frequency hopping

Lucent Technologies -- ProprietaryThis document contains proprietary information ofLucent and is not to be disclosed or used except in

accordance with applicable agreements.

GSM FrequencyHopping and VariableInterference Planning

Engineering Guideline

EG: GSMVIP401-380-365

Issue 1.2June 1999

GSM Frequency Hopping and VIP Engineering Guideline

Lucent Technologies - PROPRIETARYUse pursuant to Company instructions

ii Issue 1.2 – June 1999

Copyright © 1999 Lucent TechnologiesUnpublished and Not for Publication

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by any entity, (either internal or external to Lucent Technologies),except in accordance with applicable agreements, contracts or licensing,

without the express written consent of theCustomer Training and Information Products organisation

and the business management owner of the material.

For permission to reproduce or distribute, please contact:

The Manager, RF Systems & Capacity Engineering Group01793 883275 (domestic)

(44) 1793 883275 (international)


Contents

Lucent Technologies -- ProprietaryThis document contains proprietary information ofLucent and is not to be disclosed or used except in

accordance with applicable agreements.

Issue 1.2 – June 1999 iii

1. ABOUT THIS GUIDE 1

2. INTRODUCTION TO FREQUENCY HOPPING 3

2.1. Frequency hopping overview 3

2.2. Why use frequency hopping 5Multipath fading 5Interference 5

2.3. Hopping sequences 6Cyclic hopping 6Random hopping 6

2.4. Hopping at the base station 6Baseband hopping 6Synthesiser hopping 7

2.5. GSM network implementation 8Sequence generation 8Common control channels 8Reception level measurements 9Quality measurements 9Frequency redefinition procedure 10Mobile stations 10

2.6. Key benefits 11Frequency diversity 11Impact on network planning 16Interference diversity 16Associated techniques 19Impact on network planning 23

3. INTRODUCTION TO VIP 25

3.1. VIPone 25VIPone properties 28VIPone examples 28

3.2. VIPtwo 29VIPtwo properties 29VIPtwo examples 30


Contents


iv Issue 1.2 – June 1999

3.3. VIPone and VIPtwo compared 31Combined plans 31

4. CONFIGURING FREQUENCY HOPPING 33

4.1. Base station hardware 33Base model 33Antenna coupling equipment 34Fill-sender and phantom-RTs 34

4.2. Software release support 36

4.3. Configuration 36FHS configuration rules 36Other limitations 41

4.4. Feature activation and system parameters 41BTS hopping mode 41BSS feature enabling 41OMC parameter configuration 41Feature activation 43

4.5. Fault management 44Baseband hopping 44Synthesiser hopping: 44

4.6. DTX 44Uplink DTX 44Downlink DTX 45

4.7. Dynamic power control 45

5. VIP DEPLOYMENT 47

5.1. Introduction 47When to use VIP 47Implementation strategy 47

5.2. Choosing the right plan 48More than three transceivers per cell 48Three or fewer transceivers per cell 49Large spectrum allocation 49Microcells 49Planning for future capacity 49


Contents


Issue 1.2 – June 1999 v

5.3. Planning the frequencies and the HSN 49VIPone 49VIPtwo 50VIPone/VIPtwo 50Microcells 50BCCH planning 51

5.4. Collecting performance data 51Collection equipment 52Performance data types 52

5.5. Deployment results 56Activating frequency hopping 56

5.6. Optimising performance 60Quality-based handovers 60Quality-based power control 60Hopping over two frequencies 60DTX measurement accuracy 60Other scenarios 61

6. WORKED EXAMPLES 63

6.1. Scenario 1 63Existing configuration 63Objectives 64VIP plan choice 64Planning the frequencies 64Mapping the frequency plan to OMC settings 65



6.4. Scenario 4 70Existing configuration 70


Contents


vi Issue 1.2 – June 1999

Objectives 70VIP plan choice 70Planning the frequencies 70Mapping the frequency plan to OMC settings 71


7. LIST OF ACRONYMS 75

COMMENTS FORM 79


About this Guide

1


Issue 1.2 – June 1999 1

1. About this Guide

This guide provides a detailed description of the Frequency Hopping and Variable InterferencePlanning (VIP) solutions offered by Lucent Technologies for GSM 900 and 1800 networks. Itcontains the following chapters:

• Chapter 2 Introduction to Frequency HoppingAn overview of frequency hopping concepts and techniques, and their benefits and networkimpacts.

• Chapter 3 Introduction to VIPAn overview of VIP concepts and techniques, and their benefits and network impacts.

• Chapter 4 Configuring Frequency HoppingDescribes how to configure and activate frequency hopping in a network, from theequipment point of view.

• Chapter 5 Implementing VIPDescribes when and how to implement VIP into a network.

• Chapter 6 Worked ExamplesExamples of different scenarios and suggested implementations.

• Chapter 7 List of AcronymsDefinitions of the acronyms used in this guide.



2 Issue 1.2 – June 1999

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Introduction to FrequencyHopping

2


Issue 1.2 – June 1999 3

2. Introduction to Frequency Hopping

This chapter describes the main concepts of frequency hopping and its implementation in GSM.

2.1. Frequency hopping overview

In frequency hopping systems, each call hops between a defined set of frequencies. So poorsignal quality on any specific frequency affects only a small portion of the transmission. Thismakes it much easier to recreate any lost bits and so preserve overall call quality.

Frequency hopping is the principal component of the Variable Interference Planning solutionsoffered by Lucent Technologies and is supported in both GSM 900 and GSM 1800 networks.

GSM networks use “slow” frequency hopping; a hop occurs before each time slot is transmitted(every 4.615 millisecond, or 217 hops per second). This distinguishes it from fast frequencyhopping systems, which use several hops per symbol.



4 Issue 1.2 – June 1999

time

frequency

Figure 1 Slow frequency hopping in the time frequency domain

Frequency hopping exploits two underlying GSM error correction techniques:

• Channel coding

• Interleaving

These coding and interleaving techniques are illustrated below:

Rate 1/2 convolutional coding

456 bits

Class 1 bits Class 2 bitsP+T

182 bits 78 bits3+4

Segmentationand

interleaving

57 bits

Normal burst - 1 Normal burst - 2 Normal burst - 8

57 bits

Normal GSM burst

Tail3 bits

Information57 bits

SF1 bit

SF1 bit

Information57 bits

Tail3 bits

TS 26 bits

...

Normal GSM burst

57 bits

Tail3 bits

Information57 bits

SF1 bit...

Figure 2 GSM coding and interleaving

Channel coding takes the digital message flow (speech or data) and divides the bit stream intoblocks. Control bits used to detect and correct transmission errors are applied to the start and



Issue 1.2 – June 1999 5

end of each block. Each block of message bits and control bits is known as a “code word”.Code words for speech are 456 bits long.

Interleaving divides each code word into chunks of 57 bits at a time and mixes (interleaves)them with chunks from adjoining code words. Splitting the bit stream in this way prevents errorsoccurring across entire code words. This improves channel coding correction rates, as it ismuch easier to correct isolated bit errors than bursts of errors.

2.2. Why use frequency hopping

Frequency hopping mitigates two problems with transmission quality over the air interface:

• Multipath fading

• Interference

Multipath fading

Usually a radio signal is received as scattered signals travelling over separate paths. When thesignals combine, they produce an interference pattern of fading. For a given position the fadingdepends on the transmission frequency. This multipath fading particularly impacts slow movingmobiles, as they may stay in one position and hence a fade long enough to suffer informationloss (interleaving can only spread a code word over a limited number of time slots).

With frequency hopping, because the frequencies change, so do the fading patterns associatedwith them. Transmissions on a frequency that is subject to multipath fading will move out of thefade at the next hop (“frequency diversity”).

Frequency diversity, combined with interleaving and channel coding, improves transmissionquality - in particular for slow moving mobiles.

Interference

Any given call may suffer interference from calls on neighbour cells transmitting on or close toits frequency. This interference has a continuous impact on transmissions because it exists forthe duration of the interfering calls.

Frequency hopping mitigates this effect by spreading, or averaging, the interference acrossmultiple calls (“interference diversity”). This prevents a situation where one call hasunacceptable levels of interference and others have very good levels. When coupled withchannel coding and interleaving, it increases the probability that all calls will have acceptablequality, rather than some having very good quality and others having unacceptable quality.

Interference diversity has another advantage. It ensures that consecutive bursts of informationare received under different interference conditions, reducing the risk of large sequentialinformation loss.



6 Issue 1.2 – June 1999

2.3. Hopping sequences

In frequency hopping systems, the hopping sequence between the frequencies assigned to aparticular transmission can be either cyclic or random.

Cyclic hopping

Frequencies are used in fixed rotation.

For example: f1, f2, f3, f4, f1, f2, f3, f4, f1, f2, f3, f4, f1, f2, f3, f4, f1, f2, f3,...

Random hopping

Frequencies are used in a pseudo-random sequence.

For example: f2, f4, f1, f3, f4, f2, f3, f2, f4, f1, f1, f4, f3, f4, f2, f1, f3, f2, f2, ...

When using the same set of frequencies with random hopping, the probability of two calls usingthe same frequency in the same time slot is 1/N, where N is the number of hopping frequencies.

For example: Mobile Station 1: f1, f4, f4, f2, f1, f3, …; Mobile Station 2: f2, f1, f4, f3, f2, f1, …

2.4. Hopping at the base station

Frequency hopping can be generated in two ways:

• Baseband hopping

• Synthesiser hopping

Baseband hopping

In baseband hopping, each transceiver within a base station operates on fixed frequencies.

A transceiver provides the functionality of eight channels, according to the GSM air interface.Transceivers perform both baseband signal processing (channel coding, interleaving,encryption, and TDMA burst information) and RF signal processing (generation of RF signaland modulation of TDMA bursts).

In Lucent transceivers (also known as TRXs or RTs) the DRCC (Digital Radio Codec andControl) unit does the baseband processing and the RFU (Radio Frequency Unit) does the RFprocessing.

With baseband hopping, the digitised, or baseband, speech signal generated at the DRCC isswitched between the RFUs of the transceiver before transmission. Each frame of eight time



Issue 1.2 – June 1999 7

slots is input to a different RFU and so to a different frequency. In this way the transceivers donot need to retune to different frequencies, but each channel effectively hops over the availablefrequencies.

The primary limitation of baseband frequency hopping is that the number of hoppingfrequencies is limited to the number of RTs (Radio Terminals) in the cell.

D R C CD R C C

D R C CD R C C

R F UR F U

R F UR F U

ff 11

ff 22

Figure 3 Baseband hopping

Synthesiser hopping

With synthesiser hopping, each RFU within a transceiver retunes to a different frequency(following a defined hopping sequence) before transmitting a frame. So unlike basebandhopping, the output of each baseband processing section is always connected to the sameRFU. This allows each transceiver to hop over as many frequencies as desired, regardless ofthe number of transceivers in the cell.

However, traditional filter combiners (which are frequency specific) cannot be used withsynthesiser hopping because they are too slow in changing frequency. Hybrid combiners (whichcan operate across a frequency range and hence are also known as wide-band combiners)must be used instead. Because hybrid combiners have much higher insertion losses than filtercombiners, the maximum number of radios per cell is reduced.

D R C CD R C C

D R C CD R C C

R F UR F U

R F UR F U

ff 11

ff 22

ff 22

ff 11

Figure 4 Synthesiser hopping



8 Issue 1.2 – June 1999

2.5. GSM network implementation

This section describes in overview how frequency hopping is implemented in GSM networks.

Sequence generation

Each call has its time slots transmitted in sequence across a defined set of hoppingfrequencies. The sequence is derived from an algorithm (see GSM Recommendation 05.02).Frequency hopping occurs between time slots: a mobile station transmits or receives on a fixedfrequency during one time slot, then changes frequency before the time slot on the next TDMAframe.

The total number of available hopping sequences is 64 multiplied by the number of hoppingfrequencies (64xN). Hopping sequences are described per channel by two network parameters:

• HSN (Hopping Sequence Number): defines a number that is fed into the frequency hoppingalgorithm to generate the hopping sequence. Values can be 0 to 63. Value 0 defines cyclichopping; all other values generate a random sequence

• MAIO (Mobile Allocation Index Offset): defines the starting frequency, or offset, thetransmission will start on within a hopping sequence. The value can be 0 to N-1 where N isthe number of allocated frequencies

Two channels with the same HSN but different MAIO never use the same frequency at thesame time.

Example:

• HSN = 1, MAIO = 0: f2, f4, f1, f3, f4, f2, f3, f2, f4, f1, f1, f4, f3, f4, f2, f1, f3, f2, f2, ...

