is also pegged in the target cell when rejecting a handover due to a lack of resources. AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of SDCCH failures due to congestion that are acceptable in normal system operations. If the specified threshold is exceeded, the 1. CELL: Attempt at allocating an SDCCH failed - PM alarm will be generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Reference GSM 12.04, 3.1.0, B.1.3.4. GSM 4.08, 3.8.0, 3.3.1.2.2, 3.4.3.3, 3.4.4.4, 9.1.4, 9.1.16 and 9.1.19.

Usage

RF loss. Congestion. Quality of service monitoring: Call set-up. Network planning. Fault finding. Optimization. Ladder: Immediate assignment blocked, Connection establishment. Key statistic : SDCCH_CONGESTION.

Basis Cell.

Type Counter.

Alarm Major.

ALLOC_TCHDescriptionThis statistic provides the number of successful TCH allocations within a cell for both call originations and hand ins. PeggingThis statistic is pegged for successful TCH allocations within a cell as a result of a call establishment or hand in attempt:

Successful allocation due to call establishment including successful allocations due to directed retries.

Successful allocation due to intra-cell hand in.

Successful allocation due to inter-cell/intra-cell hand in. Successful allocation due to inter-BSS hand in.

This statistic is pegged prior to the transmission of the assignment/handover command to the MS and, therefore, does not take into account the success or failure of the assignment/hand in from an RF perspective. AnalysisThis statistic can be used for trend analysis of TCH allocations.

Usage

Quality of service monitoring: Call setup. Network planning. Ladder: Intra-BSS handover, Inter-BSS handover, Intra-cell handover, Assignment to TCH. Key statistics: TCH_CONGESTION, TCH_MEAN_ARRIVAL_RATE, TCH_MEAN_HOLDING_TIME, CELL_TCH_ASSIGNMENTS, TCH_RF_LOSS_RATE.

Basis Cell.

Type Counter.

ALLOC_TCH_FAILDescriptionThis statistic provides the number of unsuccessful allocations of a TCH within a cell for both call origination and hand in. Cases involving Immediate Assignment Reject are also included in the peg count. PeggingThis statistic is pegged when an attempt to allocate a TCH in a cell fails due to a lack of resources:

Unsuccessful allocation due to call establishment including unsuccessful allocations due to directed retries.

Successful allocation due to intra-cell hand in. Unsuccessful allocation due to inter-cell/intra cell hand in. Unsuccessful allocation due to inter-BSS hand in.

AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of TCH failures due to congestion that are acceptable in normal system operations. If the specified threshold is exceeded, the 25. CELL: Attempt at allocating an TCH failed - PM alarm will be generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Usage

Quality of service: Call set-up. Fault finding. Optimization. Network planning Ladder: Inter-BSS handover, Intra-BSS handover, Assignment to TCH. Key statistics: TCH_CONGESTION, TCH_MEAN_ARRIVAL_RATE.

Basis Cell.

Type Counter.

Alarm Major.

AVAILABLE_SDCCHDescriptionThis is a gauge statistic indicating the average number of available SDCCHs that are in use or available for use. The SDCCH is available when its administrative state is "unlocked" or "shutting down" and the operational state is "enabled", and is unavailable when its administrative state is "locked" and an operational state of "disabled". PeggingThis statistic is pegged when a change in SDCCH availability is detected. Analysis

This statistic can be used for trend analysis of SDCCH availability. By comparing the number of available SDCCHs to the total number of SDCCHs in a system, a percentage of utilization can be calculated. An analysis of the utilization information can be used to determine the requirement for additional resources before problems occur.

Reference GSM 12.04, 4.2.1, B.2.1.21

Usage Congestion. Network planning.

Basis Cell.

Type Gauge.

AVAILABLE_TCHDescriptionThis is a gauge statistic indicating the average number of available TCHs that are in use or available for use. The TCH is available when its administrative state is "unlocked" or "shutting down" and the operational state is "enabled", and is unavailable when its administrative state is "locked" and an operational state of "disabled". PeggingThis statistic is pegged when a change in TCH availability is detected. AnalysisThis statistic can be used for trend analysis of TCH availability. By comparing the number of available TCHs to the total number of TCHs in a system, a percentage of utilization can be calculated. An analysis of the utilization information can be used to determine the requirement for additional resources before problems occur.

Reference GSM 12.04, 4.2.1, B.2.1.10

Usage Congestion. Network planning.

Basis Cell.

Type Gauge.

BAD_HO_REFNUM_MSDescriptionThis statistic tracks the number of times a MS has accessed a channel with a Handover Reference Number that the BSS was not expecting. PeggingThis statistic is pegged once for each time a MS has accessed a channel with a Handover Reference Number that the BSS was not expecting. This field is an unformatted random number. The BSS only compares what it received with what it expected, for example, potentially up to four times for each logical handover detect for the SDCCH processed at the BSS. The MS will continue to attempt access until a good Handover Reference Number is received, therefore, the handover can be successful and the statistic will still peg. This statistics pegs if the handover fails and recovers, or fails and loses the mobile. AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of MS accesses of channels with Handover Reference Numbers that the BSS was not expecting that are acceptable in normal system operations. High values reported for this statistic may not be because a handover access meant for one cell can be detected by another, but the channel at the target cell is susceptible during the time between channel activation and when the MS actually arrives. This is similar to "Phantom RACHs." If the specified threshold is exceeded, the 19. CELL: Bad HO reference numbers from the MS - PM alarm will be generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Reference GSM 4.08, 3.8.0, 3.4.4.2.1, 3.4.4.2.2 and 9.1.13.

Usage Handover. Fault finding.

Basis Cell.

Type Counter.

Alarm Warning.

BUSY_SDCCHDescriptionThis statistic tracks the number of SDCCHs allocated during an interval. This is a weighted distribution statistic and will produce a mean value indicating the average number of SDCCHs in use during the interval. The bin ranges may be changed using the chg_stat_prop command. The default bin ranges for the BUSY_SDCCH statistic are shown in Table 4-1.

Table 4-1 Default bin ranges chg_stat_prop

Bin Range

0 0 - 0

1 1 - 2

2 3 - 4

3 5 - 6

4 7 - 8

5 9 - 10

6 11 - 12

7 13 - 14

8 15 - 16

9 17 - 400

PeggingThe individual bin values are incremented by the length of time that the number of allocated SDCCHs fall within the bin range of values. Bin values are pegged each time a SDCCH is allocated or de-allocated. If a TCH is reconfigured as an SDCCH, only the SDCCH statistics will be pegged. AnalysisThis statistic can be used for trend analysis of the amount of RF signalling traffic in the cell SDCCHs. An analysis of the allocation information can be used to determine the requirement for additional resources before problems occur.

Reference GSM 12.04, 3.1.0, B.1.3.2.

Usage

RF loss. Congestion. Quality of service monitoring. Network planning. Optimization. Key statistic: SDCCH_MEAN_HOLDING_TIME, SDCCH _TRAFFIC.

Basis Cell.

Type Weighted Distribution.

BUSY_TCH

Description:This statistic records the number of TCHs allocated during an interval. This is a weighted distribution statistic and will produce a mean value indicating the average number of TCHs in use during the interval. It is an indication of the average capacity for additional traffic in the cell. This statistic includes TCHs used as a Dedicated Control CHannel (DCCH) in immediate assignment mode. The BUSY_TCH time includes the guard time (the time allowed between ending a call and being allowed to start another). Since the channel is not available to be used again till the guard timer expires, it is not considered to be free until then. The bin ranges may be changed using the chg_stat_prop command. The default bin ranges for the BUSY_TCH statistic are shown in Table 4-2.

Table 4-2 Default bin ranges for BUSY_TCH

Bin Range

0 0 - 0

1 1 - 2

2 3 - 4

3 5 - 6

4 7 - 8

5 9 - 10

6 11 - 12

7 13 - 14

8 15 - 16

9 17 - 400

PeggingThe individual bin values are incremented by the length of time that the number of allocated TCHs fall within the bin range of values. Bin values are pegged each time a TCH is allocated or de-allocated. If a TCH is reconfigured as an SDCCH, only the SDCCH statistics will be pegged. AnalysisThe value of this statistic is not dependent on the number of Radio Channel Units (RCU). rather it is dependent on the number (however large) of simultaneously busy channels. The only dependence on the number of RCUs is that the value reported may be larger if more traffic channels are used simultaneously. This statistic can be used for trend analysis of the amount of RF signalling traffic in the cell TCHs. An analysis of the allocation information can be used to determine the requirement for additional resources before problems occur.

Reference GSM 12.04, 3.1.0, B1.1.1.GSM 4.08, 3.8.0, 3.3.1.2.1.

Usage

RF loss. Congestion. Quality of service monitoring. Network planning. Optimization. Key statistic: TCH_MEAN_HOLDING_TIME, TCH_TRAFFIC.

Basis Cell.

Type Weighted Distribution.

CALLS_QUEUEDDescription

This statistic counts only the Assignment Requests that are queued during an interval, not handovers. If queuing has been allowed and no resources exist, the CRM queues an assignment request and informs the RRSM with a force queue message. PeggingThis statistic is pegged when queueing is enabled in an assignment request and available queue blocks are used to queue the call successfully. AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of calls queued for each cell on the BSS that are acceptable in normal system operations. If the specified threshold is exceeded, the 20. CELL: Number of calls queued - PM alarm will be generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Reference GSM 3.01, 3.1.1, 4.1. GSM 4.08, 3.8.0, 5.2.1.1.10 and 5.2.2.8. GSM 8.08, 3.9.2, 3.1.17.

Usage

Quality of service monitoring: queuing delay. Network planning. Optimization. Fault finding. Ladder: BSS initiated call clearing.

Basis Cell.

Type Counter.

Alarm Warning.

CHAN_REQ_CAUSE_ATMPTDescriptionThis is a counter array statistic that counts the number of the Channel Request messages received by the network from a MS by cause. A total count of all successful results is also maintained.

Bin Cause Description

0 ORIGINATING_CALL Number of MS requests for originating service.

1 EMERGENCY_CALL Number of MS requests for emergency call service.

2 CM_REESTABLISH Number of failed MS requests for service in which the call recovered to the original cell.

3 LOCATION_UPDATE Number of MS location update requests.

4 PAGE_RESPONSE Number of MS page request responses.

PeggingThis statistic pegs the cause of each Channel Request message received by the network from a MS. AnalysisThis statistic can be used for trend analysis of the type of Channel Request messages that are received from MSs. This information can be used to used to determine the requirement for additional resources before problems occur.

Reference GSM 12.04, 4.21, B.2.1.6. GSM 4.08,v4.15.0, 9.1.8.

Usage Radio resource allocation. Quality of service monitoring. Network planning.

Basis Cell.

Type Counter array.

CHAN_REQ_MS_BLKDescriptionThis statistic counts the number of times a MS has been refused access to a channel. An Immediate Assignment Reject message was sent to the MS on CCCH. This event indicates that the MS has requested allocation of a dedicated channel, and the network has responded by refusing immediate access to a channel. Pegging of this counter indicates that there are no channels available for assignment in the cell. The counter pegs once for each Channel Request Reference in the message announcing the blocking. The cell is therefore fully used, and it will not be possible to hand over existing calls into the cell or set up new calls in the cell. If adjacent cells that are candidates for handover are unable to provide the MS with a radio resource, it will be impossible for the MS to initiate a call. Handover of calls may be prevented. This will result in call blocking and loss. PeggingThis statistics is pegged when an Immediate Assignment Reject message is sent to the MS on CCCH. Indicates there are no channels available for assignment in the cell. AnalysisThis statistic can be used for trend analysis of the number of times channel access has been denied. This information can be used to used to determine the requirement for additional resources.

Reference GSM 4.08, 3.8.0, 4.5.1.1, 8 and 9.1.27a

Usage

Radio resource and allocation. Fault finding. Network planning. Quality of service monitoring : Call setup. Ladder : Immediate assignment blocked

Basis Cell.

Type Counter.

CHAN_REQ_MS_FAILDescriptionThis statistic is the number of times that the BSS times out waiting for the MS to establish on the SDCCH that was assigned to it during the immediate assignment procedure (see Chapter 6, Carrier and timeslot statistics, click hereCHAN_REQ_MS_FAIL) CIPHER_MODE_FAILDescriptionThis statistic is pegged when an internal Motorola-defined timer in the BSS expires to indicate that the MS did not respond to the Cipher Mode Command message with the Cipher Mode Complete message within the allowable time period or responded improperly. It indicates that the MS probably did not switch correctly into encrypted communication mode when commanded to do so. PeggingThis statistic is pegged when the MS fails to respond to a Cipher Mode Command from the BSS. AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of times that MS failures to respond to BSS Cipher Mode Command messages are acceptable in normal system operations. If the specified threshold is exceeded, the 6. CELL: Cipher mode command from MSC failed - PM alarm will be generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Reference GSM 4.08, 3.8.0,8 and 9.1.9. GSM 8.08, 3.9.2, 3.1.14 and 3.2.1.30.

Usage Classmark update and cipher mode. Quality of service monitoring.

Fault finding. Ladder: BSS initiated call clearing.

Basis Cell.

Type Counter.

Alarm Minor.

CLASSMK_UPDATE_FAILDescriptionThis statistic is the number of classmark updates containing errors. The classmark update procedure allows the MS to inform the network of a change to its classmark. PeggingThis statistic is pegged when the validation of the Classmark Update message from the MS fails. AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of classmark updates containing errors that are acceptable in normal system operations. If the specified threshold is exceeded, the 4. CELL: Class-mark update from MS protocol error - PM alarm will be generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Reference GSM 4.08, 3.8.0,8 and 10.5.1.5-6. GSM 8.08, 3.9.2, 3.1.13 and 3.2.1.29.

Usage Classmark update and cipher mode. Quality of service monitoring. Fault finding.

Basis Cell.

Type Counter.

Alarm Minor.

CLR_REQ_TO_MSCDescriptionThis statistics counts the number of Clear Request messages sent to the MSC. GeneratingThe Clear Request message may be generated due to one of the following:

No radio resources available to allocate from the BSS, call queuing is enabled, and the maximum call delay has been exceeded.

Radio interface message failure. Protocol error. Ciphering algorithm not supported.

SendingAll Clear Requests are sent per GSM 8.08 specifications. The Clear Request messages are sent to the MSC because of radio channel failure or a timeout during a procedure. In this case a timeout means a required message was not received during a procedure. The five procedures are:

Immediate Assignment. Assignment. Channel Mode Modify. Ciphering. Handover - Intra-cell, Inter-cell, and External.

PeggingPegged when the BSS sends a Clear Request message to the MSC. CLR_REQ_TO_MSC is pegged for numerous reasons related to timer expiry. It is a mix of both GSM and internal Motorola timers. Normal release of a call either from a MS or Land site will never cause it to be pegged. A call must have at least been assigned to an SDCCH or TCH in order for this to be pegged, it is not pegged when on an AGCH overload. RF loss is the major cause of pegging.

The CLR_REQ_TO_MSC statistic equals the total number of lost calls and is pegged when any of the following statistics are pegged:

RF_LOSSES_SD. INTRA_BSS_HO_LOSTMS. RF_LOSSES_TCH. INTRA_CELL_HO_LOSTMS. CIPHER_MODE_FAIL.

CLR_REQ_TO_MSC is not pegged when the CONN_REFUSED statistic is pegged. However, there are a number of cases where CLR_REQ_TO_MSC is pegged and no associated statistic is pegged. For example, if during an assignment the BSS determines that the cell is barred, CLR_REQ_TO_MSC is pegged but no associated statistic is pegged. AnalysisThis statistic can be used for trend analysis of the number of Clear Request messages sent to the MSC. This information can be used to identify and correct problems before they become serious.

Reference GSM 4.08, 3.8.0, 5.4.3. GSM 8.08, 3.9.2, 3.2.1.20. and 3.1.9.2.

Usage

Radio resource allocation. Quality of service monitoring. Fault finding. Network planning. Ladder : BSS initiated call clearing, handover (Intra-cell) fail/lost, handover (Intra-BSS) fail/lost.

Basis Cell.

Type Counter.

CONGEST_EXIST_HO_ATMPTDescriptionThis statistic measures the number of attempted inter-cell handovers (internal or external to the BSS) of calls on a TCH due to TCH congestion from the source cell. PeggingThis statistic is pegged each time a congestion relief handover is attempted. This statistic is pegged for both internal inter-cell and external handovers. AnalysisThis statistic can be used for trend analysis of the number of internal and external handovers attempts due to TCH congestion from source cells.

Reference GSM 8.08, 4.90. GSM 3.09, 4.50. GSM 4.08, 4.11.0.

Usage Optimization. Network planning.

Basis Cell.

Type Counter.

CONGEST_STAND_HO_ATMPTDescriptionThis statistic measures the number of attempted handovers of a MS due to standard directed retry procedure caused by TCH congestion from the source cell. These inter-cell handovers (internal or external to the BSS) involve changing the channel mode during the handover. PeggingThis statistic is pegged each time a directed retry handover procedure is attempted, for both inter-cell and external handovers. Analysis

This statistic can be used for trend analysis of the number of attempted internal or external handovers due to standard directed retry mechanisms caused by TCH congestion.

Reference GSM 8.08, 4.90. GSM 3.09, 4.50. GSM 4.08, 4.11.0.

Usage Optimization. Network planning.

Basis Cell.

Type Counter.

CONN_REFUSEDDescriptionThis statistic records the number of SCCP connection refused (CREF) messages received from the MSC. PeggingThis statistic is pegged when the MSC sends the BSS an SCCP Connection Refused (CREF) message. The MSC may send an SCCP Connection Refused message in response to a BSS generated SCCP Connection Request (CR) message, as a result of

MS access for a location update. MS originated call establishment. MS terminated call establishment. SMS activity. Supplementary service activity. IMSI detach. Call re-establishment.

For example, if an association already exists for the identity of the MS requesting a connection, the connection request will be refused, and this statistic will peg. Analyse MSC statistics to determine why the CONN_REFUSED statistic is pegged. AnalysisThis statistic can be used for trend analysis of the reasons for connections refused from the MSC. This is an MSC procedure.

Reference GSM 8.06, 3.5.0, 6.1. GSM 8.08, 3.9.2, 3.1.16.

Usage

Originated call and service/connection request. Quality of service monitoring. Optimization. Network planning. Ladder: Connection establishment failure.

Basis Cell.

Type Counter.

CONN_REQ_TO_MSCDescriptionThis statistic records the number of connection requests that the BSS sends to the MSC. PeggingThe statistic will only be pegged for connection establishments initiated by the BSS. This message is sent by the MS when it is requesting set-up of the connections for several types of transactions, including location updates, connection establishments (including re-establishment on failure, short message transfer, and supplementary service requests), and paging responses. AnalysisThis statistic can be used for trend analysis of the number of connection requests sent from the BSS to the MSC.

Reference GSM 8.06, 3.5.0, 6.1. GSM 8.08, 3.9.2, 3.1.16.

Usage Originated call and service/connection request. Network planning. Ladder: Connection establishment successful, connection establishment failure.

Basis Cell.

Type Counter.

DYNET_ASSIGN_FAILDescriptionThis is a per-BSS counter which counts the number of times assignments have failed due to insufficient terrestrial backhaul resources on a BTS Dynamic Allocation Network. Pegging

Each time an assignment procedure fails, due to the lack of terrestrial backing resources. AnalysisA statistic to indicate the number of times assignments have failed due to insufficient terrestrial backhaul resources on a BTS Dynamic Allocation Network.

Usage Quality of service monitoring. Radio resource allocation. Network planning.

Basis BSS.

Type Counter.

Alarm None.

DYNET_CALL_REJECTSDescriptionThis is a per-BTS network counter array which counts the number of times calls have been blocked or preempted due to insufficient terrestrial backhaul resources on a BTS Dynamic Allocation Network. PeggingBin Procedures Description

0 NON_EMRG_CALL_BLK A nonemergency call is not allocated a TCH due to the lack of terrestrial backing resources in a BTS network.

1 EMRG_CALL_BLK An emergency call is not allocated a TCH due to the lack of terrestrial backing resources in a BTS network.

2 NON_EMRG_RESERVED A nonemergency call is not allocated a TCH due to the cell exceeding its reserve amount of terrestrial backing resources and no further backing resources are available.

3 NON_EMRG_PREEMP A nonemergency call is preempted to provide terrestrial backing resources for an emergency call.

4 LOSS_OF_RESOURCES An emergency or nonemergency call is preempted due to the loss of of terrestrial backing resources.

AnalysisThe appropriate bin is incremented when specific calls are rejected or preempted.

Usage Quality of service monitoring. Networking planning. Radio resource allocation.

Basis DYNET.

Type Medium counter array.

Alarm None.

ER_INTRA_CELL_HO_ATMPTDescriptionThis is a counter array statistic that counts the number of attempted intra-cell handovers between normal range and extended range channels in an extended range cell. A total count of attempted handover is also maintained. This statistic may only be enabled if the Extended Range Cell feature is unrestricted and enabled for the cell.

Bin Procedures Description

0 NORM_TO_EXT_HO Number of intra-cell handover attempts from normal range channels to extended range channels.

1 EXT_TO_NORM_HO Number of intra-cell handover attempts from extended range channels to normal range channels.

PeggingThis statistic is pegged each time a handover is attempted between a normal channel and an extended channel in an extended range cell. AnalysisThis statistic can be used for trend analysis of the attempted intra-cell handovers between normal and extended range channels in an extended range cell. The number of attempted extended range intra-cell handovers can be used to determine the number of extended range timeslots to configure in an extended range cell.

Reference None.

Usage Quality of service. Network planning. Radio resource allocation.

Basis Cell.

Type Counter array.

ER_INTRA_CELL_HO_SUCDescriptionThis is a counter array statistic that counts the number of successful intra-cell handovers between normal range and extended range channels in an extended range cell. The values reported for this statistic can be used to configure normal and extended range traffic channels in a cell. A total count of attempted handover is also maintained. This statistic may only be enabled if the Extended Range Cell feature is unrestricted and enabled for the cell.

Bin Procedures Description

0 NORM_TO_EXT_HO Number of successful intra-cell handovers from normal range channels to extended range channels.

1 EXT_TO_NORM_HO Number of successful intra-cell handovers from extended range channels to normal range channels.

PeggingThis statistic is pegged each time a handover is successful between a normal channel and an extended channel in an extended range cell. No other intra_cell handover statistic will be pegged when one of the bins of the ER_INTRA_CELL_HO_SUC statistic is pegged. AnalysisThis statistic can be used for trend analysis of the successful intra-cell handovers between normal and extended range channels in an extended range cell. The number of successful extended range intra-cell

handovers can be used to determine the number of extended range timeslots to configure in an extended range cell.

Reference None.


Basis Cell.

Type Counter array.

FLOW_CONTROL_BARREDDescriptionThis statistic measures the duration for which access classes are barred as a result of flow control. This statistic gives an indication when flow control actions have been taken to prevent overload. PeggingThe statistic duration is started when any of the access classes are barred due to flow control and stopped when the last access class which was barred due to flow control is unbarred. AnalysisThis statistic can be used for trend analysis of the access classes barred as a result of flow control.

Usage Quality of Service. Network Planning. Optimization.

Basis Cell.

Type Duration (total time).

HO_FAIL_NO_RESOURCESDescriptionThis is a counter array statistic that tracks the number of handover failures due to a lack of available resources.

