44890235 Frequency Planning Guidelines

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Frequency Planning GuidelinesT-Mobile USA

Frequency Planning Guidelines

Document Title: xxxx_Frequency Planning

Confidential and Proprietary Page 1T-Mobile Document Ref: ENG / RF / TGU - 47553527.doc


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Document Subject: Frequency Planning Guidelines

Document Author: Mark Cosgrove

Authors Manager: Yasmin Karimli

Company: T-Mobile

Document Category: RF Standards

Document Keywords: ENG / RF / TGU

Document Comments: Release for Comments and Approval

Document Web Location: http://rf.eng.voicestream.com/Library/.doc

Date: August 30th, 2002

Document Approved: Mark Cosgrove, Dir. RF Systems Engineering

Document Authorized: Mark Cosgrove



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Table of Contents

1 ScopeThis document outlines the Radio Frequency (RF) channel assignment strategy for

existing and new markets. Reference material and background information has been includedas an informative annex.

2 Introduction

Each market has available a set band of frequencies as defined by the relevant FCC

license. The licensed frequency band is used to support discrete 200KHz wide channels. GSMoperates on a predefined numbering scheme such that each 200KHz channel has a specificchannel number assigned. The defined channels and band information for GSM NorthAmerican (GSM-NA) is shown in table 1.

FCC PCSBand

B/W (Uplink andDownlink)

MS TX Band(MHz)

BTS TX(MHz)

GSM CH

A 30 MHz 1850.0 1930.0 512-586

D 10 MHz 1865.0 1945.0 587-611

B 30 MHz 1880.0 1960.0 612- 686

E 10 MHz 1885.0 1965.0 687-711F 10 MHz 1890.0 1970.0 712-736

C 30 MHz 1895.0 1975.0 737-811Table 1 FCC bands and GSM Channel Allocations

GSM requires that each cell (a site being made up of one or more sectored cells) haveone frequency that is used to broadcast network and cell control information and act as a pilotfrequency. This frequency is defined as the Broadcast Common Control Channel (BCCH)frequency. Timeslot 0 of the BCCH frequency carries the logical BCCH channel and additionalchannels that are used for paging, synchronization, and initial system access. The BCCHfrequency is required to transmit constantly at a set frequency and at full power. We will explainwhy the 7/21 re-use pattern is the strategy we recommend to use for the BCCH layer

Non-BCCH frequencies can adapt on a timeslot-by-timeslot basis, being able to changefrequency (Called hopping) and transmit power levels (Power control). The Non-BCCHchannels are therefore able to achieve better performance in terms of tolerance of interferenceand noise than the BCCH frequency. This better performance is exploited by reusing non-BCCH frequencies more often within a given area such that the traffic supported per MHz offrequency is increased. This document is aimed at promoting the use of frequency hopping 1/1strategy for markets with challenging terrain and 1/3 for markets in which the site are spread outwith regular azimuth.



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This document discusses the fundamental concepts of frequency reuse, varioustechniques for maximizing capacity, spectrum partitioning schemes, and finally to providerecommendations for partitioning of 5MHz, 10MHz, 15MHz and 20MHz spectrum (understand5,10,15 or 20MHz in uplink band and 5,10,15 or 20 MHz in downlink band). The Houston trialresults are provided in Appendix.

We recommend to local RF team to use this document as a frequency planning strategyguide. The recommended frequency planning strategies are summarized in therecommendation part located at the end of this document.

This document is intended for networks that have implemented GSM and GPRStechnologies, as it is the present case for all Voicestream markets.



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3 Frequency re-use theory

Frequency reuse, as the name suggests, involves reusing the same frequencyrepeatedly in a cellular network. It is this fundamental concept that enable cellular systems to

provide the necessary traffic carrying capacity to support its subscriber base.

3.1 Cellular network conceptsIn a cellular network, a group of contiguous cells all using different frequencies are

grouped into clusters. A term commonly used to describe a frequency re-use scheme is N,where N denotes the number of cells in a cluster, and thus the total number unique frequencies.

Here is how a cellular pattern, here with N=7 (i=2 and j=1) is displayed:

As shown on this figure the re-use pattern has to follow the 2 arrows directions (or one

arrow if j=0) to be regular and this is the reason why N has to verify that ijjiN ++= 22 . Where

i and j are integers.

That implies 1, 3, 4, 7, 9, 12, 13, 16, 19, 21, 27 are usually the values that we usuallyconsider in GSM.

