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Capacity Enhancement Core Techniquesfor GSM 900 and 1800

Author : RF Systems & Capacity Group Issue: 1.0

Date: 9th April 98 Page: 1 of 40

Capacity Enhancement

Core Techniques

for GSM 900 and 1800

Technical Memorandum


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Document History

Version Date Author(s) Change Description

0.0 27th Feb 98 S. Martin-Leon First Draft

0.1 3rd Mar 98 S. Martin-Leon Revision

0.2 5th Mar 98 S. Martin-Leon Various adition

0.3 11th Mar 98 S. Martin-Leon Revision

0.4 6th Apr. 98 S. Martin-Leon Revision

1.0 9th Apr. 98 S. Martin-Leon Revision




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CONTENTS

1. INTRODUCTION...........................................................................................5

2. DISCONTINUOUS TRANSMISSION............................................................6

2.1 Brief description.....................................................................................................6

2.2 GSM application.....................................................................................................62.2.1 Implementation...................................................................................................62.2.2 Common channels..............................................................................................72.2.3 Measurements....................................................................................................7

2.3 Advantages. Effects on planning...........................................................................8

2.4 Lucents solution....................................................................................................82.4.1 Feature activation and parameters......................................................................8

2.4.1.1 Uplink DTX.................................................................................................82.4.1.2 Downlink DTX............................................................................................8

2.4.2 Measurements....................................................................................................9

3. DYNAMIC POWER CONTROL...................................................................10

3.1 Brief description...................................................................................................10

3.2 GSM application...................................................................................................103.2.1 Implementation.................................................................................................103.2.2 Common channels............................................................................................113.2.3 Measurements..................................................................................................11

3.2.3.1 Power control and frequency hopping.......................................................113.2.3.2 Quality measurements................................................................................12

3.3 Advantages. Effects on planning.........................................................................13

3.4 Lucents solution..................................................................................................143.4.1 Functional split.................................................................................................143.4.2 Process.............................................................................................................153.4.3 Feature activation and parameters....................................................................163.4.4 Algorithm optimisation.....................................................................................17

4. SLOW FREQUENCY HOPPING.................................................................18

4.1 Brief description...................................................................................................184.1.1 Cyclic vs. random hopping...............................................................................184.1.2 Baseband vs. synthesiser hopping ...................................................................18




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4.2 GSM application...................................................................................................194.2.1 Implementation.................................................................................................194.2.2 Sequence generation.........................................................................................19

4.2.3 Common channels............................................................................................194.2.4 Measurements..................................................................................................20

4.2.4.1 Power control and frequency hopping.......................................................204.2.4.2 Quality measurements................................................................................20

4.2.5 Frequency redefinition procedure.....................................................................204.2.6 Mobile stations.................................................................................................20

4.3 Advantages. Effects on planning.........................................................................204.3.1 Frequency diversity..........................................................................................20

4.3.1.1 Description.................................................................................................204.3.1.2 Number of frequencies...............................................................................23

4.3.1.3 Frequency spacing......................................................................................244.3.1.4 Antenna diversity.......................................................................................244.3.1.5 Effects on planning....................................................................................25

4.3.2 Interference diversity........................................................................................254.3.2.1 Description.................................................................................................254.3.2.2 Random vs. cyclic hopping .......................................................................264.3.2.3 4/12 and 3/9 reuse patterns........................................................................264.3.2.4 1/3 reuse pattern and fractional loading.....................................................274.3.2.5 Multiple reuse patterns...............................................................................284.3.2.6 Multiple reuse patterns and fractional loading...........................................294.3.2.7 Fractional reuse patterns............................................................................30

4.3.2.8 Concentric cells..........................................................................................304.3.2.9 Control and traffic channels.......................................................................304.3.2.10 Effects on planning..................................................................................31

4.4 Lucents solutions.................................................................................................334.4.1 Base station equipment.....................................................................................334.4.2 Antenna coupling equipment............................................................................334.4.3 Fill-sender and phantom RTs..........................................................................344.4.4 Possible hopping configurations.......................................................................34

4.4.4.1 Limitations.................................................................................................344.4.4.2 Recommended configurations....................................................................35

4.4.4.3 Mixed operation of hopping and non-hopping modes...............................364.4.5 Feature activation and parameters....................................................................364.4.6 Fault defence mechanisms................................................................................37

5. CONCLUSION.............................................................................................38




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1. INTRODUCTION

In general there are two strategies to increase the capacity of a GSM network using itsexisting spectrum allocation:

1. Reduce the cell size.2. Increase the capacity per cell.

These two strategies do not exclude each other. The second type of strategies might be usedto free up some frequencies that might then be used for a microcell layer in a hierarchical networkstructure.

Cell size reduction on its own can be very effective. It produces the greatest capacityincrease: up to more than 3 times the capacity of a network coverage limited. It also improves thecoverage of the network, in terms of, for example in building penetration, and cold spot elimination.

However, it implies the introduction of new cell sites which has major drawbacks in terms of siteacquisition and management costs.

The second type of strategy provides a means to increase the capacity of the existing networkin a more cost effective way.

The aim of this document is to provide some insight into three of the techniques used.

These, so called core techniques, are options supported by the GSM specifications andtherefore, mandatory for GSM mobiles:

discontinuous transmission (DTX),

dynamic power control (PC),

frequency hopping (FH or SFH).

The first two, discontinuous transmission and dynamic power control, when used on theirown, do not provide a significant capacity increase.They were devised with a different aim: to extend the mobile battery life by minimising the batterycurrent requirements.However, when used in conjunction with frequency hopping, they provide a powerful means toincrease the capacity of the network.

This document analyses the advantages in terms of quality and capacity increase derived from thecore techniques, their implementation using Lucents equipment is discussed, concentrating onLM4.0, and, where appropriate, guidelines for their use are given.

Separate sections are included to describe each of the techniques.The benefits of their joint use are studied in the section devoted to frequency hopping.




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2. DISCONTINUOUS TRANSMISSION

2.1 Brief description

It is of common knowledge that telephone traffic is subject to alternating silence and activityperiods. Typical activity factors of telephone conversations (i.e. the fraction of time a given user isactually talking) are around 0.4. It is reasonable to expect even much lower activity factors for certaindata transmissions.

Discontinuous transmission is a technique that takes advantage of this fact by inhibiting thetransmission of the radio signal when there is no information to send (voice or data).

2.2 GSM application

2.2.1 Implementation

Discontinuous transmission, also referred to as DTX, is only relevant to some of GSMtransmission modes, in particular speech and non-transparent data, simply because in the other cases(transparent data) it is difficult to assess when user data transmission can be suspended withoutdegrading the service.

With discontinuous transmission enabled, the goal is to encode speech at 13 kbit/s when theuser is talking, and at a bit rate around 500 bit/s when not talking.500 bit/s is sufficient to encode background noise so that the listener does not think that theconnection is broken (this is the notion of comfort noise).The low rate of encoding results in a decrease in effective radio transmissions and therefore co-channel interference reduction.

In order to implement such a mechanism, the source must be able to indicate whentransmission is required. In the case of speech, the vocoder must detect whether or not there is somevocal activity. This function is called Voice Activity Detection, or VAD.At the reception side, the listeners ear must not be disturbed by the sudden silence and the decodermust therefore be able to generate some comfort noise when no speech signal is received.

The discontinuous transmission mode affects the transmit operation of the mobile station andthe Transcoder and Rate Adaptor Unit (TRAU or STF for Lucents equipment).The BTS is obviously concerned, but derives its behaviour dynamically from data coming from themobile station (uplink) and from the TRAU (downlink). The distinction between comfort noise framesand speech frames can be done on the basis of the frame contents. Then the BTS decides whether totransmit the frame on the radio interface so that the minimum bit rate is met.

DTX is an optional feature and must therefore be managed. Moreover, discontinuoustransmission may be applied independently to each direction, so that it must take into account twocomponents: the uplink mode and the downlink mode.