Bin Scenario Description

0 INTRA_BSS Number of intra-BSS handover failures due to a lack of available resources.

1 INTER_BSS Number of inter-BSS handover failures due to a lack of available resources.

2 INTRA_CELL Number of intra-cell handover failures due to a lack of available resources.

3 INTRA_BSS_DYNET Number of BTS Dynamic Allocation intra-BSS handover failures due to a lack of available resources.

4 INTER_BSS_DYNET Number of BTS Dynamic Allocation inter-cell handover failures due to a lack of available resources.

5 INTRA_CELL_DYNET Number of BTS Dynamic Allocation intra-cell handover failures due to a lack of available resources.

PeggingThese individual bins are incremented on a per scenario basis. AnalysisThis statistic can be used for trend analysis of the handover failures due to a lack of resources, such as:

The target cell is barred for some reason, such as hardware failure. The channel is unavailable, ie there is congestion at the target cell and the queue timer is

expired. Incoming handovers are turned off at the target.

The speech algorithm at the target BSS is not supported by the MS. An SCCP connection is not allocated, ie CRM runs out of SCCP numbers.

Reference Metrica.

Usage Radio resource allocation. Quality of service monitoring. Network planning.

Basis Cell.

Type Counter array.

HO_REQ_MSC_FAILDescriptionThis statistic is the number of handover request failure messages sent to the MSC, except for no channel resources available. This message is sent to notify the MSC that the BSS could not reserve the radio resource that the MSC requested in the Handover Request message. PeggingThe HO_REQ_MSC_FAIL statistic is pegged in the source or target cell depending on where the failure occurs. This statistic will be pegged in the source cell if the handover failure originates from the MS. This indicates that the MS has reverted back to its original channel. This statistic will be pegged in the target cell if the handover failure message originates from the target cell. This could indicate one of the following problems: terrestrial circuit unavailable, terrestrial circuit in use, cyphering algorithm not supported, or radio resources unavailable. This statistic will not be pegged if resources are not available in the target cell. AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of handover failures for any reason other than lack of channel resources that are acceptable in normal system operations. If the specified threshold is exceeded, the 18. CELL: HO failure to the MSC due to all possible errors except no channels - PM alarm will be generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Reference GSM 8.08, 3.9.2, 3.1.5.3.2, 3.2.1.8 and 3.2.1.16.

Usage Handover. Fault finding. Ladder: Handover (Inter-BSS) blocked.

Basis Cell.

Type Counter.

Alarm Warning.

HO_REQ_MSC_OKDescriptionThis statistic is the number of handover acknowledge messages sent to the MSC by the destination cell in an inter-BSS (external) handover. PeggingThis statistic is pegged when the BSS sends a Handover Request Acknowledge message to the MSC. This message is sent to notify the MSC that the BSS has reserved the radio resource that the MSC asked for in the Handover Request message. AnalysisThis statistic can be used for trend analysis of successful radio resource reservations by the BSS in response to MSC Handover Request messages.

Reference GSM 8.08, 3.9.2, 3.1.5.3.1, 3.2.1.8 and 3.2.1.10.

Usage Handover. Network planning. Ladder: Handover (Inter-BSS) successful, handover (Inter-BSS) fail/recovered.

Basis Cell.

Type Counter.

IN_INTER_BSS_HODescriptionThis is a counter array statistic that tracks incoming inter-BSS handover procedures on a per scenario basis. A total value for this statistic is not meaningful and will not be displayed.


0 IN_INTER_BSS_HO_SUC Number of successful incoming inter-BSS handovers.

1 IN_INTER_BSS_MS_NO_SEIZE Number of times an MS does not seize an allocated channel. This statistic is not currently pegged.

2 IN_INTER_BSS_EQUIP_FAIL Number of incoming inter-BSS handovers that fail due to equipment failure.

3 IN_INTER_BSS_HO_CLEARED

Number of incoming inter-BSS handovers aborted due to call clearing. This scenario corresponds to the receipt of a Clear Command, SCCP Released, or Release Done (internal message) during the handover procedure.

PeggingThese individual bins are incremented on a per cause basis. AnalysisThis statistic can be used for trend analysis of incoming inter-BSS handover procedures on a per scenario basis.

Reference Metrica, GSM 12.04, 3.1.0, B.1.1.7.

Usage

Radio resource allocation. Quality of service monitoring. Network planning. Fault finding. Handover.

Basis Cell.

Type Counter array.

IN_INTRA_BSS_HODescriptionThis is a counter array statistic that tracks incoming intra-BSS handover procedures on a per scenario basis. A total value for this statistic is not meaningful and will not be displayed.


0 IN_INTRA_BSS_HO_SUC Number of successful incoming intra-cell handovers.

1 IN_INTRA_BSS_HO_LOSTMS Number of failed incoming intra-BSS handovers that also failed to recover to the original cell.

2 IN_INTRA_BSS_EQUIP_FAIL Number of failed incoming intra-BSS handovers that fail due to equipment failure.

3 IN_INTRA_BSS_HO_RETURN Number of failed incoming intra-BSS handovers that recover to the original cell/channel.

4 IN_INTRA_BSS_HO_CLEARED

Number of incoming intra-BSS handovers aborted due to call clearing. This scenario corresponds to the receipt of a Clear Command, SCCP Released, or Release Done (internal message) during the handover procedure.

PeggingThese individual bins are incremented on a per scenario basis. AnalysisThis statistic can be used for trend analysis of incoming intra-BSS handover procedures on a per scenario basis.

Reference Metrica. GSM 8.08, 3.9.2, 3.1.7, 3.1.5.2.1, 3.2.1.12.

Usage

Radio resource allocation. Quality of service monitoring. Network planning. Service retainability. Fault finding. Handover.

Basis Cell.

Type Counter array.

INTERBAND_ACTIVITYDescriptionThis is a counter array statistic that measures inter-band handover attempts in the BSS. Assignment and handover failures due to incorrect or unsupported frequency information are also tracked. The Multiband option must be enabled for this statistic to be enabled, disabled, or displayed.


0 PGSM_HO_ATMPT Attempts to handover to base GSM band.

1 EGSM_HO_ATMPT Attempts to handover to extended GSM band.

2 DCS1800_HO_ATMPT Attempts to handover to DSC1800 band.

3 PCS1900_HO_ATMPT Attempts to handover to PCS1900 band.

4 PGSM_HO_FAIL Handover failures to base GSM band.

5 EGSM_HO_FAIL Handover failures to extended GSM band.

6 DCS1800_HO_FAIL Handover failures to DCS1800 band.

7 PCS1900_HO_FAIL Handover failures to PCS1900 band.

8 INVALID_FREQ_ASGN Frequency not implemented for assignment request.

9 INVALID_FREQ_HO Frequency not implemented for handover.

PeggingThis individual bins are incremented on a per cause basis. For inter-band handovers, the bin of the destination frequency will be pegged. AnalysisThis statistic can be used for trend analysis of inter-band handover attempts in the BSS on a per cause basis.

Reference None.

Usage Handover. Quality of service monitoring.

Network planning. Service retainability.

Basis Cell.

Type Counter array.

INTRA_CELL_HODescriptionThis is a counter array statistic that tracks intra-cell handovers on a per scenario basis. A total value for this statistic is not meaningful and will not be displayed.


0 INTRA_CELL_HO_REQ Number of times an intra-cell handover is considered the best option by the handover evaluator process.

1 INTRA_CELL_HO_ATMPT Number of assignment commands sent to MS during an intra-cell handover.

2 INTRA_CELL_HO_SUC Number of successful intra-cell handovers.

3 INTRA_CELL_HO_LOSTMS Number of failed intra-cell handovers that also failed to recover to the original cell.

4 INTRA_CELL_HO_RETURN Number of failed intra-cell handovers that recovered to the original cell/channel.

5 INTRA_CELL_EQUIP_FAIL Number of attempted intra-cell handover failures due to equipment failure.

6 INTRA_CELL_HO_CLEARED

Number of incoming intra-cell handovers aborted due to call clearing. This scenario corresponds to the receipt of a Clear Command, SCCP Released, or Release Done (internal message) during the handover procedure.

PeggingThese individual bins are incremented on a per scenario basis. AnalysisThis statistic can be used for trend analysis of intra-cell handovers on a per scenario basis.

Reference

Metrica. GSM 12.04, 3.1.0, B.1.1.6. GSM 8.08, 3.9.2, 3.1.6, 3.2.1.25. GSM 4.08, 3.8.0, 9.1.3.

Usage

Radio resource allocation. Quality of service monitoring. Network planning. Fault finding. Service retainability.

Basis Cell.

Type Counter array.

INV_EST_CAUSE_ON_RACHDescriptionThis statistic counts the number of RACHs with invalid establishment cause. This helps to more fully account for occurrences of Phantom RACHs. PeggingThis statistic is pegged when the establishment cause is not validated. It is possible for invalid Channel Requests not to be pegged by this statistic because the Channel Requests are precisely formatted to prevent being caught by all the validation routines (channel

coder, RSS Layer 1, RSS Abis). These invalid Channel Requests that are not pegged are due to phantom RACHs. AnalysisThis statistic can be used for trend analysis of RACHs with invalid establishment causes.

Reference GSM 4.08, 3.8.0, 3.3.1.1, 3.3.2.2, 4.5.1.6, 5.5.4, 7.1.2, 9.1.8, and 9.2.4. GSM 4.03, 3.03, and 4.3.2.1.

Usage

Handover. Quality of service monitoring. Fault finding. Ladder: Immediate Assignment Failure.

Basis Cell.

Type Counter.

MA_CMD_TO_MSDescriptionThis statistic tracks the number of Assignment Commands sent to the MS. PeggingThis statistic pegs when the BSS initiates the channel assignment procedure by sending an Assignment Command to the MS on the main DCCH. Assignment Command is sent to the MS at call setup (traffic channel assignment) and at intra-cell handovers. This causes the MS to:

Release its link layer connections. Disconnect from its current physical channels. Switch to the assigned channels. Initiate re-establishment of the lower layer connections.

AnalysisThis statistic can be used for trend analysis of BSS initiated channel assignment procedures to the MS.

Reference GSM 4.08, 3.8.0, 3.4.3.1 and 9.1.2.

Usage

Radio resource allocation. Call setup. Ladder: TCH assignment successful, TCH assignment fail-recovered to SDCCH, handover (Intra-cell) successful, handover (Intra-cell) fail-recovered, handover (Intra-cell) fail-lost.

Basis Cell.

Type Counter.

MA_COMPLETE_TO_MSCDescriptionThis statistic provides the number of successful TCH seizures. PeggingThis statistic pegs when the "Assignment Complete" message from the MS is forwarded on to the MSC. AnalysisThis statistic can be used for trend analysis of successful TCH seizures.

Reference GSM 12.04, 4.2.1, B.2.1.15

Usage

Originated call and service/connection request. Quality of service: Call set-up. Network planning. Ladder: TCH assignment successful.

Basis Cell.

Type Counter.

MA_FAIL_FROM_MSDescriptionThis statistic is the number of Assignment Failure messages received from MSs. An Assignment Failure message can be received at call setup (traffic channel assignment) and at intra cell handovers. PeggingThis statistic is pegged when the BSS receives an Assignment Failure message from the MS. If the MS detects a lower layer failure on the new channels before the Assignment Complete message has been sent:

It deactivates the new channels. Reactivates the old channels. Reconnects any TCHs. Triggers establishment of the main signalling link. It then sends the Assignment Failure message on the main DCCH. This statistic pegs for

channel assignment failures at intra-cell handovers for new calls and channel assignment failures at handover.

AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of assignment failures for MSs that are acceptable in normal system operations. If the specified threshold is exceeded, the 23. CELL: Mobile assignment failure from MS - PM alarm will be generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Reference GSM 8.08, 3.9.2, 3.2.1.3 and 3.2.2.5. GSM 12.07, 3.0.2, 2.8.6.c. GSM 4.08, 3.8.0, 3.4.3.3 and 9.1.4.

Usage

Radio resource allocation. Quality of service monitoring: Call setup. Fault finding. Ladder: TCH assignment fail-recovered to SDCCH, handover (Intra-cell) fail-recovered.

Basis Cell.

Type Counter.

Alarm Warning.

MA_REQ_FROM_MSCDescriptionThis statistic measures the number of times the MSC asked the BSS to allocate radio resources. The MSC sends the BSS an Assignment request message containing details of the type of resource required. The type of resource is based on the MSC analysis of the call control information received from the MS. PeggingThis statistic pegs the radio resource request made to a BSS by the MSC. AnalysisThis statistic can be used for trend analysis of MSC request to the BSS to allocate radio resources.

Reference GSM 8.08, 3.9.2, 3.1.1.1 and 3.2.1.1.

Usage

Radio resource allocation. Quality of service monitoring: Call setup. Network planning. Ladder: TCH assignment successful, TCH assignment fail-recovered to SDCCH, TCH assignment blocked. Key Statistic: TCH_CONGESTION.

Basis Cell.

Type Counter.

MA_CMD_TO_MS_BLKDDescriptionThis statistic tracks the number of times traffic channels are congested. Assignment Command is sent to the MS at call setup (traffic channel assignment). PeggingThis statistic is pegged by the BSS when the MSC requests the BSS to allocate radio resource to an MS, and the BSS finds it has no channels available to allocate and the Assignment Request is NOT queued. The pegging of this statistic indicates no channels were available in the currently allocated cell. It is indicated by an Assignment Failure message with cause: No radio resource available. AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of times traffic channels that may be congested due to a lack of radio resources that are acceptable in normal system operations. If the specified threshold is exceeded, the 2. CELL: Mobile assign command to MS blocked (no channel available) - PM alarm will be generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Reference GSM 8.08, 3.9.2, 3.2.1.3 and 3.2.2.5.

Usage

Radio resource allocation. Quality of service monitoring : Call setup and handover. Fault finding. Network planning. Optimization. Ladder: TCH assignment blocked. Key statistic: TCH_CONGESTION.

Basis Cell.

Type Counter.

Alarm Minor.

MS_ACCESS_BY_TYPEDescriptionThis is a counter array statistic that tracks the number of system accesses by the various types of MSs. The type of MS is included in the MS Classmark 3 information. This statistic may only be enabled if the Multiband feature is unrestricted and enabled for the cell.

Bin MS Type Description

0 MS_PGSM_ONLY Base GSM band only.

1 MS_DCS1800_ONLY DCS1800 band only.

2 MS_PCS1900_ONLY PCS1900 band only.

3 MS_PGSM_EGSM Base and extended GSM bands.

4 MS_PGSM_DCS1800 Base GSM and DCS1800 bands.

5 MS_PGSM_EGSM_DCS1800 Base GSM, extended GSM, and DCS1800 bands.

6 MS_PGSM_PCS1900 Base GSM and PCS1900 bands.

7 MS_PGSM_EGSM_PCS1900 Base GSM, extended GSM, and PCS1900 bands.

PeggingA bin is pegged when a corresponding MS type accesses the system, at the beginning of a call. AnalysisThis statistic can be used for trend analysis of system accesses by MS type.

Usage Quality of Service monitoring. Network planning.

Basis Cell.

Type Counter array.

MS_TCH_USAGE_BY_TYPEDescriptionThis is a counter array statistic that tracks the length of time spent on TCHs by the various types of MSs. Duration is measured in deciminutes, that is, one deciminute equals six seconds. For example, if MS_TCH_USAGE_BY_TYPE = 31, the call duration is 3.1 minutes (186 seconds). The type of MS is included in the MS Classmark 3 information. This statistic may only be enabled if the Multiband feature is unrestricted and enabled for the cell.


0 MS_PGSM_ONLY Base GSM band only.

1 MS_DCS1800_ONLY DCS1800 band only.

2 MS_PCS1900_ONLY PCS1900 band only.

3 MS_PGSM_EGSM Base and extended GSM bands.

4 MS_PGSM_DCS1800 Base GSM and DCS1800 bands.

5 MS_PGSM_EGSM_DCS1800 Base GSM, extended GSM, and DCS1800 bands.

6 MS_PGSM_PCS1900 Base GSM and PCS1900 bands.

7 MS_PGSM_EGSM_PCS1900 Base GSM, extended GSM, and PCS1900 bands.

PeggingEach bin is updated by the cell resource manager when the duration for an active channel is recorded. A total duration is calculated which will be the sum for all channels usage time by MSs in a given cell. AnalysisThis statistic can be used for trend analysis of active channel usage. This information can be used to identify problems before they can affect service.

Usage Service accessibility. Network planning.

Basis Cell.

Type Counter array.

NUM_EMERG_ACCESSDescriptionThis statistic indicates the number of emergency calls that access the system. PeggingThis statistic is pegged after the receipt of an emergency RACH. AnalysisThis statistic can be used for trend analysis of emergency call accesses.

Usage Quality of Service monitoring. Network planning.

Basis Cell.

Type Counter.

NUM_EMERG_REJECTEDDescription

This statistic counts the number of times an emergency call accesses the system and has to be immediately rejected because there are no resources and another call cannot be pre-empted to allow it access. PeggingThis statistic pegs the number of emergency calls that are rejected because all channels were occupied by other emergency or high priority calls. AnalysisThis statistic can be used for trend analysis of emergency call accesses that failed because all channels were occupied by other emergency or high priority calls.

Usage Quality of Service. Network planning.

Basis Cell.

Type Counter.

NUM_EMERG_TCH_KILLDescriptionThis statistic counts the number of times a call is lost due to emergency call pre-emption and not to other factors, for example, RF losses. The active channel with the lowest priority is the one that releases a TCH. Additionally, emergency calls will not be pre-empted by other emergency calls. PeggingThis statistic pegs the number of active calls that are terminated in order to make a TCH available for an emergency call. This situation will arise if all TCHs are occupied and an emergency access requires a channel. This statistic will only be pegged if the emergency pre-emption feature is enabled. AnalysisThis statistic can be used for trend analysis of the number of times that calls are lost due to emergency call pre-emption only.


Basis Cell

Type Counter.

NUM_EMERG_TERM_SDCCHDescriptionThis statistic is the number of emergency accesses terminated after SDCCH congestion. Used to indicate that an emergency call was allocated an SDCCH but could not be allocated a TCH. PeggingThis statistic is pegged each time an emergency call is assigned to an SDCCH but must be terminated because no TCHs are available to complete the call. This statistic will be pegged regardless of the status of the emergency preemption feature. AnalysisThis statistic can be used for trend analysis of the number of times that emergency accesses are terminated after SDCCH congestion.


Basis Cell.

Type Counter.

OK_ACC_PROCDescriptionThis is a counter array statistic that counts successful accesses by procedure. A total count of all successful accesses is also maintained.

Bin Procedure Description

0 CM_SERV_REQ_CALL Number of MS requests for originating service.

1 CM_SERV_REQ_SMS Number of MS requests for SMS service.

2 CM_SERV_REQ_SUPP Number of MS requests for supplementary services.

3 CM_SERV_REQ_EMERG Number of MS requests for emergency call service.

4 CM_REESTABLISH Number of failed MS requests for service in which the call recovered to the original cell.

5 LOCATION_UPDATE Number of MS location update requests.

6 IMSI_DETACH Number of received IMSI detach messages.

7 PAGE_RESPONSE Number of MS page request responses.

PeggingThis statistic pegs the total number of successful results per procedure. AnalysisThis statistic can be used for trend analysis of the number of successful accesses by procedure.

Reference GSM 12.04, 4.21, B.3.1.2. GSM 4.08, 4.12, 3.15, 3.3.2.2, 4.4.1, 4.4.4.1, 4.5.1.1, 4.5.1.1, 4.5.1.6, 5.5.4, 7.3.1, 9.2.9, 9.2.12, 9.2.15, and 9.45.

Usage

Radio resource allocation. Quality of service monitoring: Call setup. Network planning. Ladder: Immediate assignment successful. Key Statistic: RF_LOSS_RATE, TCH_ASSIGN_SUCCESS_RATE, SDCCH_MEAN_HOLDING_TIME, SDCCH_MEAN_ARRIVAL_RATE, SDCCH_RF_LOSS_RATE.

Basis Cell.

Type Counter array.

OK_ACC_PROC_SUC_RACHDescriptionThis statistic counts successful Channel Request messages on the RACH of a cell. This message is used by the MS to request allocation of a dedicated channel (to be used as a SDCCH) by the network, in response to a paging message (incoming call) from the network or as a result of an outgoing call/supplementary short message service dialled from the MS. PeggingThis statistic pegs what it believes to be a valid Channel Request (RACH). This does not necessarily result in a seizure attempt of an SDCCH. AnalysisThis statistic can be used for trend analysis of the number of successful channel requests on the RACH of a cell.

Reference GSM 4.08, 3.8.0, 3.3.1. 3.3.2.2, 4.5.1.6, 5.5.4, 9.1.8 and 9.2.4.GSM 12.04, 3.1.0, B.1.2.1.

Usage Radio resource allocation.Ladder: Immediate assignment successful, Immediate assignment blocked, Immediate assignment failure.

Basis Cell.

Type Counter.

OUT_HO_CAUSE_ATMPT

DescriptionThis statistic monitors the number of handover attempts per cell per cause. The handover attempts include cases involving outgoing inter-cell handovers.


0 UPQUAL Number of times an outgoing inter-cell handover is attempted due to the Uplink signal quality in source cell.

1 UPLEVEL Number of times an outgoing inter-cell handover is attempted due to the Uplink signal level in source cell.

2 DOWNQUAL Number of times an outgoing inter-cell handover is attempted due to the Downlink signal quality in source cell.

3 DOWNLEVEL Number of times an outgoing inter-cell handover is attempted due to the Downlink signal level in source cell.

4 DISTANCE Number of times an outgoing inter-cell handover is attempted due to a weakening signal strength to/from MS because of the distance from signal source.

5 UPINTERF Number of times an outgoing inter-cell handover is attempted due to the Uplink signal interference in source cell.

6 DOWNINTERF Number of times an outgoing inter-cell handover is attempted due to the Downlink signal interference in source cell.

7 POWERBDGT Number of times an outgoing inter-cell handover is attempted due to the power budget assessment of signal strength in the source cell.

8 CONGESTION Number of times an outgoing inter-cell handover is attempted due to traffic channel (TCH) congestion in the source cell.

9 ADJ_CHAN_INTF Number of times an outgoing inter-cell handover is attempted due to adjacent channel interference.

10 BAND_RE_ASSIGN Number of times an outgoing inter-cell handover is attempted due to band reassignment in source cell.

11 BAND_HANDOVER Number of times an outgoing inter-cell handover is attempted due to band handover.

PeggingPegs the total number of handover attempts out of a specific cell and the cause for each attempt. This statistic is pegged at the source cell.

Reference GSM 12.04, 4.2.1, B.1.1.11

Usage

Handover. Quality of service. Network planning. Ladder: Intra-BSS handover. Optimization. Fault finding.

Basis Cell.

Type Counter array.

OUT_INTER_BSS_HODescriptionThis is a counter array statistic that counts a range of handover actions associated with outgoing inter-BSS handovers. A total value for this statistic is not meaningful and will not be displayed.


0 OUT_INTER_BSS_REQ_TO_MSC Number of outgoing inter-BSS handover requests to the MSC.

1 OUT_INTER_BSS_HO_ATMPT Number of assignment commands sent to MS during an inter-BSS handover.

2 OUT_INTER_BSS_HO_SUC Number of successful inter-BSS handovers.

3 OUT_INTRA_BSS_HO_LOSTMS Number of failed intra-BSS handovers in which the MS also failed to recover to the original BSS. This statistic is not currently pegged.

4 OUT_INTER_BSS_HO_RETURN Number of failed inter-BSS handovers in which the MS recovered to the original BSS/channel.

5 OUT_INTER_BSS_EQUIP_FAIL Number of attempted inter-BSS handover failures due to equipment failure.

6 OUT_INTER_BSS_HO_CLEARED

Number of outgoing inter-BSS handovers aborted due to call clearing. This scenario corresponds to the receipt of a Clear Command, SCCP Released, or Release Done (internal message) during the handover procedure.

PeggingThis statistic is pegged for a range of handover actions that are typically involved in the outgoing inter-BSS handover scenarios that occur. AnalysisThis statistic can be used for trend analysis of inter-BSS handovers on a per handover scenario basis.