• HSN = 1, MAIO = 1: f3, f1, f2, f4, f1, f3, f4, f3, f1, f2, f2, f1, f4, f1, f3, f2, f4, f3, f3, ...

Two channels using the same frequency list and the same time slot, but with a different and non0 HSN, will interfere in 1/Nth of bursts, as if the sequences were chosen randomly.

Channels in the same cell using the same hopping frequency set should have the same HSN,and different MAIO, to avoid co-channel interference within the cell.

If random hopping is used, each channel in distant cells using the same frequency set shouldhave a different HSN; this optimises the benefits of interference diversity.

Common control channels

In order to ease initial synchronisation acquisition, the following common channels must use afixed frequency:

• FCCH (Frequency Correction Channel)



Issue 1.2 – June 1999 9

• SCH (Synchronization Channel)

• BCCH (Broadcast Control Channel)

• PAGCH (Paging Access Grant Channel)

• RACH (Random Access Channel)

Common channel extension sets (CCCH) must use the same fixed frequency as the primarygroup. This avoids the need to transmit their frequency organisation description on the BCCH.

Note: Traffic channels on the rest of the time slots in BCCH transceivers can hop. Only thecommon channels cannot hop.

Reception level measurements

If dynamic power control is in use, and frequency hopping occurs on the BCCH frequencyamong other frequencies, reception level measurement accuracy is an issue in traffic channelsthat use this combination.

Power control cannot be applied on the BCCH frequency, which must transmit at constantpower in the downlink. This means that power control applies to a subset of bursts only. Burststhat use the BCCH frequency are sent at fixed transmission power. If reception levelmeasurements in the downlink were averaged on all frequencies, including the BCCH, themeasurements would not be accurate for the power control algorithm.

To alleviate this problem, the power control indicator (PWRC) tells the mobile station to ignoreBCCH frequency slots in reception level estimations. The indicator is sent to the mobile stationat connection if the following conditions are met:

• The channel hops on at least two frequencies

• One of those frequencies is the BCCH frequency

• Dynamic power control is in use on the downlink transmission

Quality measurements

Mapping of RXQUAL (Received Signal Quality) measurements to subjective speech qualityvaries with the propagation environment. This is because it is a measure of the raw bit errorrate, estimated by backward coding the decoded bit sequence and comparing it to the receivedbit sequence. Hence it does not consider the varying efficiency of channel coding, interleavingand bit error correction under different environmental conditions.

So under different conditions the same RXQUAL values can result in different actual speechquality and calls with different RXQUAL values can have the same speech quality.



10 Issue 1.2 – June 1999

The interference diversity property of frequency hopping means that interference conditionsvary from time slot to time slot. This means that, with frequency hopping, even in the samepropagation environment, calls with the same RXQUAL can have different speech quality andvice-versa.

Accordingly, when frequency hopping is used, RXQUAL is not a reliable measurement ofconnection quality. Hence, in order to assess the quality of the network with frequency hopping,the operator should use other quality indicators, such as FER (Frame Erasure Rate) orsubjective voice quality indicators. These indicators are only available in drive test equipment.

Frequency redefinition procedure

This procedure is used both in dedicated and group transmit mode, to minimise disruption tocalls when channel frequencies and hopping sequence allocations change in the network.When this happens, the network sends a FREQUENCY_REDEFINITION message to themobile stations that are currently in call. This contains the new parameters and a start timeindicator. Parameters that can be updated are the cell channel description, mobile allocation,and MAIO.

At the indicated time slot, the base station and assigned mobile stations update their allocatedfrequencies and hopping sequences to match the new parameters. So, this time slot is the firstto use the new parameters. No other functions are normally disturbed by the change. However,some calls may be lost in the following circumstances:

If the MSC requests a handover channel, and the request is acknowledged with theactual channel information. Then, if a redefinition procedure subsequently starts for thatchannel and the mobile station is handed over to the channel at the same time, the callis lost.

Mobile stations

Currently there are some unresolved problems with certain types of older mobile stationmodels:

• Some do not support the frequency redefinition procedure

• Some cannot hop on SDCCH channels, or have problems when using frequency hopping inconjunction with Discontinuous Transmission (DTX) on the downlink, or with dynamic powercontrol



Issue 1.2 – June 1999 11

2.6. Key benefits

This section describes in more detail the primary benefits of frequency hopping:

• Frequency diversity

• Interference diversity

Frequency diversity

Multipath fading is speed and frequency dependent. The high speed of some mobile is enoughto allow GSM error correction to overcome its effects. For slower moving users, the correctionmechanisms are insufficient on their own. However, by using frequency hopping, the sameperformance levels can be obtained for slow moving users.

Figure 5 compares the required carrier to noise (C/No) ratio as a function of vehicle speed for abit error ratio (BER) of 0.5% (considered acceptable for speech) in the 900 MHz band, first at afixed frequency allocation, and then using ideal frequency hopping. Ideal frequency hoppingoccurs when hopping takes place on uncorrelated frequencies. That is, their fades areindependent of each other.

4

6

8

10

12

14

0 50 100 150 200

v [km/h]

C/N

o [

dB

] fo

r B

ER

=0.

5%

Without FH

FH

Figure 5 Required C/No against vehicle speed (BER 0.5%) – 900 MHz

Without frequency hopping the performance of the system depends on the vehicle speed. Thefaster the mobile, the better the error correction mechanisms work and the lower the minimumsignal to noise ratio is to achieve a certain BER.

With ideal frequency hopping (infinite number of frequencies and infinite separation betweenthem), optimum transmission quality is obtained at almost all vehicle speeds.

Note: Slight degradation occurs with frequency hopping at very high vehicle speeds. This iscaused by a significant change in the multipath profile at the time slot level that cannot betracked by the equaliser.



12 Issue 1.2 – June 1999

Similar improvements are gained in co-channel or adjacent-channel interference. Figure 6compares the required carrier to interference (C/I) ratio in terms of current vehicle speed forfixed frequency, and ideal frequency hopping operation. The dependence is even more markedthan for noise interference, as here the power of the interference signal also fluctuates with thespeed.

6

8

10

12

14

16

18

0 50 100 150 200

v [km/h]

C/I

[dB

] fo

r B

ER

=0.

5%

Without FH

FH

Figure 6 Required C/I against vehicle speed (BER 0.5%) – 900 MHz

Optimising frequency diversity

Frequency diversity optimisation is governed by two factors:

• Number of frequencies

• Frequency spacing

Number of frequencies

Ideal frequency diversity requires that a different frequency is used for each time slot within aninterleaved code word.

If this is not the case, at least two of the time slots over which a code word is spread aretransmitted at the same frequency. The fading effect is strongly correlated for them at lowvelocity, thus reducing the gain.

In cyclic frequency hopping, to achieve ideal frequency hopping, the hopping period must be atleast as long as the interleaving depth (eight time slots for speech). This ensures a differentfrequency in each time slot. A longer period does not provide additional gains.

Figure 7 uses bit error curves to illustrate the likely performance losses at low vehicle speed (5km/h). Note that hopping over just four frequencies comes as close as 1 dB to the gain ofhopping over eight frequencies.



Issue 1.2 – June 1999 13

0.001

0.01

0.1

1

0 2 4 6 8 10 12 14

C/No [dB]

BE

R

Without FH

2 freqs

4 freqs

8 freqs

Figure 7 Effect of number of frequencies on BER (v=5 km/h)

With random hopping, the probability of using the same radio frequency channel within theinterleaving depth is depth/N, where N is the number of frequencies in the hopping sequence.This means that the fading decorrelation within one interleaving block is never optimal,regardless of the number of hopping frequencies.

The following table shows the C/No required for a BER of 10-2 using both cyclic and randomfrequency hopping over different numbers of frequencies. Note that the frequency diversity gainwith eight frequencies is 1 dB to 2 dB lower for random hopping than for cyclic.

Similar results would be expected with interference.



14 Issue 1.2 – June 1999

Cyclic hopping Random hopping

C/No for GrossClass 11

C/No forFER2=2%

C/No for GrossClass 1

C/No forFER=2%

No. offrequencies

Level

[dB]

Gain

[dB]

Level

[dB]

Gain

[dB]

Level

[dB]

Gain

[dB]

Level

[dB]

Gain

[dB]

1 9.5 0.0 11.5 0.0 9.5 0.0 11.5 0.0

2 7.0 2.5 8.5 3.0 7.5 2.0 9.5 2.0

3 6.0 3.5 7.5 4.0 6.5 3.0 8.5 3.0

4 5.0 4.5 6.5 5.0 6.0 3.5 8.0 3.5

8 4.0 5.5 5.5 6.0 5.5 4.0 7.5 4.0

12 4.0 5.5 5.5 6.0 5.0 4.5 7.0 4.5

Table 1 Frequency diversity gains

The results in this table cannot be compared directly with the previous figures because differentpropagation conditions apply in different environments, particularly in typical urban (TU) areas.

Note: Frequency hopping gains may be smaller than predicted, due to the diminished severityof multipath propagation when compared to flat fading. In normal environments the differentpaths arrive at different times, thus cancelling out some of the fading impact.

Frequency spacing

Frequency spacing must be sufficient to ensure that uncorrelated fading affects differentfrequencies.

The coherence bandwidth can be defined as the frequency separation required for propagationpaths, and hence fading, to be considered totally independent. In most outdoor environments,coherence bandwidths of less than 1 MHz can be expected, so 1 MHz (5 GSM channels) is

1 Bits produced by the GSM encoder are ranked in importance as Class 1 and Class 2. Class 1 bits areprotected by redundancy codes.

2 FER (frame error rate) is the fraction of entire speech frames erased by the speech decoder becauseof irrecoverable bit errors.



Issue 1.2 – June 1999 15

recommended as the minimum frequency spacing for outdoor systems. However, in TUenvironments, channel separation of 400 kHz to 600 kHz (2 to 3 GSM channels) is enough.

Indoor environments are generally characterised by large coherence bandwidths. Typically, anindoor frequency hopping system gives lower frequency diversity gains than an outdoor systemwith the same hopping bandwidth. However, as indoor users are generally slow moving, there isstill potential for frequency diversity gains from frequency hopping.

Simulations show that although the gain achieved is smaller, it is still significant: Assuming 5MHz bandwidth and an FER of 2%, the gain in C/No is between 1.7 dB and 3.3 dB (compared to5 dB in typical urban areas).

Antenna diversity

Antenna diversity is another technique used to combat multipath fading. Like frequencyhopping, it achieves gains in conjunction with channel encoding and interleaving, but since ituses space rather than frequency diversity, the gain is independent of vehicle speed.

Combining frequency hopping with antenna diversity produces significantly increased gains.However, the total gain does not equal the sum of the individual gains.

The following table illustrates the likely gain in C/No (in dB) for an FER of 2% when usingantenna diversity with frequency hopping:

No FH Ideal FH No FH Ideal FH

Level[dB]

Gain[dB]

Level[dB]

Gain[dB]

Level[dB]

Gain[dB]

Level[dB]

Gain[dB]

No diversity 12.5 0.0 5.5 7.5 15.5 0.0 7.3 8.2

Ideal diversity 5.8 6.6 1.8 10.7 8.0 7.5 3.2 12.2

Table 2 Effects of antenna diversity

Note: In tests, antenna diversity gains are high in TU environments but drop in other testconditions such as rural areas and hilly terrain.



16 Issue 1.2 – June 1999

Impact on network planning

Existing radio network planning is generally based on poor transmission conditions at slowvehicle speed. Frequency hopping compensates for this degradation in transmission quality,making it largely independent of vehicle speed. Potentially, the smaller C/No and C/I values formedium speed vehicles can be used when planning for areas of significant pedestrian use.

However, because the BCCH carrier cannot hop, the reduction in C/No that arises fromfrequency diversity does not translate into reduced sensitivity values.

Similarly, the improvement in C/I values does not translate into a tighter reuse pattern for BCCHcarriers. However, planning gains are obtained for TCH carriers. Here, different C/Irequirements can be set in the frequency planning process (for example, by using TU50 valuesrather than TU3) allowing frequency diversity to be used to increase capacity.

The capacity increase depends primarily on the number of frequencies in the hoppingsequences (as discussed earlier, this affects the required C/I value). In addition, the separationbetween frequencies assigned to a cell must be appropriate for the propagation environment.

To maximise the benefits of frequency diversity, if possible, the BCCH frequency should beincluded in the hopping sequences for channels that do not occupy time slots assigned tocontrol channels.

Interference diversity

To date, interference diversity has been primarily associated with Spread Spectrum systems.Frequency hopping now enables GSM networks to exploit the benefits of interference diversity.

Example

This section illustrates the principle of interference diversity.