The choice of the strategy for applying discontinuous transmission is one of the manyconfiguration parameters which operators may use to optimise their network.Several considerations must be taken into account in this strategy. For instance, GSM mobile tomobile calls suffer a loss in quality when discontinuous transmission is applied to both radio segments(double clipping). The operator may therefore choose not to apply the downlink discontinuoustransmission mode, if MS-to-MS call numbers are significant.




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As far as the downlink discontinuous transmission mode is concerned, it must be establishedon a connection by connection basis by the Mobile Switching Centre (MSC). The MSC tells the BaseStation Controller (BSC), which configures the Base Transceiver Station (BTS), itself configuring the

TRAU.Means to change the downlink discontinuous transmission mode on an established connection alsoexist, when the transmission mode changes.

The mobile station can be ordered to use the discontinuous transmission mode in the uplinkdirection as a cell option. The cell options are regularly broadcast on the BCCH for mobile stations inidle mode, and they are also part of the general information sent to mobile stations on their SACCHwhen they are in dedicated mode. The information sent on the SACCH is specified by the BSC on atransmitter/receiver (TRX) basis. It can have three values: DTX must be applied, must not be appliedor may be applied. The decision as to whether the uplink DTX feature is to be activated for aparticular call, is determined by the mobile station on advice from that information.

2.2.2 Common channels

One peculiarity related to common channels is that the frequency they use for the downlinkmust be emitted continuously and at full power, even if no information needs to be conveyed on somebursts. This is needed because mobile stations in neighbouring cells continuously performmeasurements on this frequency.In idle mode it is used to determine the best cell they should listen to.In dedicated mode it is done to report measurements for hand-over preparation, even when thedifferent base stations are not synchronised.When there is no information transfer request, a specific pattern is emitted (the fill frames).

2.2.3 Measurements

When discontinuous transmission is applied, some slots belonging to a channel may not be

used for transmission. This is indeed the goal of it, but then measurements on these slots willobviously report a low reception level, and a bad quality.To avoid this problem, the GSM Specifications impose that at least 12 bursts are sent within eachreporting period (SACCH superframe). These bursts amount to the systematic use of the SACCH (4bursts constituting a coding block) and 8 bursts on the TCH itself. For speech, these bursts containsilence description frames (SID frame).

In addition to this minimum transmission rule, the Specifications require the BTS and themobile station to report two sets of measurements concerning the connection:

full measurements, done on all slots which may be used for transmission in the

reporting period,

sub measurements, done only on the mandatory sent bursts and blocks.

Finally, both the BTS and mobile station report for each measurement period whetherdiscontinuous transmission was used or not, in other terms, whether all bursts were transmitted, thusenabling the processes using the measurements (power control and handover) to discard the fullmeasurements when discontinuous transmission has been used.

Due to the reduced number of input values for the averaging process, the results based on thesub measurements are less accurate (reception level is averaged on 12 bursts instead of over 100bursts). That specially affects the quality measurements, which are based on estimated errorprobabilities before channel decoding, and therefore more sensitive and statistically unreliable in thecase of subset measuring, than the received level.




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Such unreliability can result in an increase in the number of dropped calls, as it has beenreported in field measurements. Therefore, for those processes in need of quality, some kind ofsolution for this problem has to be provided when discontinuous transmission is used.

2.3 Advantages. Effects on planning.

The use of DTX has two main advantages.

It decreases power consumption in mobiles thereby enhancing handset battery life.

It reduces the overall interference in the system.

This implies a quality increase. However, this quality increase cannot be translated into acapacity increase since system planning has to be done for worst case situations.Interference with DTX has an on/off nature with the maximum interference being thesame as without discontinuous transmission, and the rate of switching (rate of activityand inactivity periods) is not high enough for coding and interleaving to be effective in

averaging the variation.

In terms of quality increase, DTX has an advantage when compared with dynamic powercontrol: the interference reduction is random whereas in the case of power control it depends on thegeographical location of the mobiles.

2.4 Lucents solution

Lucents equipment offer the possibility to use discontinuous transmission both in the uplinkand downlink for speech and non-transparent data communication.

2.4.1 Feature activation and parameters

2.4.1.1 Uplink DTX

Uplink discontinuous transmission in a certain cell (BTS) can be deployed by setting, in theBase Transceiver Station window of the OMC GUI, the parameter Uplink in the DTX subsection(AUI parameterDTXof the managed object class BTS) accordingly.

It can have three values:

May be used (0).

Shall be used (1).

Shall not be used (2).

This parameter is then used to set the value of the DTX field in the cell options informationelement broadcast in the BCCH and in the associated SACCH of a communication.

2.4.1.2 Downlink DTX

Downlink DTX can be set separately for speech (system release 6.5 feature) and for non-transparent data (system release 6.7.2 feature). To do this the corresponding parameters have to be setin the BSS and the InterWorking Function (IWF) subsystems.

Speech

This feature may be either enabled or disabled on a per BTS basis via the OMC by setting, inthe Base Transceiver Station window of the OMC GUI, the parameterDownlink Speech in the DTX




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subsection (in the AUI DownlinkDtx 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 of theRecent Change and Verify program (RC/V). This applies to all the BSS supported by one MSC. Itsdefault mode is disabled.

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

Data

This feature may be either enabled or disabled on a per BTS basis via the OMC by setting, inthe Base Transceiver Station window of the OMC GUI, the parameterDownlink Data in the DTXsubsection (in the AUI DownlinkDtx attribute of the managed object class BTS contains the boolean

downlinkDtxData). The default setting is disabled (false).

If this attribute is set, the BTS acts according to the DTX commands issued by the IWF inthe received RLP frames.

To set DTX in the IWF, the IWF includes an option: DTX Mode, which can be set bychanging the value in the IWF-2 menu. Its default mode is disabled.

2.4.2 Measurements

In order to overcome the inaccuracy of measurements with discontinuous transmission thefollowing process is envisaged.

In the BSC a linear unweighed sliding window averaging is performed for all radio linkmeasurements.In the case of RXQUAL measurements, the sliding window depth is A_QUAL_RR (where RR can bePC for power control and HO for handover).A measurement where no discontinuous transmission has been used is shifted into the averagingwindow W_QUAL_RR times, whereas the number of times is just one for measurements withdiscontinuous transmission.If the discontinuous transmission flag has not been received, The sub measurements are also chosenand shifted into the averaging window once.In this way, more accurate measurements are given a higher weight in the resulting averaged value,which is then used in the power control and handover processes.This weighing process only applies to RXQUAL measurements. RXLEV measurements are shiftedinto the averaging window once no matter whether they are full or sub measurements.

Both A_QUAL_RR and W_QUAL_RR are parameters that can be set via the OMC on a BSCbasis.




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3. DYNAMIC POWER CONTROL


Dynamic power control refers to the possibility to modify the transmission power over theair, dynamically during a connection, independently both for the mobile station and the base station.

Control of the mobile station power has been used in most of the existing cellular systems, inorder to save battery power in the terminals (power consumption).

However, if the received signal level is too high, by reducing the transmission power whilekeeping a satisfactory quality level on the communication, the interference caused on other calls insurrounding areas is decreased.Since co-channel interference is one of the main capacity limiting factors in a cellular system, powercontrol can also be used to improve spectral efficiency.

Basically, in the literature, two different types of algorithms for power control have beenproposed for any kind of cellular system.One is based on the principle that the power should be reduced as the path loss is decreased. Thesimplest, and most used, of this type of algorithm keeps the received signal strength constant. It hasbeen shown that such an approach gives very little gain in capacity.A variation of this scheme is based on received signal strength, but the change in path loss is onlypartly compensated for. For this latter scheme it has been shown that there is some gain in capacity.

In the second type of algorithm the focus is instead on quality.Since most of the connections experience an excess in quality (i.e. C/I ratio), it seems natural that thepower should be controlled according to the quality of the call.In this way, the unnecessarily high C/I margin found in most calls can be converted into capacity.