Reference

GSM 8.08, 3.9.2, 3.1.5.3.1 and 3.2.1.11. GSM 4.08, 3.8.0, 9.1.16 and 3.4.4.4. GSM 3.1.5.1.1 and 3.2.1.9. GSM 12.04, 3.2.0, B.1.1.8. GSM 3.1.9.3 and 3.2.1.21. Metrica.

Usage


Basis CELL.

Type Counter array.

OUT_INTRA_BSS_HODescriptionThis is a counter array statistic that counts a range of handover actions associated with outgoing intra-BSS handovers. A total value for this statistic is not meaningful and will not be displayed.


0 OUT_INTRA_BSS_HO_REQ Number of times an intra-BSS Handover Recognised message is received. This message is sent when the system recognises a handover is needed.

1 OUT_INTRA_BSS_HO_PRI_BLK Number of times an intra-BSS handover to the primary target cell was blocked.

2 OUT_INTRA_BSS_HO_ATMPT Number of assignment commands sent to MS during an intra-BSS handover.

3 OUT_INTRA_BSS_HO_SUC Number of successful intra-BSS handovers.

4 OUT_INTRA_BSS_HO_LOSTMS Number of failed intra-BSS handovers in which the MS also failed to recover to the original BSS.

5 OUT_INTRA_BSS_HO_RETURN Number of failed intra-BSS handovers in which the MS recovered to the original BSS/channel.

6 OUT_INTRA_BSS_EQUIP_FAIL Number of attempted intra-BSS handover failures due to equipment failure.

7 OUT_INTRA_BSS_HO_CLEARED

Number of outgoing intra-BSS handovers aborted due to call clearing. This scenario corresponds to the receipt of a Clear Command, SCCP Released, or Release Done (internal message) during the handover procedure.

PeggingThis statistic is pegged for a range of handover actions that are typically involved in the outgoing intra-BSS handover scenarios that occur. AnalysisThis statistic can be used for trend analysis of intra-BSS handovers on a per (handover) scenario basis.

Reference

GSM 4.08, 3.8.0, 3.4.4.4. GSM 8.08, 3.9.2, 3.2.1.16 and 3.2.1.20. GSM 4.08, 3.8.0, 3.4.4.4 & 7.3.6 and 9.1.16. GSM 8.08, 3.9.2, 3.1.5.3.2 and 3.2.1.16. GSM 8.08, 3.9.2, 3.1.7 and 3.1.5.1.1. GSM 4.08, 3.8.0, 3.4.4. GSM 8.08, 3.9.2, 3.1.7 and 3.1.5.3.1. GSM12.04, 3.2.0, B.1.1.9. GSM 8.08, 3.9.2, 3.1.7 and 3.1.5.2.1 and 3.2.1.12 & 3.2.1.25. Metrica.

Usage


Basis CELL.

Type Counter array.

PAGE_REQ_FROM_MSCDescriptionThis statistic indicates the number of paging requests received from the MSC. This message is sent by the network when attempting to locate the MS. Each Page message refers to only one MS. The BSS in turn will transmit a paging message which may include identities for more than one MS (see ACCESS_PER_PCH). PeggingThis statistic pegs the number of Paging messages received from the MSC by the BSS. This statistic is pegged when a Paging message is received pertaining to the cell in which the MS is paged. AnalysisThis statistic can be used for trend analysis of the number of paging requests received from the MSC.

Reference GSM 4.08, 3.8.0, 3.3.2.1. GSM 8.08, 3.9.2, 3.1.10 and 3.2.1.19.

Usage Quality of service monitoring: Paging analysis.

Basis Cell.

Type Counter.

PCH_AGCH_Q_LENGTHDescriptionThis statistic measures the mean number of messages reported in the PCH-AGCH queue. PeggingThis statistic will be pegged once per paging multiframe, by summing the total number of messages in the paging queue. At the end of the interval, the mean number of messages will be calculated and reported. AnalysisThe 12.04 statistic B.2.1.1 is the mean of the number of messages in the PCH-AGCH queue.

Reference GSM 12.04, 4.2.1, B.2.1.1

Usage Quality of service: Paging analysis. Optimization. Network planning.

Basis Cell.

Type Gauge.

PCH_Q_PAGE_DISCARDDescriptionThis statistic provides the number of paging messages discarded from the PCH queue before they could be transmitted. The reasons pages may be discarded from the queue include: queue overflow, priority insertion causing an overflow, and an in_queue timer expiry. The only cause for discarding pages in the Motorola BSS is queue overflow. PeggingThis statistic is pegged each time a page from the MSC is overwritten while in the queue. AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of paging messages that may be discarded from the PCH queue that are acceptable in normal system operations. If the specified threshold is exceeded, the 24. CELL:PCH Queue Page Discard - PM alarm is generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Reference GSM 12.04, 4.2.1, B.2.1.8

Usage Quality of service: Paging analysis. Optimization. Network planning.

Basis Cell.

Type Counter.

Alarm Minor.

RF_LOSSES_SDDescriptionThis statistic is the number of calls lost while using a SDCCH. If a TCH is reconfigured as a SDCCH, only the SDCCH statistics will be configured. PeggingThis statistic pegs the number of calls that were terminated because of RF problems. This statistic will only peg for lost calls after the call has been established. This statistic will not be pegged when the CHAN_REQ_MS_FAIL statistic is pegged. AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of lost calls while using a SDCCH that are acceptable in normal system operations.

If the specified threshold is exceeded, the 0. CELL: Radio frequency losses while using an SDCCH - PM alarm will be generated. Refer to the Maintenance Information: Alarm Handling at the OMC (GSM-100-501) for troubleshooting information.

Reference GSM 12.04, 3.1.0, B.1.1.5. GSM 4.08, 3.8.0, 3.5.2 and 9.1.27a and 10.5.2.19.

Usage

RF loss. Congestion. Quality of service monitoring. Fault finding. Installation and commissioning. Key statistic: RF_LOSS_RATE, SDCCH_RF_LOSS_RATE.

Basis Cell.

Type Counter.

Alarm Major.

RF_LOSSES_TCHDescriptionThis statistics counts the number of calls lost while using a TCH. PeggingThis statistic pegs the number of calls that were terminated because of RF problems. It is composed of calls lost while using a TCH. If a TCH is reconfigured as an SDCCH, only the SDCCH statistics will be configured. AnalysisA threshold value should be assigned to this statistic which reflects the maximum number of lost calls while using a TCH that are acceptable in normal system operations.

Reference GSM 12.04, 3.1.0, B.1.1.5. GSM 4.08, 3.8.0, 3.5.2, 9.1.27a and 10.5.2.19.

Usage

RF loss. Congestion. Quality of service monitoring. Fault finding. Installation and commissioning. Key Statistic: RF_LOSS_RATE, TCH_RF_LOSS_RATE.

Basis Timeslot.

Type Counter.

Alarm Major.

SDCCH_CONGESTIONDescriptionThis statistic indicates the amount of time that no SDCCH channels were available in a cell. PeggingThis statistic records the sum of the duration of periods when all the SDCCH channels in a cell were busy. The statistic also collects the mean, maximum, and minimum time of SDCCH congestion. If a TCH is reconfigured as an SDCCH, only the SDCCH statistics will be recorded. Although the mean, maximum and minimum values are collected for this statistic they can only be viewed through the MMI using the disp_stats command. AnalysisThis statistic can be used for trend analysis of the length of time during which SDCCH channels were not available.

Reference GSM 12.04, 3.1.0, B.1.3.5.

Usage

RF loss. Congestion. Quality of service monitoring. Network planning. Optimization.

Basis Cell.

Type Duration (total time in milliseconds).

SECOND_ASSIGN_ATMPTDescriptionThis statistic indicates the number of times the network initiated a second assignment procedure on receipt of an Assignment Failure message from the MS. PeggingThe BSS only attempts (initiates) a second assignment procedure on receipt of an Assignment Failure message if the per BSS element second_asgnmnt is set.

Reference None.

Usage Congestion.

Basis Cell.

Type Counter.

SECOND_ASSIGN_SUCDescriptionThis statistic indicates the number of times that a second assignment procedure initiated by the network is successfully completed, upon receipt of an Assignment Complete message from the MS. PeggingThe BSS only attempts (initiates) a second assignment procedure on receipt of an Assignment Failure message if the per BSS element second_asgnmnt is set.

Reference None.

Usage Congestion.

Basis Cell.

Type Counter.

SMS_INIT_ON_SDCCHDescriptionThis statistic shows how many Short Message Service (SMS) transactions occurred on a cell. This statistic can be used to keep track of the number of times a Service Access Point (SAPI3) connection is created, a SMS requires a logical SAPI3 connection. Additionally, the statistic can be used to determine how SDCCHs are being utilized (for example, for SMS as opposed to location update). PeggingThis counter is incremented on one successful Service Access Point (SAPI3) connection related to a SDCCH. AnalysisThis statistic can be used for trend analysis of how many SMS transactions occurred on a cell.

Usage Originated call and service/connection request.

Basis Cell.

Type Counter.

SMS_INIT_ON_TCHDescription

This statistic shows how many Short Message Service (SMS) transactions occurred on a cell related to a TCH. This statistic can be used to keep track of the number of times a Service Access Point (SAPI3) connection is created, a SMS requires a logical SAPI3 connection. Additionally, the statistic can be used to determine how TCHs are being utilized (for example, for SMS as opposed to normal voice traffic). PeggingThis counter is incremented upon successful SAPI3 connection related to a TCH. AnalysisThis statistic can be used for trend analysis of how many SMS transactions occurred on a cell related to a TCH.

Usage Originated call and service/connection request.

Basis Cell.

Type Counter.

SMS_NO_BCAST_MSGDescriptionThis statistic indicates the number of times that a message has been broadcast on a per cell basis. PeggingThis statistic is pegged each time a message is broadcast on the CBCH. AnalysisThis statistic can be used for trend analysis of how many times a message has been broadcast on a per cell basis.

Usage Fault finding.

Basis Cell.

Type Counter.

TCH_CONGESTIONDescriptionThis statistic indicates the amount of time that no TCH channels were available in that cell (see Chapter 12, Network health reports, click hereTCH_CONGESTION). TCH_CONG_INNER_ZONEDescriptionThis statistic measures the total length of time in which all of the TCHs in the inner zone of a concentric cell are busy. The Concentric Cell option must be enabled for this statistic to be enabled, disabled, or displayed. PeggingThe timer for this statistic is started when all TCHs belonging to TCHs in the inner zone are busy. The timer is stopped when an inner zone TCH is freed. AnalysisThis statistic can be used for trend analysis of the length of time in which all of the TCHs in the inner zone of a concentric cell are busy.

Reference None.

Usage Network planning.

Basis Cell.

Type Duration.

TCH_DELAYDescriptionThis statistic records the mean delay and the statistical distribution of the delay between an assignment request or handover request and a TCH being allocated for each cell on the BSS.

As more calls (in proportion to the total number of calls) are queued, the mean delay will be longer. Queueing is only done if the assignment or Handover Request indicates that queueing is allowed for that request. This statistic will measure the queueing delay effectively. It is possible to have assignments and handovers for which there is no delay in resource allocation. These no-delay assignments are included in the mean delay statistic. PeggingA timer is started when the request is queued and stopped when request is allocated. AnalysisThis statistic can be used for trend analysis of the queueing delay for a traffic channel being allocated to a cell.

Reference

GSM 12.04, 3.1.0, B.1.1.3. GSM 3.01, 3.1.1, 4.1. GSM 8.08, 3.9.2, 3.1.17. GSM 4.08, 3.8.0, 5.2.1.1.10 and 5.2.2.8.

Usage Quality of service monitoring: Queueing delay. Network planning. Optimization.

Basis Cell.

Type Normal distribution.

TCH_Q_LENGTHDescriptionThis statistic provides the arithmetic mean of the number of queued TCH assignment procedures. Queueing is done due to the Call Queueing feature, Emergency Call Pre-emption, EGSM, and Directed Retry. PeggingThis measurement is obtained by sampling the TCH queue length and reporting the current length. The mean TCH queue length includes the number of queued assignment and external handover requests. AnalysisThis statistic can be used for trend analysis of the number of queued TCH assignment procedures.

Reference GSM 12.04, 4.2.1, B.2.1.15

Usage Quality of service: Network accessibility. Network planning. Optimization.

Basis Cell.

Type Weighted distribution.

TCH_USAGEDescriptionThis statistic gives an indication of the total amount of traffic carried by the cell. Active means connected (capable of transmitting circuit mode user data) and Activated, for example, used as a DCCH. The TCH_USAGE statistic will be used only for outer zone resources when TCH usage is being measured for Concentric Cells. The TCH_USAGE statistic will only measure the usage of normal range channels in Extended Range Cells. PeggingThis statistic is the sum of the time that each TCH was active with a call for all TCHs on the cell. If a TCH is reconfigured as an SDCCH, only the SDCCH statistics will be configured. AnalysisThis statistic can be used for trend analysis of the total amount of traffic carried by the cell.

Reference GSM 4.08, 3.8.0, 3.1.4.

Usage

RF loss. Congestion. Quality of service monitoring. Network planning. Optimization.

Basis Cell.

Type Counter.

TCH_USAGE_EXT_RANGEDescriptionThis statistic is the number of extended range traffic channels in use. This statistic may only be enabled if the Extended Range Cell feature is unrestricted and enabled for the cell. PeggingThis statistic is pegged each time an extended range traffic channel is used. AnalysisThis statistic can be used for trend analysis of the number of extended range traffic channels in use.

Reference None.


Basis Cell.

Type Counter.

TCH_USAGE_INNER_ZONEDescriptionThis statistic measures the usage of TCHs belonging to carriers in the inner zone of a concentric cell. The Concentric Cell option must be enabled for this statistic to be enabled, disabled, or displayed. The TCH_USAGE statistic will be used only for outer zone resources when the Concentric Cells feature is unrestricted. PeggingThis statistic is pegged when a TCH belonging to a carrier in the inner zone is used. AnalysisThis statistic can be used for trend analysis of the usage of TCHs belonging to carriers in the inner zone of a concentric cell.

Reference None.


Basis Cell.

Type Counter.

TOTAL_CALLSDescriptionThis statistic counts the total number of calls originated for each cell on the BSS. PeggingThis statistic indicates the number of circuit oriented calls that are originated in the cell. It is pegged only once per connection at the time of the first successful TCH assignment procedure. Subsequent channel changes, for example voice to data, are not counted.

This statistic does not necessarily peg all successful calls. It only pegs calls that have been assigned to the TCH. For example, if the call fails to send a Direct Transfer Application Part (DTAP) Connect Acknowledge, it still looks successful to the base. AnalysisThis statistic can be used for trend analysis of the total number of calls originated for each cell on the BSS.

Reference GSM 4.08, 3.8.0, 5.2.

Usage

Originated call and service/connection request. Quality of service monitoring : Call setup. Network planning. Ladder: TCH assignment successful. Key Statistics: RF_LOSS_RATE, TCH_ASSIGN_SUCCESS_RATE, TCH_MEAN_HOLDING_TIME, TCH_MEAN_ARRIVAL_RATE, TCH_RF_LOSS_RATE.

Basis Cell.

Type Counter.

ZONE_CHANGE_ATMPTDescriptionThis is a counter array statistic that is used to track attempts of each type of concentric cell specific handover. The Concentric Cell option must be enabled for this statistic to be enabled, disabled, or displayed.


0 INNER_TO_OUTER_ZONE Number of handover attempts from inner zone to outer zone.

1 OUTER_TO_INNER_ZONE Number of handover attempts from outer zone to inner zone.

2 INTRA_ZONE Number of intra-zone handover attempts.

3 TCH_ASSIGN_TO_INNER_ZONE Number of TCH assignment attempts to inner zone cells.

4 IN_INTER_CELL_HO_TO_IN_ZONE Number of internal inter-cell handover attempts to inner zone.

PeggingEach bin is pegged when a corresponding concentric cell handover is attempted. AnalysisThis statistic can be used for trend analysis of the attempts of each type of concentric cell specific handover.

Reference None.


Basis Cell.

Type Counter array.

ZONE_CHANGE_SUCDescriptionThis is a counter array statistic that is used to track each type of successful concentric cell specific handover. The Concentric Cell option must be enabled for this statistic to be enabled, disabled, or displayed.


0 INNER_TO_OUTER_ZONE Number of successful handovers from inner zone to outer zone.

1 OUTER_TO_INNER_ZONE Number of successful handovers from outer zone to inner zone.

2 INTRA_ZONE Number of successful intra-zone handovers.

3 TCH_ASSIGN_TO_INNER_ZONE Number of successful TCH assignments to inner zone cells.

4 IN_INTER_CELL_HO_TO_IN_ZONE Number of successful internal inter-cell handovers to inner zone.

PeggingEach bin is pegged when a corresponding concentric cell handover is successful. AnalysisThis statistic can be used for trend analysis of each type of successful concentric cell specific handover.

Reference None.


Basis Cell.

Type Counter array.

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BSS cell planningPlanning requirements

Quality of Net

When planning a mobile telephone system, the aim is to create a communications network that fulfils the following requirements:

Provides the desired capacity. Offers good frequency efficiency. Implemented at low cost. High grade of service.

These requirements, when analyzed, actually conflict with one another. Therefore the operating network is always a solution achieved through compromise. The cost of different network configurations can vary considerably. From an engineering point of view it would be worth while using the most frequency efficient solutions despite their high cost, but a mobile telephone network is so huge an investment that the financial factors are always going to limit the possibilities. The effect of limited funds is particularly obvious when the first stage of the network is being built. Consequently, economical planning is a condition for giving the best possible service from the start. The use of the GSM900, EGSM, and DCS1800 frequency bands, create many propagation based problems. Because the channel characteristics are not fixed, they present design challenges and impairments that must be dealt with to protect MS telephone users from experiencing excessively varying signal level and lack of voice quality. It is important to be able to predict the RF path loss between the BTS and the MS within the coverage area in different types of environment. To do this it is necessary to have knowledge of the transmitter and receiver antenna heights, the nature of the environment and the terrain variations.

Planning factors When planning the network there are a number of major factors which must be considered to enable the overall system requirements to be met.

1. click here Planning tools. 1. click here GSM frequency spectrum:

Modulation techniques and channel spacing.1. click here Traffic capacity:

Unit of measure and grade of service.1. click here Capacity calculations:

Typical call parameters.1. click here Control channel calculations:

Number of CCCH per BTS cell.Number of SDCCH per BTS cell.Control channel configurations.

1. GPRS effective load. 1. click here Propagation effects on GSM frequencies:

Introduction to decibels.Fresnel zoneRadio refractive index.Environmental effects on propagation.Multipath propagation.Free space loss.Plane earth loss.Antenna gain.Clutter factor.Power budget and system balance.

1. click here Frequency re-use: Re-use patterns.Carrier to interference ratio.Co-channel interference.Adjacent channel interference.Sectorization of sites.

1. click here Overcoming adverse propagation effects: Frequency/baseband/synthesizer hopping.Block and diagonal interleaving.Directional antennas, sectorization.Uplink and downlink power control.Discontinued transmissions.Receive diversity.Equalization.

1. click here Subscriber environment: Environment.Future planning.

1. click here The microcellular solution. Planning tools IntroductionIn order to predict the signal strength in a cell area it would be necessary to make many calculations, at regular intervals, from the BTS. The smaller the interval the more accurate the propagation model. Also the calculations would need to be performed at regular distances along each radial arm from the BTS, to map the signal strength as a function of distance from the BTS. The result, is the necessity to perform hundreds of calculations for each cell. This would be time consuming in practice, but for the intervention of the software planning tool. This can be fed with all the details of the cell, such as:

Type of terrain. Environment. Heights of antennas.

It can perform the necessary number of calculations needed to give an accurate picture of the propagation paths of the cell. Several planning tools are available on the market, such as Netplan or planet, and it is up to the users to choose the tool(s) which suit them best. After calculation and implementation of the cell, the figures should then be checked by practical measurements. This is because, with all the variable factors in propagation modelling, an accuracy of 80% would be considered excellent. GSM frequency spectrum

The GSM900 frequency spectrumThe original GSM frequency spectrum was allocated in 1979. This consisted of two sub-bands 25 MHz wide. The frequency range is:


It is usual for the uplink frequencies - mobiles transmit to the BTS - to be on the lowest frequency band . This is because there is a lower free space path loss for lower frequencies. This is more advantageous to the mobile as it has a reduced transmit output power capability compared to the BTS. The two bands are divided into channels, a channel from each band is then paired with one of the pair allocated for uplink and one for the downlink. Each sub-band is divided into 124 channels, these are then given a number known as the Absolute Radio Frequency Channel Number (ARFCN). So a mobile allocated an ARFCN will have one frequency to transmit on and one to receive on. The frequency spacing between the pair is always 45 MHz for GSM. The spacing between individual channels is 200 kHz and at the beginning of each range is a guard band. It can be calculated that this will leave 124 ARFCNs for allocation to the various network operators. These ARFCNs are numbered 1 to 124 inclusive To provide for future network expansion more frequencies were allocated to GSM as they became available. An extra 10 MHz was added on to the two GSM bands and this became known as Extended GSM (EGSM). The EGSM frequency range is:-


This allows another 50 ARFCNs to be used bringing the total to 174. These additional ARFCNs are numbered 975 to 1023 inclusive. One thing to note is that original Phase 1 MSs can only work with the original GSM frequency range and it requires a Phase 2 MS to take advantage of the extra ARFCNs. As the operator cannot guarantee that his network will have a significant number of Phase 2 MS, care must be taken when using EGSM frequencies not to make holes in the network for Phase 1 MSs.

The DCS1800 frequency spectrumAs GSM evolved it was decided to apply the technology to the Personal Communications Networks. This required changes to the air interface to modify the frequency range over which it operates. The modified frequency range is:


This provides 374 ARFCNs with a frequency separation of 95 MHz between uplink and downlink frequencies. In the UK these ARFCNs have been shared out between the four network operators, refer to Figure 3-1. Two of these, Orange and One to One operate exclusively in the DCS1800 range while the other two, Vodafone and Cellnet have been allocated DCS1800 channels on top of their GSM900 networks. ARFCNs are numbered from 512 to 885 inclusive The portion at the top of the band is used by Digital enhanced Cordless telephony (DECT).

Figure 3-1 UK network operatorsThe PCS1900 frequency spectrumThis is another adaptation of GSM into the 1900 MHz band. It is used in the United States where the Federal Communications Commission has divided the band into 300 ARFCNs and issued licences to various operators to implement GSM networks. The frequency separation is 80 MHz. The frequency range is :


Absolute radio frequency channel capacityEach RF carrier supports eight time division multiplexed physical channels and each of these is capable of supporting speech or signalling information. The maximum number of RF carriers at any one BTS site is 24 for M-Cell6 and 25 for BTS6. Therefore the maximum number of physical channels available at a BTS site is 24 x 8 = 192, for M-Cell6 and 25 x 8 = 200, for BTS6.