Figures 8 and 9 show a GSM system with two sectors that use the same set of fourfrequencies. At a given time there are four mobile stations communicating in each of the cells.

In cell A, the mobile on f1 is suffering high interference levels, because the interfering mobile incell B is very near to the border (Figure 8). Speech quality is poor as a result.

The other mobiles in cell A are subject to lower interference levels (Figure 9). The actualinterference level and resulting speech quality varies across the mobiles, but, unlike the mobileon f1, all yield acceptable speech quality.



Issue 1.2 – June 1999 17

Figure 8 Example configuration without frequency hopping

Figure 9 Example configuration without frequency hopping



18 Issue 1.2 – June 1999

Figure 10 shows the effect of switching on random frequency hopping between the fourassigned frequencies. The mobile in cell A that previously had high interference levels, now hasvarying interference levels, because the interference from the mobiles in cell B varies with eachtime slot. The rest of the mobiles in cell A, which previously had better quality, are in a similarinterference situation. This is interference diversity.

Figure 10 Effect of switching on random frequency hopping

Because the GSM channel coding and interleaving algorithm can correct interference errors intime slots, the result is that all four mobiles in cell A now have acceptable speech quality.

In summary, the previous peaks and troughs in quality within the system are averaged out toproduce acceptable quality across the whole system.

Note: If cyclic frequency hopping was used in this scenario, there would be no interferencediversity effect since the interfering mobile would always be the same.



Issue 1.2 – June 1999 19

Associated techniques

This section describes three techniques that can be used with frequency hopping to maximisethe benefits of interference diversity:

• Discontinuous transmission (DTX)

• Dynamic power control

• Fractional loading

DTX

Telephone traffic has alternating periods of silence and activity. The typical activity factor fortelephone conversations (the fraction of time a given user is actually speaking) is around 40%.Data transmissions over switched circuits generally have an even lower activity factor.

In certain GSM transmission modes (in particular speech and non-transparent data) DTXexploits this fact by inhibiting transmission of the radio signal when there is no information tosend (voice or data).

In the case of speech, the optimum goal is to encode speech at a bit rate of 13 kbps when theuser is talking, and at around 500 bps during silences (sufficient to generate background noiseso that the listener does not think the connection is broken).

Low encoding rates during silences result in decreased radio transmissions with acorresponding reduction in channel interference levels, and improvement in quality.

Using DTX alone, this improvement in quality levels cannot be translated into increasedcapacity since system planning must be done on a worst case basis. DTX is characterised byan on/off nature. Peaks in interference levels are the same whether or not DTX is used, and therate of switching (between periods of activity and silence) is not high enough for channel codingand interleaving to average out the variations.

However, when DTX is used with frequency hopping, the peaks in interference levels arelevelled out. So the quality increase produced by the lower interference levels can now betranslated into increased capacity.



20 Issue 1.2 – June 1999

Dynamic power control

Dynamic power control (or simply “power control”) regulates transmission power levelsdynamically during a connection. The mobile station and the base station can independentlyreduce their power level when the received signal strength on the other end exceedsrequirements.

This conserves battery power in the mobile stations. But also, and importantly for frequencyhopping, by reducing overall power levels it reduces channel interference.

The following figure illustrates the typical situation in the downlink without power control. Itshows the C/I ratio perceived by a mobile station as a function of the distance to the basestation normalised to the distance between interfering base stations. Note that to ensureacceptable quality at the cell borders, significant power is wasted when the mobile station isnear the base station.

-10

0

10

20

30

40

50

60

70

0 0.2 0.4 0.6

d/D

C/I

Target C/I

Cell edge

Wasted power

Figure 11 C/I ratio as a function of normalised distance (without power control)

Let’s now see what happens with power control:

The interference in this case depends on the location of the interferer mobile station (the mobilestation the interfering base station transmits to). This is illustrated in Figure 12, where the C/Iratio is plotted, again with and without power control, for different interferer locations.

When the mobile station is at the cell border, base stations generally transmit at maximumpower3, with or without power control (C in C/I remains the same). So, in systems with powercontrol, a mobile station at the cell border only perceives the same C/I as without power control

3 Power control parameters should generally be set so that mobiles at the cell edge transmit at fullpower. This is to prevent unwanted interference effects that would take place if a mobile on the cell edge,which starts its transmission at full power, had to regulate. In the time the mobile would take to reduce itspower, it would be causing high interference levels on mobiles that are already transmitting at therequired power



Issue 1.2 – June 1999 21

when the interferer mobile station is also at the cell border. In the other cases, the interferenceis lower and hence the perceived C/I is higher.

Mobile stations near the base stations receive a lower signal strength (C) than without powercontrol. In some cases, this will result in a lower C/I ratio (when the interferer mobile station isnot near its base station and the reduction of power is less). However, as these mobile stationshad very good quality before, the degradation is not noticeable.

-10

0

10

20

30

40

50

60

70

0 0.2 0.4 0.6

d/D

C/I

without PC

PC, average

PC, interf. at cellborder

PC, interf. nearbase-station

Figure 12 C/I ratio as a function of normalised distance (with power control)

Something very similar happens in the uplink.

In summary, power control improves global quality (fewer calls suffer from bad C/I values)which can be translated into a capacity increase.

The gain, however, is not enough to allow a jump from a 4/12 reuse factor to a 3/9 reuse factor,but its effect might be noticeable with automatic planning tools that take power control intoconsideration.

When used with frequency hopping, power control generates more variation between theinterference signals, improving the performance of the averaging properties of frequencyhopping. This is illustrated on page 23.

Fractional loading

Networks are typically planned for full load on the busy hour. The aim is to assign just sufficientresources to handle busy hour traffic, and no more (so that the minimum possible number offrequencies are needed).

Fractional loading changes this planning model by assigning more bandwidth (frequencies) toeach base station, than is strictly necessary to handle busy hour traffic.

The fractional load of a system is then defined as the average percentage of frequency usage,that is, the traffic/no. of traffic channels that the assigned frequencies can hold.



22 Issue 1.2 – June 1999

When using frequency hopping with fractional load, even when the network is operating atmaximum traffic level, some frequencies will suffer from no interference at all. Thanks to thechannel coding and interleaving error correction algorithms, time slots on these frequencies canbe used to correct errors in time slots that are suffering interference. The result is that thethreshold C/I value (the C/I value for the given FER or BER required for marginal quality) isreduced, allowing tighter frequency reuse.

The effect of fractional loading on a frequency hopping system is illustrated in the followingfigure. It shows a system hopping over four frequencies, but only one call per cell:

Figure 13 Fractional loading and frequency hopping

Note: In frequency hopping systems, the transceiver carrying the BCCH does not generallyoperate in hopping mode (because the BCCH frequency must transmit continuously on thedownlink). With fractional load, the only way to ensure continuous transmission on a frequencyin the hopping set is to add an extra transceiver (know as a “fill-sender”) to the cell. This is notan efficient use of resources.

Fractional loading can be implemented by either of the following methods:

• Implementing an admission control procedure

• Installing fewer transceivers than allocated frequencies and using synthesiser frequencyhopping

Admission control procedures

If the number of transceivers were to equal the number of allocated frequencies, the networkwould respond to overload conditions by reducing the quality of all calls, rather than blockingcalls. This could result in more dropped calls, the effect of which is worse for subscribers than ablocked call.



Issue 1.2 – June 1999 23

Admission control procedures could potentially minimise dropped calls by allowing moreeffective handling of local traffic peaks. As a large number of channels are temporarily availablein a sector (provided that the load in surrounding co-channel sectors is low) the admissioncontrol procedures could utilise them.

However, suitable algorithms have not yet been found and in the meantime, fractional loadingshould be used only in conjunction with synthesiser frequency hopping, by installing fewertransceivers than allocated frequencies.

Fewer transceivers than frequencies

In this case, the fractional load is often calculated as the number of transceivers divided by thenumber of assigned frequencies. This is not the actual fractional load of the frequencies, as itdoes not take into account the blocking of the system. However, since it is simple to calculateand widely used, this definition of fractional load is used in this document, unless otherwisestated.

Also, finer granularity of the levels of fractional load can be achieved by disabling some timeslots in the transceivers.

Impact on network planning

In high traffic areas such as large cities, the capacity of a cellular system is limited by its owninterference caused by frequency reuse.

Most systems aim to satisfy as many customers as possible, so the system is planned on thebasis that only a given small proportion of calls at the cell edge (usually around 10%) may sufferbad quality due to interference. With this “worst case” method, the capacity of a systemincreases if the statistical spread of the C/I around its mean value is as small as possible.

This is illustrated in the following figure:

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30 35 40 45 50

C/I [dB]

12 dB

7 dB

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 5 10 15 20 25 30 35 40 45 50

C/I [dB]

12 dB

7 dB

STANDARDDEVIATION

Figure 14 Example of C/I distributions



24 Issue 1.2 – June 1999

The diagram on the left shows the C/I distribution for systems with equal average C/I value butdifferent deviation. The diagram on the right shows how a smaller deviation allows a loweraverage C/I value for the same planning objectives. The interference diversity property offrequency hopping has exactly this effect: it averages quality across the network or, to put itanother way, it decreases the deviation. By reducing the average C/I value in this way, networkoperators can plan for tighter frequency reuse.

Two factors optimise the averaging effect of interference diversity:

• High numbers of interference sources for frequency hopping to switch over

• Low correlation between the interference they cause (that is, interference variation)

The number of hopping frequencies governs the first factor. The higher the number the better.The second factor depends partly on the locations of the interference sources. In the examplein Figure 8 the interferers are different mobile stations assigned to the same cell. In the samescenario, but in the downlink, although the interfering communication is different in every timeslot, the interference source is always the same (the same base station; different frequencies).Hence the correlation is high and the averaging effect is small. This is illustrated in the followingfigure:

Figure 15 No interference diversity in the downlink

The variable interference planning solutions from Lucent Technologies are designed tocounteract this problem. These techniques maximise levels of variable interference in thenetwork, particularly in the downlink, in order to exploit fully the benefits of frequency hopping.

Variable interference planning techniques are described in the following chapters.


Introduction to VIP

3


Issue 1.2 – June 1999 25

3. Introduction to VIP

This chapter describes the variable interference planning solutions offered by LucentTechnology:

• VIPone

• VIPtwo

3.1. VIPone

VIPone is based on variable reuse patterns. Variable reuse patterns implement different reusepatterns within the same cellular network. Typically, a loose reuse pattern such as 4/12 is usedfor the transceiver that holds the BCCH control channel. A progressively tighter reuse is appliedto the second and third TCH transceivers, and so on.

One way to implement variable reuse patterns is to divide the allocated spectrum into sub-bands, each containing a different number of separately planned carriers. One or morefrequencies from each sub-band is allocated to each sector. For example: a 12 reuse for theBCCH transceiver, and a 9 and 3 reuse for the second and third TCH transceiver respectively.The result gives a total average reuse of 8 ((12+9+3)/3 = 8)4. This reuse pattern is illustrated inthe following figure:

4 This is just an example. It is not achievable in a real network



26 Issue 1.2 – June 1999

.. . .. .

B C C H1 21 2

T C H 199

T C H 233

Figure 16 Variable reuse pattern 12/9/3

Variable reuse patterns can also be achieved with automatic frequency planning tools such asGRAND, by planning each of the transceivers in a cell for progressively lower C/I thresholdlevels. In this way network irregularities are catered for, something that cannot be done usingregular reuse patterns.

VIPone uses frequency hopping in conjunction with variable reuse patterns in order to:

• Produce the necessary interference variation in the downlink

• Improve the existing interference variation in the uplink

Downlink

As described in the previous chapter, with regular reuse patterns there is no interferencevariation in the downlink because the interfering source is always the same.

With variable reuse patterns the interfering base station is different for each time slot. Eachbase station also belongs to a different “tier” of interferers; each tier corresponding to a differentreuse pattern. This produces interference diversity in the downlink, so that the averaging affectof frequency hopping can work. The following figure illustrates downlink diversity:



Issue 1.2 – June 1999 27

Figure 17 Interference diversity with variable reuse - downlink

Uplink

In the uplink, the effect of variable reuse patterns is that the interfering mobile stations areassigned to different base stations belonging to different tiers. This results in a higherdecorrelation of the interference signals, and again, a better averaging effect with frequencyhopping than in a network with a regular reuse pattern.

Figure 18 Interference diversity with variable reuse - uplink



28 Issue 1.2 – June 1999

VIPone properties

Since the number of hopping frequencies is always equal to the number of hoppingtransceivers, VIPone can be implemented using baseband frequency hopping.

