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Interference Diversity Gain

in Frequency Hopping GSMHik an Olofsson,

Jonas Naslund, Johan Skold

Ericsson Radio SystemsABS-16480 Stockholm

Sweden

Abstract - Wideband random frequency hopping is one of thekey properties of GSM in an evolution towards PCS PersonalCommunication Services). Random frequency hopping intro-duces diversity on the transmission link, which improves theperformance. In this paper, the impact of interference diversityin GSM is analyzed by mean s of bit level simulations of the GSM

full rate speech channel.The analysis shows that the gain of interference diversity ishighly system load dependent. The interference diversity gain ismost striking in a system with low channel utilization. Theseresults, together with system level simulations of frequency hop-ping GSM, show that random frequency hopping has implica-tions for frequency planning. They indicate that GSM systemswith simple frequency planning ca n be designed without sacrific-ing the overall system quality.

I . I N T R O D U C T I O N

Today, GSM (Global System for Mobile com munications)is established in over50 countries in most parts of the world.Within GSM , new services are continuously developed andstandardized in an evolution towardsPCS. In North America,GS M is often referred to asPCS 1900.

As the number of subscribers grow, the network operatorsmay eventually get capacity constraints, since the frequencyspectrum is limited. Therefore, the need fo r techniques whichimprove the sector capacity will be of increasing importancein the near future.

One of these techniques is frequency hopping, whichisincluded in the GSM Specifications [11. Random frequencyhopping introduces two kinds of diversity which make thetransmission less sensitive to fading and interference.

This paper presents an evaluationof one of the conse-quences of frequency hopping: the reduced sensitivity to

interference, called interference diversity. The impact ofinterference diversity on the transmission quality is empha-sized. Transmission link performance with and without inter-ference diversity is studied, and the question in focus is: Whatquality in terms of Frame Erasure Rate (FER) can beexpected on a transmission link w ith a certainC/I ratio?

How ever, first of alla few words about frequency hoppingin general and its implementation inGSM.

11. FRE QU EN CYOPPING

Frequency hopping consists in changing the frequencyused for transmission at regular intervals. It has been intro-duced in GSM mainly in order to deal withtwo specific prob-lems w hich affect the tranqmission quality:

Rayleigh fading: The radio signals are subject to fading,which is approximately Rayleigh distributed. Fora slowmobile, a fading dip may cause a severe informationloss. Frequency hopping will spread sequential informa-tion in the frequency domain. Since the Rayleigh fadingis frequency selective, frequency ho pping will cause fre-quent changes of the fading, thus reducing the risk oflosing sequential information. Introducing thisfrequencydivers& will, together with the interleavingand coding,improve the transmission link quality, for slow mobilesin particular.

Integerence: Without frequency hopping, strong signalsfrom neighbor cells transmitting on or close toa carrier

frequency will affect the carrier signal continuously,which may negatively affect the transmission perfor-mance. Frequency hopping will cause different signals tointerfere with the carrier at different times,a propertycalled interference diversi9. On system level, the resultis a “smearing” of the interference between users, aneffect often called in teference averaging. Furthermore,interference diversity gives a second effect, which isemphasized in this paper. Since sequential bursts ofinformation are received under different interferenceconditions, the risk of a sequential informationloss isreduced. This, together with the interleaving and coding,will improve the transmission link quality.

In GSM, a frequency hop takes place every new burst(every 4.615 ms), resulting in217 hops per second. This isknown as slow frequency hopping as opposed to f a s t fre-quency hopping, where there are in the order of one hop persymbol transmitted. maximu m of 64 hopping frequencies isallowed.

The hopping can be either cyclic or random. Cyclic hop-ping means that all interfering sectors use the same hopping

0-7803-2742-XI95 4.00 995 IEEE 102

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I

Figure 1.

experiences interference from the BTS in sector2. If random hopping isemployed, and two mobilesare connected to the BTS in sector2, the interfer-ence will not be constant, but alternate betweenI,, I, and 0.

sequence, which implies that the sam e channels will interferewith one another at all times. Hence, any interference diver-sity will not occur.

With random hopp ing, the sectors use uncorrelated pseu do-random hopping sequences, and the interference will there-fore vary randomly for each burst transmitted. Withina sectorall channels use the same sequence, shifted in the frequencydomain by a channel individual frequency offset. Th is makesthe channels disjoint, and thereby intracell co-channel inter-ference is avoided.

detailed description of frequency hopping in GSM isbeyond th e scope of this paper. Mor e informa tion is providedin [ 2 ] .