Figure 3-2 Eight TDMA timeslots per RF carrierModulation techniques and channel spacingThe modulation technique used in GSM is Gaussian minimum shift keying. This works by shaping the data to be modulated with a Gaussian filter. The filter removes some of the harmonics from the data square wave producing a more rounded shape. When this is applied to a phase modulator the result is a modified envelope shape at the output of the modulator. The bandwidth of this envelope is narrower than that of a comparable one produced from non-filtered data. With each modulating carrier occupying a narrower bandwidth, more efficient use can be made of the overall bandwidth available. The bandwidth allocated to each carrier frequency in GSM is 200 kHz. The actual bandwidth occupied by a transmitted GSM carrier is far greater than 200 kHz, even with Gaussian filtering. The signal therefore overlaps into surrounding frequencies, as illustrated in Figure 3-3. If two carriers from the same or adjacent cells are allocated adjacent frequencies or channel numbers they will interfere with each other because of the described overlapping. This interference is unwanted signal noise. All noise is cumulative, so starting with a large amount by using adjacent channels our wanted signal will soon deteriorate below the required quality standard. For this reason adjacent frequencies should never be allocated to carriers in the same or adjacent cells. Figure 3-3 illustrates the fact that the actual bandwidth of a GMSK modulated signal is considerably wider than the 200 kHz channel spacing specified by GSM. At the channel overlap point the signal strength of the adjacent channel is only -10 dB below that of the wanted signal. While this just falls within the minimum carrier to interference ratio of 9 dB, it is not insignificant and must be planned around so that allocation of adjacent frequencies in adjacent cells never occurs. One other consideration about channel spacing that must be considered is when using combiners. If a cavity combining block is used the frequencies for combining must be separated by at least three ARFCNs otherwise it could cause intermodulation products and spurious frequency generation. These could interfere with other carriers further away in the radio spectrum, possibly in adjacent cells, so they would not necessarily be a problem to the home cell so the source of interference becomes more difficult to locate.

Figure 3-3 Modulation techniques and channel spacingTraffic capacityDimensioningOne of the most important steps in cellular planning is system dimensioning. To dimension a system correctly and hence all the supporting infrastructure, some idea of the projected usage of the system must be obtained (for example; the number of people wishing to simultaneously use the system). This means traffic engineering. Consider a cell with N voice channels, the cell is therefore capable of carrying N individual simultaneous calls. The traffic flow can be defined as the average number of concurrent calls carried in the cell. The unit of traffic intensity is the Erlang, traffic defined in this way can be thought of as a measure of the voice load carried by the cell. The maximum carried traffic in a cell is N Erlangs, which occurs when there is a call on each voice channel all of the time. If during a time period T (seconds), a channel carries traffic is busy for t (seconds), then the average carried traffic, in Erlangs, is t/T. The total traffic carried by the cell is the sum of the traffic carried by each channel. The mean call holding time is the average time a channel is serving a call. Channel blockingThe standard model used to dimension a system is the Erlang B model, which models the number of traffic channels or trunks required or a given grade of service and given offered traffic. There will be times when a call request is made and all channels or trunks are in use, this call is then blocked. The probability of this happening is the grade of service of the cell. If blocking occurs then the carried traffic will be less than the offered traffic. If a call is blocked, the caller may try again within a short interval. Repeated call attempts of this type increase the offered traffic above the level if there had been an absence of blocking. Because of this effect the notion of offered traffic is somewhat confused, however, if the blocking probability is small, it is reasonable to ignore the effect of repeated call attempts and assume that blocked calls are abandoned. The number of calls handled during a 24 hour period varies considerably with time. The figure opposite shows the type of traffic load that might be expected on a typical call. There are usually two peaks during week days, although the pattern can change from day to day. Across the typical day the

variation is such that a one-hour period shows greater usage than any other. From the hour with the least traffic to the hour with the greatest traffic, the variation can exceed 100:1. To add to these fairly regular variations, there can also be unpredictable peaks caused by a wide variety of events (for example; the weather, natural disasters, conventions, sports events). In addition to this, system growth must also be taken into account. There are a set of common definitions to describe this busy hour traffic loading. Busy Hour: The busy hour is a continuous period during which traffic volume or number of call attempts is the greatest. Peak Busy Hour: The busy hour each day it is not usually the same over a number of days. Time Constant Busy Hour: The one-hour period starting at the same time each day for which the average traffic volume or call attempts count is greatest over the days under consideration. Busy Season Busy Hour: The engineering period where the grade of service criteria is applied for the busiest clock hour of the busiest weeks of the year. Average Busy Season Busy Hour: The average busy season busy hour is used for trunk groups and always has a grade of service criteria applied. For example, for the Average Busy Season Busy Hour load, a call requiring a circuit in a trunk group should not encounter All Trunks Busy (ATB) no more than 1% of the time. Peak loads are of more concern than average loads when engineering traffic routes and switching equipment. Traffic flowIf mobile traffic is defined as the aggregate number of MS calls (C) in a cell with regard to the duration of the calls (T) as well as their number, then traffic flow (A) can be defined as: Traffic Flow (A) = C x T

Where: C is: the calling rate per hour.

T the average holding time per call. Suppose an average hold time of 1.5 minutes is assumed and the calling rate in the Busy Hour is 120, then the traffic flow would be 120 x 1.5 = 180 call-minutes or 3 call hours. One Erlang of traffic intensity on one traffic channel means a continuous occupancy of that particular traffic channel. Considering a group of traffic channels, the traffic intensity in Erlangs is the number of call-seconds per second or the number of call-hours per hour. As an example; if there were a group of 10 traffic channels which had a call intensity of 5 Erlangs, then half of the circuits would be busy at the time of measurement. Grade of serviceOne measure of the quality of service is how many times a subscriber is unsuccessful in setting up a call (blocking). Blocking data states what grade of service is required and is given as a percentage of the time that the subscriber is unable to make a call. Typical blocking for the MS-BSC link is 2% with 1% being acceptable on the BSC-MSC link. There is a direct relationship between the grade of service required and the number of channels. The customers desired grade of service has a direct affect on the number of channels needed in the network. Capacity calculations IntroductionThis section provides information on how to determine the number of control channels required at a BTS. This information is required for the sizing of the links to the BSC, and is required when calculating the exact configuration of the BSC required to support a given BSS. Typical call parameters The number of control channels and GPROC2s required at a BTS depend on a set of call parameters; typical call parameters for BTS planning are given in Table 3-1.

Table 3-1 Typical parameters for BTS call planning

Parameter Assumed Value

Call duration T = 120 seconds

Ratio of SMSs per call S = 0.1

Ratio of location updates to calls: non-border location area l = 2

Ratio of location updates to calls: border location area l = 7

Ratio of IMSI detaches to calls Id = 0

Location update factor: non-border location area (see below) L = 2

Location update factor: border location area (see below) L = 7

Number of handovers per call H = 2.5

Paging Rate in pages per second P = 3

Time duration for location update TL = 4 seconds

Time duration for SMSs TSMS = 6 seconds

Time duration for call set-ups TC = 5 seconds

Guard time for SDCCHs Tg = 4 seconds

Probability of blocking for TCHs PB-TCH < 2%

Probability of blocking for SDCCHs PB-SDCCH < 1%

The location update factor (L) is a function of the ratio of location updates to calls (I), the ratio of IMSI detaches to calls (Id) and whether the short message sequence (type 1) or long message sequence (type 2) is used for IMSI detach; typically Id = 0 (that is IMSI detach is disabled) as in the first formula given below. When IMSI detach is enabled, the second or third of the formulas given below should be used. The type of IMSI detach used is a function of the MSC. If IMSI detach is disabled:

If IMSI detach type 1 is enabled: If IMSI detach type 2 is enabled: Control channel calculations IntroductionThere are four types of air interface control channels, they are:

Broadcast control channel (BCCH). Common control channel (CCCH). Standalone dedicated control channel (SDCCH). Cell broadcast channel (CBCH), which uses one SDCCH.

There are three configurations of control channels, each occupies one radio timeslot: A combined control channel.

One BCCH plus three CCCH plus four SDCCH.or

A non-combined control channel. One BCCH plus nine CCCH (no SDCCH).

plus An SDCCH control channel.

Eight SDCCH.Each sector/cell requires a BCCH, so one of the configurations is always required. The number of air interface control channels required for a site, is dependent on the:

Number of pages. Location updates. Short message services. Call loading. Setup time.

Only the number of pages and access grants affects the CCCH. The other information uses the SDCCH. GPRS control channel RF provisioning

Control channels can be equipped to a GPRS carrier or to a circuit switched GSM carrier to support GPRS traffic channels. If the control channel timeslot(s) are assigned to a GPRS carrier, this reduces the number of available GPRS timeslots from eight to a smaller number in direct proportion to the number used as control channels. Alternatively, by equipping the control channels to the circuit switched GSM carrier, all eight timeslots on the GPRS carrier remain available for use as GPRS timeslots. The network planner needs to combine the GSM circuit switched signalling requirements with the GPRS signalling requirements in order to plan the appropriate level of control channel support. This planning guide provides the planning rules that enable the network planner to evaluate whether a combined BCCH can be used, or if a non-combined BCCH is required. The decision to use a non-combined BCCH is a function of the combined GPRS and GSM signalling load on the PAGCH ,and on the number of SDCCH channels required to support the GSM circuit switched traffic. The use of a combined BCCH is desirable because it may permit the use of only one timeslot on a carrier that is used for signalling. A combined BCCH can offer 4 more SDCCH blocks for use by the GSM circuit switched signalling traffic. If more than an average of three CCCH blocks, or more than four SDCCH blocks, is required to handle the signalling load, more control channel timeslots are required. The planning approach for GPRS/GSM control channel provisioning is to determine whether a combined BCCH is possible, given the combined GPRS and GSM load on the CCCH control channel. When more than three CCCH blocks and less than nine CCCH blocks are required to handle the combined load, the use of a combined BCCH is not possible. When more than nine CCCH blocks are needed, one or more timeslots are required to handle the CCCH signalling. In this case, it may be advantageous to use a combined BCCH again, depending on the CCCH and SDCCH load. The determination of how many CCCH and SDCCH blocks are required to support the circuit switched GSM traffic is deferred to the network planning that is performed with the aid of the relevant planning information for GSM. The network planning that is performed using the planning information determines how many CCCH and SDCCH blocks are required, and subsequently how many timeslots in total are required, to support the CCCH and SDCCH signalling load. The downlink control channels are: FCCH, SCH, BCCH, PAGCH. The Paging Access Grant CHannel (PAGCH) consists of paging messages and access grant messages. The downlink control channel load is determined by evaluating the combined GSM circuit switched signalling traffic load and the GPRS signalling traffic load on the PAGCH. The uplink control channel is the Random Access CHannel (RACH). It is assumed that by adequate provisioning of the downlink portion of the Common Control CHannel (CCCH), the uplink portion is implicitly provisioned with sufficient capacity. The provisioning of the Paging Access Grant CHannel (PAGCH) is estimated by calculating the combined load from the GPRS pages, GSM pages, GPRS access grant messages, and GSM access grant messages. The calculation is performed by adding the estimated GPRS and GSM paging blocks for the BTS cell to the estimated number of GPRS and GSM access grant blocks for the BTS cell, and dividing that sum by the CCCH utilization factor. Equation 19 should be evaluated to determine whether the number of PAGCHs is greater than three. If the evaluation is greater than three, three CCCH blocks are not sufficient: a non-combined BCCH must be used, independent of the number of SDCCH channels that are calculated as part of the BSS GSM circuit switched planning. If more than nine CCCH blocks are needed, more non-combined timeslots may be required. Example control channel configurations are shown in Table 3-2.

Table 3-2 Control channel configurations

Timeslot 0 Other timeslots Comment

1 BCCH + 3 CCCH + 4 SDCCH

N x 8 SDCCH One combined BCCH. The other timeslot may or may not be required depending on the support of circuit switched traffic, where the value of N can be >=0.

1 BCCH + 9 CCCH N x 8 SDCCH Non-combined BCCH. The value of N is >=1.

1 BCCH + 9 CCCH

N x 8 SDCCH, 9 CCCH

Non-combined BCCH. This is an example of one extra timeslot of CCCHs added in support of GPRS traffic. The value of N is >= 1.

The number of GPRS and GSM paging blocks are summed together in Equation 20.

Equation 19

Each term in the above equation is determined as per Equation 21 and Equation 22. Where: NPAGCH is: The average number of paging / Access Grant blocks rounded up to an integer.

NPCH The average number of paging blocks required at a cell.

NAGCH The average number of Access Grant blocks required at a cell.

UCCCH This is a utilization factor based on the percentage of the CCCH bandwidth that can be reliably used. A typical value for UCCCH is 30%.

The number of GPRS and GSM paging blocks are summed together in Equation 20. Equation 20

Each term in the above equation is determined as per Equation 21 and Equation 22.

Where: NPCH is: The average number of paging blocks in support of GPRS and GSM traffic required at a cell.

NPCH_GPRS The average number of paging blocks in support of GPRS traffic.

NPCH_GSM The average number of paging blocks in support of GSM traffic. Equation 21

Where: NPCH_GPRS is: The average number of paging blocks in support of GPRS traffic required at a cell.

GPRS_Page_Rate The number of GPRS pages transmitted to a BTS cell per second.

Equation 22

Where: NPCH_GSM is: The average number of paging blocks in support of GSM traffic required at a cell.

GSM_Page_Rate The number of GSM pages transmitted to a BTS cell per second. Where the denominator factor of 1.5 in Equation 21 and Equation 22 reflects that one page can be used for an average of 1.5 mobiles. The factor of 4.25 is the number of paging messages per second supported by one CCCH block. The factors of 1.5 in Equation 21 and in Equation 22 take into account the paging message packing efficiency experienced at the cell. The number of GPRS and GSM access grant channel blocks is summed in Equation 23. Equation 23

Where: NAGCH is: The average number of access grant blocks required at a cell.

NAGCH_GPRS The average number of GPRS access grant blocks required at a cell.

NAGCH_GSM The average number of GSM access grant blocks required at a cell. Each term in Equation 23 above is determined by Equation 24 and Equation 25 respectively. Equation 24Where: NAGCH_GPRS is: The number of GPRS access grant blocks required at a cell.

lBURST_GPRS

This number includes all downlink bursts per second in support of all uplink and downlink GPRS temporary data flow (TBF) originations. GPRS data traffic includes all SMS traffic carried by the GPRS infrastructure. Additionally, this factor includes routeing area updates and cell updates.

Before the GPRS network is operational, the above values in Equation 24 must be determined by the operator. Once the network is operational, these values can be obtained by inspecting the BSS busy hour statistics. Equation 25The factors in the above Equation 25 are defined as follows.

Where: NAGCH_GSM is: The average number of GSM access grant blocks required at a cell.

(lambda)CALL_GSM The call arrival rate per second.

(lambda)L_GSM The location update rate per second.

(lambda)S_GSM The number of SMS messages per second. Number of CCCH per BTS cell The following factors should be considered when calculating the number of CCCH per BTS cell:

The CCCH channels comprise the paging and access grant channel (PAGCH) in the downlink, and the random access channel (RACH) in the uplink. The PAGCH is subdivided into access grant channel (AGCH) and paging channel (PCH).

If the CCCH has a low traffic requirement, the CCCH can share its timeslot with SDCCHs (combined BCCH). If the CCCH carries a high traffic a non-combined BCCH must be used:

o Combined BCCH (with four SDCCH). Number of CCCH blocks = 3.Number of CCCH blocks reserved for AGCH ag_blks_res is 0 to 2.Number of CCCH blocks available for PCH/AGCH is 3 to 1.

Non combined BCCH. Number of CCCH blocks = 9.Number of CCCH blocks reserved for AGCH ag_blks_res is 0 to 7.Number of CCCH blocks available for PCH is 9 to 2.

When a non-combined BCCH is used, it is possible to add additional CCCH control channels (in addition to the mandatory BCCH on timeslot 0). These additional CCCH control channels are added, in order, on timeslots 2, 4, and 6 of the BCCH carrier. Thus creating cells with 18, 27, and 36 CCCH blocks. These configurations would only be required for very high capacity cells or in large location areas with a large number of pages.

Each CCCH block can carry one message. The message capacity of each CCCH block is 4.25 messages/second.

The AGCH is used to send immediate assignment and immediate assignment reject messages. Each AGCH immediate assignment message can convey channel assignments for up to two MSs. Each AGCH immediate assignment reject message can reject channel requests from up to four MSs.

The PCH is used to send paging messages. Each PCH paging message can contain pages for up to four MSs using TMSI or two MSs using IMSI. If no paging messages are to be sent in a particular CCCH block, then an immediate assignment/immediate assignment reject message can be sent instead. The current Motorola BSS implementation applies the following priority (highest to lowest) for downlink CCCH messages:

Paging message (if not reserved for AGCH). Immediate assignment message. Immediate assignment reject message.

Thus, for example, if for a particular PAGCH sub-channel there are always paging messages (that is high paging load) waiting to be sent, no immediate assignment or immediate assignment reject messages will be sent on that PAGCH sub-channel. Hence the option to reserve CCCH channels for AGCH.

It can normally be assumed that sufficient capacity exists on the uplink CCCH (RACH) once the downlink CCCH (PAGCH) is correctly dimensioned.

A number of other parameters may be used to configure the CCCH channels. Some of these are:

o Number of paging groups. Each MS is a member of only one paging group and only needs to listen to the PCH sub-channel corresponding to that group. Paging group size is a trade off between MS idle-mode battery life and speed of access (for example, a lot of paging groups, means the MS need only listen very occasionally to the PCH but as a consequence it takes longer to Page that MS resulting in slower call setup as perceived by a PSTN calling party).

o Number of repetitions for MSs attempting to access the network on the RACH. o Time MS must wait between repetitions on the RACH.

Precise determination of the CCCH requirements may be difficult; however, a number of statistics can be collected (for example ACCESS_PER_PCH, ACCESS_PER_AGCH by the BSS and these may be used to determine the CCCH loading and hence perform adjustments.

Calculate the number of CCCHs per BTS cell The following planning actions are required:

Determine the number of CCCHs per BTS. The average number of blocks required to support AGCH and PCH is given by:

The average number of blocks required to support AGCH only is given by:The average number of blocks required to support TMSI paging only is given by:The average number of blocks required to support IMSI paging only is given by:The access grant rate is given by:

Where: UCCCH is: the CCCH utilization.

lAGCH the access grant rate (per second).

P the paging rate per second.


lL the location update rate per second.

lS the number of SMSs per second. Number of SDCCH per BTS cell Determining the SDCCH requirement is an important part of the planning process. The SDCCH is where a large portion of call setup messaging takes place. As the number of calls taking place in a BTS increases, greater demand is placed on the control channel for call setup. The following factors should be considered when calculating the number of SDCCH per BTS cell:

To determine the required number of SDCCHs for a given number of TCHs per sector, the call, location update, and SMS (point to point) rates must be determined. Refer to the equations below for information on calculating these rates. Once these rates are determined, the required number of SDCCHs for the given number of TCHs can be determined. Refer to the equations below for information on calculating the required number of SDCCHs.

The rates for SMS are for the SMSs taking place over an SDCCH. For MSs involved in a call, the SMS may take place over the TCH, and may not require the use of an SDCCH.

Calculating the number of SDCCHs required is necessary for each cell at a BTS site. The equation below for NSDCCH is used to determine the average number of SDCCHs. The

number of Erlangs, e, is the number of Erlangs supported by a given sector based on the number of TCHs in that sector. To determine the number Erlangs support by a sector use Erlangs B. Use Erlang B to determine the required number of SDCCHs necessary to support the desired grade of service.

The number of location updates will be higher for sites located on the borders of location areas, as compared to inner sites of a location area. See Figure 3-4.

Figure 3-4 Location area diagramCalculate the number of SDCCHs per BTS cell The following planning actions are required:

Determine the number of SDCCHs per BTS. The average number of SDCCHs is given by:

The call rate (calls per hour) is given by:The location update rate (LU per hour) is given by:The SMS rate (SMS per hour) is given by:

Where: NSDCCH is: the average number of SDCCHs.


Tc the time duration for call setup.

lLU the location update rate.

TL the time duration of location updates.

Tg the guard time for SDCCH.

lS the number of SMSs per second.

TS the time duration of SMS (short message service setup).

e the number of Erlangs per cell.

T the average call length in seconds.

L the ratio of location updates to calls.

S the ratio of SMSs to calls.

Control channel configurations Table 3-3 and Table 3-4 give typical control channel configurations based on the typical BTS planning parameters given in Table 3-1. Control channel configurations for non-border location areaTable 3-3 is for the non-border location area cell, where the ratio of location updates to calls is 2.

Table 3-3 SDCCH planning for typical parameters (non-border location area)

Number of

RTFs

Number of

TCHsNumber of




1 7 2.94 4 1 BCCH + 3 CCCH + 4 SDCCH

2 14 8.20 8 1 BCCH + 9 CCCH 8 SDCCH

3 22 14.9 8 1 BCCH + 9 CCCH 8 SDCCH



6 45 35.6 16 1 BCCH + 9 CCCH 2 x 8 SDCCH

7 53 43.1 16 1 BCCH + 9 CCCH 2 x 8 SDCCH




11

12

Control channel configurations for border location areaTable 3-4 is for the border location area cell, where the ratio of location updates to calls is 7.

Table 3-4 SDCCH planning for typical parameters (border location area)

Number of

RTFs

Number of

TCHsNumber of




1 6 2.28 8 1 BCCH + 9 CCCH 8 SDCCH


3 21 14.0 16 1 BCCH + 9 CCCH 2 x 8 SDCCH


5 36 27.3 24 1 BCCH + 9 CCCH 3 x 8 SDCCH




9 66 55.3 40 1 BCCH + 9 CCCH 5 x 8 SDCCH


The GPRS planning processOverview of the GPRS planning processThe GPRS planning process documentation has the following structure:

Introduction to the planning process. GPRS network traffic estimation and key concepts. Air interface planning process.

Introduction to the GPRS planning processOverview the GPRS planning process introductionThe Introduction to the GPRS planning process has the following structure:

Determination of expected load or overload. Network planning flow.

Determination of expected loadThe planning process begins by determining the expected GPRS load (applied load) to the system. The next step is to determine the effective load to the system by weighting the applied load by network operating parameters. These parameters consist of the expected BLock Error Rate (BLER) based on the cell RF plan, the protocol overhead (GPRS protocol stack, that is TCP/IP, LLC, SNDCP, RLC/MAC), the expected advantage from V.42bis compression and TCP/IP header compression, and the multislot operation of the mobiles and infrastructure. The effective load at a cell is used to determine the number of GPRS timeslots required to provision a cell. The provisioning process can be performed for a uniform load distribution across all cells in the network or on an individual cell basis for varying GPRS cell loads. The number of GPRS timeslots is the key piece of information that drives the BSS provisioning process in support of GPRS. The planning process also uses network generated statistics, available after initial deployment, for replanning a network. The statistics fall into two categories: PCU specific statistics, and GSN (SGSN + GGSN) statistics. In a later section of this document, all of the statistics collected from the GPRS infrastructure are listed. The statistics that are expected to be useful for network replanning are identified. In this planning document, the statistics used for planning purposes are grouped into four categories: Stats_A, Stats_B, Stats_C, and Stats_D, as indicated in Figure 3-5.

Figure 3-5 GPRS network planning flowchartNetwork planning flowThe remaining chapters of the planning guide are presented in support of the GPRS network planning flowchart (Figure 3-5). The network planning flow is as follows:

GPRS network traffic estimation and key concepts: This text is intended to introduce the key concepts involved in planning a network. Because GPRS introduces the concept of a switchable timeslot that can be shared by both the GSM circuit switched infrastructure and by the GPRS infrastructure, much of the following text is dedicated to the discussion of this topic.

Customer inputs to the planning process: This chapter provides a table of inputs that can serve as a guide in the planning process. In subsequent planning sections, references are made to parameters in this table of inputs. A key piece of information that is needed for the planning process is the RF cell plan. This subsection discusses the impact of different cell

plans on the GPRS provisioning process, and how to use this information in order to determine the number of GPRS timeslots that are required on a per cell basis.

BSS planning: The hardware and communication link provisioning rules are detailed in this section based on the number of timeslots required. The number of timeslots is determined from the applied cell load requirements (cell throughput) that are provided by the network planner.

GSN complex planning: The hardware and communication links are determined in this section.

GPRS network statistics for network replanning: The statistics collected by the BSS and GSN are listed in tabular form, and the statistics that could be valuable for network replanning are identified.