And because interference diversity is already achieved simply by the difference in reuse, VIPone

can be used with both cyclic and random hopping. The choice will generally depend on thenumber of hopping frequencies. For small numbers of frequencies (such as two) cyclic hoppingshould be used because it achieves better spectrum utilisation.

Field tests in live networks show that an average frequency reuse factor of as low as 7.5 ispossible without impacting network quality. And by applying power control and DTX in thedownlink, the average reuse can be decreased below 7.

Variable reuse patterns can also be used to handle unevenly distributed traffic, (that is, differentnumbers of transceivers per cell). This is illustrated in the next section.

Another potential benefit of VIPone is to free up frequencies for the initial deployment of amicrocell layer.

VIPone examples

Scenario 1: Unevenly distributed traffic

In the previous 12/9/3 example, the operator might not need a third transceiver in all cellsinitially. This means the effective reuse on the third sub-band will be less than 3, and there willbe less interference in the network. But as capacity need increases, a third transceiver can beinstalled in more cells, providing a progressively tighter average reuse without the need torecalculate the frequency plan.

Scenario 2: VIPone plan in a real network

Another illustration is a 12/8/6/4 frequency plan, requiring 30 carriers (already in use incommercial GSM networks). This allows an operator to assign up to 4 transceivers per cell,roughly double the capacity with a standard 4/12 reuse pattern.

Scenario 3: Freeing up frequencies for the microcell layer

In another network, an initial reuse factor of 16.9 requiring 40 carriers was tightened to a14/10/6/2 configuration (average reuse of 12.87 and 32 carriers) and even to a 12/10/4/2configuration (reuse 11.26 and 28 carriers). There was no change in the number of droppedcalls. Some degradation of perceived speech quality occurred in the second case, but that wasidentified as relating to interference in the tighter BCCH band.

Note: All reuses quoted were achieved in capacity limited networks with an existing cell layoutoptimised for capacity (almost homogeneous antenna height, orientation, and location).



Issue 1.2 – June 1999 29

3.2. VIPtwo

The capacity of a GSM network is generally limited by one of the following:

• Number of traffic channels (hard blocking)

• Interference from neighbour cells (soft blocking)

From a hard blocking point of view, small reuse factors give better performance than higherfactors (“trunking efficiency”). However, small reuse factors are limited by soft blocking(interference) and cannot accept more than a given amount of traffic. This means that theywould need to be planned with a certain degree of fractional loading. As a result, the maximumcapacity will lie somewhere between a high and a low reuse factor.

Traditionally, networks have been planned to be limited by hard blocking. That is, the frequencyreuse has been set high so that only very few calls suffer from bad interference conditions. Themaximum capacity of the system is defined by the hard blocking limit set by the restrictednumber of frequencies.

However, especially with interference diversity, this is not the optimum way.

To illustrate this, various reuse schemes with frequency hopping in the traffic carriers have beensimulated (COST 231) and their maximum capacity has been identified as the minimum of thecapacity that hard and soft blocking allow. The soft blocking limit was set so that less than 10%of the calls were subject to an average C/I lower than 9 dB (as specified in GSMRecommendation 05.05).

For an operator with 36 TCH frequencies (9.8 MHz), ideal power control, and DTX with a voiceactivity factor of 50%, the maximum capacity per site was obtained for a sectorised base stationand a frequency reuse factor of 1/3 with a real fractional load of 30% (no. of transceivers/ no.frequencies ≈ 38% - see page 21).

These results were obtained using a regular site lay out and homogeneous propagationconditions. In practice, “off-grid” placements, irregular propagation conditions, and uneventraffic loads will produce additional interference variation, allowing a higher fractional load.Fractional loads of up to 50% (with DTX and power control) have been used in real networkswithout a noticeable decrease in quality.

VIPtwo is based on these ideas. It consists of using very tight reuse patterns, typically 1/3 or 1/1,and fractional loading to introduce the required interference variation.

VIPtwo properties

VIPtwo uses fractional loading and so requires synthesiser frequency hopping.

As mentioned before, 1/3 reuses allow fractional loads of up to 50%. Field trials show thatfractional loads of 15%-20% are possible with a 1/1 reuse.



30 Issue 1.2 – June 1999

Like VIPone, it can cater for unevenly distributed traffic, just by setting different fractional loads todifferent base stations. It can also be used to free up frequencies to be used in a microcellularlayer.

VIPtwo also provides the following benefits:

• It eases the planning effort, since the whole pool of frequencies is assigned to each site oreach cell, and only the control frequencies detailed careful planning

• The network can be planned with VIPtwo from the beginning, even if the number oftransceivers required per cell is initially low. Further transceivers can be added asnecessary without modifying the frequency plan. Quality will not be compromised as one ofthe advantages of frequency hopping is its ability to smoothly trade-off quality and capacitydepending on the traffic load

VIPtwo examples

Scenario 1: 1/3 reuse

An operator with 7.5 MHz (37 frequencies) could achieve 3-3-3 configurations by using a typical4/12 reuse factor, supporting 14.9 Erlangs/cell (2% blocking). If VIPtwo were used, 12frequencies would be assigned to the transceivers containing the control channels, using a 4/12reuse. The rest of the frequencies could be planned with a 1/3 reuse. This means, 8 hoppingfrequencies per sector and a spare frequency for optimisation. 4-4-4 configurations can then beachieved with 1 control transceiver and 3 traffic transceivers per sector. The result is 30 trafficchannels and a traffic level per cell of 21.9 Erlangs: a 47% capacity increase. The fractionalload is 3/8= 37.5%.

Scenario 2: Greater capacity

Increasing the fractional load to 50% increases the number of traffic transceivers to four. Thismeans 37 traffic channels (2 dedicated control channels are now used) and a traffic level percell of 28.3 Erlangs: a 90% capacity increase.

Scenario 3: 1/1 reuse

With lower fractional load, 1/1 reuses are also possible. In the example above, it would involvehopping over 24 frequencies. This allows four traffic transceivers in all cells, and up to five inselected cells.

Scenario 4: Network irregularities

The configuration in scenario 1, with a 1/3 reuse at a 2-sector site (typically used for highwaycoverage) means that each sector can be assigned 8 hopping frequencies each, taken from thetotal pool of 24. This means that some of the frequencies on the pool will not be used. Theinterference averaging capabilities of frequency hopping allows the system to exploit that, byimproving perceived quality levels.



Issue 1.2 – June 1999 31

3.3. VIPone and VIPtwo compared

The choice between VIPone and VIPtwo will be governed by spectrum allocation and the radioequipment in use.

For operators with a large spectrum allocation and a high number of transceivers per cell, VIPone

is the natural choice. It allows the use of lower loss filter combiners at the base stations, thuspreserving the coverage footprint while taking advantage of the system gain provided byfrequency hopping. VIPone is typically used by operators either with filter combiners inwidespread use, or with a more generous spectrum allocation and a need for high configurationcells.

In contrast, for operators with a small amount of spectrum, or with a base station infrastructurethat is already equipped with hybrid combiners, VIPtwo is potentially a more flexible approach,thanks to the ease of frequency planning.

Combined plans

It is also possible to combine VIPone and VIPtwo, by using fractional loading in conjunction withvariable reuses. Typically this might be used by operators with high spectrum allocation andwideband antenna coupling equipment. This combined plan allows the operator to take intoaccount future growth. The reuse strategy can be set tight from the beginning to cater for futuretraffic increases. Transceivers can be added to the sites as needed, without changing thefrequency plan. Initially, the low fractional load (transceivers/assigned frequencies) ensures highquality, which will then be traded off for capacity, as the need arises.

Scenario: Live VIPone/VIPtwo configuration

A configuration that is in use in a live network, is a 12/8/5/4 frequency plan with 40 carriers anda reuse of 6.2, with 5 or 6 hopping frequencies (reuses 8, 5, 5, 4, 4, and 4 every other cell). TheBCCH channel is non-hopping. An average of 4 transceivers per cell would mean a 61%fractional load. The average number of transceivers could be later increased to 4.5 (68%fractional load) or even to 5.4 (82% fractional load) while retaining good network quality.



32 Issue 1.2 – June 1999



Configuring FrequencyHopping

4


Issue 1.2 – June 1999 33

4. Configuring Frequency Hopping

This chapter details the hardware and software configurations required to support frequencyhopping in a GSM network, and the parameters required to activate it. The chapter alsodiscusses DTX and dynamic power control deployment.

4.1. Base station hardware

Base model

• RBS-900 family (900 band) supports baseband hopping only

• BTS-2000 family (900 and 1800 bands) – also known as RBS-918:

− Bosch RFUs: These vary depending on model type, as denoted by the secondletter of the equipment code. B denotes support for baseband hopping; S denotessupport for both baseband and synthesiser hopping

− Lucent SRFUs support both synthesiser and baseband hopping

− The BTS-2000/2C (or “CUBE”) supports synthesiser hopping only, and only in itssecond transceiver



34 Issue 1.2 – June 1999

Antenna coupling equipment

• Filter configurations (with TXFU09/TXFU18 filter combiners) support baseband frequencyhopping only

• Hybrid and diplexer configurations (which use TXHU09/THDU18 hybrid combiners,TXDU09/TXDU18 diplexers, or both) support baseband and synthesiser hopping

Note: Due to the hybrid combiner losses (each hybrid layer introduces a 3 dB loss into theoverall combining loss), hybrid configurations of up to 4 transceivers are available with twoantennas, and 6 transceivers with three antennas.

Fill-sender and phantom-RTs

The BCCH frequency transmits continuously to enable mobile stations to monitor it. This is noproblem for baseband hopping since there are as many RFUs as frequencies, and one of themis always transmitting the BCCH frequency.

However, for synthesiser hopping and fractional loading, traffic channels assigned to thetransceiver that includes the BCCH cannot hop unless an additional transmitter is used.(Otherwise the BCCH frequency would not be transmitted continuously.) This extra transmitteris known as the “fill-sender” (although fill-transmitter would be more accurate). A fill-sender is anRFU or SRFU used to fill the gaps so that the BCCH is continuously transmitted on the BCCHfrequency.

To implement hopping channels on the BCCH transceiver, the fill-sender RFU or SRFU and theBCCH-RT must be located in the same DRCC (Double Radio Codec and Control5) in the BTS.So fill-senders cannot be used on the BTS-2000/6od or the BTS-2000/2C as these do not havethe DRCC in their physical configuration. The following figure illustrates a BTS with fill-senderconfiguration:

5 A logical association of two slots powered by the same power supply unit



Issue 1.2 – June 1999 35

In baseband hopping, there is potential to hop over more frequencies, by adding a “phantom-RT” (an RT is a transceiver). Like a fill-sender, this is an extra RFU set on a different frequency,which can be included in the hopping sequences, but does not carry additional traffic.

Both a fill-sender and a phantom-RT take up the physical space of a standard transmitter, sotheir suitability must be weighed against the fact that the equipment could otherwise be used tosupport another eight traffic channels.

6 CCB: Channel Codec Board. FCO: Flash Controller Board

Fill Sender

CCB

F1F2F3

F0

DRCC

SRFU0(synth hopping)

SRFU1(synth hopping)

FCO FCOCCB

F1F2F3

F0

Figure 19 BTS with fill-sender configuration6



36 Issue 1.2 – June 1999

4.2. Software release support

• LM3.0 software release (GSM release 6.0) supports baseband hopping only

• LM4.0 (GSM release 7.0) supports baseband and synthesiser

4.3. Configuration

Frequency hopping configuration in Lucent equipment is based on the concept of a FrequencyHopping System (FHS). An FHS consists of:

• A set of hopping frequencies (from the pool of frequencies that are available at the cell)

• An HSN (Hopping Sequence Number)

The HSN is used to generate the hopping sequence in which the set of allocated hoppingfrequencies is used. Allowable values are 0 to 63. Value 0 generates a cyclic hoppingsequence; all other values generate a pseudo-random sequence.

Each channel (defined as a transceiver and time slot pair) must have an associated FHS thatdetermines the frequencies the channel hops on and the hopping sequence.

Additionally, the MAIO (Mobile Allocation Index Order) is automatically generated by the systemto prevent Um interface collision (channels using the same frequency at the same time) betweenchannels belonging to the same cell. The value can be 0 to N-1 where N is the number ofhopping frequencies.

FHS configuration rules

To avoid adjacent channel interference within a cell, the frequencies in an FHS should generallyobey a minimum co-site spacing rule: there should be a separation of 2 or 3 GSM carriersbetween them. The configuration must also comply with the following rules:

• The maximum number of frequencies in an FHS is 8

This means that a channel can hop on a maximum of 8 frequencies. In future releases, anew feature known as “Improved Frequency Hopping” will overcome this limitation andallow hopping on up to 18 frequencies in the case of synthesiser hopping.