Analysis has shown that a large capacity gain is obtained by using a power control scheme giving thesame quality to all users (roughly 5-7 dB interference level reduction in threshold value).The problem with such an algorithm is that it needs centralised control. Thus it is, in its presentversion, of little practical use in a real cellular environment.Different distributed algorithms have been devised, most of them showing better capacity results thanthe ones based on signal level. Mixed algorithms, where both the signal level and the quality are takeninto account, are also possible.

3.2 GSM application


In GSM, both uplink and downlink power control may be applied independently from eachother; furthermore they are applied independently for each mobile station.

The range specified for uplink power control lies between 20 and 30 dB, in 2 dB steps,depending on the mobile station power class. The range used for downlink power control ismanufacturer dependent and may be up to 30 dB, also in 2 dB steps.

The control of the transmission power is a network option, i.e. the operator may choose toapply it or not, in one direction, or in both. All mobile stations though, must support the feature.

Power control on both directions is managed by the BSS. The transmission power of themobile station is chosen by the BSS, and the commands to regulate it are issued to the mobile station.




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The BSS computes the required MS transmission power through reception level measurementsperformed by the BTS, taking into account the MS maximum transmission power as well as qualitymeasurements done by the BTS.

This last parameter helps to ensure that transmission quality is kept above some acceptance threshold.For the downlink direction, the BTS transmission power is also computed by the BSS for eachconnection, based on the measurements performed by the mobile station and reported regularly to theBTS.

Inside the BSS, the split of tasks between BTS and BSC is basically an option for themanufacturer. The specification of the Abis interface is basically adapted to the implementation ofpower control on the BSC, but implementation in the BTS is also possible.

At the start of a connection, the initial value of the transmission power (both for mobilestation and BTS) is chosen by the BSC.In the case of an initial assignment, the information available to choose this power is at best verysmall. Therefore, in GSM, the initial power level to be used by a mobile station for the first messagessent on the new dedicated channel is fixed on a cell-per-cell basis, and is the same level as used for

sending random access bursts.The value of this level is broadcast on the BCCH, to be known by all mobile stations before any accessattempt. A mobile station whose power level is below the broadcast value shall simply use itsmaximum power level instead.

Except at the start of a channel connection, a command to change the transmission powerdoes not trigger an immediate transition to the ordered value in the mobile station. The maximumvariation speed is of 2 dB each 60 ms. That means that a high jump in the power control commandswill be answered gradually.


See section 2.2.2.

3.2.3 Measurements

3.2.3.1 Power control and frequency hopping

The combination of power control with frequency hopping using the BCCH carrier raises aproblem about measurement accuracy in traffic channels using such combination.Power control can be applied on any frequency except for the BCCH frequency, which must betransmitted with a constant power in the downlink.The result for the channels under consideration is that power control applies only to a subset of thebursts, whereas other bursts (those using the BCCH frequency) are sent with a fixed transmissionpower.

This could lead to inaccurate reception level measurements.

In order to alleviate this problem, the mobile station is requested in such cases not to takeinto account the slots falling on the BCCH frequency in the reception level estimation.This is controlled by an indicator, the PWRC indicator, sent on a connection basis to the mobilestation when the following conditions are met:

the channel hops on at least two different frequencies,

one of those frequencies is the BCCH frequency and

downlink transmission power control is in use.

This problem does not apply to quality measurements which are performed as usual.




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3.2.3.2 Quality measurements

In the GSM system the only way to get information about the current interference level, to be

used in the power control algorithm, is the received quality (i.e. RXQUAL) which is related to the C/Iratio.However, according to the NTC, the eight available quality levels (RXQUAL=0..7) represent a C/Iinterval which in the worse case can be as small as 9 dB1(see Figure 1). Outside this interval the levelof interference cant be determined.

Because of the small C/I window which can be measured by RXQUAL, the direct C/I controlwould cause harmful quality problems in combination with shadow fading.For example when the power control has reached a target of RXQUAL=2, an attenuation increasecaused by shadowing would break down the quality immediately (e.g. to RXQUAL=6).The power control wont be able to react fast enough, because it has not been able to detect thelowering of the quality until it is too near the level where it deteriorates to unacceptable values.Therefore pure C/I control is impossible in the current GSM system.

Another point to be taken into account is the different mappings of C/I to RXQUAL on TCHchannels depending on the propagation scenario. Because RXQUAL represents the estimated errorprobabilities before channel decoding, it does not consider the varying efficiency of coding,interleaving and bit error correction under different environmental conditions.The following figure shows the result of simulations using the TRASIM simulation tool to model theGSM receiver.

1 This case happened at receiver sensitivity level for a static channel model.



0 5 10 15 20.

0.2%

0.4%

0.8%

1.6%

3.2%

6.4%

12.8%

25.6%

BER ->

CIR [dB] ->

nowi

TU50 no FHTU50 with FHRA250 no FH

0

1

2

3

4

7

RX

Figure 2 C/I to RXQUAL mapping for differentscenarios

RXQUAL

0

7

CIR9 - 18 dB

Figure 1 Qualitative relationship between RXQUAL and CIR


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TU3 and TU50 refer to a typical urvan environment and mobile stations with a speed of 3 and 50km/h respectively. RA250 refers to a rural area environment and a mobile at 250 km/h.

3.3 Advantages. Effects on planning

The following figure shows the C/I ratio perceived by a mobile station as a function of thedistance to the base station normalised to the distance between interfering base stations. It can be seenthat there is a lot of power wasted when the mobile is near the base station to ensure that, when it isnear the cell border, the C/I ratio and hence the quality of the communications, remains good enough.

In a system where all mobiles transmit at maximum output power, the total interference isgreater than if some mobiles regulate their power.

At the cell border, the mobiles transmit at maximum power in both regulating and nonregulating systems. Hence in the non regulating system, the base station receives lower C/I values forthe mobile near the cell border than in the regulating system.The mobiles near the cell border also produce the lowest C/I value in both systems.

When regulating, the mobiles near the base station will transit at lower power and hencesignal strength will be lower than in the non regulating system. In some cases that will result in lowerC/I ratios (when the interferer is at the cell border).However, these mobiles already have good conditions, i.e. they are in the upper region of the C/Icurve, and do not suffer from degraded performance.

These results can be better understood with the following figure. In it the C/I ratio is plottedagainst the distance between the desired mobile and its base station normalised to the distance

between interfering base stations, without and with downlink power control. In the latter case the C/Iratio depends on the position of the interfering mobile station. Two different cases and the averagevalue are shown.



-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 3C/I ratio as a function of the normalised distance


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Therefore the use of power control provides an increase in the global quality of the system(less calls in interference conditions) which can be translated into a capacity increase by planning thenetwork with lower average C/I values2.

The gain is not enough to be able to jump from a 4/12 reuse factor to a 3/9 reuse factor, butits effect might be noticeable when using automatic planning tools that take power control intoconsideration.

3.4 Lucents solution

Lucents equipment implements level and quality based dynamic power control.The multi-step threshold comparison technique proposed as an option in GSM Rec. 05.08, AppendixA, Section 3.2, is not supported. Instead, a similarly powerful technique is provided which uses

sliding window measurement averaging in conjunction with a single-step threshold comparisonprocess.

3.4.1 Functional split

The functional split for the overall power control process in Lucents BSS subsystem is asfollows:

BTS:

All measurement results necessary for Power Control, i.e. both the downlink measurement results

reported by the MS, and the uplink measurement results reported by the RT, are acquired by thecorresponding BTS, and transmitted to the BSC without any preprocessing.

BSC:

Measurement averaging (pre-processing) as well as the entire uplink and downlink power controlprocessing is performed in the BSC. This is done by independent processes for all TCH andSDCCH channels. In particular, for each dedicated channel, uplink and downlink processesoperate independently of each other.