Planning examples: A planning example is provided for both the BSS and GSN portions of the GPRS infrastructure.

Recommended planning guidelines: Based on the network planning rules, a few recommended planning guidelines are provided in this section.

GPRS network traffic estimation and key conceptsOverview of the GPRS network traffic estimation and key conceptsThe GPRS network traffic estimation and key concepts section has the following structure:

Introduction to the GPRS network traffic estimation and key concepts. Dynamic timeslot mode switching. Carrier timeslot allocation examples. BSS timeslot allocation methods. Provisioning the network with switchable timeslots. Recommendation.

Introduction to the GPRS network traffic estimation and key conceptsThe GPRS network planning is fundamentally different from the planning of circuit switched networks. One of the fundamental reasons for the difference is that a GPRS network allows the queuing of data traffic instead of blocking a call when a circuit is unavailable. Consequently, the use of Erlang B tables for estimating the number of trunks or timeslots required is not a valid planning approach for the GPRS packet data provisioning process. The GPRS traffic estimation process starts by looking at the per cell GPRS data traffic profile such as fleet management communications, email communications, web browsing, and large file transfers. Once a typical data traffic profile mix is determined, the required network throughput per cell can be calculated as measured in kbits per second. The desired network throughput per cell is used to calculate the number of GPRS timeslots required to support this throughput on a per-cell basis. The estimated GPRS network delay is derived based on computer modeling of the delay between the Um interface and the Gi interface. The results are provided in the planning guide. The network delay can be used to determine the mean or average time it takes to transfer a file of arbitrary length. In order to simulate the delay, the following factors are considered: traffic load per cell, mean packet size, number of available GPRS carrier timeslots, distribution of CS-1 and CS-2 rate utilization, distribution of Mobile Station (MS) multislot operation (1,2,3, or 4), and BLER. Use of timeslotsThe use of timeslots on a GPRS carrier is different from how they are used in the GSM circuit switched case. In circuit switched mode, an MS is either in the idle mode or dedicated mode. In the dedicated mode, a circuit is assigned through the infrastructure whether or not a subscriber is transporting voice or data. In the Idle mode, the network knows where the MS is, but there is no circuit assigned. In the GPRS mode, a subscriber uses the infrastructure timeslots for carrying data only when there is data to be sent. However, the GPRS subscriber can be attached and not sending data and this still presents a load to the GSN portion of the GRPS system, and must be accounted for when provisioning the GPRS infrastructure, that is, in state 2 as explained below. The GPRS mobile states and conditions for transferring between states are provided in Table 3-5 and shown in Figure 3-6 in order to specify when infrastructure resources are being used to transfer data. The comment column specifies what the load on the infrastructure equipment is for that state and only in state 3 does the infrastructure equipment actually carry user data. The infrastructure equipment is planned such that many more MSs can be attached to the GPRS network, that is in state 2, than there is bandwidth available to simultaneously transfer data. One of the more significant input decisions for the network planning process is to determine and specify how many of the attached MSs are actively transmitting data in the Ready state 3. In the Standby state 2, no data is being

transferred but the MS is using network resources to notify the network of its location. The infrastructure has equipment limits as to how many MSs can be in state 2. When the MS is in state 1, the only required infrastructure equipment support is the storage of MS records in the HLR. Network provisioning requires planning for traffic channels and for signalling channels also referred to as control channels. The BSS GSR 4.1 release combines the circuit switched and GPRS control channels together as BCCH/CCCH. This planning guide provides a planning procedure in a later section for determining the BCCH/CCCH control channel capacity needed.

Table 3-5 MM State Model of MS

Present state #

Present state Next state Condition for state

transfer Comments (Present state)

1 IDLE READY(3) GPRS Attach

Subscriber is not monitored by the infrastructure, that is not attached to GPRS MM, and therefore does not load the system other than the HLR records.

2 STANDBY READY(3) PDU Transmission

Subscriber is attached to GPRS MM and is being actively monitored by the infrastructure, that is MS and SGSN establish MM context for subscriber IMSI, but no data transmission occurs in this state.

3 READY IDLE(1) GPRS Detach Data transmission through the infrastructure occurs in the Ready state

3 READY STANDBY(2)

Ready timer expiry or force to Standby (The network or the MS can send a GMM signalling message to invoke force to Standby.)

The ready timer (T3314) default time is 32 seconds. The timer value can be modified during the signalling process by MS request. 2-60 sec. in 2 sec. increments or 61-1800 sec. in 60 sec. increments.

Figure 3-6 MM state models for MS and SGSNDynamic timeslot mode switchingThis section proposes a network planning approach when utilizing dynamic timeslot mode switching of timeslots on a GPRS carrier. The radio interface resources can be shared dynamically between the GSM circuit switched services and GPRS data services as a function of service load and operator preference. The timeslots on a GPRS carrier can be reserved for GPRS use, for circuit switched use only, or allocated as switchable. Motorola uses the term switchable to describe a timeslot that can be dynamically allocated for GPRS Data service or for circuit switched service. The timeslot allocation is performed such that the GPRS reserved timeslots are allocated for GPRS use before switchable timeslots. GSM circuit switched timeslots are allocated to the circuit switched calls before switchable timeslots. The switchable timeslots are allocated with priority given to circuit switched calls. Motorola has a BSS feature called Concentration at BTS. This feature enables the terrestrial backhaul resources to be dynamically assigned over the E1 links between the BSC and BTS. The terrestrial backhaul resources are managed and allocated in increments of 16 kbit/s. When the concentration-at-BTS feature is enabled, it is important to have a sufficient level of terrestrial backhaul resources provisioned. This feature has the concept of reserved and switchable BSC-to-BTS resources. This concentration-at-BTS feature allows the network planner to allocate dedicated or reserved backing pools to reserved GPRS timeslots so that there is a guaranteed level of terrestrial backing available to GPRS traffic. It is recommended that the reserved backing pool is made large enough to serve the expected busy hour GPRS traffic demands on a per BTS site basis. It is possible for the circuit switched portion of the network to be assigned all of the switchable terrestrial backing under high-load conditions and, in effect, block GPRS access to the switchable timeslots at the BTS. In addition, the reserved GPRS pool of backing resources can be taken by the circuit switched portion of the network when BSC-to-BTS E1 outages occur, and when emergency

pre-emption type of calls occur and cannot be served with the pool of non-reserved resources. The concentration-at-BTS feature does not take the last switchable backhaul timeslot until all of the GPRS traffic has be transmitted, in the case when there are no provisioned reserved GPRS timeslots at the cell site. Provisioning rules for the concentration-at-BTS feature are described in the planning information. Background and discussionThe initial Motorola BSS GPRS infrastructure product permits up to one carrier per cell to be provisioned as a GPRS carrier. The GPRS carrier can also be the BCCH/CCCH carrier. Alternatively, the GPRS carrier can be specified to use all eight timeslots for GPRS traffic and one of the GSM circuit switched carriers in the cell can be designated as the BCCH/CCCH carrier. The GPRS carrier can be provisioned to carry a mix of circuit switched traffic and GPRS traffic. There are three provisioning choices:

Reserved GPRS timeslots allocated only for GPRS use. Switchable timeslots dynamically allocated for either GSM circuit switched traffic or GPRS

traffic (designated as switchable timeslots by Motorola). Remaining GPRS carrier timeslots, if any, only for circuit switched use.

The BSS supports a user definable number of GPRS timeslots and reserved GPRS timeslots. The BSS calculates the number of switchable timeslots by taking the number of operator allocated GPRS timeslots minus the number of operator allocated reserved GPRS timeslots. The number of circuit switched timeslots on a non-BCCH GPRS carrier is equal to eight timeslots minus the number of GPRS timeslots, that is GPRS timeslots include reserved plus switchable timeslots. The network planner may have some of the following network planning goals in mind when trying to determine when to use reserved timeslots versus and when to use switchable:

Use reserved timeslots to guarantee a minimum GPRS quality of service. Use switchable timeslots to provide low circuit mode blocking and high GPRS throughput

when the voice busy hour and the GPRS busy hour do not coincide. Use switchable timeslots to provide higher GPRS throughput without increasing the circuit

switched blocking rate. If all the GPRS carrier timeslots are provisioned as switchable, the last available timeslot is not given to a circuit switched call until transmission of all the GPRS traffic on that last timeslot is completed. Therefore, there is a circuit switched blocking on that last timeslot until the timeslot becomes free.

Use switchable timeslots to provide some GPRS service coverage in low GPRS traffic volume areas.

Use switchable timeslots to provide extra circuit switched capacity in spectrum limited areas. In order to make the decision on how to best allocate reserved and switchable timeslots, the network planner needs to have a good idea of the traffic level for both services. The proposal in this planning guide is to drive the allocation of switchable timeslots and reserved GPRS timeslots from a circuit switched point of view. Start by looking at the circuit switched grade of service objectives and the busy hour traffic level, as measured in Erlangs. Once the circuit switched information is known, the potential impact on switchable timeslots can be analysed. The GPRS quality of service can be planned by counting the number of available reserved GPRS timeslots, and by evaluating the expected utilization of the switchable timeslots by the circuit switched portion of the network during the GPRS busy hour. Carrier timeslot allocation examplesThe following two-carrier configuration examples explore different ways a two-carrier system may provision switchable and reserved GPRS timeslots. All blank timeslots in the following figures are available only for circuit switched traffic use. The BSS starts the reserved GPRS timeslot allocation at the top of the carrier (timeslot 7), and then allocates the switchable timeslots, followed by circuit-switched-use-only timeslots. When GPRS and GSM signalling requirements are added together to be served by a two-carrier cell, it is highly likely that one timeslot will be used for BCCH and another timeslot allocated as an SDCCH timeslot. Therefore, the following examples A to example E assume that there is an extra timeslot allocated as an SDCCH timeslot (SD) for GSM signalling purposes. In Example A, Figure 3-7, only four timeslots are used for GPRS on carrier 1; two are reserved GPRS timeslots (R), and two are switchable timeslots (S). One timeslot is used for BCCH (B) and another timeslot for SDCCH (SD), and two timeslots for circuit- switched-only use (blank).

In Example B, Figure 3-8, the GPRS signalling information is carried on the BCCH (B) of carrier 1 and SDCCH GSM signalling on a separate timeslot (SD). A separate carrier (carrier 2) is used to carry the GPRS data traffic. In this example, three timeslots are reserved GPRS timeslots and two are switchable. The remaining three timeslots on the second carrier are for circuit-switched-only use(blank). In Example C, Figure 3-9, all GPRS timeslots are configured as switchable timeslots on the BCCH carrier 1 and no reserved GPRS timeslots are configured. Again, one timeslot is assigned for SDCCH signalling use. In Example D, Figure 3-10, all GPRS timeslots are configured as switchable timeslots on the non-BCCH carrier, carrier 2. In Example E, Figure 3-11, all eight GPRS timeslots are configured as reserved timeslots on the non-BCCH carrier, carrier 2. Timeslot allocation for examples A and BB: BCCH/CCCH timeslot for GPRS/GSM signalling SD: SDCCH timeslot for GSM signalling R: Reserved GPRS timeslot S: Switchable timeslot Blank: Circuit-switched-use-only timeslots Figure 3-7 provides a timeslot allocation example A.

Figure 3-7 Example AFigure 3-8 provides a timeslot allocation example B.

Figure 3-8 Example BTimeslot allocation for examples C, D, and EB: BCCH/CCCH for GPRS/GSM signalling SD: SDCCH for GSM signalling R: Reserved PDCH S: Switchable PDCH Blank: Circuit-switched-use-only timeslots Figure 3-9 provides a timeslot allocation example C.

Figure 3-9 Example CFigure 3-10 provides a timeslot allocation example D.

Figure 3-10 Example DFigure 3-11 provides a timeslot allocation example E.

Figure 3-11 Example EBSS timeslot allocation methodsThe BSS algorithm that is used in order to determine allocation of switchable timeslots gives priority to circuit switched calls. Consequently, if a switchable timeslot is being used by a GPRS mobile and a circuit switched call is requested after all other circuit switched timeslots are used, the BSS takes the timeslot away from the GPRS mobile and gives it to the circuit switched mobile. The switchable timeslot can be re-allocated back to the GPRS mobile when the circuit switched call ends. The number of reserved GPRS timeslots can be changed by the operator in order to guarantee a minimum number of dedicated GPRS timeslots at all times. The operator provisions the GPRS carrier by selecting the number of timeslots that are allocated as reserved and switchable, and not by specifically assigning timeslots on the GPRS carrier. Motorola has implemented an idle circuit switched parameter that enables the operator to strongly favour circuit switched calls from a network provisioning perspective. By setting the idle parameter to 0, this capability is essentially turned off. The use of the idle circuit switched parameter is as follows. When a circuit switched call ends on a switchable GPRS timeslot and the number of idle circuit switched timeslots is greater than an operator settable threshold, the BSS re-allocates the borrowed timeslot for GPRS service. When the number of idle timeslots is less than or equal to a programmable threshold, the BSS does not allocate the timeslot back for GPRS service, even if it is the last available timeslot for GPRS traffic. If the BSS needs to use the last switchable timeslot in a cell for a circuit switched call when all of the timeslots are allocated as switchable, re-allocation of the timeslot to circuit switched must wait until there is no GPRS traffic in the cell. There is no GPRS traffic in the cell when all of the GPRS uplink and downlink BSS infrastructure queues are empty. At this point, the BSS can re-allocate the last switchable timeslot back as a circuit switched timeslot. If one or more timeslots in a cell are allocated

as reserved, the last switchable timeslot is allocated immediately on demand for a circuit switched call. Multislot mobile operation requires that contiguous timeslots are available. The BSSl takes the lowest numbered switchable timeslot in such a manner as to maintain contiguous GPRS timeslots for multislot GPRS operation. The BSS attempts to allocate as many timeslots as requested in multislot mode, and then backoff from that number as timeslots are not available. For example, suppose that timeslots 3 and 4 are switchable, and timeslots 5,6, and 7 are GPRS reserved (see Figure 3-12). When the BSS needs to re-allocate a switchable timeslot from GPRS mode to circuit switched mode, the BSS assigns timeslot 3 before it assigns timeslot 4 for circuit switched mode. Timeslot allocation for Figure 3-12B: BCCH/CCCH for GPRS/GSM signalling SD: SDCCH for GSM signalling R: Reserved PDCH S: Switchable PDCH Blank: Circuit-switched-use-only timeslots Figure 3-12 provides a timeslot allocation with reserved and switchable timeslots.

Figure 3-12 GPRS carrier with reserved and switchable timeslotsIf the Emergency Call Pre-emption feature is enabled, the BSS selects the air timeslot that carries the emergency call from the following list: (most preferable listed first)

1. Idle circuit switched. 1. Idle or in-service switchable GPRS timeslot (from lowest to highest). 1. In-service circuit switched. 1. Idle or in-service reserved GPRS timeslot (from lowest to highest).

Provisioning the network with switchable timeslotsProvisioning the network with switchable timeslots can offer flexibility in the provisioning process for combining circuit switched and GPRS service. This flexibility is in the form of additional available network capacity to both the circuit switched and GPRS subscribers, but not simultaneously. Because the BSS favours circuit switched use of the switchable timeslots, the network planner should examine the demand for switchable timeslots during the circuit switched busy hour and during the GPRS busy hour. Normally the operator provisions the circuit switched radio resource for a particular Grade Of Service (GOS) such as 2%. This means that 2 out of 100 circuit switched calls are blocked during the busy hour. If the operator chooses to use the new switchable timeslot capability, it is now possible to share some GPRS carrier timeslots between the circuit switched calls and the GPRS calls. During the circuit switched busy hour, the circuit switched use of these switchable timeslots may dominate their use. The circuit switched side of the network has priority use of the switchable timeslots, and attempts to provide a better grade of service as a result of the switchable timeslots being available. The example in Table 3-6 assumes that the planning is being performed for a cell that has two carriers. The first carrier is for circuit-switched-only use as shown in Table 3-6. The second carrier is a GPRS carrier; all eight timeslots are configured as switchable as shown in Figure 3-13. The table was created using the Erlang B formula in order to determine how many circuit switched timeslots are required for a given grade of service. The table covers the range of 2 Erlangs to 9 Erlangs of circuit switched traffic in order to show the full utilization of two carriers for circuit switched calls. The purpose of the table is to show how the circuit switched side of the network allocates switchable timeslots during the circuit switched busy hour in an attempt to provide the best possible GOS, assumed to be 0.1% for the purposes of this example. The comments column in the table is used to discuss what is happening to the availability of switchable timeslots for GPRS data use as the circuit switched traffic increases, as measured in Erlangs. This example does show some Erlang traffic levels that cannot be adequately served by two carriers at the stated grade of service listed in the tables. This occurs at the 7 and 8 Erlang levels for 0.1% GOS. In these cases, all of the switchable timeslots are used up on the second carrier in an attempt to reach a 0.1% GOS. For the 9 Erlang traffic level, 2 carriers is not enough to serve the circuit switched traffic at a 2% GOS. This would indicate a need for a second circuit switched carrier, in addition to the first circuit switched carrier and the GPRS carrier. Timeslot allocation for B: BCCH/CCCH for GPRS/GSM signalling

SD: SDCCH for GSM signalling R: Reserved PDCH S: Switchable TCH Blank: Circuit-switched-use-only timeslots Assumptions: 2 Carrier site. Figure 3-13 shows one circuit switched carrier with one BCCH/CCCH timeslot, one SDCCH timeslot, and six TCH timeslots.

Figure 3-13 1 circuit switched carrier with 1 BCCH/CCCH timeslot, 1 SDCCH timeslot and 6 TCH timeslots

Figure 3-14 shows one GPRS carrier with all timeslots (eight TCHs) designated as switchable. Figure 3-14 One GPRS carrier with all timeslots (eight TCHs) designated as switchable

Table 3-6 shows part of the switchable timeslot utilization.

Table 3-6 Switchable timeslot utilization (part A)

GOSPlanned circuit


Total number of circuit switched

timeslots required including

switchable



Comments

2% 2 6 0

During off busy hour time periods, the GPRS carrier most likely carries only GPRS traffic. Therefore, GPRS network planning should be performed assuming there are 8 timeslots available for GPRS traffic.

0.1% 2 8 2

During circuit switched busy hour at least 2 of the switchable timeslots are occasionally used by the circuit switch side of the network in an attempt to provide the best possible GOS - assumed to be on the order of 0.1%.

2% 3 8 2

During the circuit switched busy hour, 2 of the switchable timeslots are occasionally used by the circuit switch side of the network in an attempt to provide the 2% GOS.

0.1% 3 10 4

During the circuit switched busy hour, 4 of the switchable timeslots are occasionally used by the circuit switch side of the network in an attempt to provide the best possible GOS - assumed to be on the order of 0.1%.

2% 4 9 3

0.1% 4 12 6

2% 5 10 4

0.1% 5 14 8 All of the switchable timeslots are occasionally used to satisfy the 0.1% GOS.

Table 3-7 shows more switchable timeslot utilization.

Table 3-7 Switchable timeslot utilization (part B)

GOS Planned circuit switched

Total number of circuit switched timeslots


Comments

Erlangs/cell required including switchable


2% 6 12 6


2% 7 13 7


2% 8 14 8

All of the switchable timeslots are occasionally used to satisfy the 2% GOS.


2% 9 15 9 There are not enough switchable timeslots to reach 2% GOS


RecommendationThe following recommendation is offered when using switchable timeslots. It is important to determine the GOS objectives for circuit switched traffic and QoS objectives for GPRS traffic prior to selecting the number of switchable timeslots to deploy. During the circuit-switched-busy-hour, potentially all switchable timeslots are occasionally used by the circuit switched calls. The circuit switched timeslot allocation mechanism continues to assign switchable timeslots as circuit switched timeslots as the circuit switched traffic continues to increase. Therefore, if there is a minimum capacity requirement for GPRS services, the network planner should plan the GPRS carrier with enough reserved timeslots in order to handle the expected GPRS data traffic. This ensures that there is a minimum guaranteed network capacity for the GPRS data traffic during the circuit switched busy hour. During the circuit-switched-off-busy-hours, the switchable timeslots could be considered as available for use by the GPRS network. Therefore, in the circuit switched off busy hours potentially all switchable timeslots could be available for the GPRS network traffic. The BSS call statistics should be inspected to determine the actual use of the switchable timeslots by the circuit switched services. The circuit-switched-busy-hour and the GPRS-busy-hour should be monitored to see if they overlap when switchable timeslots are in use. If the busy hours overlap, an adjustment may be needed to the number of reserved timeslots allocated to the GPRS portion of the network in order to guarantee a minimum GPRS quality of service as measured by GPRS throughput and delay. Furthermore, one or more circuit switched carriers may need to be added to the cell being planned or replanned so that the switchable timeslots are not required in order to offer the desired circuit switched grade of service. In conclusion, assume switchable timeslots are occasionally unavailable for GPRS traffic during the circuit switched portion of the network busy hour. Provision enough reserved timeslots for GPRS traffic during the circuit switched busy hour to meet the desired minimum GPRS quality of service objectives, as measured by GPRS data throughput. The following step-wise process is proposed when determining how best to allocate GPRS carrier timeslots. AssumptionsThe process assumptions are:

A GPRS carrier can be added to a cell in addition to circuit switched carriers. A circuit switched carrier can be used to provide the control channels (BCCH/CCCH/SDCCH)

on one or more timeslots as needed. The number of circuit switched timeslots are determined as part of the BSS planning effort

prior to the GPRS planning effort. When the concentration-at-BTS feature is enabled, a sufficient pool of reserved backing

resources is provisioned in support of the number of reserved GPRS timeslots in order to meet the GPRS QoS objectives.

Step 1Determine how many reserved GPRS timeslots are needed on a per-cell basis in order to satisfy a GPRS throughput QoS. The GPRS reserved timeslots should equal the sum of the active and standby timeslots that are allocated to a carrier. Step 2If there are any timeslots left on the GPRS carrier after step 1, consider using them as switchable timeslots. The use of switchable timeslots can potentially offer increased capacity to both the GPRS and circuit switched traffic if the traffic is staggered in time. Step 3If there is a need to use some timeslots on the GPRS carrier to satisfy the circuit switched GOS objectives and the timeslot requirement overlaps with the number of reserved GPRS timeslots, consider adding another circuit switched carrier to the cell. Step 4After deploying the GPRS carrier, review the network statistics listed in the Network statistics section on a continuous basis in order to determine whether the reserved GPRS timeslots, switchable GPRS timeslots, and circuit switched timeslots are truly serving the GOS and QoS objectives. As previously discussed, the use of switchable timeslots can offer network capacity advantages to both circuit switched traffic and GPRS traffic as long as the demand for these timeslots is staggered in time. GPRS Air interface planning processOverview of the GPRS air interface planning process structureThe air interface planning process is documented as follows:

Introduction to the air interface planning process. Air interface interface throughput. Throughput estimation process: step 1. Throughput estimation process: step 2. Throughput estimation process: step 3. Throughput estimation process: step 4.

Introduction to the GPRS air interface planning processThe air interface planning process uses the range of values listed in Table 3-8 to Table 3-13. If network values are not available at the time a network is planned, typical or recommended values are provided where appropriate. The minimum values are given for the maximum capacity of a minimum system, and the typical values are used as standard model parameters.

Table 3-8 Air interface planning inputs (part A)


Typical value


CS rate ratio, CS-1/CS-2

Approx. 0 % 10% 100 %

CS rate ratio is determined by the Cell plan, mean TBF size and use of Acknowledge mode. Refer to cell plan tables: Table 3-14, Table 3-15 and Table 3-16.

V.42 bis compression ratio

1 2.5 4

A ratio of 1 means there is no compression and a ratio of 4 is the theoretical maximum, which is most likely never realized. Most users see a compression advantage in the range of 2-to-3 over the air interface between the MS and the SGSN. The compression ratio is used in Equation 3.