• The maximum number of FHSs using the same frequency is 2 in BTS-2000 (RBS-918) and1 in RBS-900

For the RBS-900 this means that either the transceiver holding the BCCH must be left tonon-hopping, or time slots 0 (and 2, 4, and 6 if additional CCCH channels are present) of alltransceivers must be non-hopping.



Issue 1.2 – June 1999 37

Example 1: BTS with 4 RTs, 1 additional CCCH, baseband hopping

BCCH non hopping:

CHN0 CHN1 CHN2 CHN3 CHN4 CHN5 CHN6 CHN7

RT0 BCCH -- CCCH -- -- -- -- --

RT1 FH1 FH1 FH1 FH1 FH1 FH1 FH1 FH1



FH0 no FH0

FH1 RT1,RT2, RT3

BCCH hopping (RBS-900):


RT0 BCCH FH1 CCCH FH1 FH1 FH1 FH1 FH1

RT1 -- FH1 -- FH1 FH1 FH1 FH1 FH1



FH0 no FH0

FH1 RT0, RT1, RT2, RT3

BCCH hopping (BTS-2000/RBS-918):


RT0 BCCH FH1 CCCH FH1 FH1 FH1 FH1 FH1




FH0 RT1, RT2, RT3

FH1 RT0, RT1, RT2, RT3



38 Issue 1.2 – June 1999

• The maximum number of physical channels using the same FHS is 42

In baseband hopping this restriction limits the maximum number of hopping frequencieseven further.

Example 2: BTS with 6 RTs, baseband hopping

BCCH non hopping:


RT0 BCCH -- -- -- -- -- -- --






FH0 no FH0

FH1 RT1, RT2, RT3, RT4, RT5



RT0 BCCH FH1 FH1 FH1 FH1 FH1 FH1 FH1

RT1 -- FH1 FH1 FH1 FH1 FH1 FH1 FH1





FH0 no FH0

FH1 RT0, RT1, RT2, RT3, RT4, RT5



Issue 1.2 – June 1999 39









FH0 RT1, RT2, RT3, RT4, RT5

FH1 RT0, RT1, RT2, RT3, RT4, RT5

Example 3: BTS with 7 RTs, baseband hopping

BCCH non hopping:


RT0 BCCH -- -- -- -- -- -- --







FH0 no FH0

FH1 RT1, RT2, RT3

FH2 RT4, RT5, RT6



40 Issue 1.2 – June 1999










FH0 no FH0

FH1 RT1, RT2, RT3

FH2 RT4, RT5, RT6










FH0 no FH0

FH1 RT1, RT2, RT3

FH2 RT4, RT5, RT6

• The maximum number of FHS in a BTS is 8

• The maximum number of FHS in a BSS is 48



Issue 1.2 – June 1999 41

Other limitations

• Intra-cell handover is disabled when frequency hopping is active on a channel. In the caseof mixed configurations (hopping and non-hopping channels) intra-cell handovers will takeplace between the non-hopping channels and from non-hopping channels to hoppingchannels, but not from hopping channels

• Frequency hopping is allowed with concentric cells, as long as hopping is betweenfrequencies assigned to the same zone

• For dual band operation, frequency hopping is only allowed between frequencies belongingto the same band

4.4. Feature activation and system parameters

Once the hopping configuration is defined, frequency hopping must be configured and activatedin the requisite network elements:

BTS hopping mode

In the RBS-900, the hopping mode (which is baseband only) is implicitly defined by the BTS-HW configuration.

In the BTS-2000 family, which allows both types of hopping, the hopping mode (baseband orsynthesiser) is set via the RBT-2000 (Radio Base Station Tester) software in the IMW-20008.The default value is baseband.

Note: If a fill-sender is used, the corresponding RT is configured via the RBT-2000.

BSS feature enabling

Frequency hopping is a purchased option, and a factory access code is required to enablefrequency hopping in a BSS. This is specified in the Freq_Hopp_Enabled variable in the BSSlocal configuration data.

OMC parameter configuration

The following OMC objects must be created or modified (internal parameter names are used,followed in brackets by the OMC GUI and AUI parameter names respectively).

8 A notebook PC with dedicated software for BTS-2000 and BCF-2000 administration



42 Issue 1.2 – June 1999

BTS: Each frequency in the FHS must be defined in the cellAllocation (CellAllocation,CELLALLOC) attribute of the BTS object.

RT: For baseband hopping, an RT (Radio Terminal) object must be created for each frequencyto be used in an FHS. The frequency is defined by the attribute initialFrequency(InitialFrequency, INITFREQ).

For synthesiser hopping there is no relationship between the initialFrequency and thefrequencies belonging to the FHS. However care has to be taken to ensure that the back-up RThas the initialFrequency set to a value that will ensure good operation if the BCCH RT fails.

To create a phantom-RT, the following attributes are dropped from the appropriate RT object:AbisServiceProvider; AbisSigHDLCInfo; AbisTrafSlotInfo; BackupObject.

FH: A Frequency Hopping (FH) object must be created for each required hopping sequence.The following attributes must be defined for each FH object:

• allocatedFrequencies (AllocatedFrequencies, ALLOCFREQ): frequencies belonging to thehopping sequence must be defined here (entries must match the cellAllocation attribute ofthe BTS object)

• HSN (SequenceNumber, HOPSEQNO): defines the HSN to be used by the hoppingsequence generator. Specify 0 for cyclic hopping; or a number in the range 1 through 63 forrandom hopping

CHN: Each channel (CHN) object must be defined as hopping or non-hopping via thefreqHoppRelationship (FHRelationship, FREQHOPREL) attribute. This specifies an associatedFH object (hopping channels) or is left empty (for non-hopping channels).

A CHN object with channelType (Channel Type) of CCCH cannot be defined as a hoppingchannel.

The RT that the channel belongs to must not be a phantom-RT.

A MAIO will be generated internally for the channel (according to GSM Recommendation05.02). A maximum of 42 channels can be associated with the same FHS.

Automatic parameter update in the OMC

All the necessary changes required to install frequency hopping in a selected BTS can beautomated using the site independent OMC script inst-fhs.r78. The script installs frequencyhopping systems in the selected BTS depending on:

• Number of RTs connected to the BTS

• Hopping type (baseband or synthesiser)

• Presence of a fill-sender (synthesiser hopping)

• Presence of CCCH(s)



Issue 1.2 – June 1999 43

Additionally, the Automatic Network Modification for Frequency Hopping feature (omc-cm093)of OMC-2000 release 4.5 has automated the process of frequency replanning with frequencyhopping. This feature allows the frequency planner to provide the OMC operator with frequencyand frequency hopping plans in electronic format.

The OMC reads the plan, validates the data, and generates a set of AUI scripts that will updatethe existing OMC data to match the new plan. The scripts may be executed immediately orscheduled for later execution.

The procedure is as follows:

The OMC operator requests a frequency plan report in raw format, via the Configuration ReportGenerator implemented in the OMC. This report can be exported to an off-line PC where thefrequency planner can modify the frequency plan. The OMC operator can then import the databack to the OMC. Using the Receive Plan Option on the Expert AUI window, an AUI script isgenerated that contains the modifications required to change the OMC data to the dataspecified in the file.

For more information, refer to the OMC-2000 System Operator’s Guide - OMC Release 4.5.

Feature activation

When the FHSs are activated, the reconfiguration process involves two steps:

1. The reallocation procedure provides the BTS with the necessary information, and instructsit to reconfigure its hopping behaviour at a specified start time.

2. The frequency redefinition procedure triggers the call handling function to start thefrequency redefinition in the mobile stations (see Frequency redefinition procedure on page10).

Note: Activation and deactivation of FH takes time because as said it involves a frequencyredefinition procedure that takes up to 3.5 minutes per hopping system. However, thisprocedure has little impact on established calls (see Frequency redefinition procedure on page10), that is, it causes no down time of RTs or base stations. Only modification of RTs’ InitialFrequency produces RT downtime, and that is regardless of whether or not the system hops.



44 Issue 1.2 – June 1999

4.5. Fault management

Frequency hopping is automatically disabled in the following situations:

Baseband hopping

If the number of available hopping frequencies used by the FHS falls below a given thresholddue to severe RT faults. This threshold is calculated by multiplying the number of frequenciesby a percentage defined in the LMB Enable/Disable CH options of the BCE or BCF localconfiguration area.

Because the BSS does not redefine the list of allocated frequencies when an RT fails, thethreshold must be set to 100% (the default) to avoid bad quality connections due to frequencyloss.

Note: The Frequency redefinition procedure (see page 10) that is triggered with the deactivationof frequency hopping, prevents all the calls in the base station from being dropped when an RTfails. Only the calls served by the affected RT are dropped (as happens in a non-hoppingsystem). The quality of the other calls will decrease for as long as the Frequency redefinitionprocedure takes place. The level of degradation depends on the number of hoppingfrequencies: the greater the number, the lower the degradation. This behaviour is howevertypical of any baseband hopping system, irrespective of the vendor, and is due to the waybaseband frequency hopping is generated.

Synthesiser hopping:

If a fill-sender is used, frequency hopping is disabled if the fill-sender fails.

4.6. DTX

Uplink DTX

Uplink discontinuous transmission is set on a per-BTS basis. To do this, set the Uplink in theDTX parameter in the BTS Detail View of the OMC GUI (AUI parameter DTX of the managedobject class BTS).

The parameter can have three values:

• May be used (0)

• Shall be used (1)

• Shall not be used (2)



Issue 1.2 – June 1999 45

Downlink DTX

Downlink DTX can be set independently for speech (system release 6.5) and for non-transparent data (system release 6.7.2). To do this the corresponding parameters have to beset in the BSS, the MSC and the InterWorking Function (IWF).

Speech

This feature is enabled or disabled on a per BTS basis via the OMC. To do this, set theDownlink Speech in the DTX parameter in the BTS Detail View of the OMC (the AUIDownlinkDtx attribute of the managed object class BTS contains the booleandownlinkDtxSpeech).The default setting is disabled (false).

In the MSC, the switch option Downlink DTX Mode in the WBOPM (Wireless Base OfficeParameters Miscellaneous) view, can be enabled and disabled in the corresponding windows ofthe Recent Change and Verify (RC/V) program. This applies to all the BSS supported by oneMSC.The default setting is disabled.

DTX is permitted for the connection if DTX is requested by the MSC and enabled by the OMC.

Data

This feature is enabled or disabled on a per BTS basis via the OMC. To do this, set theDownlink Data in the DTX parameter in the BTS Detail View of the OMC (the AUI DownlinkDtxattribute of the managed object class BTS contains the boolean downlinkDtxData).The default setting is disabled (false).

If this parameter is enabled, the BTS acts according to the DTX commands issued by the OWFin the received RLP frames.

To enable DTX in the IWF, the IWF option DTX Mode is set by changing the value in the IWF-2menu.The default mode is disabled.

4.7. Dynamic power control

Power control for communications through a given BTS can be deployed independently in thedownlink and the uplink via the parameters EN_MS_PC (uplink) and EN_BS_PC (downlink) ofthe POWER object associated with the BTS.

Before doing that the following parameters of the POWER object should be set to their propervalues:

• Maximum transmit power values:



46 Issue 1.2 – June 1999

− MS_TXPRWR_MAX: defines the maximum TX power an MS is permitted to useon a dedicated control channel or a traffic channel within the serving cell

• Averaging measurement parameters:

− A_LEV_PC: defines the averaging window size for receive power levelmeasurements

− A_QUAL_PC: defines the averaging window size for quality measurements

− W_QUAL_PC: defines the weighting of full-set quality measurements with respectto sub-set quality

• Threshold levels:

− L_RXLEV_UL_P, U_RXLEV_UL_P, L_RXQUAL_UL_P, U_RXQUAL_UL_P:defines the uplink lower (L) and upper (U) RX_LEV and RX_QUAL thresholds

− L_RXLEV_DL_P, U_RXLEV_DL_P, L_RXQUAL_DL_P, U_RXQUAL_DL_P:defines the downlink lower (L) and upper (U) RX_LEV and RX_QUAL thresholds

• Power step sizes:

− POW_INCR_STEP_SIZE, POW_RED_STEP_SIZE: defines the step sizes usedwhen increasing or decreasing the MS and BTS transmit power

• Timer values:

− P_CON_ACK: defines the power control acknowledge time

− P_CON_INTERVAL: defines the minimum interval between successivemodifications of the radio frequency power level


VIP Deployment

5


Issue 1.2 – June 1999 47

5. VIP Deployment

5.1. Introduction

When to use VIP

There are two main reasons why an operator might implement VIP and frequency hopping:

• To improve quality in an area with interference problems

• To increase capacity in an already saturated area (in terms either of a need for moretransceivers to meet traffic loads, or a need to free up frequencies in the existing spectrumfor use in other layers)

Frequency hopping should not be used to try to improve poor quality in networks where theunderlying cause is poor coverage, network planning or tuning. In such cases, frequencyhopping can cause further deterioration in performance.