The following description of the process and parameters relates to LM4.0. Differences with LM5.0are also highlighted.

2 Something very similar happens with interference diversity. See 4.3.2.1.



-10

0

10

20

30

40

50

60

70

0 0.2 0.4 0.6

d/D

C/I

w ithout PC

PC, average

PC, interf. at cell

border

PC, interf. near

base-station

Figure 4 C/I ratio as a function of the normalised distance


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3.4.2 Process

The power control algorithm implemented can be split into the following basic steps:

1. PC measurement averaging: both RXLEV and RXQUAL measurements areaveraged using average window sizes and weighting factors: AV_LEV_PC, A_QUAL_PC,W_QUAL_PC.

2. PC threshold comparisons: the averaged level and quality measurements arecompared with upper and lower limits and a decision is made on whether an increase ordecrease in transmit power is required (Figure 5).

1. PC execution : in the case the actual transmit power is not at its minimum ormaximum value, a fixed step power increase or decrease is ordered.

2. PC disabling : the total time between a new power command and the effect in themeasurements is 2-3 SACCH multiframes in the downlink and 3-4 SACCH multiframes inthe uplink.Because of the delay caused by the dead time and the process of averaging measurements, theeffect of the command takes some time to be noticed by the BSC, and a power changecommand could be sent after a previous power change command, even when the power is setto the correct value.This decreases the stability of the power control loop.To avoid incorrect power commands, power command disabling is used. The power controlalgorithm waits for a power command acknowledge, and after this, an extra interval of timeto ensure the power control command has started influencing measurements.If the first timer expires before the acknowledgement has been received, the process willconsider the current transmit power the one that should be set at the moment. It will wait forthe extra interval and then the threshold comparison process will be resumed.

This process takes into account the different transmitter characteristics of the systems GSM900 and GSM 1800.

The dynamic range of Lucents BTSs is 30 dB.



1

2

L_RXQUAL_XL_PC

U_RXQUAL_XL_PC

L_RXLEV_XL_PC

U_RXLEV_XL_PC

7 0

-110 dBm (0)

-48 dBm (63) 1 - Range of

operation without

Power Control

2 - Targetrange

ofoperation with

Power Control

RXLEV

RXQUAL

Figure 5 Operation of Power Control


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Handovers only take place when the mobile is transmitting at its maximum power, except inthe case of hierarchical structures. In this case, the power control process varies in that first it has tobe decided whether a handover of a power control step is the right alternative.

In addition to the regular PC process, a procedure is implemented which immediately directsMS and BTS to use their maximum power, if:

there is a Radio Link Failure warning (Such a warning is produced by the BTS as soon as

it detects that a radio communication on the uplink is about to break-down);

the radio link control process is started while power control is disabled;

the allocated channel is on the BCCH carrier of a non-hopping system;

a mandatory handover in the lower cell layer of a hierarchical cell structure can be

prevented.

In LM5.0, the threshold comparison and execution processes are modified to include avariable step size for emergency power control, which is used to influence the recovery time of the PCloop.

In an emergency, i.e. when the receive level measurement (not the averaged one) is well outside thenormal range of operation, a step size is calculated so that receive values are directed in one steptowards that range.

3.4.3 Feature activation and parameters

Power control for communications through a certain BTS can be enabled in the downlinkand uplink independently by setting parameters EN_MS_PC (uplink) and EN_BS_PC (downlink) ofthe POWER object associated with the BTS accordingly.

Before doing that the following parameters should have been set to their proper values:

Maximum transmit power values:

MS_TXPRWR_MAX: maximum TX power a MS is permitted to use on a dedicated

control channel or a traffic channel within the serving cell.

Averaging measurement parameters:

A_LEV_PC: averaging window size for receive power level measurements.

A_QUAL_PC: averaging window size for quality measurements.

W_QUAL_PC: weighting of full-set quality measurements with respect to sub-set quality.

Threshold levels:

L_RXLEV_UL_P, U_RXLEV_UL_P, L_RXQUAL_UL_P, U_RXQUAL_UL_P: uplink

lower (L) and upper (U) RX_LEV and RX_QUAL threshold.

L_RXLEV_DL_P, U_RXLEV_DL_P, L_RXQUAL_DL_P, U_RXQUAL_DL_P: the

same for the downlink.

Power step sizes:

POW_INCR_STEP_SIZE, POW_RED_STEP_SIZE: step sizes used when increasing or

decreasing the MS and BTS transmit power.

Timer values:

P_CON_ACK: power control acknowledge time.

P_CON_INTERVAL: minimum interval between successive modification of the radio

frequency power level.

The following table shows recommended values.




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3.4.4 Algorithm optimisation

Presently, work is being undertaken within Lucent in order to specify an optimised powercontrol algorithm that improves the performance of the current one.Its stability for certain range of the parameters and its speed to adapt to a fast changing environmentwill be enhanced.Also new methods are being studied to better estimate the C/I value with the range of RXQUALavailable in GSM.



O&M parameter Range Default value

A_LEV_PC 1-31 (SACCH multiframes) 6

A_QUAL_PC 1-31 6

W_QUAL_PC 1-3 1 (if no DTX is used)L_RXLEV_DL_P 0-63 (0-110 dB;63-48 dB) 25

L_RXLEV_UL_P 0-63 25

L_RXQUAL_XL_P 0-7 3

U_RXLEV_DL_P 0-63 35

U_RXLEV_UL_P 0-63 35

U_RXQUAL_XL_P 0-7 1

POW_INCR_STEP_SIZE 0-2 (2-6 dB) 2 (6 dB)

POW_RED_STEP_SIZE 0-1 (2-4 dB) 1 (4 dB)

P_CON_ACK 0-31 (2 SACCH multiframes) 4

P_CON_INTERVAL 0-31 4


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4. SLOW FREQUENCY HOPPING


Frequency hopping consists of changing the frequency used for transmission at regular intervals.In GSM a frequency hop takes place every new burst (every 4.615 ms), resulting in 217 hops persecond. This is known as slow frequency hopping (SFH) as opposed to fast frequency hopping, wherethere are several hops per symbol transmitted.

SFH has been introduced in GSM mainly in order to deal with two specific problems which affectthe transmission quality:

Multipath fading: The radio signals are subject to multipath fading, which is space and frequency

selective. A slowly moving mobile may stay in a fading dip long enough to suffer severeinformation loss.

Frequency hopping combats multipath fading by exploiting its frequency selectivity: changingfrequencies also means changing fading patterns.

Introducing frequency diversity will, together with the interleaving and coding, improve thetransmission quality of the link, for slow moving users in particular.

Interference: Without frequency hopping, strong signals from neighbour cells transmitting on or

close to a carrier frequency will affect the carrier signal continuously, which may have a negativeeffect on the transmission performance.

Frequency hopping can cause different signals to interfere with the carrier at different times, aproperty called interference diversity.

At a system level, the result is a smearing of the interference levels between users, an effect oftencalled interference averaging.

Furthermore, interference diversity gives a second effect: since consecutive bursts of information are

received under different interference conditions, the risk of a sequential information loss isreduced. This, together with the interleaving and coding, will improve the transmission linkquality.

4.1.1 Cyclic vs. random hopping

The hopping can be either cyclic or random.

Cyclic hopping means that all mobiles use frequencies consecutively from the set allocated tothe cell, e.g f1, f2, f3, f4, f1, f2, .

With random hopping, mobiles use uncorrelated pseudo-random hopping sequences.In the case of a regular assignment pattern, the probability of two mobiles in interfering cells usingthe same frequency in the same time slot is 1/N, where N is the number of frequencies assigned toeach cell, e.g. MS1: f1, f4, f4, f2, f1, f3, ; MS2: f2, f1, f4, f3, f2, f1, .

4.1.2 Baseband vs. synthesiser hopping

There are two methods to produce frequency hopping by the base station: baseband hoppingand synthesiser hopping.

