1. Um interface   The "air" or radio interface standard that is used for exchanges between a mobile (ME) and a base station (BTS / BSC). For signalling, a modified version of the ISDN LAPD, known as LAPDm is used.

2. Abis interface   This is a BSS internal interface linking the BSC and a BTS, and it has not been totally standardised. The Abis interface allows control of the radio equipment and radio frequency allocation in the BTS.

3. A interface   The A interface is used to provide communication between the BSS and the MSC. The interface carries information to enable the channels, timeslots and the like to be allocated to the mobile equipments being serviced by the BSSs. The messaging required within the network to enable handover etc to be undertaken is carried over the interface.

4. B interface   The B interface exists between the MSC and the VLR . It uses a protocol known as the MAP/B protocol. As most VLRs are collocated with an MSC, this makes the interface purely an "internal" interface. The interface is used whenever the MSC needs access to data regarding a MS located in its area.

5. C interface   The C interface is located between the HLR and a GMSC or a SMS-G. When a call originates from outside the network, i.e. from the PSTN or another mobile network it ahs to pass through the gateway so that routing information required to complete the call may be gained. The protocol used for communication is MAP/C, the letter "C" indicating that the protocol is used for the "C" interface. In addition to this, the MSC may optionally forward billing information to the HLR after the call is completed and cleared down.

6. D interface   The D interface is situated between the VLR and HLR. It uses the MAP/D protocol to exchange the data related to the location of the ME and to the management of the subscriber.

7. E interface   The E interface provides communication between two MSCs. The E interface exchanges data related to handover between the anchor and relay MSCs using the MAP/E protocol.

8. F interface   The F interface is used between an MSC and EIR. It uses the MAP/F protocol. The communications along this interface are used to confirm the status of the IMEI of the ME gaining access to the network.

9. G interface   The G interface interconnects two VLRs of different MSCs and uses the MAP/G protocol to transfer subscriber information, during e.g. a location update procedure.

10. H interface   The H interface exists between the MSC the SMS-G. It transfers short messages and uses the MAP/H protocol.

11. I interface   The I interface can be found between the MSC and the ME. Messages exchanged over the I interface are relayed transparently through the BSS.

GMSK basics

GMSK modulation is based on MSK, which is itself a form of phase shift keying. One of the problems with standard forms of PSK is that sidebands extend out from the carrier. To overcome this, MSK and its derivative GMSK can be used.

MSK and also GMSK modulation are what is known as a continuous phase scheme. Here there are no phase discontinuities because the frequency changes occur at the carrier zero crossing points. This arises as a result of the unique factor of MSK that the frequency difference between the logical one and logical zero states is always equal to half the data rate. This can be expressed in terms of the modulation index, and it is always equal to 0.5.

Signal using MSK modulation

A plot of the spectrum of an MSK signal shows sidebands extending well beyond a bandwidth equal to the data rate. This can be reduced by passing the modulating signal through a low pass filter prior to applying it to the carrier. The requirements for the filter are that it should have a sharp cut-off, narrow bandwidth and its impulse response should show no overshoot. The ideal filter is known as a Gaussian filter which has a Gaussian shaped response to an impulse and no ringing. In this way the basic MSK signal is converted to GMSK modulation.

Spectral density of MSK and GMSK signals

Generating GMSK modulation

There are two main ways in which GMSK modulation can be generated. The most obvious way is to filter the modulating signal using a Gaussian filter and then apply this to a frequency modulator where the modulation index is set to 0.5. This method is very simple and straightforward but it has the drawback that the modulation index must exactly equal 0.5. In practice this analogue method is not suitable because component tolerances drift and cannot be set exactly.

Generating GMSK using a Gaussian filter and VCO

A second method is more widely used. Here what is known as a quadrature modulator is used. The term quadrature means that the phase of a signal is in quadrature or 90 degrees to another one. The quadrature modulator uses one signal that is said to be in-phase and another that is in quadrature to this. In view of the in-phase and quadrature elements this type of modulator is often said to be an I-Q modulator. Using this type of modulator the modulation index can be maintained at exactly 0.5 without the need for any settings or adjustments. This makes it much easier to use, and capable of providing the required level of performance without the need for adjustments. For demodulation the technique can be used in reverse.

Block diagram of I-Q modulator used to create GMSK

Advantages of GMSK modulation

there are several advantages to the use of GMSK modulation for a radio communications system. One is obviously the improved spectral efficiency when compared to other phase shift keyed modes.