Implementation strategy

The interference averaging effect of frequency hopping generally diminishes the number ofinterference problems in the network. However, those problems that remain will be more difficult



48 Issue 1.2 – June 1999

to resolve. For this reason, we strongly recommend that the process of increasing capacity withfrequency hopping and VIP should be implemented in stages as follows:

1. Switch on frequency hopping.

2. Tighten the frequency reuse or the fractional load step by step, as and when new capacityis needed.

3. When the capacity gains from frequency hopping have been exhausted (but not before),implement DTX and power control.

Each step should be deployed in a small trial area first, with extensive data collection made ateach stage in order to assess accurately the impact of the new plan on the network. No twonetworks behave the same when frequency hopping is switched on, so it is important thatdetailed results data is collected for each network.

The conclusions drawn from the initial deployment can then be used in the overall deploymentto minimise the initial impact and the subsequent optimisation work. In particular they can beused to optimise the frequency plan and the radio link control parameter setting.

5.2. Choosing the right plan

This section describes how to identify the appropriate VIP plan for the implementation area.

More than three transceivers per cell

Areas with typical configurations of more than three transceivers per cell can use either VIPone orVIPtwo. The choice will depend on the type of antenna coupling equipment already in place, theinvestment that the operator is prepared to make, and the operator’s requirements for flexibilityand future growth. Specific factors that might influence the final choice include:

• VIPtwo has the big advantage of eliminating the need for frequency planning of the trafficcarriers in a network. So a VIPtwo plan is very flexible when it comes to introducing new basestations

• The main disadvantage of VIPtwo is that it requires hybrid or diplexer antenna combinerequipment, which might not be in place

• For network areas with existing filter-type combining equipment, consider carefully beforemaking any decision to swap-out existing equipment. This is because the increase ininsertion losses can affect the performance of the network, particularly for in-buildingcoverage

• If for whatever reason the use of hybrid combiners is not considered feasible, then VIPone isthe appropriate plan



Issue 1.2 – June 1999 49

Three or fewer transceivers per cell

Areas with typical configurations of three or fewer transceivers per cell can use VIPtwo only.

Large spectrum allocation

For areas where synthesiser frequency hopping is possible, but a high number of availablefrequencies means that 1/3 or 1/1 reuse patterns are not possible, a mixed VIPone/VIPtwo plan isrecommended.

Microcells

In microcellular environments (where configurations are normally low) VIPtwo is the best optionfor the non-BCCH transceivers. It allows frequency reuse from the macro layer, with no need totake into account interference from other micro cells if the fractional load is low enough. Thismeans that capacity can be added to the micro layer with minimal impact on the existingfrequency plan.

Planning for future capacity

Implementing DTX and dynamic power control in the downlink can produce further capacitygains. However, remember that they should be introduced in stages; not at the same time.

5.3. Planning the frequencies and the HSN

VIPone

In areas with an average number of transceivers per cell of more than three, a quality increasein terms of interference can be expected just by switching on random frequency hopping overthe existing assigned frequencies.

The more irregular the existing frequency plan, and hence the higher the levels of variableinterference, the greater the improvement. However, the gains may not be noticeable innetworks with existing high quality levels.

For cells with only two transceivers, it is best to either enable cyclic hopping or leave the cell asnon-hopping. These cells will still benefit from the interference diversity caused by surroundinginterfering cells randomly hopping over the same frequency set.

If VIPone has been chosen primarily for capacity gains, the first stage in the design process is tocalculate the average reuse factor required to handle the proposed capacity increase. Note thateven if additional capacity is needed in a few cells only, the calculation has to be done as if allcells in the area under consideration were to be upgraded.



50 Issue 1.2 – June 1999

For example, in a network with 3-3-3 configurations, only certain sites will be upgraded to 4-4-4.Calculations will be done as if all the sites would be upgraded to 4-4-4.

Once the average reuse factor is determined, a variable reuse plan should be devised thatspreads the reuse factors around the average value to as great an extent as possible (takinginto account the number of transceivers within the plan).

For example: for three transceivers and 24 frequencies (average reuse of 24/3=8) a 12/8/4 planwould work better than a 12/6/6.

Reuse capability depends greatly on the reuses allowed by the network infrastructure.Homogeneous networks (grid site locations, regular antenna orientation and height) can supporta reuse value of 12, while others may require values as high as 15 or even 18.

If the number of hopping frequencies is 2 the HSN should be set to 0. Otherwise, it should beset to an integer in the range 1 to 63, ensuring that all values are evenly distributed across thearea.

VIPtwo

VIPtwo always requires frequency re-planning, whether it is implemented to achieve qualityimprovement or capacity gains.

The first step is to decide on the reuse factor (1/1 or 1/3). If the number of available frequenciesis low and a 1/3 reuse would mean hopping over fewer than six frequencies, a 1/1 reuse shouldbe used.

In other cases the operator can choose between the two options, taking into account themaximum number of hopping frequencies available.

The HSN should be set to a value between 1 and 63, ensuring that it is different for basestations using the same set of hopping frequencies. Especially in the case of a 1/1 reuse it isimportant to set the HSN of co-sited base stations to different values.

VIPone/VIPtwo

In mixed plans, the number of hopping frequencies should be set to the maximum of eight, andthe VIPone plan should be designed assuming eight transceivers per cell.

As in the case of VIPone, the HSN should be set to an integer in the range 1 to 63, ensuring thatall values are evenly distributed across the area.

Microcells

Each microcell that requires additional capacity must be allocated a set of eight frequenciesfrom the traffic transceivers in the macro layer. A propagation prediction tool can be used to



Issue 1.2 – June 1999 51

select the frequencies with the lowest probability of interference within the area covered byeach given microcell. Random frequency hopping can then be activated.

The fractional load will ensure that interference from the macro and micro layer does not havean adverse impact on transmission quality.

The HSN should be set to an integer in the range 1 to 63, ensuring that all values are evenlydistributed across the microcell area.

BCCH planning

Unless the capacity of the network is already stretched to its limits, the BCCH transceiver isbest left to non-hopping. In any case it is generally better if it is planned separately, usingfrequencies specifically set aside for the BCCH.

This approach has the following benefits:

• The high levels of interference generated by the BCCH transceiver downlink are limited to aspecific band. (As the BCCH transceiver must transmit continuously, even when there is noinformation to transfer, dynamic power control and DTX cannot be applied to it.)

• Gains from implementing dynamic power control and DTX elsewhere in the network aremaximised

• Control channel behaviour is separated from the traffic load. This is required to ensuresuccessful cell selection, handover, locating, access, and paging activities. BaseTransceiver Station Identity Code (BSIC) decoding on the SCH is especially important forhandover performance (poor handover performance causes more dropped calls)

• Capacity in existing cells can be increased without having to replan the BCCH

Generally the BCCH transceiver will only be set to hopping in the case of a VIPone plan wherethe number of hopping frequencies would otherwise be less than three.

5.4. Collecting performance data

To assess the benefits of deploying a VIP plan into the network, performance data must becollected before and after the deployment.

First collect performance data for the current network configuration and frequency. This has twopurposes:

• To provide a data source for optimisation and tuning purposes

• To provide a performance benchmark for comparison of data collected under the new plan



52 Issue 1.2 – June 1999

Collection equipment

To collect the optimal range of performance data, the following equipment is required:

• GSM drive test equipment:

− Test handsets

− Data collection kit, preferably with reverse path measurement capability

− Scanner

− Post processing/analysis tool

− Voice quality measurement equipment

• Performance management tool (such as the OMC-PMS)

• Abis link monitor and protocol analyser

• Coverage prediction and frequency planning tool

Performance data types

This section details the various types of data that ideally should be collected for performancemeasurement purposes. Each data type can be categorised as one of the following:

• Global information

• Drive test information

Note: When frequency hopping is switched off, ideally the performance data should differentiatebetween BCCH and non-BCCH transceivers in cases where the BCCH is non-hopping. Thismay not be possible with global information, but drive test information should allow it.

Global information

This type of performance data is usually obtained via the OMC-PMS. The ideal collectionmethod is to collect the data on a per cell basis both for all cells within the deployment trial areaand for cells surrounding the trial area.

Global information includes both traffic-related and quality-related data:

Traffic-related data

As traffic load is a major factor in frequency hopping performance, traffic data should becollected before and after frequency hopping is implemented. This enables accurate analysisand comparison of subsequent quality measurement results.



Issue 1.2 – June 1999 53

We recommend that the following traffic data is collected, as a minimum for the busy hour, andideally also on a daily and historical basis:

• Busy hour (the hour segment with the largest TCH traffic value)

• TCH seizure attempts

• TCH seizures

• TCH seizure blocks

• % TCH blocking

• TCH traffic in Erlangs

• Mean TCH holding time

• SDCCH seizures

• SDCCH seizure blocks

• % SDCCH blocking

• SDCCH traffic in Erlangs

• Mean SDCCH holding time

Quality-related data

Quality data is used to compare performance results before and after frequency hopping isimplemented. Accurate analysis of the before and after performance data requires the followingconditions for the data collection:

• Values should be per Erlang wherever possible

• Traffic conditions before and after the implementation should be sufficiently similar toensure no significant variation in interference and GOS (Grade of Service) levels. The trafficdata described in the previous section should be used to ensure equivalent traffic

Note: These conditions apply to both hopping and non-hopping cells, and to the cellssurrounding the deployment area.



54 Issue 1.2 – June 1999

We recommend that the following quality data is collected, as a minimum for the busy hour, andideally also on a daily and historical basis:

Dropped calls

• TCH seizures dropped for radio reasons

• % dropped TCH

• TCH dropped calls/Erlang

• SDCCH seizures dropped for radio reasons

• % dropped SDCCH

• SDCCH dropped calls/Erlang

Handovers

• Total number of handover attempts

• Intracell handover attempts

• Intracell handover failures

• % intracell handover failures

• Intercell handover attempts

• Intercell handover failures

• % intercell handover failures

• Uplink quality handovers

• % uplink quality handovers

• Uplink level handovers

• % uplink level handovers

• Downlink quality handovers

• % downlink quality handovers

• Downlink level handovers

• % downlink level handovers



Issue 1.2 – June 1999 55

RXQUAL statistics

If possible, RXQUAL statistics should be obtained (this will need an Abis protocol analyser).Measurement should be performed at least over the busy hour.

Ideally, all Abis links for base stations in both the deployment area and surrounding areas shouldbe monitored. However, the equipment may restrict the number of links that can be monitored.

Drive test information

Drive tests should be performed over the most significant routes, including the main trafficroutes and, if possible, routes with known or potential conflict problems. In-building walk testsare also very useful in order to assess the impact of frequency hopping on in-building quality.

Ideally, the drive tests should be performed during the busy hour (both before and afterimplementation).

If possible they shall also be repeated a number of times to ensure no external events influencethe results.

The following data should be collected:

• BSIC, BCCH frequency and TCH frequency of the serving cell

• BSIC, BCCH frequency and RXLEV of neighbouring cells

• Downlink RXLEV and RXQUAL measurements

• Downlink co-channel and adjacent channel C/I (if this measurement is not available in thedrive test equipment, a scanner can be used). In the case of frequency hopping, the C/Ishould be obtained for each hopping frequency

• FER on the downlink and, if possible, on the uplink

• Voice quality on the downlink and uplink (in the uplink take care not to introduce externalsources of quality degeneration)

• Handover, power control, and dropped call events and their causes (this requires calltracing capabilities in the Abis monitor)



56 Issue 1.2 – June 1999

5.5. Deployment results

This section describes the results that can be expected at each VIP implementation stage:

1. Activating frequency hopping

2. Tightening frequency reuse

3. Implementing DTX

4. Implementing dynamic power control

Activating frequency hopping

The expected results of activating frequency hopping are:

• Dropped calls and failed handovers will decrease. In a medium loaded network, frequencyhopping may reduce the number of dropped calls by about 20%

• RXQUAL statistics will show an increase in the reported RXQUAL values (see RXQUALbehaviour below). The increase is generally about one unit. Under normal circumstances(no frequency hopping) this would imply serious degradation of transmission quality, but it isnot the case with frequency hopping

• As a consequence of the increase in RXQUAL values, the percentage (and possiblyabsolute numbers) of quality based handovers will increase (see Figure 20)

• The number of intra-cell handovers will be very small (mixed hopping and non-hoppingconfigurations) or 0 (only hopping configurations)

• FER and voice quality (as measured in the drive tests) will improve

• FER/voice quality versus RXQUAL/carrier to interference ratio will show improvement buthigher deviation

These improvements are expected to be higher in the uplink than in the downlink. The downlinkwill be the capacity-limiting link, with better quality in the uplink than the downlink.