Table 3-9 Air interface planning inputs (part B)

Variable Minimum value Typical value Maximum

value Assumptions/ variable use

BLER 0 10% 100%

The BLock Error Rate (BLER) is largely determined by the cell RF plan. The typical value is an average rate. There are separate BLERs for CS-1 and CS-2 rates that are RF plan specific.

FTD 0.7 second

3 seconds for a 3 kbyte file, subject to network load and multislot operation.

File size dependent

This is the File Transit Delay (FTD) objective measured in seconds from the Um interface to the Gi interface. The minimum delay is the approximate delay for a RLC block of 23 bytes or less, which is the minimum system limit with only one user on the system. The FTD value is determined by Equation 4.

The number of GPRS timeslots per cell


This number can represent reserved and/or switchable timeslots as explained from Figure 3-7 to Figure 3-14.

Number of active GPRS timeslots per PCU with redundancy


This is the number of timeslots simultaneously in use with N+1 redundancy. This number is used to calculate the number of PRP and PICP boards to equip at the PCU using the PCU planning rules tabled in Chapter 5.


Table 3-10 Air interface planning inputs (part C)


Typical value


Number of GPRS users monitored at the PCU with redundancy


This is the number of mobiles that can be monitored in addition to the mobiles actually using timeslots. This value reflects N+1 redundancy. This number reflects the coverage capability of the PCU.

Number of active GPRS timeslots per PCU without redundancy


This is the number of timeslots simultaneously in use without N+1 redundancy. This number is used to calculate the number of PRP and PICP boards to equip at the PCU using the PCU planning rules tabled in Chapter 5.

Number of GPRS users monitored at the PCU without redundancy


This is the number of mobiles that can be monitored in addition to the mobiles actually using timeslots without N+1 redundancy. This number reflects the coverage capability of the PCU.


Table 3-11 Air interface planning inputs (part D)



Mean LLC PDU 20 435 1,580 This parameter is used in

packet size (bytes)

determining the cell and subscriber throughput capacities.

Data traffic /subscriber (peak)

0 98 kbytes/hour


This parameter is the expected GPRS load of a subscriber. This figure should include the SMS traffic carried as GPRS data.


Table 3-12 Air interface planning inputs (part E)



Total number of GPRS pages per attached subscriber

0 0.6


This effects the signalling traffic load over the SGSN-to-PCU (Gb) interface, the PCU-to-BSC interface(GSL), and the BSC-to-BTS interface(RSL). The GPRS paging traffic must be added to the circuit switched signalling traffic at the BSC in order to determine the total signalling traffic between the BSC and reporting BTSs. This parameter is also used to determine the GPRS load on the CCCH.

Number of data transfers per hour per subscriber

0

No maximum limit other than what the network is provisioned to support

This number is used to determine the provisioning of the control channels (CCCH provisioning).

Number of BSCs supporting GPRS per OMC-R serving area


This establishes how many PCUs are required per OMC-R serving area. The size of the PCU is determined from the GPRS subscriber profile. (Provision 1 PCU per BSC.)


Table 3-13 Air interface planning inputs (part F)


Typical value


Equipment redundancy (BSS PCU & GSN)

No Yes More equipment can be deployed when redundancy is desired.

E1 redundancy No Yes

Extra E1 lines are deployed for GSL, GDS, GBL, and Gi links when redundancy is desired. The extra E1 lines provide logical redundancy because the traffic is load shared over the redundant links.

Air interface throughputThe GPRS data throughput estimation process given in this planning guide is based upon the Poisson process for determining the GPRS mobile packet transfer arrivals to the network and for determining the size of GPRS data packets generated or received by the GPRS mobiles. A number of wired LAN/WAN traffic studies have shown that packet interarrival rates are not exponentially distributed. Recent work argues that LAN traffic is much better modelled using statistically self-similar processes instead of Poisson or Markovian processes. Self-similar traffic pattern means the interarrival rates appear the same regardless of the timescale at which it is viewed

(in contrast to Poisson process, which tends to be smoothed around the mean in a larger time scale). The exact nature of wireless GPRS traffic pattern is not known due to lack of field data. In order to minimize the negative impact of underestimating the nature of the GPRS traffic, it is proposed in this planning guide to limit the mean GPRS cell loading value to 50% of the system capacity. Using this cell loading factor has the following advantages:

Cell overloading due to the bursty nature of GPRS traffic is minimized. The variance in file transit delay over the Um-to-Gi interface is minimized such that the delay

can be considered a constant value for the purposes of calculating the time to transfer a file of arbitrary size.

LAN/WAN wireline studies have also shown that even when statistically valid studies are performed, the results come out very different in follow-up studies. It turns out that web traffic patterns are very difficult to predict accurately and, therefore, it is highly recommended that the network planner makes routine use of the GPRS network statistics. About the stepsThe following steps 1 and 2 are used for dimensioning the system. Step 1 needs to be performed prior to step 2 in order to calculate the number of GPRS timeslots that should be provisioned on a per cell basis. Steps 3 and 4 are optional. These steps are included in this section so that an over-the-air file transfer time can be calculated for any size file. The results from steps 3 and 4 are dependent upon the choices made in steps 1 and 2. Step 1: throughput estimation processChoose a cell plan in order to determine the expected BLER and percentage of time data is transferred at the CS-1 rate and at the CS-2 rate. The cell plan that is chosen for GPRS may be determined by the plan currently in use for the GSM circuit switched portion of the network. However, it may be necessary to change an existing cell plan used for GSM circuit switched in order to get better BLER performance for the GPRS portion of the network. After the cell plan is chosen, the network planner can move on to step 2. The PCU dynamically selects the best CS-1 or CS-2 rate in order to maximize the GPRS data throughput on a per mobile basis. The CS-1 and CS-2 rate selection is performed periodically during the TBF. Simulations were performed (see Impact of the Radio Interface on GPRS System Dimensioning - a Simulation Study, Draft 0.1 of June 1999) for two typical frequency hopping cell configurations; results for a 1x3 cell reuse pattern with 2/6 hopping are shown in Table 3-14 (which is hopping on 2 carriers over 6 frequencies) and results for a 1x1 cell reuse pattern with 2/18 hopping are shown in Table 3-15 (which is hopping on 2 carriers over 18 frequencies). Results for a non-hopping cell configuration with a TU-3 model is shown in Table 3-17 provide a chart of the cell coverage area and cell C/I performance for the non-hopping case. The following tables were created, based on the simulations, in order to indicate the percentage of the time the CS-2 rate would be chosen over the CS-1 rate and at what mean BLER. The simulation results indicate that the higher data rate of the CS-2 more than offsets its higher BLER rate in the majority of the cell coverage area, resulting in the CS-2 rate being chosen most of the time. Reviewing the following tables it can be seen that under good cell C/I conditions, better throughput may be obtained by provisioning the GPRS timeslots on the BCCH carrier as indicated by Table 3-16.



% Rate chosen 10 90

% Mean BLER 50 20



% Rate chosen 10 90

% Mean BLER 56 14

Table 3-16 Non-hopping TU-3 model


% Rate chosen 0 100

% Mean BLER 10 3

Table 3-17 provides the cell C/I performance, as measured in dBs, as a function of cell area coverage for the TU-3 model.

Table 3-17 Cell coverage versus carrier-to-interface (C/I)

% cell coverage 90 80 70 60 50 40

C/I 12 16 18 20 22 24

The cell plans assume a regular cell reuse pattern for the geographic layout and for the allocation of frequencies. The computer simulation generated the above cell plan using a typical urban 3 kph model, a propagation law with a Radius (R) exponent of -3.7 and a shadowing function standard deviation of 5 db. If non-regular patterns are used, a specific simulation study may be required to match the particular cell characteristics. The simulation process is outside the scope of this planning guide and the network planner should contact Motorola for additional simulation results. Step 2: throughput estimation processStep 2 determines the number of GPRS timeslots that need to be provisioned on a per cell basis. Timeslot provisioning is based on the expected per-cell mean GPRS traffic load, as measured in kbit/s. The GPRS traffic load includes all SMS traffic routed through the GSN. The SMS traffic is handled by the GPRS infrastructure in the same manner as all other GPRS traffic originating from the PDN. The cell BLER and CS rate characteristics chosen in step 1 provide the needed information for evaluating the following Equation 1. Equation 1Equation 2

Where: is:

Mean_trf_ld The mean traffic load, as measured in kbit/s, is defined at the LLC layer therefore all the higher layer protocol overheads (for example, TCP, UDP, IP, SNDCP, LLC) are encapsulated in this load figure.

Denom_1 Denominator 1 is used in Equation 1.

PDCH The number of timeslots per cell, maximum 8.

%CS1 The percent of time data transmission occurs using the CS-1 coding scheme.


%C2S The percent of time data transmission occurs using the CS-2 coding scheme.




Mean_ld_f The mean load factor for the number of active timeslots to provision at a cell. The recommended value is 50% of the number GPRS timeslots provisioned at a cell.

TBF SETUP REL factor

TBF SETUP and Release Factor. The recommended value 0.45. This factor is an interim solution whilst the Overlapping TPF feature is being completed.

The number of PDCH timeslots calculated in Equation 1 includes the number of active timeslots and the number of standby timeslots. The Mean_load_factor of 50% determines the ratio of active timeslots to standby timeslots. For example, if Equation 1 evaluated to 8 timeslots, 4 timeslots would be counted as active timeslots and 4 timeslots as standby timeslots. It is important to differentiate between the required number of active timeslots and the required number of standby timeslots because it directly effects the provisioning of the communication links and the PCU hardware. The active timeslots are timeslots that are simultaneously carrying data. The standby timeslots are timeslots that are being monitored by the PCU for an uplink or downlink timeslot request. A request on a standby timeslot for an active timeslot is granted for an active timeslot as soon as one becomes available at the PCU. For example, when the PCU is provisioned to handle 30

active timeslots and all of them are in use, at least one of these 30 active timeslots must become available in order to move a standby timeslot to active state. The use of active timeslots and standby timeslots enables several cells to share the PCU resource. While one cell is experiencing a high load condition, using all eight GPRS timeslots for instance, another cell operating below its mean load averages out the GPRS traffic load at the PCU. The E1s between the BTS and BSC must be provisioned to handle the number of timeslots calculated in Equation 1 because all of the timeslots can become active under high load conditions. Throughput estimation process: step 3 (optional)Step 3 is optional, and the results can be used in optional step 4. Step 3 is intended to be used as an aid in determining the size of a file that is to be transferred as an LLC PDU from the mobile to the SGSN, by using Equation 3. The file size consists of the application file to be transferred, which includes any application-related overhead. In addition to the application file, there is transport and network layer protocol overhead, TCP and IP. Finally, there is GPRS Link Layer Control (LLC) and SubNetwork Convergence (SNDCP) protocol overhead. The application file plus all of the protocol overhead summed together makes up the one or more LLC_PDU frames that constitute the file to be transferred. The percentage of protocol overhead depends on the transport layer used, such as TCP or UDP. For example, the TCP/IP protocol overhead is 40 bytes when TCP/IP header compression is not used. When TCP/IP header compression is used, the TCP/IP header can be reduced to 5 bytes from 40 bytes after the first LLC frame is transferred. The use of header compression continues for as long as the IP address remains the same. Figure 3-15 illustrates the typical LLC_PDU frame with the user application payload and all of the protocol overhead combined for the case of no TCP/IP header compression.

Figure 3-15 LLC PDU layoutIf V.42bis application data compression is used, the effective file size for transmission is reduced by the data compression factor which can range from 1 to 4. Typically, V.42bis yields a 2.5 compression advantage on a text file, and close to no compression advantage (factor=1) on image files and very short files. Equation 3Where: File_size_LLC is: The files size in bytes to be transferred measured at the LLC layer.

Appln The user application data file size measured in bytes.

LLC_payload The maximum LLC PDU payload of 1527 bytes.

protocol_overhead The protocol overhead for TCP/IP/SNDCP/LLC/CRC is 53 bytes without header compression and 18 bytes with header compression.

V.42bis_factor Application data compression is over the range of 1 to 4, a typical value is equal to 2.5.

ExampleA 3 kbytes application file transfer requires the following number of bytes to be transferred at the LLC_PDU layer: Application= 3 kbytes Assume V.42bis_factor = 1, that is no application data compression No header compression: File_size_LLC = 3000 + roundup (3000/1527) x 53 = 3106 bytes With header compression: The first LLC_PDU the header is not compressed, and all subsequent LLC_PDUs are compressed. For this size file of 3000 bytes, only 2 LLC_PDU transmissions are required so the File_size_LLC is: File_size_LLC = 3000 + 53+18 = 3071 bytes Throughput estimation process: step 4 (optional)The network planner can use step 4 to determine how long it takes to transfer a file of an arbitrary size over the Um-to-Gi interface. The application file is segmented into LLC PDU frames as illustrated previously. The File Transit Delay (FTD) is calculated in Equation 4 by using the following information: total number of RLC blocks of the file, BLER, number of timeslots used during the transfer, and mean RLC Transit Delay (RTD) value. Equation 4 does not include the effects of acknowledgement messages. The reason is that the largest effect is in the uplink direction, and it is expected that the downlink direction will dominate the cell traffic. The DL sends an acknowledgement message on an as-needed basis, whereas the uplink

generates an acknowledgement message every 2 out of 12 RLC_Blocks. It is expected that the downlink acknowledgement messages will not significantly effect the file transit delay in the downlink direction. Equation 4Where: FTD is: The file transit delay measured in seconds.

RTD

This is the transit delay time from the Um interface to the Gi interface for a file size of only 1 RLC/MAC block of data. RTD is estimated to be 0.9 seconds when system running at 50% capacity. This parameter will be updated when field test data is available.

RLC_Blocks This is the total number of RLC blocks of the file. This can be calculated by dividing file_size_LLC by the corresponding RLC data size of 20 bytes for CS-1 and 30 bytes for CS-2.

mslot This is the mobile multislot operating mode; the value can be from 1 to 4.

CSBLER This is the BLER for the specific CS rate. The value is specified in decimal form. Typical values range form 0.1 to 0.2.

The RTD parameter is directly correlated to the system utilization and the mean packet size. When the cell approaches its throughput capacity limit, the RTD value increases dramatically, and the infrastructure starts to drop packets. Simulation data indicates that when traffic load is minimal, the RTD value is at a minimum limit of 0.7 seconds. At a cell throughput capacity of 50%, the RTD increases to 0.9 seconds. It is recommended that cell throughput provisioning be performed at the mean cell capacity level of 50%. Provisioning for a mean cell throughput greater than 50% greatly increases the likelihood of dropped packets, and RTD values of over 2.6 seconds can occur. The assumptions used in the simulation to determine the RTD value at a mean cell throughput level of 50% are: 25% of the cell traffic at the CS-1 rate and 75% of the cell traffic at the CS-2 rate, BLER 10%, mobiles multislot distribution 1:2:3:4 = 20:50:20:10, 8 PDCH, DL, mean LLC_PDU packet size of 435 bytes. For example, a 3 kbyte application file transit time at the CS-2 rate, using one timeslot, BLER = 10%, and no header or V.42 bis compression is: 3 Kbyte file transit time over Um-to-Gi interface = 0.9 + Roundup (3106/30) x 0.02 x 1.1 / 1 = 3.2 seconds

Where: File_size_LLC is: = 3106 bytes (as calculated in the previous example)

CS-2 payload = 30 bytes

Air time for one RLC/MAC block = 0.02 seconds

(1+CSBLER) = 1.1

Multislot operation = 1 Propagation effects on GSM frequencies

Propagation productionMost of the methods used to predict propagation over irregular terrain are actually terrain based, since they are designed to compute the diffraction loss and free space loss based upon the path profile between the transmitter and the receiver. A widely used technique in the United Kingdom is the prediction method used by the Joint Radio Committee of the Nationalized Power Industries (JRC). This method utilizes a computerized topographical map in a data base, providing some 800,000 height reference points at 0.5 km intervals covering the whole of the UK. The computer predicts the received signal level by constructing the ground path profile between the transmitter and receiver using the data base. The computer then tests the path profile for a line of sight path and whether Fresnel-zone clearance is obtained over the path. The free space and plane earth propagation losses are calculated and the higher value is chosen. If the line of sight and Fresnel-zone clearance test fails, then the programme evaluates the loss caused by any obstructions and grades them into single or multiple diffraction edges. However, this method fails to take any buildings into account when performing its calculation, the calculations are totally based upon the terrain features. Although the use of topographical based calculations are useful when designing mobile communication systems, most mobile systems are centred around urban environments. In these urban environments, the path between transmitter and the receiver maybe blocked by a number of

obstacles (for example; buildings), so it is necessary to resort to approximate methods of calculating diffraction losses since exact calculations for each obstacle then become extremely difficult.

Introduction to decibels Decibels are used to express power output levels, receiver input levels and path losses. The reason they are used is to simplify the calculations used when planning radio systems. Any number maybe expressed as a decibel (dB). The only requirement is that the original description and scale of unity is appended to the dB, so indicating a value which can be used when adding , subtracting, or converting dBs. For example for a given power of 1 mW it may be expressed as 0 dBmW, the mW refers to the fact that the original scale of measurement was in thousandths of a watt. For a power of 1 W the equivalent in dBs is 0 dBW. The decibel scale is logarithmic and this allows very large or very small numbers to be more easily expressed and calculated. For example take a power of 20 watts transmitted from a BTS which was .000000001 W at the receiver. It is very difficult to accurately express the total power loss in a simple way. By converting both figures to decibels referenced to 1 mW, 20 W becomes 32 dBmW and .000000001 W is -60 dBmW. The path loss can now be expressed as 92 dBmW. Multiplication and division also become easier when using dBs. For figures expressed as dBs to multiply them together simply add the db figures together. This is the equivalent in decimal of multiplying. For division simply take one dB figure from the other. Another example is for every doubling of power figures the increase in dBs is 3 dB and for every halving of power the decrease is 3 dB. Table 3-18 gives examples of dB conversions.

Table 3-18 dBmW and dBW to Power conversion

dBmW dBW Power dBmW dBW Power

+ 59 29 800 W + 7 - 23 5 mW

+ 56 26 400 W + 4 - 26 2.5 mW

+ 53 23 200 W + 1 - 29 1.25 mW

+ 50 20 100 W 0 - 30 1 mW

+ 49 19 80 W - 3 - 33 0.5 mW

+ 46 16 40 W - 6 - 36 0.25 mW

+ 43 13 20 W - 9 - 39 0.125 mW

+ 40 10 10 W - 10 - 40 0.1 mW

+ 39 9 8 W - 20 - 50 0.01 mW

+ 36 6 4 W - 30 - 60 1 mW

+ 33 3 2 W - 40 - 70 0.1 mW

+ 30 0 1 W - 50 - 80 0.01 mW

+ 27 - 3 500 mW - 60 - 90 1 nW

+ 24 - 6 250 mW -70 -100 0.1 nW

+ 21 - 9 125 mW - 80 - 110 0.01 nW

+ 20 - 10 100 mW - 90 - 120 1 pW

+ 17 - 13 50 mW - 100 - 130 0.1 pW

+ 14 - 16 25 mW -103 - 133 0.01 pW

+ 11 - 19 12.5 mW - 106 - 136 0.001 pW

+ 10 - 20 10 mW

Fresnel zone The Fresnel (pronounced Fresnel) actually consists of several different zones, each one forming an ellipsoid around the major axis of the direct propagation path. Each zone describes a specific area depending on the wavelength of the signal frequency. If a signal from that zone is reflected of an obstacle which protrudes into the zone, it means that a reflected signal as well as the direct path signal will arrive at the receiver. Radio waves reflected in the first Fresnel zone will arrive at the receiver out of phase with those taking the direct path and so combine destructively. This results in a very low received signal strength. It is important when planning a cell to consider all the radio paths for obstacles which may produce reflections from the first Fresnel zone because if they exist it is like planning permanent areas of no coverage in certain parts of the cell. In order to calculate wether or not this condition exists the radius of the first Fresnel zone at the point where the object is suspected of intruding into the zone must be calculated. The formula, illustrated in Figure 3-16, is as follows:

Where: F1 is: the first Fresnel zone.

d1 distance from Tx antenna to the obstacle.

d2 distance from Rx antenna to the obstacle.

l wavelength of the carrier wave.

d total path length. Once the cell coverage has been calculated the radio path can be checked for any objects intruding into the first Fresnel zone. Ideally the link should be planned for no intrusions but in some cases they are unavoidable. If that is the case then the next best clearance for the first Fresnel zone is 0.6 of the radius. When siting a BTS on top of a building care must be taken with the positioning and height of the antenna to ensure that the roof edge of the building does not intrude into the first Fresnel zone.

Figure 3-16 First Fresnel zone radius calculationRadio refractive index It is important when planning a cell or microwave radio link to have an understanding of the effects changes in the Radio Refractive Index (RRI) can have on microwave communications, also what causes these changes. RRI measurements provide planners with information on how much a radio wave will be refracted by the atmosphere at various heights above sea level. Refraction, Figure 3-17, is the changing of direction of propagation of the radio wave as it passes from a more dense layer of the atmosphere to a less dense layer, which is usual as one increases in height above sea level. It also occurs when passing from a less dense layer to a dense layer. This may also occur under certain conditions even at higher altitudes.

Figure 3-17 RefractionThe main effect to cell planners is that changes in the RRI can increase or decrease the cell radius depending on conditions prevailing at the time. The RRI is normally referenced to a value n at sea level. The value will vary with seasons and location but for the UK the mean value is 1.00034. This figure is very cumbersome to work with so convention has converted n to N.

Where: N is: (n-1) x 10 to the power of 6. The value of N now becomes 340 units for the UK. The actual seasonal and global variations are only a few 10 s of units at sea level. The value of N is influenced by the following :

The proportion of principle gasses in the atmosphere such as nitrogen, oxygen, carbon dioxide, and rare gasses. These maintain a near constant relationship as height is increased so although they affect the RRI the affect does not vary.

The quantity of water vapour in the atmosphere. This is extremely variable and has significant effects on the RRI.

Finally the temperature, pressure, and water vapour pressure have major effects on the RRI. All the above will either increase or decrease the RRI depending on local conditions, resulting in more or less refraction of a radio wave. Typically though for a well mixed atmosphere the RRI will fall by 40 N units per 1 km increase in height above sea level.

Measurement of the RRIThere are two main ways of measuring the RRI at any moment in time. Firstly by use of Radio Sonds. This is an instrument which is released into the atmosphere, normally attached to a balloon. As it rises it measures the temperature, pressure, and humidity. These are transmitted back to the ground station with a suitable reference value. The measurements of pressure are made every 35 m, humidity every 25 m, and temperature every 10 m. These together provide a relatively crude picture of what the value of the RRI is over a range of heights. The second method is a more serious means of measuring the RRI. It uses fast response devises called refractometers. These maybe carried by a balloon , aircraft, or be spaced apart on a high tower. These instruments are based upon the change in resonant frequency of a cavity with partially open ends caused by the change in RRI of air passing through the cavity. This gives a finer measurement showing variations in the RRI over height differences of a little over one meter. This is illustrated by the graph in Figure 3-18. The aircraft mounted refractometer can give a detailed study over several paths and heights.