A further advantage of GMSK is that it can be amplified by a non-linear amplifier and remain undistorted This is because there are no elements of the signal that are carried as amplitude

variations. This advantage is of particular importance when using small portable transmitters, such as those required by cellular technology. Non-linear amplifiers are more efficient in terms of the DC power input from the power rails that they convert into a radio frequency signal. This means that the power consumption for a given output is much less, and this results in lower levels of battery consumption; a very important factor for cell phones.

A further advantage of GMSK modulation again arises from the fact that none of the information is carried as amplitude variations. This means that is immune to amplitude variations and therefore more resilient to noise, than some other forms of modulation, because most noise is mainly amplitude based.

GMSK highlights

GMSK modulation is a highly successful form of modulation, being used in GSM cellular technology, and as a result, its use is particularly widespread. It is also used in other radio communications applications because of its advantages in terms of spectral efficiency, resilience to noise and its ability to allow the use of efficient transmitter final amplifiers. Even though other radio communications systems utilise other forms of modulation, GMSk is an ideal choice for many applications.

GSM slot structure and multiple access scheme

GSM uses a combination of both TDMA and FDMA techniques. The FDMA element involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart as already described.

The carriers are then divided in time, using a TDMA scheme. This enables the different users of the single radio frequency channel to be allocated different times slots. They are then able to use the same RF channel without mutual interference. The slot is then the time that is allocated to the particular user, and the GSM burst is the transmission that is made in this time.

Each GSM slot, and hence each GSM burst lasts for 0.577 mS (15/26 mS). Eight of these burst periods are grouped into what is known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it forms the basic unit for the definition of logical channels. One physical channel is one burst period allocated in each TDMA frame.

There are different types of frame that are transmitted to carry different data, and also the frames are organised into what are termed multiframes and superframes to provide overall synchronisation.

GSM slot structure

These GSM slot is the smallest individual time period that is available to each mobile. It has a defined format because a variety of different types of data are required to be transmitted.

Although there are shortened transmission bursts, the slots is normally used for transmitting 148 bits of information. This data can be used for carrying voice data, control and synchronization data.

GSM slots showing offset between transmit and receive

It can be seen from the GSM slot structure that the timing of the slots in the uplink and the downlink are not simultaneous, and there is a time offset between the transmit and receive. This offset in the GSM slot timing is deliberate and it means that a mobile that which is allocated the same slot in both directions does not transmit and receive at the same time. This considerably reduces the need for expensive filters to isolate the transmitter from the receiver. It also provides a space saving.

GSM burst

The GSM burst, or transmission can fulfil a variety of functions. Some GSM bursts are used for carrying data while others are used for control information. As a result of this a number of different types of GSM burst are defined.

Normal burst   uplink and downlink Synchronisation burst  downlink Frequency correction burst  downlink Random Access (Shortened Burst)   uplink

GSM normal burst

This GSM burst is used for the standard communications between the basestation and the mobile, and typically transfers the digitised voice data.

The structure of the normal GSM burst is exactly defined and follows a common format. It contains data that provides a number of different functions:

1. 3 tail bits:   These tail bits at the start of the GSM burst give time for the transmitter to ramp up its power

2. 57 data bits:   This block of data is used to carry information, and most often contains the digitised voice data although on occasions it may be replaced with signalling information in the form of the Fast Associated Control CHannel (FACCH). The type of data is indicated by the flag that follows the data field

3. 1 bit flag:   This bit within the GSM burst indicates the type of data in the previous field. 4. 26 bits training sequence:   This training sequence is used as a timing reference and for

equalisation. There is a total of eight different bit sequences that may be used, each 26 bits long. The same sequence is used in each GSM slot, but nearby base stations using the same radio frequency channels will use different ones, and this enables the mobile to differentiate between the various cells using the same frequency.

5. 1 bit flag   Again this flag indicates the type of data in the data field. 6. 57 data bits   Again, this block of data within the GSM burst is used for carrying data. 7. 3 tail bits   These final bits within the GSM burst are used to enable the transmitter power

to ramp down. They are often called final tail bits, or just tail bits. 8. 8.25 bits guard time   At the end of the GSM burst there is a guard period. This is

introduced to prevent transmitted bursts from different mobiles overlapping. As a result of their differing distances from the base station.

GSM Normal Burst

GSM synchronization burst

The purpose of this form of GSM burst is to provide synchronisation for the mobiles on the network.