For example, N=3, denotes a 3 sites cluster, each cell with a unique frequency group.Repeating this cluster over the geographic area of coverage forms the cellular network. In asectorized reuse scheme, a site number / cluster size convention is commonly used to denotethe reuse pattern. For instant 3/9, denotes a 3 site / 9 cell cluster (3 sectors per site).

The frequency-repeat pattern determines the maximum number of radios that can bedeployed in each cell, thus the maximum amount of traffic carried.

A cellular network may consist of omni sites or sectorized sites or a combination of both.

Given the same total number of channels, the capacity of a sectorized site is less than thecapacity of an omni site, as the example below illustrates.

Site type Total TCHavailable

Traffic Capacity (2% blocking)

Omni 45 35.6 Erlangs


i

j

A

A

AAA A

Ai

j


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3 Sector 45 3 x 9.01 = 27.0 Erlangs

However, sectorization allows higher frequency re-use with smaller number of sites aseach site contains 3 cells instead of one, and thus higher overall network capacity and that ismaking much more sense economically, therefore all GSM networks use sectorized sites. Forboth site types, several frequency re-use schemes are possible with varying levels of carrier to

interference ratio (C/I).For any re-use pattern, the ratio of co-channel cell site to the cell radius is:

NRD 3/ =

This comes from the fact that we have, here shown for N=4:

Lets call R the radius from the center of the hexagon to the middle of a side of thehexagon. We have:

RRR ==2

3)30cos('

The distance D between the middle of and hexagon and the middle of the next one thatuses the same frequency verifies:

ijjiRNRD++

==

22

'2'2

Therefore:

NRD 3/ =

From this value we can estimate the theoretical interference created by the first ring offrequency re-use. The hexagon grid implies that six first ring cells always surround a cell (Onefor each side of the hexagon). Lets illustrate how this work in the case of omni-directional site

I

S

I

Cm

6=

If we estimate that the propagation of the signal is proportional to the distance power theattenuation factor n, we have at the edge of the cell (i.e. worst case C/I):

R

RD

I

Cn

n

6

)(

=

So in dB we have:

)6

)13(log()

6

)1(

log(

=

=

NRD

I

C

nn

We usually assume a value of n=3.5 for the attenuation.


R

D


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3.2 Examples GSM frequency re-use patterns

Lets illustrate this theory with few examples of plan that can be used for TCH and/orBCCH. But we should notice that any BCCH plan may be implemented in TCH but the reverseis not true:

(1) 1/1 frequency reuse

Minimum reuse distance to cell radius ratio; Co-channel interference; worst case C/Ic = 0dB Each neighboring cell use the same frequency group, this can only be used with

frequency hopping in GSM

Figure 3.1: 1/1 frequency reuse pattern

The example below illustrates the 3 re-use schemes for sectorized sites, and theapproximate C/I ratio that can be achieved with a homogeneous network of cells using a 120degree beam width antenna.

(2) 1/3 frequency reuse

Minimum reuse distance to cell radius ratio; D/R = 1.732, Co-channel interference; worst case C/Ic = 5.5dB Adjacent channel interference; every neighboring cell uses an adjacent channel, this

can only be used with frequency hopping in GSM


1

3 2

1

3 2

1

3 21

3 2

1

3 21

3 2

1

3 2

1

1 1

1

1 1

1

1 11

1 1

1

1 11

1 1

1

1 1


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Figure 3.4: 4/12 frequency reuse pattern 2 adjacent channel neighbors

Figure 3.5: 4/12 frequency reuse pattern no adjacent channel neighbor

From the above example, we can see that interference reduces as N increases; this isdirectly connected to the theoretical C/I calculation illustrated before in the omni site case. Buton another hand the number of frequencies available in each group also decreases as Nincreases.

Although these grid patterns are often used for initial site planning, in practice however,it is the site acquisition process, which would ultimately determine the cellular pattern. Thereare many factors, which would significantly influence the network topology, these include:

Terrain (hilly or flat) Large water bodies Budgetary constraint Site placement constraints

All these factors make it difficult to achieve the ideal network topology.

Irregularities of site coverage will increase the carrier to interference ratios. In general,areas consisting of hilly terrain and large water bodies pose the most difficult frequencyplanning problems.

The theoretical minimum C/I for which GSM is designed to work is 9 dB (GSM rec05.05). However in reality it is not very efficient if we do not reach C/I = 12dB.


