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WCDMA Network Performance in

Variable Repeater Hotspot Traffic Cases

P. Lhdekorpi, J. Niemel, J. Borkowski, J. Lempiinen

Institute of Communications EngineeringTampere University of Technology

P.O. Box 553 FI-33101 TAMPERE FINLANDTel. +358 3 3115 4552, Fax. +358 3 3115 3808

Email: {panu.lahdekorpi, jarno.niemela, jakub.borkowski, jukka.lempiainen}@tut.fi

Keywords: hotspots, repeaters, WCDMA.

AbstractThe target of the paper is to illustrate the impact of hotspottraffic distributions on the WCDMA network performancewhen repeaters are used for providing service to the hotspotusers. The paper concentrates on providing a detailed system-and cell-level analysis of the impact of repeaters by usingstatic Monte Carlo simulations. Moreover, the effects ofdifferent repeater configurations are presented. Simulationswere made by using different repeater gains, different hotspottraffic densities, and two different repeater distances to findout a planning guideline for WCDMA repeaters. The resultsshow that the optimum repeater gain does not depend on the

amount of traffic in hotspots, but mainly on the repeaterconfiguration, and on the distance from the mother cell.Moreover, cell-level analysis reveals that repeaters haveremarkable effect on the interference levels in cellssurrounding the repeater cells. The results also show, how thedownlink capacity can considerably be improved by usingrepeaters.

1 Introduction

WCDMA (Wideband Code-Division Multiple Access)repeaters are used as an amplifier unit between the motherbase station and mobile stations. Correct installation of a

repeater results in increased signal level under repeaterservice area. Repeaters are deployed to help mobile stationstogether with the mother base station to use lower transmitpowers and thus to reduce interference propagated to thesurrounding cells. Hence, repeater is a very attractive choice,e.g., when hotspots (areas with increased traffic density) areintroduced.

Repeaters, and their effect on the network performance inhotspot cases, have been studied in [1] and [2]. Clear increasein downlink capacity was observed in both cases. However,the uplink direction seems to be more problematic in sense ofcapacity and interference as illustrated in [1]. In addition,repeater field measurements also indicate the increase indownlink capacity at properly adjusted repeater gains [3].

This paper presents the expected WCDMA networkperformance (in system-level and in cell-level), when

repeaters are installed near hotspot areas. Moreover, theeffects of repeaters on the nearby cells are analysed. Thispaper also clarifies what happens in the network especially incases with different hotspot traffic loads, and what is theeffect to the overall network performance in these casescompared to the case without repeaters. Finally, the effects ofdifferent repeater distances are studied to illustrate theimportance of correct repeater configuration.

2 Simulations in brief

Analysis in the following chapters is based on Monte Carlosimulations with same simulation parameters already

presented in [1]. Support for repeaters and hotspots wasimplemented in the static network simulator (NPSW [4]).Simulations were performed with two repeater configurationsand by using voice users only. In the scenario 1, repeaterswere located at the distance of 500 m (path loss value 100 dB)from the mother base station. In the scenario 2, repeaters werebrought to the distance of 333 m (path loss value 96 dB).Figure 1 shows the simulation scenario 2. Table 1 defines theterms used in the cell-level analysis.

Figure 1: Network layout with 19 3-sectored sites and 6 repeaters serving 6hotspots in the centre. Site spacing is 1000 m and repeaters are located 333 maway from the mother base station (scenario 2).

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The repeater loss ( GT ) value is used to define the repeaterconfiguration from the mother base station antenna to therepeater service antenna [5]:

[dB] REP DONOR BS T GG LGG ++= , (1)

where G BS is the mother base station antenna gain and G DONOR is the repeater donor antenna gain. Different repeaterconfiguration scenarios can be simulated by using differentpath loss ( L) between the repeater and mother base station andvarying repeater gain ( G REP ).

Hotspot traffic density (the density of the users in hotspotarea) is defined by the hotspot density factor ( HSDF ). Hotspottraffic ( T HS ) is defined by the traffic density in the overallnetwork area per square km multiplied by the HSDF factor:

HS HS A D HSDF T = , (2)

where D is the user density of the whole network area(users/km 2) and the A HS is size of the hotspot area (km

2).

HSDF values between 0.001 and 20 were used in thesimulations. Very small HSDF value (such as 0.001) meansthat no hotspot traffic is present. A special traffic case wasalso included, where the homogenous overall traffic layer wasremoved, including user traffic only in the hotspots. In this

case the amount of users in the whole simulated area equals tothe total number of users in the hotspots.