Baseband hopping is realised by switching the output from each of the baseband processingsections between the inputs of the RF portions of each RT.In this way, the RTs do not need to retune, but each channel effectively hops over the availablefrequencies.




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The primary limitation of this type of frequency hopping is that the maximum number of hoppingfrequencies equals the number of RTs in that particular cell.

Synthesiser hopping implements frequency hopping by retuning the RF portion (transmit andreceive) of each TRX (RT) in a defined hopping pattern.Therefore, the output of each baseband processing section is always connected to the same RF portion(unlike in baseband hopping). This allows each RT to hop over as many frequencies as desired,independently of the number of RTs in the cell.However wideband hybrid combiners must be used with this type of hopping, and not filter combiners,which take some time to change their frequencies.Hybrid combiners have much higher insertion losses than filter combiners and, therefore, the numberof radios is limited in a cell.

4.2 GSM application


In GSM with frequency hopping every mobile transmits its time slots according to asequence of frequencies that it derives from an algorithm. The frequency hopping occurs between timeslots and, therefore, a mobile station transmits (or receives) on a fixed frequency during one time slotand then must change frequency before the time slot on the next TDMA frame.

4.2.2 Sequence generation

For a set of N frequencies, up to 64xN different hopping sequences can be built. They aredescribed by two parameters:

the MAIO (Mobile Allocation Index Offset) which may take as many values 1 ..N,

the HSN (Hopping Sequence Number) which may take 64 different values.

Two channels bearing the same HSN but different MAIO never use the same frequency onthe same burst.Two channels using the same frequency list and the same time slot, but bearing different HSN,interfere 1/Nth of the bursts, as if the sequences were chosen randomly.The sequences are indeed pseudo-random, except for the special case of HSN=0, where thefrequencies are used one after the other in order (cyclic hopping).

Channels in one cell using the same set of frequencies bear the same HSN and differentMAIO to avoid interference between channels inside a cell.. If random hopping is used, different HSNshould be used in distant cells using the same frequency set, in order to gain from interferencediversity.


Common channels (FCCH, SCH, BCCH, PAGCH and RACH) must use a fixed frequency.This constraint is meant to ease initial synchronisation acquisition.Similarly, extension sets of common channels are also forbidden from hopping and use the samefrequency as the primary group, so that there is no need to transmit the description of their frequencyorganisation on the BCCH or time slot 0.




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4.2.4 Measurements

4.2.4.1 Power control and frequency hopping

See section 3.2.3.

4.2.4.2 Quality measurements

As it was already said in section , the mapping of RXQUAL to C/I varies depending on thepropagation environment.In the case of frequency hopping, when its interference diversity effect is used (see 4.3.2.2), themapping also varies call to call, depending on the changing interference conditions.That means that RXQUAL is not a reliable measurement of the quality of the connection when usedin such conditions.

4.2.5 Frequency redefinition procedure

In dedicated mode and group transmit mode, this procedure is used by the network to changethe frequencies and hopping sequences of the allocated channels, when the network frequency plan ischanged, so that as few calls as possible are disturbed.

The network sends to the MS a FREQUENCY_REDEFINITION message containing thenew parameters together with a starting time indication. When receiving such a message, the MSmodifies the frequencies/hopping sequences it uses at the exact indicated time-slot, i.e. the indicatedtime slot is the first with new parameters.All other functions are not disturbed by this change. New parameters can be the cell channeldescription, the mobile allocation and the MAIO. Other parameters describing the allocated channelare identical to the current parameters.

Despite this procedure, some calls may be lost. This can happen when the MSC asks for achannel for handover, and the request is acknowledged with the actual channel information. Ifafterwards, a redefinition procedure is started for this channel, and the MS is handed over to thatchannel at the same time, the call is lost.

4.2.6 Mobile stations

There are certain mobiles which cannot cope with the redefinition procedure.It has also been reported that others are not able to hop on the SDCCH channels or have problemswhen using SFH in conjunction with DTX downlink and/or dynamic power.

4.3 Advantages. Effects on planning

4.3.1 Frequency diversity

4.3.1.1 Description

Multipath fading is speed and frequency dependent. For speech services, a speed of 35 km/hin the 900 MHz band (17.5 in the 1800 band) is enough to overcome its effects. For more slowlymoving users, the GSM error correcting mechanisms are not enough . However, using frequencyhopping the same performance can be obtained.




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Figure 6 shows the necessary C/No ratio as a function of vehicle speed at fixed frequencyallocation (900 MHz band) and with the use of an ideal frequency hopping for a bit error ratio (BER)of 0.5% which is regarded as tolerable for speech transmission.

With ideal frequency hopping, the best possible transmission quality is obtained at almostevery vehicle speed. A slight degradation is observed at very high vehicle speeds. The reason for thisis a significant change in the multipath profile even within one time slot that cannot be solved by theequaliser.

A similar situation is observed for co-channel and/or adjacent-channel interference. Figure 7contains an impression of the necessary C/I ratio in terms of the current vehicle speed. Thedependence is even more clearly marked than for noise interference, as here the power of theinterference signal also fluctuates in accordance with speed.



4

5

6

7

8

9

10

11

12

13

0 50 100 150 200

v [km/h]

C/No[dB]forBER=0.5

%

Without FH

FH

Figure 6 Required C/No against vehicle speed for a BER of 0.5%


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The two causes of non-ideal frequency hopping are: a low number of frequencies and afrequency spacing that is too small. They will be studied in the following sections.

4.3.1.2 Number of frequencies

The decisive factor for the minimum length of the hopping period for ideal frequencydiversity is that a different frequency has to be used for every slot inside the interleaving block.In the case of cyclic frequency hopping the hopping period must be at least as long as the interleavingdepth (8 for speech), and a greater period will not cause additional gain.In the case of a shorter hopping period at least two of the time slots, over which a code word is spread,are transmitted at the same frequency so the fading processes are strongly correlated for them at a lowvelocity. That will lower the gain.

The bit error curves in Figure 8provide an impression of the losses to be anticipated at lowvehicle speed (5 km/h). It is notable that hopping over only 4 frequencies comes 1 dB closer to idealhopping.

For 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 sequence. That means thatthe fading decorrelation within one interleaving block is never optimal. The following table shows the



6

8

10

12

14

16

0 50 100 150 200

v [km/h]

C/I[dB]forBER=0.5

%

Without FH

FH

Figure 7 Required C/I against vehicle speed for a BER of 0.5%

0.001

0.01

0.1

1

0 2 4 6 8 10 12 14

C/No [dB]

BER

Without FH

2 freqs

4 freqs

8 freqs

Figure 8 Effect of number of frequencies on bit error ratio (BER), v=5km/h


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C/No required for a BER of 10-2 in the case of using cyclic and random frequency hopping for different

number of frequencies. It can be seen that the frequency diversity gain with 8 frequencies is 1-2 dBlower for random hopping.

The results in this table cannot be directly compared with the results of the previous figuresbecause different propagation conditions are used (in particular typical urban).

Measurements indicate that the gains realised from frequency hopping can be smaller thanpredicted due to the diminished severity of multipath propagation when compared to flat fading. Innormal environments the different paths arrive a different times so the cancelling is not complete.

With respect to interference, the same statements apply.

4.3.1.3 Frequency spacing

The frequency spacing has to be large enough to ensure that different frequencies sufferuncorrelated fading. For example, in a typical urban environment (TU) a channel separation of 400-600 kHz (2-3 GSM channels) would be needed. In most environments coherence bandwidths of lessthan 1 MHz can be expected, and therefore 1 MHz (5 GSM channels) can be recommended as theminimum frequency spacing for outdoor scenarios

Indoor systems, however, are generally characterised by large coherence bandwidths whichindicates that a lower frequency diversity gain would be achieved than in outdoor systems with thesame hopping bandwidth.On the other hand, indoor users usually have lower velocity than outdoor users indicates potential fora higher frequency diversity gain with frequency hopping. Simulations have shown that even thoughthe gain achieved is smaller (for a 2% FER 1.7 to 3.3 dB gain in C/N o instead of the 5 dB in a TUscenario with 5 MHz hopping bandwidth), it is still significant.