An illustration of interference diversity. The mobile in s ector1

111. QUALITY A I NDUE TO INTERFERENCEDIVERSITY

The principle of interference diversity is easily illustratedby means of a simple example. Considera GSM system withonly two sectors, which use the same setof 4 frequencies(Figure 1). In one of the time slots, a mobile communicateswith the base station in sectorI . In the downlink the receivedcarrier power isC. Since the frequencies are common fo r bothsectors, the mobile w ill experience interfere nce from sector2.In this example , the channel utilization in sector2 is 50 , i.e.2 out of 4 channels are used on the studied time slot. If ran-dom frequency hopping is employed, the mobile will ran-domly experience interferenceI,, IB or no inte rference at all.Thus, different bursts will be received under different C/Iconditio ns, and the result is interferen ce diversity.

The time varying interference is well suited for theGSMinterleaving and d ecoding schemes. Sincea speech frame isinterleaved and transmitted over8 bursts [4], one or two ofthe bursts may suffer from stron g interference without affect-ing the decoded frame.

In order to evaluate the gain of interference diversity, theFrame Erasure Rates (FER) at certain C/I ratios have been

compared with the IFERs for a system without interferencediversity. Ho wev er, ffora random frequency hopping system,the CA ratio is not a well defined entity, since the interf erencesituation varies for each burst. Therefore, to be ableto carrythrough the comparison,a mean CA ratio has been defined.

In the definition of the mean C/I, the time over which themean is calculated is chosen so that the fading can beneglected. The Rayleigh fading is removedby averagingwhile the shadowing is assumed constant. In the exampleabove, the expression for the meanC/I ratio would be

1 = 1

From this equatio n it is clear that various interfere nce con-figurations can result in the sa me meanCA ratio. The interfer-ence diversity obviously depends on the n umber of hoppingfrequen cies, the channe l utilization and the interfe rence levelsfrom the interfering sector. Considera case whe re the utiliza-tion is 100% and the interfer ence levels are equaI; this would

result in no interfe rence diversity.In a real system with m ultiple sites, alln sectors using the

same set o ff frequeincies will interfere with the carrier. Themean C/I ratio is then definedas

(3 .2 )' c c k iJ k = l i = l

where Iki is the interferen ce energy from interferer in sectork . The equation mig ht seem confu sing, but it is merelya gen-eralization of Figure 1, and can easily be understood intu-itively.

The interference diversity still depends on the number offrequencies as well as the average channel utilization, fromnow on referred toas system load.

IV. SIMULATIONODEL

The effects of interference diversity have been evaluatedby means of bit level simulatio ns of theGSM full rate speechchannel. The simulation environment consisted of blocks forchannel coding, interleaving etc, modellinga GSM trans-ceiver (Figure 2). The simulator was used to measure thedownlink quality fora mobile in d ifferent interference config-urations. The dow nlink was chosen, since it lacks the receiverantenn a diversity of the uplink, and therefo re generally limitsthe system. For each interference case,9.3 seconds was simu-

lated, which corresponds to transmission of2025 speechframes.

The key blocks of the simulato r need a closer explanation:

A ropagation model

The channel propagation model used was Typical Urban,specified in [5]. The shadow fading was assumed constant

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RADIONTERLEAVING

FORMAlTlNGANOBURST - MODULATION - OEMODULATIONHANNEL

CODING

OUTEQUALIZATION EINTERLEAVING DECODING

NTERFERENCEI I

I I

'INTERFERING RADIO

JGENERATOR

BURST MoDuLAT oN PROPAGATION

INTERFERINGBURST

GENERATOR

Figure 2.tors and eventually received and dec oded. The decision whethera frame should be erased or not is made by the decoder.

The simulation environm ent. Each speech frame is encoded , transmitted. propagated through the air, exposed to co-cha nnel interference from9 sec-

during a simulation run. Uncorrelated Rayleigh fadingbetween successive bursts was assumed, which occurs whenthe hopping frequencies are well spread in the frequencydomain or when the mobiles move fast. However, duringaburst fading was assumed constant, which is a reasonableassumption in GSM.

B. Intederence model

The system was considered limited by co-channel interfer-ence. Noise and adjacent channel interference wereneglected.

Interference configurations were obtained by collectingstatistics from a simulated cell plan with a large number ofclusters and randomly distributed mobiles. When creating thecell plan and determining the interference levels, parametersin Table I were used. Only the interference from the ninestrongest interfering sectors was taken into account. Interfer-ence from oth er sectors was small enough to be consideredinsignificant. Only the mobiles witha mean C/I ratio of lessthan 10 dB w ere studied, since the others turned out to haveFER < 0.1%.

Since the number of hopping frequencieswas assumed tohave an impact on interference diversity, the number of fre-quencies was varied between1 and 12 per sector. The systemload, i.e. the average channel utilization in the system, wasvaried between 25 and 75%.

Radio propagation model Okum ura-Hata

Path loss slope 3.5

Lognormal fading standard dev.