3 Cell-level analysis

Cell-level analysis was performed with different hotspottraffic densities and with both simulation scenarios. Repeatercells and regular cells were analysed separately.

Figures 2 and 3 show averaged uplink other-to-own cellinterferences ( iUL) as a function of the repeater gain for thescenarios 1 and 2. They illustrate how other cell interferenceof the regular cells increases rapidly at high repeater gains. Atthe same time, own cell interference in regular cells isreduced because of smaller cell dominance area and thussmaller number of users in the cell. These two figures alsoindicate that the repeater gain can not be increased aftercertain limit without heavily affecting interference levels inthe surrounding cells. Dashed lines in Figure 2 and 3 indicatelower uplink other-to-own cell interference in repeater cell athigh repeater gains because the cell dominance area expands(own cell interference increases) when repeater gainincreases.

Figure 2 indicates also that when the density of traffic inhotspots is increased, the effect to the uplink other-to-owncell interference is more sensitive. Use of too high repeater

gains can be observed as the cell blocking phenomenon as inFigure 4.

Figure 2: UL other-to-own cell interference in different hotspot trafficdensities when using the scenario 1 (repeater distance 500 m) with 2000homogenous overall users. Solid lines indicate neighbouring regular cells(averages) and dashed lines indicate repeater cells (averages).

Figure 4: Downlink throughput in cell-level in the scenario 2. Networkparameters: HSDF = 6, repeater gain = 75 dB. White color indicates blockedcells.

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Repeater Gain [dB]

i U L

HSDF 0.001 (regular cells)HSDF 1 (regular cells)HSDF 6 (regular cells)HSDF 10 (regular cells)HSDF 0.001 (repeater cells)HSDF 1 (repeater cells)HSDF 6 (repeater cells)HSDF 10 (repeater cells)

Figure 3: UL other-to-own cell interference in different hotspot trafficdensities when using the scenario 2 (repeater distance 333 m) with 2000homogenous overall users. Solid lines indicate neighbouring regular cells(averages) and dashed lines indicate repeater cells (averages).

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Repeater Gain [dB]

i U L

HSDF 0.001 (regular cells)HSDF 1 (regular cells)HSDF 6 (regular cells)HSDF 10 (regular cells)HSDF 0.001 (repeater cells)HSDF 1 (repeater cells)HSDF 6 (repeater cells)HSDF 10 (repeater cells)

Name Definition Related cellsRepeater cell A cell with

repeater installedBS2, BS3, BS8,BS11, BS18, BS21

Regular cell A cell with norepeater installed

BS1, BS4, BS7,BS14, BS15, BS19

Table 1: Cell definitions.

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4.2 MS transmit power

As mentioned before, mobile station transmit power is acrucial parameter in determining the efficiency of a repeaterconfiguration. Figures 8 and 9 present the network-levelaveraged mobile station transmit powers for the scenario 1and scenario 2 when different hotspot traffic cases aresimulated.

In Figures 8 and 9, the impact of repeaters on the mobilestation transmit powers is most clearly seen from the traffic

case with only hotspot traffic present. When using the casewith only hotspot traffic, the resulting transmit power value isstill an average from the whole network. However, this valuerepresents quite well an average taken only from hotspotusers (i.e., repeater cell users). This is due to the fact, thatmost of the connections are made by using a repeater cell. Areduction of almost 5 dB in MS transmit power average isobserved in this particular case when using the scenario 1. Incases of low hotspot traffic present, the phenomenon ismainly insignificant, because the uplink interferencedominates and raises the MS transmit power average value.This, together with the service probability results from Figure5 and 6, basically indicates that repeaters are useless in case

of low hotspot traffic situations. However, in case of high

hotspot traffic, maximum average gain of 5 dB is expected inmobile station transmit powers.

When comparing the Figures 8 and 9, the change in the GT value (also in the optimum repeater gain value) is againclearly visible. MS transmit power curves rise 4-5 dBs earlierwhen using the scenario 2 instead of using the scenario 1,thereby indicating major uplink interference at high gains.

4.3 Capacity evaluation

Network-level capacity evaluations were made for scenarios 1and 2 to determine the optimum repeater gain setting and DLcapacity gain for each hotspot traffic case. The results fromthese evaluations are presented in Figures 10 and 11.

Capacity gains in Figures 10 and 11 are calculated bycomparing the averaged network throughputs when repeatersare swithced on and off at a certain network load point. Thesecalculations were made separately to each of the hotspottraffic cases.