4.3.1.4 Antenna diversity

Another method generally used to overcome the effects of multipath fading is antennadiversity. This technique also achieves gains in conjunction with channel encoding and interleaving,but, since it uses space diversity, the gain is independent of vehicle speed.Both methods can be used together to provide a further considerable increase in gain.The total gain, however, does not amount to the sum of the individual gains. The following table willillustrate this effect:



Cyclic SFH Random SFH

C/No for GrossClass 1

BER=10-2

C/No forFER=2%

C/No for GrossClass 1

BER=10-2

C/No forFER=2%

No. of frequencies 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 2.0 7.5 2.0 9.5 2.0

3 6.0 3.5 7.5 3.0 6.5 3.0 8.5 3.0

4 5.0 4.5 6.5 4.0 6.0 3.5 8.0 3.5

8 4.0 5.5 5.5 5.0 5.5 4.0 7.5 4.0

12 4.0 5.5 5.5 5.0 5.0 4.5 7.0 4.5


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It must be noted that the antenna diversity gain is very high for TU environments, but itdrops in other test conditions (rural area or hilly terrain).

4.3.1.5 Effects on planning

Planning of a radio network must be based on the unfavourable transmission conditions atslow vehicle speed. Frequency hopping offers a possibility to compensate for the degradation intransmission quality, making it largely independent of vehicle speed. That could mean that thesmaller C/No and C/I values for medium speed vehicles can be used in the planning process of areaswhere a meaningful percentage of pedestrian users are expected.

The reduction of the required C/No that frequency diversity provides cannot be translated intoa reduction in sensitivity values because the BCCH carrier cannot hop.

In terms of interference reduction, something very similar happens.The improvement in the required C/I levels cannot be translated into a tighter reuse pattern in theBCCH carriers.In this case, however, TCH carriers can be planned differently, i.e. different C/I requirements can beset in the frequency planning process (use TU50 values instead of TU3, for example), and frequencydiversity can be used as a means for increase capacity.The increase will largely depend on the number of frequencies that are set in the hopping sequences,which as seen have an effect on the required C/I value. Care must be taken to ensure that the

separation between frequencies assigned to a cell is appropriate to the propagation environment.If possible, the BCCH frequency shall be included in the hopping sequences for those channels that donot occupy time-slots assigned to control channels.

In general, just by switching on frequency hopping in an existing network with the currentfrequency plan, an increase in the quality perceived by slow moving users will be noticed.

4.3.2 Interference diversity

4.3.2.1 Description

Interference diversity is a property that is generally associated Spread Spectrum systems, and

GSM can also take advantage of it by using frequency hopping.

In high traffic areas, such as large cities, the capacity of a cellular system is limited by itsown interference caused by frequency reuse.Since the aim of a system is usually to satisfy as many customers as possible, its maximum capacity iscalculated based on a given small proportion (generally around 10%) of calls subject to a noticeabledecrease in quality due to interference. Because of this concept of worst case, the capacity of asystem is better when the statistical spread around this mean value is as small as possible. That can beseen in the following figures. In the first one ( Figure 9, left) we can see the C/I distribution for



C/No for FER=2% C/I for FER=2%

No SFH Ideal SFH No SFH Ideal SFH

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


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systems with equal average C/I value but different deviation. A smaller deviation means that you canplan the system for a lower average C/I value without loss in quality (Figure 9, right)

In a system where the interference level perceived depends on the mean of interference levelaveraged among all the time-slots, which is the case of GSM thanks to its channel encoding andinterleaving, the spread reduction in the interference suffered by a call can be achieved by changingthe interferers, and therefore the interference, slot by slot. The greater this number of interferers for agiven total mean, the better the system. This is how interference diversity operates.

Interference diversity also allows the system to take advantage of the fact that traffic peaks donot occur at the same time in all cells. The averaging effect will spread the interference caused by thehighly loaded cells among the rest, causing only a small degradation in the quality of the system.

Frequency hopping produces interference diversity if it is ensured that the interference ineach of the hopped frequencies is different. Also, the greater the number of hopping frequencies, thegreater the gain, because the interference is averaged among more interferers.

4.3.2.2 Random vs. cyclic hopping

Traditionally interference diversity has been connected with random frequency hopping. Thatis because the effect of SFH has been studied in regular frequency assignment pattern scenarios (4/12,1/3, ). In them, with random frequency hopping, cells use uncorrelated pseudo-random hoppingsequences that make the interference vary randomly for each burst transmitted.If cyclic hopping were to be used in such scenario, the same hopping frequencies, and the samehopping pattern, would be used in the next cochannel cells, and there would initially be no advantagein terms of interference diversity.

However, if there is a different frequency reuse pattern for every set of hopping frequencies,the sources of co-channel interference will switch from time slot to time slot, even if cyclic hopping is

used.Thus there is an interference diversity effect no matter the kind of hopping used.

Accordingly, two kinds of strategies can be followed:

tightening frequency reuse factor with random hopping

using multiple reuse factor patterns with both random and cyclic hopping.

4.3.2.3 4/12 and 3/9 reuse patterns



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

STANDARD

DEVIATION

Figure 9 Example of C/I distributions


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Simulation results show that frequency hopping in the traffic carriers used with a 4/12 reusepattern increases the quality in the network due to a reduction in interference, only if used inconjunction with downlink discontinuous transmission, and downlink power control.

The reason for this is that in the downlink, frequency hopping on its own produces little benefit. Allpotential interferers from any given cell (base stations) are at almost the same distance, due to theregular cell structure. Consequently they have similar transmission paths and there is littleinterference variation.The benefit on the mobile to base link is achieved due to the geographical distribution of interferers.Discontinuous transmission and power control produce the necessary variation.

The increase in quality can be traded with capacity by tightening the reuse factor to 3/9.

These results were obtained with 100% traffic load. In reality such load is never achievedbecause networks are planned for a certain percentage of blocking during busy hours. In practisedifferent interference conditions between slots due to traffic can then be expected in the downlink.Irregular propagation conditions will also affect the interference conditions, enabling the downlink tobenefit from frequency hopping even if power control and DTX were not used.

4.3.2.4 1/3 reuse pattern and fractional loading

The capacity of a GSM network is limited by either the number of traffic channel (hardblocking) or the interference from the neighbour cells (soft blocking).

It can be seen that, from a hard blocking point of view, small reuse factors give a superiorperformance to higher reuse factors, due to the so called trunking efficiency. The small reuse factorswill though be limited by soft blocking, i.e. interference, and will not be able to accept more than agiven amount of traffic.It is therefore expected that the maximum capacity will be somewhere between a high reuse factor anda small reuse factor. In the case of interference diversity the soft blocking limit will be a lot lower,allowing more traffic load for a given reuse factor.

To illustrate this, various reuse schemes with frequency hopping in the traffic carriers havebeen simulated (COST 231) and their maximum traffic load has been found for an operator with 36TCH frequencies (9.8 MHz), ideal power control, and DTX with voice activity factor of 50%.The maximum capacity per site was obtained for a sectorised base station and a frequency reuse factorof 1/3 with a traffic load of 30% (DTX lowers the occupation of the traffic channels to approx. 15%of the time)3. The capacity increase of a random SFH network was approx. 74 % compared to a nonSFH network with a frequency reuse scheme of 4/12.For omni-directional configurations, a 3 reuse scheme offered the greatest capacity increase.In general the capacity increase depends on the spectrum allocation: the higher the allocation (i.e. thenumber of hopping frequencies), the higher the increase.

To take advantage of these capacity increases, a new approach for network planning is

required. More bandwidth has to be assigned to each base station than is strictly needed if only itstraffic load were taken into account. This approach is called fractional loading.