Number of hopping frequencies

Frequency planning

6 dB

1, 3, 12 per sector

1/3 (3 sector reuse, 3 sector sites)

Power control Based on signal strengths

(112 G PI)Antennas 120 sector antennas

System load 2 5 , 1 5

It should also be noted that the choice of frequency plan-ning and method for powe r control do not significantly affectthe results. Both ha ve a slight impacton the distribution ofI ,but not en ough to affect the results.

V. SIMULATIONESULTS

A random frequency hopping system was simulated, usingthe simulation model described in the previous section. The

results are illustrated in Figures 3-6. Each cross in the dia-grams correspon ds to a mobile with a certain meanC/I on thedownlink and a resulting FER.

As a reference,a system without interferen ce diversity wasalso simulated. These results appearas a solid line in the dia-grams.

An important result is that interference diversity seems todepend mainly on two parameters: the system load and thenumber of hopping frequencies.

A . System load

Simulations were run with system loads of25 and 75%.The results, shown in Figures 3-6, clearly establisha relationbetween system load and interference diversity:as the systemload decreases, the gain of interference diversity increases. At2 FER, which is used as a quality c riterion, the requiredC/Iis approximately 5-5.8 dB with low system load (Figures 3 and 5 ) ,and around 7 dB with high system load(Figures 4 and6). The required C/I for a system without interference diver-sity is 7 5 dB (The solid line in Figures 3-6).

Another point is that fora random frequency ho pping sys-tem, there exists no distinct mapping fromC/I to FER . cer-tain mean C/I can result in various FERs, depending on theinterference configurations. Thus, FER predictions basedonmean C / I are very difficult to make.As an exam ple, the twocrosses marked A and B in Figure 4 are results of extremeinterference configurations with one dominant interferingsector with very low channel utilization.