3

11 7 1

9 5

4

128

2

10 6

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128

3

11 7 1

9 5

2

10 6 4

128


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An N=12 might not allow the C/I requested to be sufficient. Moreover, to deal with theirregularities of site coverage, certain amount additional frequencies (i.e. the value of N) shouldbe allowed. If

If the spectrum available allows it, we shall advise the implementation of N=21 (7/21pattern) for non-hopping channels. The choice of this value of 7/21 is because it is the lowest

value for which we have a regular pattern (N verifies ijjiN ++= 22 ), where N is a multiple of 3

(tri-sectorial sites) and a sufficient theoretic C/I=15.56dB.Lets point out here that strategies such as 5/15, 6/18 or 8/24 that are often use do not

respect a regular pattern.



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4 Recommended Planning Rules

4.1 Frequency planning general rules

Based on industry technical reports, vendor discussions and field trial work carried out inthe Houston market by Aerial (see Appendix I), the following recommendations have beendevised for the application of frequency hopping to increase system capacity.

1. Separate frequency channel sets will be assigned to BCCH and non-BCCH usage, this is detailed in section 52. A fractional reuse pattern with soft loading shall be used.3. For non-BCCH channels a 1/3 or 1/1 frequency hopping reuse isoptimum, the frequency hopping benefits are detailed in section 4.34. Downlink Power control and DTX need to be active.5. Average loading of high traffic area should not be more than 40% for a1/3 and 15% for a 1/1

6. The peak loading for some sectors will be 60% if the 1/3 is implementedand 30% if 1/1 is used7. Non-BCCH channels are chosen for voice in preference to the BCCHchannels.8. GPRS (and future EDGE) traffic is assigned to the BCCH in preference tothe Non-BCCH channels, this is detailed in section 7.

The Non-BCCH frequencies will be used as the first choice for carrying voice traffic.This ensures that mobiles achieve all the benefits of frequency hopping. To maximize theperformance of the traffic channels it will be necessary to minimize the number of frequenciesassigned to the BCCH plan. Although fractional reuse averages out interference from manysources, effectively eliminating the need for frequency planning on the non-BCCH channels, the

actual peak and averaging loading will depend on the quality of the traditional frequency plan,i.e. the BCCH plan.

4.2 Frequency hopping benefits

4.2.1 Interference averaging concept

In traditional cell planning, where transmit powers are constant and frequencies are non-hopping, the downlink interference pattern is also constant and is worst at the cell edges wherea low number of interferers dominate.

As the frequency reuse pattern is tightened the number of interferers increases, assurrounding sites transmissions are no longer suitably attenuated by distance, however someBTS continue to be lower interferers than others.

By letting sectors change frequency on a seemingly random basis the interferencepattern at a single point is no-longer constant but changing. Frequency hopping exploits theinterleaving and coding protection of GSM such that short bursts of very poor interference canbe tolerated.



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Interference at a point is no longer defined by a low number of dominant interferers butis now defined by the average interference received from a large number of non-dominantinterferers. This interference averaging is repeated across the sector for all mobiles.

Interference

F1

F2 F3

MS_1 MS_2 MS_3

No hopping Interference

F1

F2 F3

MS_1 MS_2 MS_3

With hopping

F1

F2

F3 F1

F2 F3

avera

Figure 1 Interference Averagingsource: Nokia

4.2.2 Frequency hopping a good interference averaging technique

Frequency hopping, a standard feature of the GSM system, provides the most effectiveinterference averaging technique. The term frequency hopping describes a technique wherethe base station and mobile station changes RF frequency between each burst of transmission.The number of RF frequencies over which to hop is called a hop-set.

By hopping over a number of frequencies, the interference created by each transceiver

(TRX) is averaged across all frequencies in the hop-set. The impact on the overall interferencelevel is additive but not focused on any one frequency or geographical area. Hopping over asufficiently large frequency range will also average out the effect of frequency selective fading,which is a characteristic of multipath environments. This is known as frequency diversity. Thisbenefit is particularly important for the downlink, as the mobile station does not have receiverdiversity.