1. 3 tail bits:   Again, these tail bits at the start of the GSM burst give time for the transmitter to ramp up its power

2. 39 bits of information:   3. 64 bits of a Long Training Sequence:   4. 39 bits Information:   5. 3 tail bits   Again these are to enable the transmitter power to ramp down.6. 8.25 bits guard time:   to act as a guard interval.

GSM Synchronization Burst

GSM frequency correction burst

With the information in the burst all set to zeros, the burst essentially consists of a constant frequency carrier with no phase alteration.

1. 3 tail bits:   Again, these tail bits at the start of the GSM burst give time for the transmitter to ramp up its power.

2. 142 bits all set to zero:   3. 3 tail bits   Again these are to enable the transmitter power to ramp down.4. 8.25 bits guard time:   to act as a guard interval.

GSM Frequency Correction Burst

GSM random access burst

This form of GSM burst used when accessing the network and it is shortened in terms of the data carried, having a much longer guard period. This GSM burst structure is used to ensure that it fits in the time slot regardless of any severe timing problems that may exist. Once the mobile has

accessed the network and timing has been aligned, then there is no requirement for the long guard period.

1. 7 tail bits:   The increased number of tail bits is included to provide additional margin when accessing the network.

2. 41 training bits:   3. 36 data bits:   4. 3 tail bits   Again these are to enable the transmitter power to ramp down.5. 69.25 bits guard time:   The additional guard time, filling the remaining time of the GSM

burst provides for large timing differences.

GSM Random Access Burst

GSM discontinuous transmission (DTx)

A further power saving and interference reducing facility is the discontinuous transmission (DTx) capability that is incorporated within the specification. It is particularly useful because there are long pauses in speech, for example when the person using the mobile is listening, and during these periods there is no need to transmit a signal. In fact it is found that a person speaks for less than 40% of the time during normal telephone conversations. The most important element of DTx is the Voice Activity Detector. It must correctly distinguish between voice and noise inputs, a task that is not trivial. If a voice signal is misinterpreted as noise, the transmitter is turned off an effect known as clipping results and this is particularly annoying to the person listening to the speech. However if noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased.

It is also necessary for the system to add background or comfort noise when the transmitter is turned off because complete silence can be very disconcerting for the listener. Accordingly this is added as appropriate. The noise is controlled by the SID (silence indication descriptor).

GSM power levels

The base station controls the power output of the mobile, keeping the GSM power level sufficient to maintain a good signal to noise ratio, while not too high to reduce interference, overloading, and also to preserve the battery life.

A table of GSM power levels is defined, and the base station controls the power of the mobile by sending a GSM "power level" number. The mobile then adjusts its power accordingly. In virtually all cases the increment between the different power level numbers is 2dB.

The accuracies required for GSM power control are relatively stringent. At the maximum power levels they are typically required to be controlled to within +/- 2 dB, whereas this relaxes to +/- 5 dB at the lower levels.

The power level numbers vary according to the GSM band in use. Figures for the three main bands in use are given below:

Power level number

Power output level dBm

2 39 3 37 4 35 5 33 6 31 7 29 8 27 9 25 10 23 11 21 12 19 13 17 14 15 15 13 16 11 17 9 18 7 19 5

GSM power level table for GSM 900

Power level number Power output level dBm 29 36 30 34 31 32

Power level number Power output level dBm 0 30 1 28 2 26 3 24 4 22 5 20 6 18 7 16 8 14 9 12 10 10 11 8 12 6 13 4 14 2 15 0

GSM power level table for GSM 1800

Power level number

Power output level dBm

30 33 31 32 0 30 1 28 2 26 3 24 4 22 5 20 6 18 7 16 8 14 9 12 10 10 11 8 12 6 13 4 14 2 15 0

GSM power level table for GSM 1900

GSM Power class

Not all mobiles have the same maximum power output level. In order that the base station knows the maximum power level number that it can send to the mobile, it is necessary for the base station to know the maximum power it can transmit. This is achieved by allocating a GSM power class number to a mobile. This GSM power class number indicates to the base station the maximum power it can transmit and hence the maximum power level number the base station can instruct it to use.

Again the GSM power classes vary according to the band in use.