Systems with a system load of8% have also been simu-lated, but the performance of these systems do not differ sig-nificantly from the25% case.

~~~~~

Table Input data for the simulated cellular envlronment

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B Number of hopping frequencies

The number of hopping frequencies was assumed to havean impact on the interference diversity. Therefore, simula-tions were run with 3 and 12 frequ encies per sector (Figures3-6). The performance was compared with the performanceof a system without interference diversity. 12 frequencies areassumed to be e nou gh to get the full benefit from interferencediversity, but if look ing at Figure 3 , one can see that already3frequencies give a significant interference diversity gain ofabout 1.5 dB.

VI. IMPLICATIONS FOR FREQUENCY LANNINGAND SYSTEM CAPACITY

The simulation results described in the previous sectionshow that the interference diversity gain from random fre-quency hopping is most striking in systems with low systemload and many fr eque ncies used for hopping.

The implications for frequency p lanning are clear: to makethe most of frequency ho pping , the system should be plannedwith as tight frequency reuseas possible. Such a system can

be designed with a low system load without loss of capacity.Furthermore, it allows hopping over many frequencies, sincethe frequencies are divided into few frequency groups.

An academ ic example (not considering the B CH channels)can be used to compare the characteristics of two frequencyreuse strategies. Consider a GSM system, to which 36 fre-quencies are assigned. Suppose that the frequency plan maybe either 3/9 (9 sector reuse, 3 sector sites) or 1/3 (3 sectorreuse, 3 sector sites). Assigning the 3 6 frequencies to the dif-ferent sectors results in 4 frequencies per sector in the 3/9case, and 12 frequencies per sector in the 1/3 case. SinceGSM has 8 time slots, this implies 32 channels per sector forthe 319 system and96 channels pe r sector for the 1/3 system.Now, assume that a maxi mum of 2 blocking is allowed.Alook in an Erlang table yields that the 3/9 system allows 23Erlang per sector, which corresponds toa system load of72 . At the same capacity (23 Erlang per sector), the 1/3 sys-tem has a system load of 24 and no blocking at all. Thus ,from a frequency ho pping po int of view, the 1 /3 system ispreferable, since it at a certain capacity has a lower systemload and m ore frequencies per sector.

However, in real systems the co-channel interference mustalso be considered. Whereas ina system with 3/9 reuse theblocking limit is reached before co-channel interferencebecomes a problem , this is not the case fora 1/3 reuse systemwhere strong co-channel interference will limit the capacity[6]. Hen ce, a “soft blocking” criterion, which accounts fo r thequality for the mob iles, has to be introduced.

Thus, in a system limited by co-channel interference, thetransmission link gain of interference diversity may be usedto increase the capacity. In the fo llowin g, sector capacities ofsystems with different frequency reuse will be calculatedusing earlier system simulations and taking into account thetransmission link gain of interference diversity.

In [6], different frequency planning strategies have beencompared with respect to sector capacity. In these systemsimulations, the effect of interference averaging has beenincluded, but the transmission link gain of interference diver-sity has not been considered.A soft blocking criterion requir-ing CL > 9 dB for 90% of the mobile s has been used.

The system simulated in[6] used random hopping over 72

frequencies, discontinuous transmission (DTX) with a voiceactivity factor0.5, and power control based o n signal strengthand quality. Four different frequency reuse cases were simu-lated: 1/1 (1 cell reuse), 1/3, 3/9 and 4/12.No receiverantenn a diversity was simulated, and the simulations weremade on 1 time slot only. A full description of the simulationsis found in [ 6 ] .

The results showed that the systems with 319 and 4/12reuse were limited by 2 blocking, and the 1/1 and 1/3 reusesystems were limited by the softC/I > 9 dB blocking crite-rion. The system loads were generally low, sinceDTXreduces the system load bya factor two.

Now, if the transmission link gain of interference diversityis taken into account, the capacity of the 1 /1 and 1/3 systems

can be increased further, sinc e they are limited by soft block-ing. The required CA decreases by approximately 2.5 dBaccording to F igure5 , which corresponds toa factor 1.8. Asthe required C/I decreases, the allowed traffic density can beincreased, also by a factor 1.8.

If the transmission link gainof interference diversity isapplied to the results from the system simulations, and thecapacities are recalculated for8 instead of 1 time slot, thecapacities in TableI1 are obtained, which indicate that1/3 fre-quency reuse is by far the most favorable choice . Mor e capac-ity results for GSM PC S19 00 are presented in [7].

Frequency planning Capacity BlockingE / sector / MHz

U1 1 cell reuse)

113 (3 sector reuse, 3 sector sites)

3/9 (9 sector reuse, 3 sector sites)

4/12 (12 sector reuse, 3 sector sites) 2.5 I 2

Table 11.systems with different frequency reuse when interference diversity is takeninto account.

Results from sirnulations compa ring the sector capacity of

VII. CONCLUSION

The transmission link gain of interference diversity inGSM has proved to be highly dependent on the system loadand the number of hopping frequencies used. For a systemwith a load of 25% and 12 hopping frequencies, the gain isapproximately 2 .5 dB, resulting in a requiredC/I ratio ofapproximately 5 dB for acceptable vo ice quality. For systemswith high load, no significant gainhas been measured.

The gain is essentially independent of the radio propaga-

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tion environment, and may be especially useful wherefre-quency hopping does not give optimal frequency diversity,due to large coherence bandw idth (e.g. indoors).

The system load requirements give implications for fre-quency planning: to mak e the most of random frequency hop-ping, the frequency reuse should be tight, since this implieslow system load and many frequencies per sector.A capacity

compa rison based on system simulation s show s that a systemwith 1/3 reuse pattern has mo re than3 times the capacityof a4/12 reuse system.

Together with the two other positive effects, frequencydiversity and interference averaging, the transmission linkgain of interference diversity makes random frequency hop-ping a useful means for further increase of the capacity inG S M P C S1900.

10

\ I F

8 9 i oi o

mean Cll ( dB)

Figure 3 25 system load, 3 frequencies used foi hopping

FER

i o

10+ ‘ , \ I

‘

:

‘ \\ 7

4 5 7 8 9 1010

mean Cll dB)

Figure 4. 75 system load,3 frequencies used for hopping

REFER.ENCES

G SM 05.02, “Multiplexing and multiple access on the radio path,”ETSI, 1993.

M.Mouly and M-B Pautet, The GSM System ,for MobileCommunicafions.1992, ISBN 2-9507190-0-7.

J.F. Whitehead, “Signal Level Based Dynamic Power Control for Co-channel Interference Management,”in Proceedings of the 43rd IEEEVehicular Techno logy Conjerence, 1993, pp. 499-502.

GSM 05.03, “Channel coding,” ETSI, 1993.

GSM 05.05.. “Radio transmission and reception,” ETSI, 1993.

C Carneheim et al, “Frequency Hopping GSM,” in Proceedings of the44th IEEE Vehicular Technology Conference, 1994, pp. 1155-1 159.

Johan Skold, Jan Fiirjh, Bjom Gudmundson, “Performance andCharacteristics of GSM-based PCS,” in Proceedings of the 45th IEEEVehicular. Technology Conference, 1995.

FER

10.

10

t \+ +

4 6 8 9 10

mean CII (dB)

Figure 5 25 system load, 12 frequencies used for hopping.

FER

10

‘4 8 9 1010-31

mean C/I (dB)

Figure 6 75 system load, 12 frequencies used forhopping

Figures 3-6. The performanc eof the GSMPCS 1900 transmission linkin a Typical Urban environmen t The solid reference line applies to cyclic hopping sys-tems and to non hopping system s with fast moving mobiles . The crosses applyto random hoppin g systems with interference diversity. Each cross correspo nds toa mobile w ith a certain interferen ce configuration, generated according to sectionIV B . The dashed lines are least square curve fittings oflog(FER). A definitionof mean C/I s given in section 111. The diagrams are commented furtherin section V

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