Frequency hopping benefits [4]:



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Better tolerance to low C/I: The GSM channel coding scheme is designedto recover lost information when the punctures are short in duration. Frequencyhopping is more resistant to interference because of the convolutionnaldecoding, this frequency diversity gain is detailed in part 4.2.3. Easier frequency planning: Once the spectrum has been partitioned andthe hop-sets identified, TRX can be added to the system with relative ease by

simply including them in the most appropriate hopping set for that cell Soft capacity and gradual degradation: rather than a small number ofusers being victimized by co-channel in a small number of areas, all users onhopping channels will be degraded somewhat. This is called soft capacity and isgenerally desirable in systems where large numbers of users share a limitedcommon resource Frequency selective fading diversity: By FH over larger frequencyranges, the coherence bandwidth can be overcome and the depth of fast fadesand BER/FER is consequently reduced. This can greatly improve channelconditions for stationary and slow moving mobile users in any environment. Theoverall benefits in terms of S/N ratio have been estimated to be around 2dB.Note that for users who are mobile, the effects of fast fading are taken care of by

the interleaved channel coding error correction mechanisms

The disadvantage includes:

Synthesized frequency hopping requires a minimum of two TRX toimplement, one for the BCCH, and the other for hopping TCH. Base-bandhopping will require as many TRX as there are frequencies in the hop-set Quality is very traffic sensitive; the quality of the network degrades veryrapidly with increase in traffic, but on the other hand a high traffic makes thefrequency planning impossible if non-hopping strategy is implemented

4.2.3 Frequency Diversity Performance Gains

Rayleigh fading causes deep notches in the received signal strength due to activecancellation of the received signals that arrive at a mobile caused by the difference in pathlengths. Rayleigh fading is most prominent in urban and suburban environments where mostradio propagation paths are non-line-of-sight. The occurrence of these deep notches, both inspace and time, are highly frequency dependent.

By changing the carrier frequency (frequency hopping) on a burst-by-burst basis, theoccurrence of these notches are spread over several transmission bursts rather than effecting agroup of consecutive bursts. Frequency hopping hence provides frequency diversity that hasthe effect of de-correlating the errors across the interleaved time-slots. To achieve de-correlation between hopping bursts frequency separations of 400KHz (2 GSM channels) is

needed.

The speech codec of GSM delivers data rate to the channel codec of 13kbps. Thechannel coding applies convolution coding and parity protection for the more important speechbits. The 456 bits of the encoded speech are divided in to 8 groups of 57 bits. Thereforeindividual encoded speech bursts are interleaved over eight time-slots, to ensure that errors

caused by radio fading are as distributed through out the speech frame as much as possible.The maximum performance of the coding is achieved when any radio, induced errors are de-correlated across the interleaved bursts.



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Moving mobiles benefit by increasing the time spread and hence de-correlation oftransmission errors; above 35km/h a correlation envelope of 0.7 is achieved. Mobiles at slowerspeed therefore have relatively poor performance when compared to the high-speed mobiles.Frequency diversity is able to provide high levels of de-correlation. As high-speed mobilesalready experience de-correlated bursts, the frequency diversity gain is only available to slow

moving mobiles.

Several simulation studies have been performed on the gain attainable from FH in anoise-limited environment.

The COST 231 study Performance of Slow Frequency Hopping in GSM, Poznan, September1995) produced the following tables from link level simulations:

Number ofFrequencies

Frequency Hopping Gain TU 3 Frequency Hopping Gain TU 50

CyclicHopping

AbsoluteLevel (dB)

Relative Gain(dB)

Absolute Level(dB)

Relative Gain(dB)

1 11.5 0.0 6.5 0.02 8.5 3.0 6.0 0.53 7.5 4.0 6.0 0.54 6.5 5.0 6.0 0.58 5.5 6.0 6.0 0.5

RandomHopping

1 11.5 0.0 6.5 0.02 9.5 2.0 6.5 0.03 8.5 3.0 6.5 0.04 8.0 3.5 6.0 0.58 7.5 4.0 6.0 0.5

12 7.0 4.5 6.0 0.5

Table 2 Frequency Hopping Gains

The link simulation shows that the gain in a noise limited environment for mobiles at amedium to high speed is less than 0.5 dB even for hopping over 12 channels. For a slowmoving mobile the gains are much higher; even for just two carrier hopping the gain is 2 to 3dB,for higher hopping sequences gains of up to 6dB can be attained. In an ideal hoppingenvironment, slow moving mobiles are able to attain the same level of performance as fastmoving mobiles.