GSM Power Class

Number

GSM 900 GSM 1800 GSM 1900

  Power level number

Maximum power output

Power level number

Maximum power output

Power level number

Maximum power output

1     PL0 30 dBm / 1W

PL0 30 dBm / 1W

2 PL2 39dBm / 8W

PL3 24 dBm/ 250 mW

PL3 24 dBm / 250 mW

3 PL3 37dBm / 5W

PL29 36 dBm / 4W

PL30 33 dBm / 2W

4 PL4 33dBm / 2W

       

5 PL5 29 dBm / 800 mW

       

GSM power amplifier design considerations

One of the main considerations for the RF power amplifier design in any mobile phone is its efficiency. The RF power amplifier is one of the major current consumption areas. Accordingly, to ensure long battery life it should be as efficient as possible.

It is also worth remembering that as mobiles may only transmit for one eighth of the time, i.e. for their allocated slot which is one of eight, the average power is an eighth of the maximum

Basic GSM frame structure

The basic element in the GSM frame structure is the frame itself. This comprises the eight slots, each used for different users within the TDMA system. As mentioned in another page of the tutorial, the slots for transmission and reception for a given mobile are offset in time so that the mobile does not transmit and receive at the same time.

GSM frame consisting of eight slots

The basic GSM frame defines the structure upon which all the timing and structure of the GSM messaging and signalling is based. The fundamental unit of time is called a burst period and it lasts for approximately 0.577 ms (15/26 ms). Eight of these burst periods are grouped into what is known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it forms the basic unit for the definition of logical channels. One physical channel is one burst period allocated in each TDMA frame.

In simplified terms the base station transmits two types of channel, namely traffic and control. Accordingly the channel structure is organised into two different types of frame, one for the traffic on the main traffic carrier frequency, and the other for the control on the beacon frequency.

GSM multiframe

The GSM frames are grouped together to form multiframes and in this way it is possible to establish a time schedule for their operation and the network can be synchronised.

There are several GSM multiframe structures:

Traffic multiframe:   The Traffic Channel frames are organised into multiframes consisting of 26 bursts and taking 120 ms. In a traffic multiframe, 24 bursts are used for traffic. These are numbered 0 to 11 and 13 to 24. One of the remaining bursts is then used to accommodate the SACCH, the remaining frame remaining free. The actual position used alternates between position 12 and 25.

Control multiframe:   the Control Channel multiframe that comprises 51 bursts and occupies 235.4 ms. This always occurs on the beacon frequency in time slot zero and it may also occur within slots 2, 4 and 6 of the beacon frequency as well. This multiframe is subdivided into logical channels which are time-scheduled. These logical channels and functions include the following:

o Frequency correction burst   o Synchronisation burst   o Broadcast channel (BCH)   o Paging and Access Grant Channel (PACCH)   o Stand Alone Dedicated Control Channel (SDCCH)  

GSM Superframe

Multiframes are then constructed into superframes taking 6.12 seconds. These consist of 51 traffic multiframes or 26 control multiframes. As the traffic multiframes are 26 bursts long and the control multiframes are 51 bursts long, the different number of traffic and control multiframes within the superframe, brings them back into line again taking exactly the same interval.

GSM Hyper frame

Above this 2048 superframes (i.e. 2 to the power 11) are grouped to form one hyperframe which repeats every 3 hours 28 minutes 53.76 seconds. It is the largest time interval within the GSM frame structure.

Within the GSM hyperframe there is a counter and every time slot has a unique sequential number comprising the frame number and time slot number. This is used to maintain synchronisation of the different scheduled operations with the GSM frame structure. These include functions such as:

Frequency hopping:   Frequency hopping is a feature that is optional within the GSM system. It can help reduce interference and fading issues, but for it to work, the transmitter and receiver must be synchronised so they hop to the same frequencies at the same time.

Encryption:   The encryption process is synchronised over the GSM hyperframe period where a counter is used and the encryption process will repeat with each hyperframe.

However, it is unlikely that the cellphone conversation will be over 3 hours and accordingly it is unlikely that security will be compromised as a result.

GSM Frame Structure Summary

GSM frame structure summary

By structuring the GSM signalling into frames, multiframes, superframes and hyperframes, the timing and organisation is set into an orderly format that enables both the GSM mobile and base station to communicate in a reliable and efficient manner. The GSM frame structure forms the basis onto which the other forms of frame and hence the various GSM channels are built.

Requirements for GSM handover

The process of handover or handoff within any cellular system is of great importance. It is a critical process and if performed incorrectly handover can result in the loss of the call. Dropped calls are particularly annoying to users and if the number of dropped calls rises, customer

dissatisfaction increases and they are likely to change to another network. Accordingly GSM handover was an area to which particular attention was paid when developing the standard.