Figure 3-18 Measurement of the RRIEffects of deviations from the normal lapse rateThe lapse rate of 40 N per km is based on clear sky readings with good atmosphere mixing. Normally a radio system is calibrated during these conditions and the height alignment in the case of a microwave point to point link is determined. It is easier to see the effects on a microwave point to point system when examining the effects of uneven variations of the RRI. Figure 3-19A shows an exaggerated curved radio path between two antennas under normal conditions. The signal is refracted by the atmosphere and arrives at the receiving antenna. Figure 3-19B illustrates the condition known as super refraction where the radio waves are not diffracted enough. This occurs when the lapse rate is less than 40 N per km. Under these conditions the main signal path will miss the receive antenna. Similar effects on a cell would increase the cell size as the radio waves would be propagated further resulting in co-channel and adjacent channel interference. The second effect is where the RRI increases greater than 40 N per km. This results in the path being refracted too much and not arriving at the receive antenna. This condition is known as sub-refraction. While this will not cause any interference as with super refraction, it could result in areas of no coverage. See Figure 3-19C. The last effect is known as ducting and occurs when the refraction of the radio wave produces a path which matches the curvature of the Earth. If this happens radio waves are propagated over far greater distances than normal and can produce interference in places not normally subjected to any.

Figure 3-19 Effects on a microwave systemEvents which can modify the clear sky lapse rateThere are four main events which can modify the clear sky lapse rate and they are as follows: Radiation nightsThis is the result of a very sunny day followed by clear skies overnight. The Earth absorbs heat during the day and the air temperature rises. After sunset the Earth radiates heat into the atmosphere and its surface temperature drops. This heat loss is not replaced resulting in air closer to the surface cooling faster than air higher up. This condition causes a temperature inversion and the RRI profile no longer has a uniform lapse rate. This effect will only occur overland and not water as water temperature variations are over a longer period of time. Advection effectsThis effect is caused by high pressure weather fronts moving from land to the sea or other large expanses of water. The result is warm air from the high pressure front covering the relatively cool air of the water. When this combination is then blown back over land a temperature inversion is caused by the trapped cool air. It will persist until the air mass strikes high ground where the increase in height will mix and dissipate the inversion. SubsidenceThis occurs again in a high pressure system this time overland when air descending from high altitude is heated by compression as it descends. This heated air then spreads over the cooler air below. This type of temperature inversion normally occurs at an altitude of 1 km but may occasionally drop to 100 m where it cause severe disruption to radio signals. Frontal systemsThis happens when a cold front approaching an area forces a wedge of cold air under the warmer air causing a temperature inversion. These disturbances tend to be short lived as the cold front usually dissipate quickly.

Although those described above are the four main causes of RRI deviations, local pressure, humidity and temperature conditions could well give rise to events which will affect the RRI. Environmental effects on propagation At the frequency range used for GSM it is important to consider the effects that objects in the path of the radio wave will have on it. As the wave length is approximately 30 cm for GSM900 and 15 cm for GSM1800, most objects in the path will have some effect on the signal. Such things as vehicles, buildings, office fittings even people and animals will all affect the radio wave in one way or another. The main effects can be summarized as follows:

Attenuation. Reflection. Scattering. Diffraction. Polarization changes.

AttenuationThis will be caused by any object obstructing the wave path causing absorption of the signal. The effects are quite significant at GSM frequencies but still depend on the type of materials and dimensions of the object in relation to the wavelength used. Buildings, trees and people will all cause the signal to be attenuated by varying degrees.

Figure 3-20 AttenuationReflectionThis is caused when the radio wave strikes a relatively smooth conducting surface. The wave is reflected at the same angle at which it arrived. The strength of the reflected signal depends on how well the reflector conducts. The greater the conductivity the stronger the reflected wave. This explains why sea water is a better reflector than sand.

Figure 3-21 ReflectionScatteringThis occurs when a wave reflects of a rough surface. The rougher the surface and the relationship between the size of the objects and the wave length will determine the amount of scattering that occurs.

Figure 3-22 ScatteringDiffractionDiffraction is where a radio wave is bent off its normal path. This happens when the radio wave passes over an edge, such as that of a building roof or at street level that of a corner of a building. The amount of diffraction that takes place increases as the frequency used is increased. Diffraction can be a good thing as it allows radio signals to reach areas where they would not normally be propagated.

Figure 3-23 DiffractionPolarization changesThis can happen any time with any of the above effects of due to atmospheric conditions and geomagnetic effects such as the solar wind striking the earths atmosphere. These polarisation changes mean that a signal may arrive at the receiver with a different polarisation than that which the antenna has been designed to accept. If this occurs the received signal will be greatly attenuated by the antenna.

Figure 3-24 PolarizationMultipath propagation Rayleigh and Rician fadingAs a result of the propagation effects on the transmitted signal the receiver will pick up the same signal which has been reflected from many different objects resulting in what is known as multipath reception. The signals arriving from the different paths will all have travelled different distances and will therefore arrive at the receiver at different times with different signal strengths. Because of the reception time difference the signals may or may not be in phase with each other. The result is that some will combine constructively resulting in a gain of signal strength while others will combine destructively resulting in a loss of signal strength. The receiving antenna does not have to be moved very far for the signal strength to vary by many tens of dBs. For GSM900 a move of just 15 cm or half a wavelength will suffice to observe a change in signal strength. This effect is known as multipath fading. It is typically experienced in urban areas

where there are lots of buildings and the only signals received are from reflections and refractions of the original signal. Rayleigh environmentThis type of environment has been described by Rayleigh. He analysed the signal strength along a path with a moving receiver and plotted a graph of the typical signal strength measured due to multipath fading. The plot is specifically for non line of sight, Figure 3-25, and is known as Rayleigh distribution, Figure 3-26.

Figure 3-25 Propagation effect - Rayleigh fading environmentFigure 3-26 Rayleigh distribution

Rician environmentWhere the signal path is predominantly line of sight, Figure 3-27, with insignificant reflections of refractions arriving at the receiver, this is know as Rician distribution, Figure 3-28. There are still fades in signal strength but they rarely dip below the threshold below which they will not be processed by the receiver.

Figure 3-27 Propagation effect - Rician environmentFigure 3-28 Rician distribution

Comparison of DCS1800 and GSM900: From a pure frequency point of view it would be true to say that DCS1800 generally has more fades than GSM900. However, they are usually less pronounced. Receive signal strengthA moving vehicle in an urban environment seldom has a direct line of sight path to the base station. The propagation path contains many obstacles in the form of buildings, other structures and even other vehicles. Because there is no unique propagation path between transmitter and receiver, the instantaneous field strength at the MS and BTS exhibits a highly variable structure. The received signal at the mobile is the net result of many waves that arrive via multiple paths formed by diffraction and scattering. The amplitudes, phase and angle of arrival of the waves are random and the short term statistics of the resultant signal envelope approximate a Rayleigh distribution. Should a microcell be employed, where part of a cell coverage area be predominantly line of sight then Rician distribution will be exhibited. Free space lossThis is the lose of signal strength that occurs as the radio waves are propagated through free space. Free space is defined as the condition where there are no sources of reflection in the signal path. This is impossible to achieve in reality but it does give a good starting point for all propagation loss calculations. Equally important in establishing path losses is the effect that the devices radiating the signal have on the signal itself. As a basis for the calculation it is assumed the device is an isotopic radiator. This is a theoretical pin point antenna which radiates equally in every direction. If the device was placed in the middle of a sphere it would illuminated the entire inner surface with an equal field strength. In order to find out what the power is covering the sphere, the following formula used:

Where: Pt is: the input power to the isotopic antenna.

d the distance from the radiator to the surface of the sphere. This formula illustrates the inverse square law that the power decreases with the square of the distance. In order to work out the power received at a normal antenna the affective aperture (Ae) of the receiving antenna must be calculated. The actual received power can be calculated as follows: Now if P is substituted with the formula for the power received over the inner surface of a sphere and Ae with its formula the result is: Free space path lossThis is the ratio of the actual received power to the transmitted power from an isotopic radiator and can be calculated by the formula: Logs are used to to make the figures more manageable. Note that the formula is dependant on distance and frequency. The higher the frequency the shorter the wavelength and therefore the greater the path loss. The formula above is based on units measured in metres. To make the formula more convenient it can be modified to use kilometre and megahertz for the distance and frequency. It becomes:

Where: d is: the distance in km.

f the frequency in MHz. Plane earth lossThe free space loss as stated was based solely on a theoretical model and is of no use by itself when calculating the path loss in a multipath environment. To provide a more realistic model, the earth in its role as a reflector of signals must be taken into account. When calculating the plane earth loss the model assumes that the signal arriving at the receiver consists of a direct path component and a reflective path component. Together these are often called the space wave. The formula for calculating the plane earth loss is: This takes into account the different antenna heights at the transmitter and receiver. Although this is still a simple representation of path loss. When this formula is used is implies the inverse fourth law as opposed to the inverse square law. So for every doubling of distance there is a 12 dB loss instead of 6 db with the free space loss calculation. The final factors in path loss are the ground characteristics. These will increase the path loss even further depending on the type of terrain, refer to Figure 3-29. The ground characteristics can be divided into three groups:

1. Excellent ground. For example sea water, this provides the least attenuation so a lower path loss.

1. Good ground. For example rich agricultural land, moist loamy lowland and forests. 1. Poor ground. For example Industrial or Urban areas, rocky land. These give the highest

losses and are typically found when planning Urban cells. Figure 3-29 Plane earth loss

Clutter factorThe propagation of the RF signal in an urban area is influenced by the nature of the surrounding urban environment. An urban area can then be placed into two sub categories; the built up area and the suburban area. The built up area contains tall buildings, office blocks, and high-rise residential tower blocks, whilst a suburban area contains residential houses, playing fields and parks as the main features. Problems may arise in placing areas into one of these two categories, so two parameters are utilised, a land usage factor describing the percentage of the area covered by buildings and a 'degree of urbanization' factor describing the percentage of buildings above storeys in the area.

Where: B is: the clutter factor in dBs.

F the frequency of RF signal.

L the percentage of land within 500m square occupied by buildings.

H the difference in height between the squares containing the transmitter and receiver.

K 0.094U - 5.9

U the percentage of L occupied by buildings above 4 storeys. A good base station site should be high enough to clear all the surrounding obstacles in the immediate vicinity. However, it should be pointed out that employing high antennas increases the coverage area of the base station. However, it will also have adverse effects on channel re-use distances because of the increased possibility of co-channel interference. Antenna gainThe additional gain provided by an antenna can be used to enhance the distance that the radio wave is transmitted. Antenna gain is measured against an isotopic radiator. Any antenna has a gain over an isotopic radiator because in practice it is impossible to radiate the power equally in all directions. This means that in some directions the radiated power will be concentrated. This concentration or focusing of power is what enables the radio waves to travel further than those that if it were possible were radiated from an isotopic radiator. See Figure 3-30.

Figure 3-30 Focusing of powerThe gain of a directional antenna is measured by comparing the signal strength of a carrier emitted from an isotopic antenna and the directional antenna. First the power of the isotopic radiator is increased so that both receive levels are the same. The emitted powers required to achieve that are then compared for both antennas. The difference is a measure of gain experienced by the directional antenna. It will always have some gain when compared to an isotopic radiator. See example in Figure 3-31.

Figure 3-31 Measurement of gain

In this example to achieve a balanced receive level the isotopic radiator must have an input power of 1000 W as opposed to the directional antenna which only requires 10 W. The gain of the directional antenna is 100 or 20 dBi.

Where: i is: for isotopic. The more directional the antenna is made the more gain it will experience. This is apparent when sectorizing cells . Each sectored cell will require less transmit power than the equivalent range omni cell due to the gain of its directional antenna, typically 14 to 17 dBi. The gain is also present in the receive path though in all cases the gain decreases as the frequency increases. That is why the uplink mobile to BTS frequency is usually the lowest part of the frequency range. This gives a slight gain advantage to the lower power mobile transmitter. Propagation in buildingsWith the increased use of hand portable equipment in mobile cellular systems combined with the increased availability of cordless telephones, it has become essential to study RF propagation into and within buildings. When calculating the propagation loss inside a building, Figure 3-32, a building loss factor is added to the RF path loss. This building loss factor is included in the model to account for the increase in attenuation of the received signal when the mobile is moved from outside to inside a building. This is fine if all users stood next to the walls of the building when making calls, but this does not happen, so the internal distance through which the signal must pass which has to be considered. Due to the internal construction of a building, the signal may suffer form spatial variations caused by the design of the interior of the building. The building loss tends to be defined as the difference in the median field intensity at the adjacent area just outside the building and the field intensity at a location on the main floor of the building. This location can be anywhere on the main floor. This produces a building median field intensity figure which is then used for plotting cell coverage areas and grade of service. When considering coverage in tall buildings, coverage is being considered throughout the building, if any floors of that building are above the height of the transmitting antenna a path gain will be experienced.

Figure 3-32 In building propagationThe Okumura methodIn the early 1960's a Japanese engineer named Okumura carried out a series of detailed propagation tests for land mobile radio services at various different frequencies. The frequencies were 200 MHz in the VHF band and 453, 922, 1310, 1430, and 1920 MHz in the UHF band. The results were statistically analyzed and described for distance and frequency dependencies of median field strength, location variabilities and antenna height gain factors for the base and mobile stations in urban, suburban, and open areas over quasi-smooth terrain. The correction factors corresponding to various terrain parameters for irregular terrain, such as rolling hills, isolated mountain areas, general sloped terrain, and mixed land/sea path were defined by Okumura. As a result of these tests carried out primarily in the Tokyo area, a method for predicting field strength and service area for a given terrain of a land mobile radio system was defined. The Okumura method is valid for the frequency range of 150 to 2000 MHz, for distances between the base station and the mobile stations of 1 to 100 km, with base station effective antenna heights of 30 to 100m. The results of the median field strength at the stated frequencies were displayed graphically. Different graphs were drawn for each of the test frequencies in each of the terrain environments (for example; urban, suburban, hilly terrain) Also shown on these graphs were the various antenna heights used at the test transmitter base stations. The graphs show the median field strength in relation to the distance in km from the site. As this is a graphical representation of results it does not transfer easily into a computer environment. However, the results provided by Okumura are the basis on which path loss prediction equations have been formulated. The most important work has been carried out by another Japanese engineer named Hata. Hata has taken Okumura's graphical results and derived an equation to calculate the path loss in various environments. These equations have been modified to take into account the differences between the Japanese terrain and the type of terrain experienced in Western Europe.

Figure 3-33 Okumura propagation graphsHata's propagation formula

Hata used the information contained in Okumura's propagation loss report of the early 1960's, which presented its results graphically, to define a series of empirical formulas to allow propagation prediction to be done on computers. The propagation loss in an urban area can be presented as a simple formula of:

Where: A is: the frequency.

B the antenna height function.

R the distance from the transmitter.

Hata using this basic formula which is applicable to radio systems is the UHF and VHF frequency ranges, added an error factor to the basic formula to produce a series of equations to predict path loss. To facilitate this action Hata has set a series of limitations which must be observed when using this empirical calculation method:

Where: Frequency range (fc) is: 100 - 1500 MHz

Distance (R) 1 - 20 km

Base station antenna height (hb) 30 - 200 M

Vehicular antenna height (hm) 1 - 10 M Hata defined three basic formulas based upon three defined types of coverage area; urban, suburban and open. It should be noted that Hata's formula predicts the actual path loss, not the final signal strength at the receiver. Urban Area: Lp = 69.55 + 26.16 log10fc - 13.82.log10hb - a (hm)# + (44.9 - 6.66. log10hb).log10R dB

Where: # is: Correction factor for vehicular station antenna height. Medium - Small City:

a(hm) = (1.1 . log10fc - 0.7).hm - (1.56.log10fc - 0.8) Large City:

a(hm) = 3.2 (log10 11.75 hm)2 - 4.97 Where: fc is: >400 MHz.

Suburban Area: Lps = Lp [Urban Area] - 2.[log10 (f/28)]2 - 5.4 dB Rural Area: Lpr = Lp [Urban Area] - 4.78.(log10fc)2 + 18.33.log10fc - 40.94 dB Power budget and system balanceIn any two-way radio system the radio path losses and equipment output powers must be taken into account for both directions. This is especially true in a mobile network where there are different characteristics for the uplink and downlink paths. These include receive path diversity gain in the uplink only, the possibility of mast head amplifiers in the uplink path, the output power capability of the mobile is a lot less than that of the BTS and the sensitivity of the BTS receiver is usually better than the mobiles. If these differences are not considered it is possible that the BTS will have a service area far greater than that which the mobile will be able to use due to its limited output power. Therefore the path losses and output powers in the uplink and downlink must be carefully calculated to achieve a system balance. One where the power required of the mobile to achieve a given range is equitable to the range offered by the power transmitted by the BTS. The output powers of the BTS and mobile are unlikely to be the same for any given distances due to the differences in uplink and downlink path losses and gains as described above. Once the area of coverage for a site has been decided the calculations for the power budget can be made. The system balance is then calculated which will decide the output powers of the BTS and mobile to provide acceptable quality calls in the area of coverage of the BTS. The BTS power level must never be increased above the calculated level for system balance. Although this seems a simple way to increase coverage, the system balance will be different and the mobile may not be able to make a call in the new coverage area. To increase the cell coverage, an acceptable way is to increase the gain of the antenna. This will affect both the uplink and downlink therefore maintaining system balance. Where separate antennas are used for transmit and receive they must be of similar gain. If the cell size is to be reduced then

this is not a problem as the BTS power can be altered and the mobiles output power is adaptive all the time. There is a statistic in the BTS that checks the path balance every 480 ms for each call in progress. The latest uplink and downlink figures reported along with the actual mobile and BTS transmit powers are used in a formula to give an indication of the path balance. GSM900 path lossFigure 3-34 and Figure 3-35 compare the path losses at different heights for the BTS antenna and different locations of the mobile subscriber between one and 100 km cell radius.

Figure 3-34 BTS antenna height of 50 m, MS height of 1.5 m (GSM900)Figure 3-35 BTS antenna height of 100 m, MS height of 1.5 m (GSM900)

Path loss GSM900 vs DCS1800Figure 3-36 illustrates the greater path loss experienced by the higher DCS1800 frequency range compared to the GSM900 band. The cell size is typical of that found in urban or suburban locations. The difference in path loss for the GSM900 band at 0.2 km compared with 3 km is 40 dB, a resultant loss factor of 10,000 compared to the measurement at 0.2 km.

Figure 3-36 Path loss vs cell radius for small cellsFrequency re-use Introduction to re-use patternsThe network planner designs the cellular network around the available carriers or frequency channels. The frequency channels will be allocated to the network provider from the GSM, EGSM, or DCS1800 band as shown below: Within this range of frequencies only a finite number of channels may be allocated to the planner. The number of channels will not necessarily cover the full frequency spectrum and there has to be great care taken when selecting/allocating the channels. Installing a greater number of cells will provide greater spectral efficiency with more frequency re-use of available frequencies. However, a balance must be struck between spectral efficiency and all the costs of the cell. The size of cells will also indicate how the frequency spectrum is used. Maximum cell radius is determined in part by the output power of the mobile subscriber (MS) (and therefore, its range) and interference caused by adjacent cells. Remember that the output power of the MS is limited in both the GSM900 and DCS1800 systems. Therefore to plan a balanced transmit and receive radio path the planner must make use of the path loss and thus the link budget. The effective range of a cell will vary according to location, and can be as much as 35 km in rural areas and as little as 1 km in a dense urban environment.

Figure 3-37 Frequency re-useRe-use patternThe total number of radio frequencies allocated is split into a number of channel groups or sets. These channel groups are assigned on a per cell basis in a regular pattern which repeats across all of the cells. Thus, each channel set may be re-used many times throughout the coverage area, giving rise to a particular re-use pattern (for example; 7 cell re-use pattern, Figure 3-38).

Figure 3-38 7 cell re-use patternClearly, as the number of channel sets increases, the number of available channels per cell reduces and therefore the system capacity falls. However, as the number of channel sets increases, the distance between co-channel cells also increases, thus the interference reduces. Selecting the optimum number of channel sets is therefore a compromise between quality and capacity. 4 site - 3 cell re-use patternDue to this increase in frequency robustness within GSM, different re-use frequency patterns can be adopted, which gives an overall greater frequency efficiency. The most common re-use pattern is 4 site with 3 cells. With the available frequency allocation divided into 12 channels sets numbered a1-3, b1-3, c1-3, and d1-3. The re-use pattern is arranged so that the minimum re-use distance between cells is at least 2 to 1. The other main advantage of this re-use pattern is if a new cell is required to be inserted in the network, then there is always a frequency channel set available which will not cause any adjacent channel interference.

Figure 3-39 4 site - 3 cell re-use pattern2 site - 6 cell re-use pattern

Another solution to possible network operators capacity problems may be an even higher frequency re-use pattern. The re-use pattern, shown in Figure 3-40, uses a 2 site - 6 cell re-use. Therefore: 2 sites repeated each with 6 cells = 2 x 6 = 12 groups. If the operator has only 24 carriers allocated for their use, they are still in a position to use 2 carriers per cell. However this may be extremely difficult and may not be possible to implement. It also may not be possible due to the current network configuration. However, the subscribers per km ratio would be improved.

Figure 3-40 2 site - 6 cell re-use pattern

Carrier/ Interference (C/I) ratioWhen a channel is re-used there is a risk of co-channel interference which is where other base stations are transmitting on the same frequency. As the number of channel sets increases the number of available channels per cell reduces and therefore capacity reduces. But the interference level will also reduce, increasing the quality of service. The capacity of any one cell is limited by the interference that can be tolerated for a given grade of service. A number of other factors, apart from the capacity, effect the interference level:

Power control (both BTS and MS). Hardware techniques. Frequency hopping (if applied). Sectorization. Discontinuous transmission (DTX).

Carrier/Interference measurements taken at different locations within the coverage of a cell can be compared to a previously defined acceptable criterion. For instance, the criterion for the C/I ratio maybe set at 8 dB with the expectation that the C/I measurements will be better than that figure, for 90% of cases (C/I90). For a given re-use pattern, the predicted C/I ratio related to the D/R ratio can be determined, to give overall system comparison. For example:

Figure 3-41 Carrier interference measurementsOther sources of interferenceAdjacent Channel Interference: This type of interference is characterised by unwanted signals from other frequency channels `spilling over' or injecting energy into the channel of interest. With this type of interference being influenced by the spacing of RF channels, its effect can be reduced by increasing the frequency spacing of the channels. However, this will have the adverse effect of reducing the number of channels available for use within the system. The base station and the mobile stations receiver selectivity can also be designed to reduce the adjacent channel interference. Environmental Noise: This type of interference can also provide another source of potential interference. The intensity of this environmental noise is related to local conditions and can vary from insignificance to levels which can completely dominate all other sources of noise and interference. There are also several other factors which have to be taken into consideration. The interfering co-channel signals in given cell would normally arise from a number of surrounding cells not just one. What effect will directional antennas have when employed? Finally, if receiver diversity is to be used, what type and how is implementation to be achieved? Sectorization of sitesAs cell sizes are reduced, the propagation laws indicate that the levels of carrier interference tend to increase. In a omni cell, co-channel interference will be received from six surrounding cells all using the same channel sets. Therefore, one way of significantly cutting the level of interference is to use several directional antennas at the base stations, with each antenna radiating a sector of the cell, with a separate channel set. Sectorization increases the number of traffic channels available at a cell site which means more traffic channels available for subscribers to use. Also by installing more capacity at the same site there is a significant reduction in the overall implementation and operating costs experienced by the network operator. By using sectorized antennas, sectorization allows the use of geographically smaller cells and a tighter more economic re-use of the available frequency spectrum. This results in better network performance to the subscriber and a greater spectrum efficiency.