In defining the link budgets for urban and suburban sites the performance differencesbetween fast and slow moving mobiles is not taken in to account. Therefore hopping

effectively helps slow moving mobiles meet the link budget assumptions and does notoffer an improvement over the link budget.



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4.2.4 Baseband or synthesized frequency hopping?

Frequency hopping is implemented in 2 ways, the baseband and the synthesizedfrequency hopping.

Baseband hopping only allows the TCH (or SDCCH) to hop on as many frequencies asthere are hopping TRX (e.g. if 3 sectors are hopping the frequency hopping would only contain3 frequencies).

Synthesized hopping allows the TCH to hop on the whole spectrum allocated forhopping with very little limitations (it is actually limited to 63 frequencies in a group and this limitis rarely reached as it corresponds to a 12.6 MHz spectrum).

Baseband hopping was actually implemented when GSM started, as some vendorswere unable to propose synthesized hopping on their early equipments. As it is much easier toplan and reinforce the performance of frequency hopping, synthesized frequency hopping is theimplementation recommended.

4.2.5 What frequency hopping pattern should be implemented?

The re-use patterns most commonly used in GSM for hopping are 1/1 and 1/3. In theorythey are quite equivalent as you can reach a 16% load (20% if optimized) of the frequencygroup if using 1/1 and 50% load of the frequency group if using 1/3. But as the frequency groupused in 1/1 (this group in that case represents the whole spectrum used for hopping channels)is three times larger than the frequency group used in 1/3 (a third of the spectrum allocated forhopping channels).

The theoretical advantage of the 1/3 reuse pattern is that the adjacent cells never use thesame channels, but it is counterbalanced by the fact that collisions with the cell using the samegroup and which is often in dense urban areas quite close are very important. The 1/3 re-usepattern is very efficient if there is a regular pattern, typically this is true for flat areas with

buildings of homogeneous sizes.The 1/1 is the hopping strategy that we recommend migrating towards in cases of more

challenging environment: As it is impossible in the real world to have a 120 degrees angle between

azimuths for all sites (building or terrain mask, unpopulated areas such as water or mountaincovered if pattern is respected, etc). And these sites that do not respect the pattern degradethe 1/3 hopping strategy. Whereas on the other hand 1/1 strategy does not suffer anydegradation from pattern not respected.

We often have a number of hopping frequencies that is not divisible by 3 sohopping groups in 1/3 might unbalanced or get a reduced number of frequencies. 1/1 strategyof course maximizes the use of the bandwidth allocated for the frequency hopping

The 1/1 strategy also reduces interferences more easily in areas that would

require a site that does not respect the pattern (bi-sectorial site to cover a highway, pico-cell inhot spots)

In case of a large spectrum allocation and a low usage other techniques such as 3*9,3*3, 4*4, 4*12, and 5*5 may be used in frequency hopping with smaller groups of frequencies,therefore the interference is limited. The issue with these techniques is that a frequency planmust be performed; these techniques will not be studied further in this document.

Recommendations for different markets, according to their spectrum allocation, will begiven in section 5.



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4.3 Average and Peak Loading

Fractional loading performance is dependent on the interference averaging achieved byhaving more transmitting frequencies available to a sector than there are TRXs. The use ofdifferent hopping patterns on similar orientated sectors randomizes the possibility of collisions.At any one time only a fraction of the possible frequencies in a reuse pattern are beingtransmitted. For two sectors using the same MA list, but different HSN, the probability of thesame frequency being transmitted at the same time is 1/(N*T), where N is the number oftransmit frequencies available, T is the number of TRX.

To maintain speech quality and drop call performance this probability needs to be keptwithin certain limits. This is called soft loading. There is no automatic way of limiting the trafficsuch that the performance of the system is not impacted; therefore the loading of a networkneeds to be periodically monitored. The loading of the system is defined in two parameters:

Average Load Peak Load.

The Peak load is computed from the highest traffic cells in an area and is often set as ahard limit, i.e the number of available TRXs are set at the Peak load, such that the systemreaches 2% blocking at the peak load.

The Average Load is computed by taking all cells in an area and computing the effectivetraffic loading and frequency reuse. There is no hard limit to the average load andhence the performance of a network must be monitored. Once the average load limit isreached cell splits will be needed. It is possible for a network to reach the average loadlimit before any one site has reached the peak load limit.

4.3.1 Process for Calculating Loading

Step 1. Measure BH Traffic for busiest cell and surrounding area,approximately 21 sites.