Localised areas with previously bad interference problems but good coverage, should showsignificant quality improvement, particularly if the channel used belongs to a hoppingtransceiver.



Issue 1.2 – June 1999 57

Figure 20 Impact of frequency hopping on handover causes

Figure 21 is a typical drive test output when using frequency hopping. It shows the RXQUAL,FER and SQI (Speech Quality Indicator - the measure of the speech quality TEMS equipmentoffer) measured by a TEMS piece of equipment before and after handing over between achannel that does not use frequency hopping and a channel that uses frequency hopping.

Figure 21 Output from TEMS when handing over between a non-hopping and a hopping channel

Frequency hopping activated



58 Issue 1.2 – June 1999

It can be seen that in the channel with no frequency hopping (shown left of the first marker inthe figure) bad RXQUAL values translate into an increasing FER (in red) and a decreasing SQI(in grey). In the channel with frequency hopping (shown right of the first marker in the figure)even worse RXQUAL values than before translate into no FER and a very slight degradation ofthe SQI.

Important: Some trial implementations of frequency hopping have reported performancedegradation in cells that hop over only two frequencies. Such cells should be monitored for thiseffect. To do this, aggregate the performance results according to the number of hoppingtransceivers in the cell and compare the performance with the results obtained when the cellswere non-hopping.

RXQUAL behaviour

The increase in reported RXQUAL values is caused by the following reason:

The RXQUAL parameter does not increase linearly with the error rate of unprotected bits.Instead it increases with its logarithm (RXQUAL increases by one unit if the BER is doubled ordecreases by two units if the BER is divided by four).

The following table shows RXQUAL values obtained in a cell after frequency hopping wasactivated over four transceivers:

RXQUALFH RXQUALTRXi

i=1,nAverage

RXQUALTRXi

Potential speechquality

5 0, 2, 7, 1 2.50 good

5 5, 5, 5, 5 5.00 fair

4 0, 6, 0, 0 1.50 excellent

2 0, 0, 4, 1 1.25 excellent

1 3, 0, 0, 0 0.75 excellent

1 0, 0, 0, 2 0.50 excellent

0 0, 1, 0, 0 0.25 excellent

Table 3 Example RXQUAL values with frequency hopping

With frequency hopping active, the BER (for unprotected bits) for the different hoppingsequence frequencies are averaged and then mapped into an RXQUALFH value for the hoppingchannel. This means the RXQUALFH value is not calculated as the arithmetical average of the



Issue 1.2 – June 1999 59

RXQUALTRXi values of the individual TRX transceivers in non-hopping mode (as illustrated in thetable above).

This logarithmic behaviour means that RXQUALFH ≥ average(RXQUALTRXi).

As the values in the previous table show, with frequency hopping there is no direct mapping orcorrelation of actual speech quality to RXQUAL.

Tightening the reuse

The expected result of tightening the average reuse is that the performance of the system, interms of dropped calls, remains constant up to a certain point. At that point, which representsthe capacity limit of the system and the current configuration, performance begins to deterioraterapidly.

However, the performance deterioration may be due to interference just in the BCCH band. Thisshould be investigated first. If the deterioration is identified as being due to BCCH interference,then careful optimisation of the allocation in this band may produce further capacity gains.

DTX implementation

When DTX is switched on, the number of dropped calls may increase. This is because, by thenature of DTX, some channel slots may not be used for transmission. Measurements on theseslots will obviously report a low reception level, and corresponding bad quality.

To avoid this problem, the GSM Recommendations specify the following requirements:

• At least 12 bursts (an SACCH superframe) must be sent within each reporting period.These bursts mirror the systematic use of the SACCH (four bursts constitute a codingblock) plus eight bursts on the TCH itself. For speech, these bursts contain silencedescription frames (SIDs)

• The BTS and the mobile station must report two distinct sets of measurements concerningthe connection:

− “full” measurements for all slots which may be used for transmission in thereporting period

− “sub” measurements for the mandatory sent bursts and blocks only

• Both the BTS and the mobile station must report for each measurement period, whether ornot discontinuous transmission was used. This allows the processes using themeasurements (power control and handover) to discard the “full” measurements in caseswhen discontinuous transmission was used

Results based on “sub” measurements are less accurate due to the reduced number of inputvalues for the averaging process (reception level is averaged on 12 bursts instead of more than100 bursts). This affects quality measurements in particular. Because they are based onestimated error probabilities before channel decoding, they are more sensitive to the statisticalunreliability introduced by subset measuring.



60 Issue 1.2 – June 1999

Particularly in the case of frequency hopping, this unreliability causes an increase in reportedRXQUAL values with a corresponding increase in dropped call rates.

Dynamic power control implementation

The RXQUAL behaviour described above means that an increase in RXQUAL reportedhandovers (intercell and intracell) can be expected.

5.6. Optimising performance

Quality-based handovers

The increase in reported RXQUAL values leads to an unwanted increase in the percentage ofquality-based handovers. The easiest way to avoid this effect is to increase the handoverquality thresholds by approximately the same amount as the increase in the average RXQUALvalue.

If the percentage of quality based handovers remains high, the RXQUAL averaging windowshould be increased, since the effect is probably due to statistical nature of the measurements.

Quality-based power control

A similar solution can be used to counteract the effect of increased RXQUAL based powercontrol commands following power control implementation. That is, increase the power controlquality thresholds by approximately the same amount as the increase in the average RXQUALvalue.

Hopping over two frequencies

Performance may deteriorate in cells that hop over only two frequencies. If this happens,frequency hopping should be switched off in the affected cells. If the number of such cells issmall, there should be an improvement in their performance compared with a non-hoppingnetwork, even though they do not hop.

DTX measurement accuracy

A weighting algorithm has been devised in Lucent equipment that overcomes the potentialmeasurement inaccuracies introduced by DTX. Full measurements are given a higher weightthan the “sub measurements” (which are more likely to be inaccurate) in the average RXQUALvalue calculation used in the power control and handover processes.

Setting the averaging parameters in this manner will improve performance in systems that useDTX.



Issue 1.2 – June 1999 61

Other scenarios

The following situations may also require investigation and optimisation:

• The global number of dropped calls and failed handovers either does not reduce, or evenincreases. There could be three reasons for this:

− Poor coverage conditions

It has been reported that frequency hopping can aggravate problems arising frompoor coverage (as yet the reasons are unclear).

This situation is indicated by unusually high percentages of mandatory handovers(good coverage networks should show a majority of power budget handovers)both with and without frequency hopping.

− Very poor quality in the network before frequency hopping was implemented

In this situation, the averaging effect of frequency hopping will degrade qualityfurther. The few good quality mobiles will decrease their quality in an attempt toimprove the bad quality mobiles. However the bad quality mobiles will remain bad.

This situation is indicated by existing high numbers of dropped calls and failedhandovers before frequency hopping is implemented.

− Strong interferers exist in the network

Depending on location, some base stations can produce much higher interferencelevels than the others in the network. For example, this often happens with basestations at a higher than average height.

With frequency hopping, this interference is spread across all channels. The bestindicator of an offending base station is a permanently high measured level ofinterference when it is scanned in drive tests. To avoid this effect, such sitesshould be treated separately in terms of frequency planning and, in extremesituations, taken out of the deployment area.

• The number of dropped calls and failed handovers in a particular cell either does notreduce, or even increases. The possible reasons are:

− Poor coverage conditions

This situation is indicated by unusually high percentages of mandatory handoversin the cell, both with and without frequency hopping.

− Locally high interference conditions

The cell may suffer localised interference from a very strong interferer. Thepropagation prediction tool can be used to pinpoint the possible interferers. Then,drive tests can be used to scan the BCCH frequency of these base stations to



62 Issue 1.2 – June 1999

determine whether received levels from them are high enough to causeinterference.

If a strong interferer is found, the frequency plan should be modified to prevent thetwo cells (the interferer and the cell suffering the interference) being used as co-channels. The same average reuse or fractional load must be maintained. Thesechanges may involve rearranging of frequencies, which can be done eithermanually or with a frequency planning tool.

Note: This situation often arises with BCCHs that are included in the hoppingsequences - because they transmit continuously, frequency hopping cannot takeadvantage of traffic variations.

In VIPtwo implementations, if frequency rearrangement is not possible, the frequenciessuffering high interference should be taken out of the hopping sequences in the affectedcell, even if this reduces the number of hopping frequencies available.


Worked Examples

6


Issue 1.2 – June 1999 63

6. Worked Examples

This chapter provides examples of VIP implementations. Each example describes the currentnetwork configuration, the objectives of the implementation, and the planning and designrequirements.

6.1. Scenario 1

Existing configuration

• Operator working in the 1800 band

• Wide-band combiner equipment

• 48 frequencies allocated

• Network still growing with an irregular network layout that is mainly coverage ridden

• To maintain good quality in the existing network, current BCCH planning requires a reuse of7/21

• Configurations are mostly 2-2-2



64 Issue 1.2 – June 1999

• Microcells in use for “cold-spot” coverage with plans to develop them into a continuousmicrocell layer

Objectives

• To improve quality in the network, especially in localised areas where propagationconditions cause high interference levels

• To generate a frequency plan that is able to cope with the rapid pace of change and growthin the network

VIP plan choice

• Since the existing combiner equipment is already wide-band, VIPtwo is the easiest and mostflexible solution to implement

• Because the network is still growing, it is recommended that the macrocell base stationBCCHs are planned on a separate sub-band. This ensures that future capacity expansion inexisting base stations, or addition of new microcells, will not require modifications to theBCCH frequency plan

Planning the frequencies

• The frequency band will be divided into three subsets of 21, 18, and 9 frequencies. The firstsub-band will be used for macro BCCH planning, the second for macro TCH, and the thirdfor micro BCCH

• The BCCH in the macro layer will be planned using a 7/21 reuse that is already known togive adequate performance in the current network conditions

• The BCCH in the micro layer, once continuous coverage is achieved, will be planned usinga 9 reuse, which is know to be adequate in a micro-cellular environment

• The additional TCH transceiver in the existing 2-2-2 configurations will be planned using a1/3 reuse. That is, assigning 18/3=6 frequencies per sector and switching on synthesiserfrequency hopping. (Reuse of 1/1 is not possible in current releases because it would implyhopping over 18 frequencies.)

• If additional capacity is subsequently required in the macro layer, it will be necessary toupgrade base stations to 3-3-3 configurations. The frequency plan will not need to bechanged since a 1/3 reuse can easily accept fractional loads of 2/6=33% without noticeableimpact on the quality of the network

• However, if even more capacity is required in the macro-layer, the frequency plan will needto be changed because a fractional load of 50% is too close to the maximum limit beyondwhich network quality may degrade. However:



Issue 1.2 – June 1999 65

− Ideally, by this time network growth in terms of base stations will have stabilisedand the network layout will have been rationalised into a more homogeneous lay-out (grid locations, similar antenna height and orientation, and so on)

− If this is the case, the BCCH will allow a much tighter reuse: probably around 15.The spectrum allocation can then be split into three bands of 15, 24, and 9. TheTCH transceivers will be assigned 24/3=8 hopping frequencies, which can accepta load of 3/8=37.5%. Hence 4-4-4 configurations would be possible

− If the eventual network layout is insufficiently homogeneous, this must becorrected. No additional capacity gains will be possible until this is done

Mapping the frequency plan to OMC settings

As stated earlier, each cell will be assigned one BCCH frequency fBCCH and 6 hoppingfrequencies, fFH1, …, fFH6.

To do this:

1. Add frequencies fFH1, …, fFH6 to the cellAllocation attribute of the BTS object that relates tothe cell.

2. Set the initialFrequency attribute of the RT (Radio Terminal) object that will hold the BCCHchannel to fBCCH.

3. As the eight CHN (Channel) objects belonging to this RT are non-hopping, thefreqHoppRelationship (FH ID) attribute is left empty.

4. Set the initial frequency of the second RT (and the third RT if using 3-3-3 configurations) toone of the hopping frequencies.

5. Create a FH (Frequency Hopping) object with attribute allocatedFrequencies equal to fFH1,…, fFH6.

Set the attribute sequenceNumber to an integer in the range 1 through 63. Ensure that thisvalue is different from the values used by FH objects of surrounding cells that have beenassigned the same set of hopping frequencies.

The ID attribute can be set to any value in the range 0 through 7.