In practise, use of the 1/3 scheme would make frequency planning trivial.

Fractional loading can be obtained by either installing fewer transceivers than allocatedfrequencies and using synthesiser frequency hopping, or implementing an admission controlprocedure.

3 Here it must be reminded again that these results were obtained with homogeneous propagation andtraffic conditions. The irregular propagation conditions and non homogeneous traffic distributionsexpected in real networks will allow higher fractional loading.




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The need of admission control procedures is due to the fact that if as many TRXs asfrequencies are installed, in the case of overload the network will not react by blocking calls, but bymaking all calls have a lower quality. This can result in more dropped calls. The effect of a dropped

call for a subscriber is worse than that of a blocked call, and therefore they should be avoided.

The advantage of this latter alternative is that local traffic peaks can be handled.A large number of channels are temporarily available in a sector, provided that the load insurrounding co-channel sectors is low, and the admission control procedures could make use of them.However, suitable algorithms are yet to be found. Until then fractional loading should only be used inconjunction with synthesiser hopping.

4.3.2.5 Multiple reuse patterns

The basic principle of multiple reuse patterns (MRP) is to divide the spectrum into sub-bands, each containing a different number of separately planned carriers. As a consequence, anetwork may operate with several reuse patterns simultaneously.

One frequency from each sub-band is allocated to each sector, e.g. a 12 reuse for the BCCH, and 9and 3 reuse for the TCH, respectively (referred to as 12/9/3). The total average reuse is then(12+9+3)/3 =8. Another possible plan is the 12/9/6/4 (reuse 7.75).

The interference may vary greatly between different frequencies due to the difference inreuse. However, the interference diversity resulting from frequency hopping ensures that high qualityis still maintained for all users, in spite of the tight reuse on some frequencies.

Because the variation in interference is already achieved by the difference in reuse, thistechnique can be used with both cyclic and random hopping. The choice will generally depend on thenumber of transceivers in the cell. For few frequencies (for example, 2) cyclic is better because the

spectrum utilisation will be better.

A valuable feature of MRP is the ability to handle unevenly distributed traffic, i.e. differentnumber of transceivers per cell.In the example given, one may not need a third transceiver in all cells initially, which means that theeffective reuse on the third sub-band will be sparser than 3. As the capacity need increases, a thirdtransceiver is installed in more cells, which results in gradual tightening of the average reuse.

It is claimed that MRP can achieve a further capacity increase compared with the 3/9 reusepattern (lower average reuse factors), and it offers the advantage of being suitable for basebandhopping, something that cannot be said of the 1/3 reuse pattern with fractional loading option.However, no studies have been done to find the most suitable MRP configuration in terms of capacityincrease while maintaining good quality.



... ...

BCCH-4/12 TCH1-3/9 TCH2-1/3

SPECTRUM ALLOCATION

Figure 10 12/9/3 multiple reuse pattern


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A 12/8/6/4 frequency plan has been used in a commercial GSM network, which roughlydoubles the capacity compared to a standard 12 reuse pattern. The high system quality was shown to

be maintained. Neither DTX nor power control were needed, because the downlink interferencevariation was achieved by the different reuse patterns.

In another trial, an initial reuse factor of 16.9 requiring 40 carriers was tightened to a14/10/6/2 configuration (reuse 12.87 and 32 carriers) and even a 12/10/4/2 (reuse 11.26 and 28carriers). The BCCH carrier was non-hopping and no DTX or dynamic power control was used.There was no change in the minutes/dropped call. Some degradation of perceived speech quality inthe second case was found, but it was shown to relate to interference in the tighter BCCH band. Thenumber of successful handovers increased and in general they went more smoothly, showing that thenew configurations had little impact on the quality.Simple calculations can be used to assess the capacity gains of these new configurations:

Without FH: reuse 16.9, 40 channels => 2.48 TRX/cell, 12.23 Erlangs/cell.

Phase 1: reuse 12.87, 40 channels => 3.1 TRX/cell, 16.7 Erlangs/cell (37% gain).

Phase 2: reuse 11.29, 40 channels => 3.54 TRX/cell, 19.3 Erlangs/cell (58% gain).

4.3.2.6 Multiple reuse patterns and fractional loading

Multiple reuse patterns can be deployed in conjunction with fractional loading to furtherreduce the average reuse factor in a network and increase its capacity, without having to jump to theaggressive 1/3 pattern.It also allows the operator to plan for the future. The reuse strategy can be set tight to cope for futuretraffic increases. TRXs will be added to the sites when they are needed without changing thefrequency plan. At the beginning the low fractional load will ensure high quality, which will then betraded off for capacity, when the latter is needed.

One example of such configurations is a 12/8/5/4 with 40 carriers and a reuse of 6.2. The

number of hopping frequencies is 5 or 6 (reuses 8, 5, 5, 4, 4, and 4 every other cell). The BCCHcarrier is non hopping. An average of 4.0 TRX per cell would mean a 61% load and 21.4 Erlangs percell. With 4.5 TRX (68% load) the traffic would increase to 24 Erlangs, and with 5.4 (82% load) to 30Erlangs.

The following table shows other configurations that have been proposed for a 40 carrieroperator:




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As it happens with MRP, no studies have yet been carried in order to find the most suitableconfigurations.

4.3.2.7 Fractional reuse patterns4

A different approach to tightening the frequency reuse are the fractional reuse patterns. Theyare obtained by dividing the set of available frequencies into overlapping subsets which are thenassigned to the different cells that form a cluster. For example an operator with twelve frequenciesand a 4/12 reuse structure, which normally would assign one frequency to each cell, can insteadassign three frequencies with a fractional reuse pattern.

This configuration is not reported to have been used in real networks because of its planningdifficulty. It can be considered as a version of multiple reuse patterns with a non regular reuse in eachfrequency.

4.3.2.8 Concentric cells

Concentric cells are used to increase capacity by enabling an operator to employ differentfrequency reuse patterns for different sets of frequencies allocated to a cell.Part of the spectrum is allocated to an outer zone for seamless contiguous coverage. This is plannedusing a conventional reuse patter.The remaining spectrum is divided into concentric zones, employing progressively tighter reusepatterns.

Combining this structure with frequency hopping permits the network capacity to beenhanced even further by allowing a more aggressive frequency reuse in the different layers.

4.3.2.9 Control and traffic channels

4 Sometimes the term fractional patterns is used to refer to a tight reuse pattern with fractionalloading. This is not the case in this document.



FL-MRP construction Frequencies per cell Average reuse

12/8/7/4 5.5 7.3

12/9/7/4 5.7 7.0

12/8/7/5/4 6.0 6.712/9/6/3 6.7 6.0

12/8/7/3 7.1 5.6

12/9/6/3 7.2 5.6

A1 B1 C1 A2 B2 C2 A3 B3 C3 A4 B4 C4

f1

f2

f3

f4

f5

f6 f7

f8

f9

f10

f11

f12


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Frequency hopping used in conjunction with power control and discontinuous transmissioncreates a potential for higher capacity in the system.Unfortunately, these features cannot be applied to frequencies used for broadcast control channels

(BCCH frequencies). Furthermore, the interference on the BCCH frequencies should be kept low toensure that cell selection, locating, access, paging, and so forth work properly.Today a 4/12 reuse factor is probably the best that can be achieved for the BCCH In many runningsystems, factor of around 15 is typically used.

Consequently, to make the most of these techniques, it is desirable to have different reusepatterns for TCH and BCCH frequencies.Two different strategies can be envisaged: in the first one the frequencies are divided into twodedicated sub-bands: one for BCCH frequencies and another one for TCH frequencies.In the second BCCH and TCH frequencies share the same spectrum, although the BCCH are given asparser reuse.