Step 2. Compute the peak cell traffic and the average cell traffic Step 3. Convert peak and average traffic levels in to channel requirements using

Erlang B tables and 0.1% blocking1 Step 4. Convert channel requirements in to TRX occupancy Step 5. Calculate the TRX loading.

Example

8 Sites, 24 sectors carrying 279 ErlangsPeak traffic is 26 Erlangs8 Frequencies per MA list

1 The measured BH traffic is carried traffic where as Erlang B tables uses offered traffic. By using a verylow level of blocking the difference between offered and carried traffic channel requirements is minimized.



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Peak AverageTraffic per cell 26 11.6TCH per cell @ 0.1% GOS 42 27Loaded Frequencies (8 TCH perfrequency)

5.25 3.4

Loading (8 Frequencies per MA

list)

65% 42%

9 freq per MA list 58% 37%

Table 3 Loading calculations

4.4 Frequency hopping implementation

4.4.1 Implementing the 1/1 frequency hopping

The frequency hopping implementation requires three elements: Frequency group definition HSN (hopping sequence number) planning

MAIO (mobile allocation index offset) planningThe frequency group definition is quite simple in the case of 1/1, as it will be composedof all the frequencies available for TCH.

The HSN planning is using all HSN except 0 (use 1 to 63). Each site shall use the sameHSN for all its cells as the cells are synchronized and therefore it allows avoiding any adjacentchannel interference with the correct MAIO implementation. There is however an exception tothis rule if the cells are collocated but not synchronized then the cells should use different HSN.This is true currently for any Nokia site larger than S666 and any Nortel site bigger than S888and for Ericsson bigger than S11_11_10 (32 TRX).

The HSN planning must be performed so that the sites using the same HSN should beas far as possible from each other, because if two sites are using the same HSN they willinterfere each other all the time, hence there will be no benefit from frequency hopping. If there

are pico-cells using frequency hopping then the planning of the HSN of these cells must beperformed after the macro layer has been done in order to prioritize the interference from thecells with longer coverage range.

The MAIO planning should be performed in order to avoid any adjacency between thedifferent cells of the same site. This is achieved this way:

Frequency ofhoppinggroup

f1 f2 f3 f4 f5 f6 f7 f8 f9 f10 f11 f12 f13

Sector AMAIO

0 6 12

Sector BMAIO

2 8

Sector CMAIO

4 10

We can see from this table that using the MAIO 0,6,12,18 TRX 1,2,3,4 of sector A,MAIO 2,8,14,20 for TRX 1,2,3,4 of sector B and MAIO 4,10,16,22 for TRX 1,2,3,4 of sectorC we are sure to avoid any adjacent channel and co-channel interference between the sitesdifferent cell.

If we summarize graphically each sites sector should look this way:



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Sector A MAIO 0,6,12,18

For all three sectors HSN=N with 1=


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Group A F1 F4 F7 F10 F13 F16 F19 F22 F25 F28Group B F2 F5 F8 F11 F14 F17 F20 F23 F26 F29Group C F3 F6 F9 F12 F15 F18 F21 F24 F27 F30

From this table we can observe that there is no adjacent channel at the same time. SoMAIO 0,2,4,6 should be used for TRX 1,2,3,4 of sector A, MAIO 1,3,5,7 used for TRX

1,2,3,4 of sector C

Graphically the HSN, MAIO implementation of 1/3 hopping is:

Sector A MAIO 0,2,4,6

For all three sectors HSN=N with 1=


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Operating the new cell at half power, building a lower height cell or using high levels of antennatilt can achieve this.

Figure 2 Cell Split process for narrow beam systems

The split cell will be reduced in area by 50% and can be left operating at full power.However with a 1/3 reuse pattern a more balanced interference environment is required. This isachieved by reducing the power on the split cell by 50% and adding two additional cell splitsites.

For narrow beam systems ideal cell splitting requires a 3:1 increase in sites.

For systems built using wide beam antennas (90 to 120 degree beamwidth), the process

is the same but the site placement is different.

For 90 degree systems the new site is still placed at an equidistance between theexisting sites, but in this case it is not at the limit of the cell but off to one side. If the cell split isplaced at the farthest cell edge the grid array is broken and further cell splits will require theremoval of the split cell.