The use of sectorized antennas allows better control of any RF interference which results in a higher call quality and an improved call reliability. More importantly for the network designer sectorization extends and enhances the cells ability to provide the in-building coverage that is assumed by the hand portable subscriber. Sectorization provides the flexibility to meet uneven subscriber distribution by allowing if required an uneven distribution of traffic resources across the cells on a particular site. This allows a more efficient use of both the infrastructure hardware and the available channel resources. Finally, with the addition of diversity techniques an improved sensitivity and increased interference immunity are experienced in a dense urban environment. Overcoming adverse propagation effects Hardware techniquesMultipath fading is responsible for more than just deep fades in the signal strength. The multipath signals are all arriving at different times and the demodulator will attempt to recover all of the time dispersed signals. This leads to an overlapping situation where each signal path influences the other, making the original data very hard to distinguish. The example opposite shows three component paths of the original signal which after demodulation should give three examples of the original data. This is not the case in reality as the output will be the result of the combination of the three inputs. As is shown in the diagram the output is very different making it difficult to decide wether the data should represent a 1 or a 0. This problem is known as inter symbol interference (ISI) and is made worse by the fact that the output from the demodulator is rarely a square wave. The sharp edges are normally rounded off so that when time dispersed signals are combined it makes it difficult to distinguish the original signal state. Another factor which makes things even more difficult is that the modulation technique Gaussian minimum shift keying, itself introduces a certain amount of ISI. Although this is a known distortion and can under normal conditions be filtered out, when it is added to the ISI distortion caused by the time delayed multipath signals it makes recovery of the original data that much harder. Frequency hoppingFrequency hopping is a feature that can be implemented on the air interface, (for example; the radio path to the MS), to help overcome the effects of multipath fading. GSM recommends only one type of frequency hopping, baseband hopping; but the Motorola BSS will support an additional type of frequency hopping called synthesizer hopping. Baseband hoppingBaseband Hopping is used when a base station has several DRCU/TCUs available. The data flow is simply routed in the baseband to various DRCU/TCUs, each of which operates on a fixed frequency, in accordance with the assigned hopping sequence. The different DRCU/TCUs will receive a specific individual timeslot in each TDMA frame containing information destined for different MSs. There are important points to note when using this method of providing frequency hopping.

There is a need to provide as many DRCU/TCUs as the number of allocated frequencies. The use of remote tuning combiners, cavity combining blocks or hybrid combiners is

acceptable in BTS6 applications. Within M-Cell equipment applications the use of either combining bandpass filter/hybrid or

cavity combining block is acceptable. Synthesizer hoppingSynthesizer hopping uses the frequency agility of the DRCU/TCU to change frequencies on a timeslot basis for both transmit and receive. The SCB in the DRCU and the digital processing and control board in the TCU calculates the next frequency and programmes one of the pair of Tx and Rx synthesizers to go to the calculated frequency. As the DRCU/TCU uses a pair of synthesizers for both transmit and receive, as one pair of synthesizers is being used the other pair are returning. There are important points to note when using synthesizer hopping:

Instead of providing as many DRCU/TCUs as the number of allocated frequencies, there is only a need to provide as many DRCU/TCUs as determined by traffic plus one for the BCCH carrier.

The output power available with the use of hybrid combiners must be consistent with coverage requirements.

Therefore as a general rule, cells with a small number of carriers will make good candidates for synthesizer hopping, whilst cells with many carriers will be good candidates for baseband hopping. There is also the other rule. There can only be one type of hopping on a BTS site, not a combination of the two. Error protection and detection

To protect the logical channels from transmission errors introduced by the radio path, many different coding schemes are used. Figure 3-43 illustrates the coding process for speech, control and data channels; the sequence is very complex. The coding and interleaving schemes depend on the type of logical channel to be encoded. All logical channels require some form of convolutional encoding, but since protection needs are different, the code rates may also differ. The coding protection schemes,shown in Figure 3-42, are: Speech channel encodingThe speech information for one 20 ms speech block is divided over eight GSM bursts. This ensures that if bursts are lost due to interference over the air interface the speech can still be reproduced. Common control channel encoding20 ms of information over the air will carry four bursts of control information, for example BCCH. This enables the bursts to be inserted into one TDMA multiframe. Data channel encodingThe data information is spread over 22 bursts. This is because every bit of data information is very important. Therefore, when the data is reconstructed at the receiver, if a burst is lost, only a very small proportion of the 20 ms block of data will be lost. The error encoding mechanisms should then enable the missing data to be reconstructed.

Figure 3-42 The coding processFigure 3-43 Error protection and detection

Speech channel encoding The BTS receives transcoded speech over the Abis interface from the BSC. At this point the speech is organized into its individual logical channels by the BTS. These logical channels of information are then channel coded before being transmitted over the air interface. The transcoded speech information is received in frames, each containing 260 bits. The speech bits are grouped into three classes of sensitivity to errors, depending on their importance to the intelligibility of speech. Class 1aThree parity bits are derived from the 50 Class 1a bits. Transmission errors within these bits are catastrophic to speech intelligibility, therefore, the speech decoder is able to detect uncorrectable errors within the Class 1a bits. If there are Class 1a bit errors, the whole block is usually ignored. Class 1bThe 132 Class 1b bits are not parity checked, but are fed together with the Class 1a and parity bits to a convolutional encoder. Four tail bits are added which set the registers in the receiver to a known state for decoding purposes. Class 2The 78 least sensitive bits are not protected at all. The resulting 456 bit block is then interleaved before being sent over the air interface. The encoded speech now occupies 456 bits, but is still transmitted in 20 ms thus raising the transmission rate to 22.8 kbit/s.

Figure 3-44 Speech channel encodingChannel coding for enhanced full rate The transcoding for enhanced full rate produces 20 ms speech frames of 244 bits for channel coding on the air interface. After passing through a preliminary stage which adds 16 bits to make the frame up to 260 bits the EFR speech frame is treated to the same channel coding as full rate. The additional 16 bits correspond to an 8 bit CRC which is generated from the 50 Class 1a bits plus the 15 most important Class 1b bits and 8 repetition bits corresponding to 4 selected bits in the original EFR frame of 244 bits. Preliminary channel coding for EFREFR Speech Frame

50 Class 1a + 124 Class 1b + 70 Class 2 = 244 bits Preliminary Coding

8 bit CRC generated from 50 Class 1a + 15 Class 1b added to Class 1b bits 8 repetition bits added to Class 2 bits

Output from preliminary coding 50 Class 1a + 132 Class 1b + 78 Class 2 = 260 bits

EFR frame of 260 bits passed on for similar channel coding as Full Rate. Figure 3-45 Preliminary coding for enhanced full rate speech

Control channel encoding Figure 3-46 shows the principle of the error protection for the control channels. This scheme is used for all the logical signalling channels, the synchronization channel (SCH) and the random access burst (RACH). The diagram applies to SCH and RACH, but with different numbers. When control information is received by the BTS it is received as a block of 184 bits. These bits are first protected with a cyclic block code of a class known as a Fire Code. This is particularly suitable for the detection and correction of burst errors, as it uses 40 parity bits. Before the convolutional encoding, four tail bits are added which set the registers in the receiver to a known state for decoding purposes. The output from the encoding process for each block of 184 bits of signalling data is 456 bits, exactly the same as for speech. The resulting 456 bit block is then interleaved before being sent over the air interface.

Figure 3-46 Control channel codingData channel encoding Figure 3-47 shows the principle of the error protection for the 9.6 kbit/s data channel. The other data channels at rates of 4.8 kbit/s and 2.4 kbit/s are encoded slightly differently, but the principle is the same. Data channels are encoded using a convolutional code only. With the 9.6 kbit/s data some coded bits need to be removed (punctuated) before interleaving, so that like the speech and control channels they contain 456 bits every 20 ms. The data traffic channels require a higher net rate (`net rate' means the bit rate before coding bits have been added) than their actual transmission rate. For example, the 9.6 kbit/s service will require 12 kbit/s, because status signals (such as the RS-232 DTR (data terminal ready)) have to be transmitted as well. The output from the encoding process for each block of 240 bits of data traffic is 456 bits, exactly the same as for speech and control. The resulting 456 bit block is then interleaved before being sent over the air interface. The encoded control information now occupies 456 bits but is still transmitted in 20 ms thus raising the transmission rate to 22.8 kbit/s.

Figure 3-47 Data channel encodingMapping logical channels onto the TDMA frame structure InterleavingHaving encoded, or error protected the logical channel, the next step is to build its bitstream into bursts that can then be transmitted within the TDMA frame structure. It is at this stage that the process of interleaving is carried out. Interleaving spreads the content of one traffic block across several TDMA timeslots. The following interleaving depths are used:

Speech - 8 blocks Control - 4 blocks Data - 22 blocks

This process is an important one, for it safeguards the data in the harsh air interface radio environment. Because of interference, noise, or physical interruption of the radio path, bursts may be destroyed or corrupted as they travel between MS and BTS, a figure of 10-20% is quite normal. The purpose of interleaving is to ensure that only some of the data from each traffic block is contained within each burst. By this means, when a burst is not correctly received, the loss does not affect overall transmission quality because the error correction techniques are able to interpolate for the missing data. If the system worked by simply having one traffic block per burst, then it would be unable to do this and transmission quality would suffer. It is interleaving that is largely responsible for the robustness of the GSM air interface, enabling it to withstand significant noise and interference and maintain the quality of service presented to the subscriber.

Table 3-19 Interleaving

TRAU Frame Type Number of GSM Bursts spread over

Speech 8

Control 4

Data 22

Diagonal interleaving - speechFigure 3-48 illustrates, in a simplified form, the principle of the interleaving process applied to a full-rate speech channel. The diagram shows a sequence of `speech blocks' after the encoding process previously described, all from the same subscriber conversation. Each block contains 456 bits, these blocks are then divided into eight blocks each containing 57 bits. Each block will only contain bits from even bit positions or bits from odd bit positions. The GSM burst will now be produced using these blocks of speech bits. The first four blocks will be placed in the even bit positions of the first four bursts. The last four blocks will be placed in the odd bit positions of the next four bursts. As each burst contains 114 traffic carrying bits, it is in fact shared by two speech blocks. Each block will share four bursts with the block preceding it, and four with the block that succeeds it, as shown. In the diagram block 5 shares the first four bursts with block 4 and the second four bursts with block 6.

Figure 3-48 Diagonal interleafing - speechTransmission - speechEach burst will be transmitted in the designated timeslot of eight consecutive TDMA frames, providing the interleaving depth of eight. Table 3-20 shows how the 456 bits resulting from a 20 ms speech sample are distributed over eight normal bursts. It is important to remember that each timeslot on this carrier may be occupied by a different channel combination: traffic, broadcast, dedicated or combined. The FACCH will steal a 456 bit block and be interleaved with the speech. Each burst containing a FACCH block of information will have the appropriate stealing flag set.

Table 3-20 Distribution of 456 bits from one 20 ms speech sample

Distribution Burst

0 8 16 24 32 40 ..........................448 even bits of burst N








Rectangular interleaving - controlFigure 3-49 illustrates, in a simplified form, the principle of rectangular interleaving. This is applied to most control channels. The diagram shows a sequence of `control blocks' after the encoding process previously described. Each block contains 456 bits, these blocks are then divided into four blocks each containing 114 bits. Each block will only contain bits for even or odd bit positions. The GSM burst will be produced using these blocks of control. Transmission - controlEach burst will be transmitted in the designated timeslot of four consecutive TDMA frames, providing the interleaving depth of four. The control information is not diagonally interleaved as are speech and data. This is because only a limited amount of control information is sent every multiframe. If the control information was diagonally

interleaved, the receiver would not be capable of decoding a control message until at least two multiframes were received. This would be too long a delay.

Figure 3-49 Rectangular interleaving - controlDiagonal interleaving - dataFigure 3-50 illustrates, in a simplified form, diagonal interleaving applied to a 9.6 kbit/s data channel. The diagram shows a sequence of `data blocks' after the encoding process previously described, all from the same subscriber. Each block contains 456 bits, these blocks are divided into four blocks each containing 114 bits. These blocks are then interleaved together. The first 6 bits from the first block are placed in the first burst. The first 6 bits from the second block will be placed in the second burst and so on. Each 114 bit block is spread across 19 bursts and the total 456 block will be spread across 22 bursts. Data channels are said to have an interleaving depth of 22, although this is sometimes also referred to as an interleaving depth of 19. Transmission - dataThe data bits are spread over a large number of bursts, to ensure that the data is protected. Therefore, if a burst is lost, only a very small amount of data from one data block will actually be lost. Due to the error protection mechanisms used, the lost data has a higher chance of being reproduced at the receiver. This wide interleaving depth, although providing a high resilience to error, does introduce a time delay in the transmission of the data. If data transmission is slightly delayed, it will not effect the reception quality, whereas with speech, if a delay were introduced this could be detected by the subscriber. This is why speech uses a shorter interleaving depth.

Figure 3-50 Diagonal interleaving - dataVoice Activity Detection - VAD VAD is a mechanism whereby the source transmitter equipment identifies the presence or absence of speech. VAD implementation is effected in speech mode by encoding the speech pattern silences at a rate of 500 bit/s rather than the full 13 kbit/s. This results in a data transmission rate for background noise, known as comfort noise, which is regenerated in the receiver. Without comfort noise the total silence between the speech would be considered to be disturbing by the listener. Discontinuous Transmission - DTX DTX increases the efficiency of the system through a decrease in the possible radio transmission interference level. It does this by ensuring that the MS does not transmit unnecessary message data. DTX can be implemented, as necessary, on a call by call basis. The effects will be most noticeable in communications between two MS. DTX in its most extreme form, when implemented at the MS can also result in considerable power saving. If the MS does not transmit during silences there is a reduction in the overall power output requirement. The implementation of DTX is very much at the discretion of the network provider and there are different specifications applied for different types of channel usage. DTX is implemented over a SACCH multiframe (480 ms). During this time, of the possible 104 frames, only the 4 SACCH frames and 8 Silence Descriptor (SID) frames are transmitted.

Figure 3-51 SACCH Multiframe (480 ms)Receive diversityIn its simplest case, multipath fading arises from destructive interference between two transmission paths. The deepest instantaneous fade occurring at the frequency for which the effective path length difference is an odd multiple of half wavelengths. If two receive antennas are mounted a defined distance apart, then it follows that the probability of them simultaneously experiencing maximum fade depth at a given frequency is very much less than for the single antenna situation. There are three ways of utilizing this concept:

The receiver can be switched between the two RF receive paths provided two antennas. The RF signals from two receive paths can be phase aligned and summed. The phasing can be made so as to minimize the distortion arising from the multipath

transmission. Each of the methods has advantages and disadvantages.

In the case of the switched configuration, its simply chooses the better of the two RF signals which is switched through to the receiver circuitry. Phase alignment has the advantage of being a continuously optimized arrangement in terms of signal level, but phase alignment diversity does not minimize distortion. The Motorola DRCU/TCU uses this diversity concept. The distortion minimizing approach, whilst being an attractive concept, has not yet been implemented in a form that works over the full fading range capabilities of the receivers and therefore has to switch back to phase alignment at low signal levels. This means a rather complex control system is required. It must be emphasized that diversity will not usually have any significant effect on the mean depression component of fading, but the use of phase alignment diversity can help increase the mean signal level received.

Figure 3-52 Receive diversityEqualizationAs mentioned in multipath fading, in most urban areas the only signals received are multipath. If nothing was done to try and counter the effects of (Inter Symbol Interference) ISI caused by the time dispersed signals, the Bit Error Rate (BER) of the demodulated signal would be far too high, giving a very poor quality signal, unacceptable to the subscriber. To counter this a circuit called an equalizer is built into the receiver. The equalizer uses a known bit pattern inserted into every normal burst transmitted, called the training sequence code. This allows the equaliser to assess and modify the effects of the multipath component, resulting in a far cleaner less distorted signal. Without this equalizer the quality of the circuit would be unacceptable for the majority of time. Training sequence codeThe training sequence code, Figure 3-53, is used so that the demodulator can estimate the most probable sequence of modulated data. As the training sequence is a known pattern, this enables the receiver to estimate the distortion ISI on the signal due to propagation effects, especially multipath reception. The receiver must be able to cope with two multipaths of equal power received at an interval of up to 16 microseconds. If the two multipaths are 16 micro seconds delayed then this would be approximately equivalent to 5-bit periods. There are 32 combinations possible when two 5-bit binary signals are combined. As the transmitted training sequence is known at the receiver, it is possible to compare the actual multipath signal received with all 32 possible combinations reproduced in the receiver. From this comparison the most likely combination can be chosen and the filters set to remove the multipath element from the received signal. The multipath element can be of benefit, once it has been identified, as it can then be recombined with the wanted signal in a constructive way to give a greater received signal strength. Once the filters have been set, they can be used to filter the random speech data as it is assumed they will have suffered from the same multipath interference as the training sequence code. The multipath delay is calculated on a burst by burst basis, as it is constantly changing.

Figure 3-53 Training sequence codeSubscriber environment Subscriber hardwareSystem quality, (for example; voice quality) system access and grade of service, as perceived by the customer, are the most significant factors in the success of a cellular network. The everyday subscriber neither knows or really cares about the high level of technology incorporated into a cellular network. However, they do care about the quality of their calls. What the network designer must remember is that it is the subscriber who chooses the type of equipment they wish to use on the network. It is up to the network provider to satisfy the subscriber whatever they choose. The output power of the mobile subscriber is limited in a GSM system to a maximum of 8 W for a mobile and a minimum of 0.8 W for a hand portable. For a DCS1800 system, the mobile subscriber is restricted to a maximum of 1 W and a minimum of 250 mW hand portable. EnvironmentNot only does the network designer have to plan for the subscribers choice of phone, the designer has to plan for the subscribers choice as to where they wish to use that phone. Initially when only the mobile unit was available, system coverage and hence subscriber use was limited to on street, high density urban or low capacity rural coverage areas. During the early stages

of cellular system implementation the major concern was trying to provide system coverage inside tunnels. However, with the advances in technology the hand portable subscriber unit is now firmly established. With this introduction came new problems for the network designer. The portable subscriber unit provides the user far more freedom of use but the subscriber still expected exactly the same service. The subscriber now wants quality service from the system at any location. This location can be on a street, or any floor of a building whether it be the basement or the penthouse and even in lifts, refer to Figure 3-54. Thus greater freedom of use for the subscriber gives the network designer even greater problems when designing and implementing a cellular system.

Figure 3-54 The subscriber environmentDistributionNot only do network designers have to identify the types of subscriber that use the cellular network now and in the future, but at what location these subscribers are attempting to use their phones. Dense urban environments require an entirely different design approach, due to considerations mentioned earlier in this chapter, than the approach being used to design coverage for a sparsely populated rural environment. Road and rail networks have subscribers moving at high speed, so this must be accounted for when planning the interaction between network entities whilst the subscriber is using the network. Even in urban areas, the network designer must be aware that traffic is not necessarily evenly distributed. An urban area may contain sub-areas of uneven distribution such as a business or industrial district, and may have to plan for a seasonal increase of traffic due to, say, a convention centre. It is vitally important that the traffic distribution is known and understood prior to network design, to ensure that a successful quality network is implemented.

Figure 3-55 Subscriber distributionMost demandingThe network designer must ensure that the network is designed to ensure a quality service for the most demanding subscriber. This is the hand portable subscriber. The hand portable now represents the vast majority of all new subscriber units introduced into cellular networks. So clearly the network operators, and hence the network designers, must recognise this. Before commencing network design based around hand portable coverage, the network designer must first understand the limitations of the hand portable unit and secondly, what the hand portable actually requires from the network. The hand portable phone is a small lightweight unit which is easy to carry and has the ability to be used from any location. The ability of the unit to be used at any location means that the network must be designed with the provision of good in-building coverage as an essential element. To further complicate the network designers job, these hand portable units have a low output power:

0.8 W to 8 W for GSM900. 0.25 W to 1 W for DCS1800.

So the distance at which these units can be used from a cell is constrained by RF propagation limitations. For practical purposes, the actual transmit power of the hand portable should be kept as low as possible during operation. This helps not only from an interference point of view, but this also helps to extend the available talk time of the subscriber unit, which is limited by battery life. Future planningNormal practice in network planning is to choose one point of a well know re-use model as a starting point. Even at this early stage the model must be improved because any true traffic density does not follow the homogeneous pattern assumed in any theoretical models. Small-sized heavy traffic concentrations are characteristic of the real traffic distributions. Another well known traffic characteristic feature is the fast descent in the density of traffic when leaving city areas. It is uneconomical to build the whole network using a standard cell size, it becomes necessary to use cells of varying sizes. Connecting areas with different cell sizes brings about new problems. In principle it is possible to use cells of different size side by side, but without careful consideration this may lead to a wasteful frequency plan. This is due to the fact that the re-use distance of larger cells is greater than that of smaller cells. The situation is often that the borders are so close to the high density areas that the longer re-use distances mean decreased capacity. Another solution, offering better frequency efficiency, is to enlarge the cell size gradually from small cells into larger cells.

In most cases the traffic concentrations are so close to each other that the expansion cannot be completed before it is time to start approaching the next concentration, by gradually decreasing the cell size. This is why the practical network is not a regular cluster composition, but a group of directional cells of varying size. Besides this need for cells of different size, the unevenness of the traffic distribution also cause problems in frequency planning. Theoretical frequency division methods applicable to homogenous clusters cannot be used. It is quite rare that two or more neighbouring cells need the same amount of channels. It must always be kept in mind that the values calculated for future traffic distribution are only crude estimates and that the real traffic distribution always deviates from these estimates. In consequence, the network plan should be flexible enough to allow for rearrangement of the network to meet the real traffic needs. ConclusionIn conclusion there are no general rules for radio network planning. It is a work of experimenting and reiterating. By comparing different alternatives, the network designers should find a plan that both fulfils the given requirements and keeps within practical limitations. When making network plans, the designers should always remember that every location in a network has its own conditions, and all local problems must be tackled and solved on a individual basis. The microcellular solutionLayered ArchitectureThe basic term layered architecture is used in the microcellular context to explain how macrocells overlay microcells. It is worth noting that when talking of the traffic capacity of a microcell it is additional capacity to that of the macrocell in the areas of microcellular coverage. The traditional cell architecture design, Figure 3-56, ensures that, as far as possible, the cell gives almost total coverage for all the MSs within its area.

Figure 3-56 Layered architectureCombined cell architectureA combined cell architecture system, Figure 3-57, is a multi-layer system of macrocells and microcells. The simplest implementation contains two layers. The bulk of the capacity in a combined cell architecture is provided by the microcells. Combined cell systems can be implemented into other vendors networks. Macrocells: Implemented specifically to cater for the fast-moving MSs and to provide a fallback service in the case of coverage holes and pockets of interference in the microcell layer. Macrocells form an umbrella over the smaller microcells. Microcells: Microcells handle the traffic from slow-moving MSs. The microcells can give contiguous coverage over the required areas of heavy subscriber traffic. Picocells: Low cost installation by using in-building fibre optics or telephone wiring with a HDSL modem, easily expanded to meet capacity requirements. Efficient use of the frequency spectrum due to low power radios causing low interference to external networks. Higher quality speech compared with external illumination of the building due to improved uplink quality.

Figure 3-57 Combined cell architectureCombined cell architecture structureA combined cell architecture employs cells of different sizes overlaid to provide contiguous coverage. This structure is shown in Figure 3-58. Some points to note:

Macrocell and microcell networks may be operated as individual systems. The macrocell network is more dominant as it handles the greater amount of traffic. Microcells can be underlayed into existing networks. Picocells can be introduced as a third layer or as part of the second layer.

Figure 3-58 Combined cell architecture structureExpansion solution As the GSM network evolves and matures its traffic loading will increase as the number of subscribers grow. Eventually a network will reach a point of traffic saturation. The use of microcells can provide high traffic capacity in localised areas. The expansion of a BTS site past its original designed capacity can be a costly exercise and the frequency re-use implications need to be planned carefully (co-channel and adjacent channel interference). The use of microcells can alleviate the increase in congestion, the microcells could be stand-alone cells to cover traffic hotpots or a contiguous cover of cells in a combined architecture. The increased coverage will give greater customer satisfaction.

Technology

Bss cell planning