Types of GSM handover

Within the GSM system there are four types of handover that can be performed for GSM only systems:

Intra-BTS handover:   This form of GSM handover occurs if it is required to change the frequency or slot being used by a mobile because of interference, or other reasons. In this form of GSM handover, the mobile remains attached to the same base station transceiver, but changes the channel or slot.

Inter-BTS Intra BSC handover:   This for of GSM handover or GSM handoff occurs when the mobile moves out of the coverage area of one BTS but into another controlled by the same BSC. In this instance the BSC is able to perform the handover and it assigns a new channel and slot to the mobile, before releasing the old BTS from communicating with the mobile.

Inter-BSC handover:   When the mobile moves out of the range of cells controlled by one BSC, a more involved form of handover has to be performed, handing over not only from one BTS to another but one BSC to another. For this the handover is controlled by the MSC.

Inter-MSC handover:   This form of handover occurs when changing between networks. The two MSCs involved negotiate to control the handover.

GSM handover process

Although there are several forms of GSM handover as detailed above, as far as the mobile is concerned, they are effectively seen as very similar. There are a number of stages involved in undertaking a GSM handover from one cell or base station to another.

In GSM which uses TDMA techniques the transmitter only transmits for one slot in eight, and similarly the receiver only receives for one slot in eight. As a result the RF section of the mobile could be idle for 6 slots out of the total eight. This is not the case because during the slots in which it is not communicating with the BTS, it scans the other radio channels looking for beacon frequencies that may be stronger or more suitable. In addition to this, when the mobile communicates with a particular BTS, one of the responses it makes is to send out a list of the radio channels of the beacon frequencies of neighbouring BTSs via the Broadcast Channel (BCCH).

The mobile scans these and reports back the quality of the link to the BTS. In this way the mobile assists in the handover decision and as a result this form of GSM handover is known as Mobile Assisted Hand Over (MAHO).

The network knows the quality of the link between the mobile and the BTS as well as the strength of local BTSs as reported back by the mobile. It also knows the availability of channels in the nearby cells. As a result it has all the information it needs to be able to make a decision about whether it needs to hand the mobile over from one BTS to another.

If the network decides that it is necessary for the mobile to hand over, it assigns a new channel and time slot to the mobile. It informs the BTS and the mobile of the change. The mobile then retunes during the period it is not transmitting or receiving, i.e. in an idle period.

A key element of the GSM handover is timing and synchronisation. There are a number of possible scenarios that may occur dependent upon the level of synchronisation.

Old and new BTSs synchronised:   In this case the mobile is given details of the new physical channel in the neighbouring cell and handed directly over. The mobile may optionally transmit four access bursts. These are shorter than the standard bursts and thereby any effects of poor synchronisation do not cause overlap with other bursts. However in this instance where synchronisation is already good, these bursts are only used to provide a fine adjustment.

Time offset between synchronised old and new BTS:   In some instances there may be a time offset between the old and new BTS. In this case, the time offset is provided so that the mobile can make the adjustment. The GSM handover then takes place as a standard synchronised handover.

Non-synchronised handover:   When a non-synchronised cell handover takes place, the mobile transmits 64 access bursts on the new channel. This enables the base station to determine and adjust the timing for the mobile so that it can suitably access the new BTS. This enables the mobile to re-establish the connection through the new BTS with the correct timing.

Inter-system handover

With the evolution of standards and the migration of GSM to other 2G technologies including to 3G UMTS / WCDMA as well as HSPA and then LTE, there is the need to handover from one technology to another. Often the 2G GSM coverage will be better then the others and GSM is often used as the fallback. When handovers of this nature are required, it is considerably more complicated than a straightforward only GSM handover because they require two technically very different systems to handle the handover.

These handovers may be called intersystem handovers or inter-RAT handovers as the handover occurs between different radio access technologies.

The most common form of intersystem handover is between GSM and UMTS / WCDMA. Here there are two different types:

UMTS / WCDMA to GSM handover:   There are two further divisions of this category of handover:

o Blind handover:   This form of handover occurs when the base station hands off the mobile by passing it the details of the new cell to the mobile without linking to it and setting the timing, etc of the mobile for the new cell. In this mode, the network selects what it believes to be the optimum GSM based station. The mobile first locates the broadcast channel of the new cell, gains timing synchronisation and then carries out non-synchronised intercell handover.

o Compressed mode handover:   using this form of handover the mobile uses the gaps I transmission that occur to analyse the reception of local GSM base stations using the neighbour list to select suitable candidate base stations. Having selected a suitable base station the handover takes place, again without any time synchronisation having occurred.