Simulations show a much better performance of the dedicated bands strategy, achieved bylimiting interference generated by BCCH frequencies to a specific band in the downlink, which is

generally the limiting link in interference limited systems (diversity can be used in the uplink).It also ensures secure control channel behaviour independent of the traffic load.This better performance has also been observed in field measurements, and is therefore commonlyused.It also has the advantage of avoiding most of the current handset problems, which arise when thecontrol channels (which use the BCCH carrier) hop.

4.3.2.10 Effects on planning

Two are the major reasons that should encourage an operator to use frequency hopping in thetraffic carriers and take advantage of its interference diversity effects:

Increase quality in a network with interference problems.

Increase the capacity of an already saturated network.

Increasing the quality

Just by switching on random frequency hopping among the traffic carriers in a network, withits existing frequency plan, a quality increase in terms of interference should be expected, unless ofcourse the quality of the network is already good enough.

The following paragraphs present the performance results from the activation of randomfrequency hopping5on an existing network.

A significant improvement in the dropped call rate was seen both in the cells where

frequency hopping was enabled and in the surrounding non-hopping cells.

RXLEV/RXQUAL statistics were also collected. An increase in RXQUAL levels (i.e. adecrease in the measured quality) were reported. However, the average FER improved compared withthe non-hopping case, i.e. there was a quality increase. This is because the channel decoder was ableto correct errored bits on bad frequencies, using correct bits on other frequencies

The increased RXQUAL levels produce an increase in the number of handover per call,which was an unwanted effect. In order to cope with it, network operators will need to alterRXQUAL-based handover thresholds in hopping cells.For example, the lower RXQUAL handover threshold can be increased by an amount equal to theincrease in average RXQUAL values.

5 Baseband FH was used because filter combiners were used in all of the networks BTSs.




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These results prove that SFH can be used to increase the QoS of an existing network.Nevertheless, frequency hopping for quality increase should not be used to hide major planning or

tuning problems, in a badly planned network, i.e. with high and extended interference levels, becausethe performance would deteriorate.

Increasing the capacity

This section includes two cases:1. An operator needs to introduce more TRXs for traffic reasons,2. Some frequencies in the existing spectrum need to be freed for other layers.

It had been seen in the previous sections that there are four ways to increase the capacity(efficiency) of the network by using random frequency hopping:

tightening the reuse factor of the traffic carriers to 3/9, with downlink PC and DTX;

using multiple reuse patterns; using multiple reuse patterns with fractional loading;

tightening the reuse factor of the traffic carriers to 1/3 with fractional loading, downlink

PC and DTX;

In reality they can all be considered steps in the process of tightening the average reuse factorin the network while maintaining sufficient quality.

The latter two give the greatest capacity increase and flexibility, but they require synthesiserhopping. Therefore, they cannot be used in cells that have filter combiners, more than 4-6 TRXs (seesection 4.4.2), or coverage restrictions that do not allow for the losses a hybrid combiner wouldintroduce, i.e. cells where only baseband hopping is available.

For networks with this type of cells the process of increasing capacity should be done insteps, gradually tightening the frequency reuse as more TRXs are needed in the network, andcontinuously monitoring the QoS attained.If homogeneous reuse patterns are used, downlink power control and discontinuous transmission willprobably be needed.Multiple reuse patterns offer in this case greater flexibility, in that they are suited for nonhomogeneous traffic conditions, and non whole number average reuse factors can be achieved. Forexample, a network can go from a 12, to a 12/9 (10.5 average reuse), a 12/9/6 (9 average reuse) and a12/9/6/3 (7.5) reuse configuration as the traffic increases.

With synthesiser hopping, two different strategies to increase the capacity of the network canbe followed.The first one is the same one used with baseband hopping: progressively tighten the reuse factor asmore capacity is needed in the network.The only difference in this case is that fractional loading techniques are available to further decreasethe average reuse factor. It must be ensured, however, that the appropriate fractional load is achieved.

The second strategy was already outlined in section 4.3.2.6. The average reuse factor couldbe initially set tight, to cope for future traffic increases, and TRXs added to the sites when they areneeded, without changing the frequency plan.The reuse strategy can be either a 1/3 or an MRP. As new TRXs are added, the QoS should bemonitored to ensure that the maximum fractional load the plan allows for is not surpassed.DTX and PC can be deployed to increase this maximum fractional load.

The choice between 1/3 and MRP is still difficult because of the lack of comparison studies.Both of them are suited to non-homogeneous traffic conditions. The advantage of the 1/3 reuse patternis that it eliminates the need for frequency planning of the traffic carriers in a network, and therefore




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it is very flexible when it comes to introducing new base stations. However, it is also a very aggressivepattern.A possible use of 1/3 patterns is the microcell layer.

In general, reuse patterns are suitable for networks with a regular layout.Most of the time this is not the case because of the irregular terrain morphology and propagationconditions, and uneven traffic distributions.This is why frequency planning is often done with the aid of automatic planning tools which basetheir frequency assignments on interference matrices.Such an approach, which is based on a certain model of the network, is however not suitable forplanning frequency hopping. New modelling techniques are required in the existing automaticplanning tools in order to make the most of frequency hopping.

Most operators already have a planning tool at their disposal. With it, frequency hoppingplanning can be tackled in different ways, depending on the chosen strategy.

Fractional loading can be achieved by assigning to each cell more frequencies than the traffic

requires, i.e. assigning more TRXs in the frequency planning tool, even though they will not be realTRXs.

An homogeneous reuse pattern can be tightened by reducing the required C/I planningthreshold.The difficulty is to determine the level to which the C/I threshold can be reduced.A possible approach is to decrease its value in steps, monitoring the network performance at eachstep.

Multiple reuse pattern can be planned by following a step approach. First, the first layer ofTRXs (which represent frequencies in the tool) is planned using the conventional C/I threshold.In the second step, the first layer of frequencies remains fixed, and an additional layer of TRXs(frequencies) is added to the base stations that require them. Frequencies are assigned using a new,

lower C/I threshold.In the third step, a new layer of TRXs (frequencies) is introduced and the same procedure is followed.Steps are repeated until all the layers have been assigned.

4.4 Lucents solutions

4.4.1 Base station equipment

RBS900 family: 900 band - will only support baseband hopping.

BTS2000 family : 900/1800 bands.

Bosch RFUs: The second letter in their code - B => baseband hopping, S =>

baseband/synthesiser hopping. Lucents SRFUs are capable of synthesiser and baseband hopping.

CUBE supports synthesiser hopping.

4.4.2 Antenna coupling equipment

Filter configurations (with TXFU09/TXFU18 filter combiners) only allow baseband frequency

hopping.

Hybrid and diplexer configurations (which either use hybrid combiners - TXDU09/TXDU18)

allow the two types.




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Due to the hybrid combiner losses (each layer of them introduces a loss of 3 dB into the overall

combining loss), hybrid configurations of up to 4 TRX are available with two antennas and 6 TRXwith three.

4.4.3 Fill-sender and phantom RTs

The BCCH carrier is always transmitting to enable mobiles to monitor it.This does not cause a problem for baseband hopping since there are as many RFUs as frequencies, andone of them is available to transmit the BCCH carrier.For synthesiser hopping, the traffic channels assigned to the carrier which includes the BCCH, willnot be able to hop unless an additional transmitter is used.

This additional transmitter is known as the fill-sender, but would be more accurately termeda fill transmitter. It is a RFU or SRFU which is used to transmit continuously on the BCCH frequency.

To implement hopping channels on the BCCH-RT the fill-sender RFU or SRFU must belocated in the same DRCC (Double Radio Codec and Control 6) as the BCCH-RT, in the BTS. Fill-senders cannot, therefore, be used on the 6od or the BTS2000/2C as these do not have the DRCC aspart of their physical configuration.

In the case of baseband hopping, there also exists the possibility of hopping over morefrequencies than RTs, by adding phantom RTs. Like the fill-sender, this is an extra RFU set on adifferent frequency, that can be included i

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