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Figure 3 Cell Split Process for Wide beam Systems

The initial addition of a cell split site reduces the area of the target cell by 38% andrequires that the split site must be operated at full power. The addition of a second new siteallows the split site to be reduced in power to balance the area and allow the coverage to bereduced to 25%.

For 90 degree systems an ideal cell split requires a 2:1 increase in sites.

In ideal systems the cell split process focuses on balancing the coverage area of thesites, which in turn balances, the interference.

In practice, traffic is not evenly distributed within a cell and additional information isneeded such that the cell split is successful in reducing the traffic levels on split sectors.

When planning a cell split, it is necessary to gather information on traffic distributionswithin a cell. Timing Advance (TA) information will provide information as to the distribution oftraffic. Each TA2 unit corresponds to 0.5km; for example all mobiles reporting a TA of 0 are with0.5 km of the site, a TA of 1 between 0.5 to 1km etc.

A distribution of TA can be collected from the OMC-R by running Cell Trace. TA shouldbe collected for the Busy Hour, the distribution calculated and the results compared to either the

predicted or known coverage area of the cell. A traditional cell split will offload the traffic in theouter 50% of the cell. If the TA distribution shows that the traffic is concentrated in theinner50% of the sector than the cell split will be ineffective and another method of increasingcapacity should be considered.

4.6 BCCH Layer Constraints

The GSM system requires that the BCCH carriers must be transmitted at constantoutput power, at all times, for all 8-time slots on the carrier. In addition, the BCCH carrier mustbe static, which implies it cannot frequency hop. This precludes the use of enhancedinterference management techniques, as well as standard power control and DTX. For thesereason, the BCCH layer is subject to a high interference level and as this TRX is essential forthe mobile access and selection of the cell, therefore this layer shall be protected as much aspossible. Note that 6 or 7 timeslots on BCCH carrier are used to carrier traffic.

The reuse schemes employed for the BCCH layer should be sufficiently relaxed toensure acceptable QoS. Typically it is recommended if the spectrum allows it to use a 7/21 forthe BCCH pattern to ensure a good quality of service. If the allocated spectrum is reduced, a4/12 pattern is the minimal re-use that should be considered to guarantee a minimal access tothe network.

2 Note that the TA is calculated based on the roundtrip delay of the radio signals and hence isthe radio path length. Where strong multipath effects exist the radio path may be significantlylonger than the true distance from the site.



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4.7 BSIC Planning

The BSIC (base station identity code) is the combination of the NCC (network colorcode) and the BCC (base color code). The BCC and NCC take values between 0 and 7. Usuallythe NCC is unique to a market and should not be changed from cell to cell. The BCC on theother hand should be dealt with carefully. The rule should be to re-use the same couples BCCHfrequency-BSIC as far apart as possible. The reason is that the couple BCCH-BSIC is used torecognize neighbors. Therefore if two cells quite close use the same couple it will confuse thehandover process. Furthermore, the OMC will prevent the creation of two neighbors using sameBCCH-BSIC couple.

4.8 Preparing the future migration plans

Until recently as Hardware limits of the older Ericsson and Nokia equipment is 6 TRX,older Nortel equipment is limited to 8 TRX. Therefore we were usually limiting the BTSexpansion to S666 in the first two case and S888 for Nortel, with the hardware being the limiting

factor.

As now all vendors offer (by end of 2002) high capacity base stations supporting 12 TRXper cabinet, it opens to Voicestream new possibilities for expansion. This enhancement will behighly needed for dense areas with a high subscriber base as well as it is required to supportbandwidth consuming data technologies such as GPRS and EDGE.

In order to support the High Capacity Base Stations markets will need to tighten thecurrent frequency assignment rules and take full advantage of fractional loading. In additiondata will eventually require separate channels as usage increases. Ultimately high data ratesmay require the deployment of 3G or similar technologies that will require spectrum to be setaside.

Markets should develop three to four year migration plans that systematically tighten thereuse for the BCCH channels and allow for higher fractional loading on the hopping trafficchannels. Markets should aim to increase the TRX per sector to the levels shown for AdvancedNetwork Designs. In many cases levels higher than the loadings shown may be obtaineddepending on the quality of the design. The ability to meet the Advanced Network Designlevels is dependent on the ability to contain interference on the BCCH plan.

This can be attained by:

Consistent height of sites Use of Electrical down tilt and narrow beam antennas Use of Static Power control Maintaining consistent antenna orientation Gird like pattern of site placement

Advanced features such as concentric cells and layered networks will increase thecapacity of the system allowing lower spectrum allocations to meet the 12 TRX limit, oralternatively allow for more spectrum to be set aside for data services. Such techniques are forfurther study.