6. Set the eight CHN objects belonging to the second RT (and the third RT if using 3-3-3-configurations) to hopping. To do this, set the freqHoppRelationship (FH ID) attribute tomatch the ID of the FH object created in the previous step.

Note: If the frequency plan is later rearranged and additional capacity introduced to support 4-4-4 configurations, the OMC process will be the same, except that eight hopping frequenciesrather than six are added to the cell allocation, and a new FH object is created with those 8frequencies.



66 Issue 1.2 – June 1999

6.2. Scenario 2



• Existing combiner equipment is all filter type


• Network already stabilised

• Average of 2.48 transceivers per cell; actual configurations vary between one and fourtransceivers per cell

• Average reuse of 16.13. It is not possible to add another carrier in the area withconventional frequency planning

• Microcellular layer will be added for capacity increase

Objectives

• To free enough frequencies to be able to plan the BCCH of the microcell layer, without anyadditional investment

VIP plan choice

• Since the existing combiner equipment is filter type and the operator is not willing to investin swapping combiners, VIPone is the choice for the initial solution

• Since the number of transceivers per cell is relatively low, the BCCH transceiver will beincluded in hopping sequences


• It is envisaged that eight frequencies will be enough for the microcell layer. Hence the newmacrocell plan will use only 32 frequencies

• Since the maximum configuration is 4-4-4, the planning assumes that all configurations are4-4-4

• Average reuse: 32/4=8

• Frequency plan obtained by spreading around 8: 14/10/6/2



Issue 1.2 – June 1999 67

• Actual average reuse: 32/2.48=12.9

• Cells with only one transceiver will be left to non-hopping

• Cells with only two transceivers will be hopping, but using cyclic hopping sequences


Each cell will be assigned n frequencies f1, …, fn where n is the number of transceivers in thatcell (in this scenario a value between 1 and 4).

In this scenario, f1 belongs to the sub-band of 14 frequencies, f2 belongs to the sub-band of 10frequencies, f3 belongs to the sub-band of 6 frequencies and f4 to the sub-band of 2 frequencies.

If n>1, the following steps are required:

1. Add frequencies f1, …, fn to the cellAllocation attribute of the BTS object that relates to thecell.

2. Set the initial frequency of each of the four RTs to fI where i is the number of the RT.

If base station is of type RBS-900 or if n=2:

3. Set all CHN objects that have an ID attribute of 0 (or 2, 4, or 6 if CCCH channels arepresent in these time slots of the BCCH transceiver) to non-hopping. To do this, leave thefreqHoppRelationship (FH ID) attribute blank. These channels belong to air interface timeslot 0 (or 2, 4, or 6 if CCCH channels are present in these time slots of the BCCHtransceiver).

4. Create a FH object with attribute allocatedFrequencies set to f1, … fn.

The attribute sequenceNumber should be set to 0 if n=2.Otherwise set it to an integer in the range 1 through 63, ensuring that all the values areused evenly across the area.

Set the ID attribute to a value in the range 0 through 7.

5. The CHN objects not included in step 3 should be defined as hopping. To do this, set thefreqHoppRelationship (FH ID) attribute to the ID of the FH object created in the previousstep.

If base station is of type BTS-2000 (RBS-918) and n>2:





68 Issue 1.2 – June 1999

Set the ID attribute to a value in the range 0 through 7.

4. All CHN objects in the non-BCCH transceivers that have an ID attribute of 0 (or 2, 4, or 6 ifCCH channels are present in these time slots of the BCCH transceivers) to hopping. To dothis, set the freqHoppRelationship (FH ID) attribute to the ID of the FH object created in theprevious step.



Set the ID attribute to a value in the range 0 through 7, but different to the one of the FHobject created in step 3.

3 The CHN objects not included in step 3 should be defined as hopping. To do this, set thefreqHoppRelationship (FH ID) attribute to the ID of the FH object created in the previousstep.

If n=1, then the only RT object will be assigned an initial frequency belonging to the subband of14 frequencies.

In the case of base stations of type RBS-900, because all channels in time slot 0 (or 2, 4, 6) arenon-hopping, they can potentially suffer from unacceptable interference levels. This is becausethey are using frequencies with a very tight reuse, particularly those using frequencies from thesub-bands of 6 and 2 frequencies. Performance on these channels should be closely monitoredand if quality is unacceptable, they should be shut down. This will imply a small loss in themacro layer capacity. However, the increase in capacity provided by the frequencies that havebeen freed for the micro-layer will more than compensate for this.

6.3. Scenario 3




• Underlay microcell layer using BTS-2000/2C (CUBE) base stations. Continuous coverageand 1 transceiver per micro

• Frequencies divided in three subsets: 18 for the macro BCCH, 19 for the macro TCHs and9 for the macro BCCH



Issue 1.2 – June 1999 69

Objectives

• To add one transceiver to all cells in the micro layer

VIP plan choice

• As CUBEs support synthesiser hopping, VIPtwo is the most appropriate plan


• In order to minimise disruption to the existing frequency plan, the frequencies belonging tothe macro TCH sub-band will be reused

• A set of eight frequencies will be chosen for each microcell. Frequencies should be selectedfrom the TCH transceivers of the macrocells that cause the least interference to themicrocell

This information can be obtained by using a scanner and doing a drive-test of the area (if itis small enough), or with a propagation prediction tool. The GRAND tool allows theprobability of interference matrices to be calculated between the macrocells and themicrocells. For each microcell, the suggested frequencies will belong to the macrocells withthe lowest probability of interfering with that microcell


Each microcell will be assigned one BCCH frequency fBCCH and eight hopping frequencies, fFH1,…, fFH8.

To do this:

1. Add frequencies fFH1, …, fFH8 to the cellAllocation attribute of the BTS object that relates tothe micro-cell.

2. Set the initialFrequency attribute of the RT that will hold the BCCH channel to fBCCH.

3. Set the eight CHN objects belonging to this RT as non-hopping (leave thefreqHoppRelationship (FH ID) attribute empty).

4. Set the initial frequency of the second RT to any of the hopping frequencies.

5. Create a FH object and set attribute allocatedFrequencies equal to fFH1, …, fFH8.

Set attribute sequenceNumber to an integer in the range 1 through 63. Ensure so far aspossible that all values are used and that they are spread evenly across the wholemicrocell layer.

Set the ID attribute to any value in the range 0 through 7.



70 Issue 1.2 – June 1999

6. Set each of the eight CHN objects belonging to the second RT as hopping. To do this, setthe freqHoppRelationship (FH ID) attribute to match the ID of the FH object created in theprevious step.

6.4. Scenario 4



• Network already stabilised. With maximised cell splitting, regular layout, and reasonably lowantennas

• 20 new frequencies acquired in the 1800 band

• Collocated 900 and 1800 base stations

• Wide-band combiners available in the 1800 band equipment

Objectives

• To maximise capacity in the small 1800 band

VIP plan choice

• Since the existing combiner equipment is already wide-band, VIPtwo is the most appropriatesolution


• The regular network layout and low antenna heights mean that the network can support4/12 reuses on the BCCH. Hence the 1800 band spectrum allocation will be divided in twobands, one of 12 frequencies to plan the BCCH; and one of 8 frequencies for the extraTCHs

• A 1/1 reuse is most appropriate given the small number of frequencies available for theTCHs

• 2-2-2 configurations imply a fractional load of 12.5% (1/8). This is below the 15-20%threshold for 1/1 patterns

• Widespread 3-3-3 configurations are unlikely to be possible, even with DTX and dynamicpower control switched on (2/8 gives a fractional load of 25%).However, subsequent extra capacity can be added to selective locations without significantimpact on quality, thanks to the averaging properties of frequency hopping.



Issue 1.2 – June 1999 71

An alternative would be to add a third transceiver in all locations but activate only some ofthe channels (up to a maximum of 3 or 4)


Each cell in the 1800 band will be assigned one BCCH frequency fBCCH and eight hoppingfrequencies, fFH1, …, fFH8.

To do this:

1. Add frequencies fFH1, …, fFH8 to the cellAllocation attribute of the BTS object that relates tothe 1800 cell.


3. Set each of the eight CHN objects belonging to this RT as non-hopping (leave thefreqHoppRelationship (FH ID) attribute empty).

4. Set the initial frequency of the second RT to any hopping frequency.

5. Create a FH object with attribute allocatedFrequencies equal to fFH1, …, fFH8.

Set attribute sequenceNumber to an integer in the range 1 through 63. Ensure so far aspossible that all values are used and that they are spread evenly across the whole 1800layer.


6. Set each of the eight CHN objects belonging to the second RT as hopping. To do this, setthe freqHoppRelationship (FH ID) attribute to match the ID of the FH object created in theprevious step.

6.5. Scenario 5



• Combiner equipment is wide-band


• Mature network that is already stabilised

• Average of 3.3 transceiver per cell



72 Issue 1.2 – June 1999

• Antenna height is quite low, so average reuse has been set to 12

Objectives

• To increase capacity in the network to allow four transceivers per cell

VIP plan choice

• Since the existing combiner equipment is wide-band, VIPtwo is the most appropriate solution

• However, leaving 12 frequencies for the BCCH means that a 1/3 reuse would requirehopping over more than 9 frequencies, which is not possible with Lucent equipment.Accordingly, a mixed VIPone/VIPtwo solution is chosen


• 12 frequencies are set aside for the BCCH, since it has already been proven that thenetwork can support that reuse. This leaves 28 remaining frequencies

• The plan will assign 6 hopping frequencies, a high enough number to benefit fromfrequency hopping

• Average reuse: 28/6=4.6

• Frequency plan obtained by spreading around 4.6: 9/6/6/3/3/1

• Actual reuse: 28/4=7


Each cell will be assigned one BCCH frequency fBCCH and 6 hopping frequencies, fFH1, …, fFH6.

To do this:

1. Add frequencies fFH1, …, fFH6 to the cellAllocation attribute of the BTS object that relates tothe cell.


3. Set the eight CHN objects belonging to this RT as non-hopping (leave thefreqHoppRelationship (FH ID) attribute empty).

4. Set the initial frequency of the second, third, and fourth RT to a hopping frequencybelonging to one of the looser reuses.

5. Create a FH object and set the allocatedFrequencies attribute equal to fFH1, …, fFH6.



Issue 1.2 – June 1999 73

Set attribute sequenceNumber to an integer value between 1 and 63. Ensure so far aspossible that all values are used and that they are spread evenly.


6. Set each of the eight CHN objects belonging to the second, third, and fourth RT as hopping,To do this, set the freqHoppRelationship (FH ID) attribute to match the ID of the FH objectcreated in the previous step.



74 Issue 1.2 – June 1999



List of Acronyms

7


Issue 1.2 – June 1999 75

7. List of Acronyms

The following acronyms are used in this document:

BCCH Broadcast Control Channel

BCE BSS Controller Equipment

BCF Base Station Controller Frame

BER Bit Error Rate

BSIC Base Transceiver Station Identity Code

BSS Base Station System

BTS Base Transceiver Station

CCB Channel Codec Board

CCCH Common Control Channel

CHN Channel



76 Issue 1.2 – June 1999

C/I Carrier to Interference (ratio)

C/No Carrier to Noise (ratio)

DRCC Double Radio Codec and Control

DTX Discontinuous Transmission

FCCH Frequency Correction Channel

FCO Flash Controller Board

FER Frame Error Rate

FH Frequency Hopping

FHS Frequency Hopping System

GOS Grade of Service

GSM Global System for Mobile Communications

HSN Hopping Sequence Number

IMW Integrated Maintenance Workstation

MAIO Mobile Allocation Index Offset

MSC Mobile Switching Centre

OMC Lucent Technologies Operations and Maintenance Centre 2000

OMC-PMS Lucent Technologies Operations and Maintenance Centre 2000 PerformanceManagement Subsystem

PAGCH Paging and Access Grant Channel

PWRC Power Control Indicator

RBT Radio Base Station Tester

RFU Radio Frequency Unit

RT Radio Terminal

RXLEV Received Signal Level

RXQUAL Received Signal Quality

SACCH Slow Access Control Channel



Issue 1.2 – June 1999 77

SCH Synchronisation Channel

SDCCH Standalone Dedicated Control Channel

SID Silence Description Frame

SQI Speech Quality Indicator

SRFU Standard Radio Frequency Unit

TCH Traffic Channel

TDMA Time Division Multiple Access

TRX Transceiver

TU Typical Urban

VIP Variable Interference Planning



78 Issue 1.2 – June 1999


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frequency hopping

Documents

frequency hopping overview

cyclic hopping

random hopping

frequency diversity

use frequency

vip deployment

network planning

gsm network implementation