Handover from GSM to UMTS / WCDMA:   This form of handover is supported within GSM and a "neighbour list" was established to enable this occur easily. As the GSM / 2G network is normally more extensive than the 3G network, this type of handover does not normally occur when the mobile leaves a coverage area and must quickly find a new base station to maintain contact. The handover from GSM to UMTS occurs to provide an improvement in performance and can normally take place only when the conditions are right. The neighbour list will inform the mobile when this may happen.

Summary

GSM handover is one of the major elements in performance that users will notice. As a result it is normally one of the Key Performance Indicators (KPIs) used by operators to monitor performance. Poor handover or handoff performance will normally result in dropped calls, and users find this particularly annoying. Accordingly network operators develop and maintain their networks to ensure that an acceptable performance is achieved. In this way they can reduce what is called "churn" where users change from one network to another.

Frequency Hopping on GSM Networks

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Last Updated: 03-Mar-2003

Before I can adequately describe Frequency Hopping and its advantages, I need to explain how GSM divides up the available resources for each caller. Back in the days of analog that task was a fairly simple: one caller got one channel. When he handed off, he moved to another channel on another site. With digital technology it's nowhere near that simple, but it isn’t too difficult to understand either.

GSM uses a form of air interface called TDMA, which stands for Time Division Multiple Access. Don’t assume that it’s roughly the same as iDEN or IS-136 just because they too are TDMA-based technologies. They are no more alike than a Yugo is to a Ferrari, even though they are both examples automotive technology. So we'll forget about those other TDMA technologies, and we'll concentrate solely upon the GSM implementation of the concept.

GSM still uses physical channels, but each of those channels is divided into 8 time slots. One user consumes one slot, thus allowing 8 users to be on a GSM channel simultaneously. Each GSM channel is 200 kHz wide, thus giving a 30 MHz license-holder (such as Microcell Connexions) a grand total of 75 physicals channels within their spectrum allotment.

Obviously 75 channels isn’t enough to spread evenly among the 200 some odd cell sites around the GTA, each of which has 3 independent sectors. A sector is an area covering 120 degrees around the site. That’s a grand total of 600 sectors and only 75 channels. Obviously the idea is to reuse channels in multiple sites, and to keep those co-channels far enough apart that they don’t interfere with one another.

The most common type of interference suffered by a dense GSM network is therefore co-channel inference. This means that your phone call is interfered with by another site operating on the same physical channel and time slot. Unlike analog, where co-channel interference would often result in you actually hearing the other conversation, that never happens in GSM.

This is because each call is encrypted, and any attempt by your phone to decrypt another call would result a stream of bits that simply cannot be turned into audio. However, the signal from that other conversation can clobber the signal that you actually want to receive. This will result in audio dropouts, and generally poor audio quality overall.

Another problem facing narrowband radio systems is multipath. This happens when large objects such as buildings reflect your desired signal. The reflection can sometimes be just as strong as the direct signal, and the two can interfere with one another. Although you might think that the reflected signal and the direct signal are actually the same, the fact is they aren’t. The reflected signal had further to travel, and it is therefore out of phase with the direct signal.

It doesn’t sound like there is any viable way to avoid these problems, but that’s where the magic of Frequency Hopping comes in. Who says that our conversation must remain on the same physical channel and time slot for the entire time we are on a particular site? If the network were able to move us from slot to slot, and from frequency to frequency, then we could randomize the effects of interference.

Consider co-channel interference. Not all of the slots are in use on all of the physical channels on

each site where they are reused, so although slot 4 on channel 522 might be clobbered by another conversation, slot 7 on channel 530 probably isn’t. So, if we can take each caller on a particular sector and jump them from slot to slot, and from frequency to frequency, then each user runs a far lower risk of suffering from co-channel interference. And when such interference does occur, chances are good that the error correction algorithms can take care of it.

Now consider multipath. Due to the very high frequency of PCS service the wavelength of the signals is extremely short (only a few inches in fact). That means the phase difference on one channel will be quite different than on another. By jumping from frequency to frequency we may only experience problematic multipath for very short periods of time, once again giving the error correction algorithms the chance to clean it up.

But does it work? From what I’ve been able to gather from testing Microcell Connexions in the areas where Frequency Hopping is implemented, and by comparing that to areas where it is not, I would have to say that it works exceedingly well. It obviously does best when site density is high, which explains why it isn't implemented everywhere. Rogers GSM also implements Frequency Hopping across their entire network.