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Both the average and peak traffic levels represent a soft limit. This limit implies thatalthough capacity is available in an area the performance of the system will be impacted byincreasing the loading of the system beyond an average of 40%. If the soft limit of the systemis exceeded then the average TRX per sector with in a given area needs to be reduced byadding additional sites, i.e. cell splitting.



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5 Spectrum Partitioning

The question here is, for a given spectral bandwidth, how many frequency channels(ARFCN) should be used as the BCCH, the remainder will thus be used as TCH? Further, whatre-use schemes should be deployed for the TCH carriers to support the projected traffic load?

5.1 Comparison of block and interleave partition

There are two basic methods for allocation of BCCH carriers, block partition andinterleaved partition. In block partition, a block of frequencies is reserved for used as BCCHcarriers, and the rest as TCH carriers. In interleaved partition, every 2nd or 3rd frequency isreserved for use as BCCH carrier. Refer to the diagram below.

Figure 5.4: Example of block and interleave spectrum partition

DowTCH

Figure 5.5: Interleave spectrum partition prevents downlink power control

The interleaved partition approach is not practical since it prevents the use of downlinkpower control, therefore the BCCH and TCH spectrum should be separated and notinterleaved. As illustrated in Figure 5.5, when the TCH is power down, there is strong adjacentchannel interference from the BCCH carrier, which is transmitted at full power.

5.2 Comparison between contiguous and split TCH

As illustrated in the diagram below, the BCCH frequencies block can be placed in thecenter of the available spectrum, thus splitting the TCH into 2 blocks of frequencies. Thisprovides larger frequency hopping range.



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However, the split TCH results in two adjacent TCH carriers. Thus increasing thepotential for adjacent interference. Using guard bands on either side of the BCCH spectrum willsolve this. It is recommended to use a guard band between TCH and BCCH band.

BCCFigure 5.6: Example of side and central spectrum partition

Alternatively, placing the BCCH at the bottom or top of the available spectrum reducesthe number of adjacent channel TCH to one, and provides a contiguous TCH block.

The penalty of this approach is that it reduces the ability of frequency hopping to combatfrequency selective fading, however it does not seem to compensate for the loss of a frequencyexcept in the case of very important spectrum allocation (typically 20 MHz uplink and downlinkof spectrum or more). Therefore we recommend in most cases to use contiguous TCH and tohave the BCCH located on top or bottom of the spectrum.



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6 Market application

6.1 Frequency planning strategy suggested for a 5 MHz market (5 MHzbandwidth uplink and downlink)

For a 5MHz market (5MHz downlink, 5MHz uplink), there is a total of 25 ARFCNs(absolute radio frequency channel number). Of these 25, there is 1 frequency block guardband, the 25th ARFCN, and 2 low power guard bands (quarter watts, 24dBm maximum), the 1st

and the 24th. Therefore it leaves only 22 ARFCN available, which is very reduced.

In order to realistically provide a sufficient quality for the signaling a 4/12 BCCH reuse isinevitable. As we have to leave a guard channel between the TCH and BCCH band, this willleave only 9 channels to use for the TCH, implemented in 1/1 hopping, which allows us to go upto S333 configuration the frequency load being 2/9=22% quite high but achievable. Otherstrategies (non-hopping, 1/3) would probably be limited to lower configuration and in the case ofnon-hopping create a very complicated cell planning.

The cell split criteria would be to split at S333 configuration in a 5 MHz market.Here is the suggested frequency planning for a 5MHz market:

Block guard Power guard BCCH TCH guard band bet TCH&BCCH

The frequency groups would be set this way for the BCCH (group 1 to 4):

sector Group1 Group2 Group3 Group4

A 2 3 4 5

B 6 7 8 9C 10 11 12 13

As for the TCH the repartition will be the following:

Group 1/1 15 16 17 18 19 20 21 22 23

The MAIO strategy used is going to be:

Sector MAIO TRX1 MAIO TRX2A 0 6B 2 8

C 4 7 (8 or 6 if the first or second cell has only 2TRX)

2 3 4 5 6 7 8 9 10 11 12 131 14 15 16 17 18 19 20 21 22 23 24 25

Documents

44890235 Frequency Planning Guidelines