The optimum repeater gain values and capacity gains arefinally put in Table 2. The optimum repeater gain values areextracted from Figures 10 and 11 by allowing a loss of 5percent units in uplink capacity. Finally, the DL capacitygains are selected by using these optimum repeater gainvalues.

Figure 8: Averaged MS transmit powers in different hotspot traffic cases forthe scenario 1.

Figure 9: Averaged MS transmit powers in different hotspot traffic cases forthe scenario 2.

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Repeater Gain [dB]

M S T X P o w e r

[ d B m

]

3000 users HSDF 0.001 REP ON (REP O FF value: 12.475)3000 users HSDF 1 REP ON (REP OFF value: 12.400)3000 users HSDF 6 REP ON (REP OFF value: 12.320)2000 users HSDF 10 REP ON (REP OFF value: 13.011)1000 users HSDF 20 REP ON (REP OFF value: 13.910)1000 users HS users only REP ON (REP OFF value: 13.192)

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M S T X P o w e r

[ d B m

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3000 users HSDF 0.001 REP ON (REP OFF value: 12.447)3000 users HSDF 1 REP ON (REP OFF value: 12.219)3000 users HSDF 6 REP ON (REP OFF value: 12.415)2000 users HSDF 10 REP ON (REP OFF value: 13.514)1000 users HSDF 20 REP ON (REP OFF value: 14.727)1000 users HS users only REP ON (REP OFF value: 13.288)

Figure 10: Evaluated uplink and downlink capacity gains for the scenario 1.

Figure 11: Evaluated uplink and downlink capacity gains for the scenario 2.

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C a p a c

i t y g a

i n [ % ]

Repeater gain [dB]

HSDF 0.001HSDF 1HSDF 6HSDF 10HSDF 20

Uplink

Downlink

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HSDF 20

Uplink

Downlink

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Results from the capacity evaluations (Figures 10 and 11,Table 2) show, that the optimum repeater gain is not largely

changing, when changing the hotspot traffic density. The pathloss reduction of 4 dBs is again visible when comparing theoptimum repeater gain values between the scenarios 1 and 2.

When looking at the capacity gain values in Table 2, thedownlink capacity is clearly increased in both scenarios whenusing higher HSDF values. Up to 35 percent unit increase isobserved in DL capacity gain when comparing the lowest andthe highest hotspot traffic density case for the scenario 1. Incase of the scenario 2, the difference is not that large due tothe increased repeater noise and interference issues.

The results in Table 2 show the importance of lowering theusers transmit powers in hotspots with high number of users,

e.g., by using a repeater. Although the small uplink capacityloss is still present, the downlink can be now more efficientlyused to, e.g., serve higher speed data users.

5 Conclusions and discussion

The simulations have indicated that repeaters are veryeffective in increasing the overall downlink capacity in allhotspot traffic cases. However, uplink direction has proven tobe the bottleneck for the system performance, when usingrepeaters. With repeater gains larger than 70 dB, the rapidincrease in uplink interference levels starts to deteriorate thesystem performance.

Optimum repeater gain for these simulation scenarios hasbeen seen to depend mostly on the repeater configuration(repeater distance). The smaller is the repeater distance, thesmaller is the optimum repeater gain (See Table 2).

Improvement up to 5 percent units in overall serviceprobability has been observed in case of high hotspot traffic.If allowing 5 % decrease in uplink capacity, a gain of 16 % indownlink capacity is observed even when no hotspots areintroduced. When the amount of hotspot traffic is increased,much higher DL capacity gains are observed.

Due to the differing behaviour of the uplink and downlink

directions, the repeater gain could be set separately for uplinkand downlink. Higher growth in downlink capacities could be

observed by using more gain in downlink than in uplink,without simultaneously raising the uplink interference.

An interesting application for outdoor repeater is theconnection to indoor WCDMA networks. Repeaters could belocated in between outdoor network and indoor network insuch a way, that the penetration loss from the wall materials isbypassed.

Acknowledgments

Authors would like to thank European CommunicationsEngineering (ECE) Ltd for helpful comments and theNational Technology Agency of Finland for funding thework.

References

[1] J. Niemel, P. Lhdekorpi, J. Borkowski, J. Lempiinen, Assessment of repeaters for WCDMA UL and DL performance in capacity-limited environment , In Proc. IST Mobile & Wireless Communications Summit ,Dresden, June 2005.

[2] M. Rahman, P. Ernstrm, Repeaters for hotspotcapacity in DS-CDMA , IEEE Trans. VehicularTechnology, issue 3, vol. 53, pp. 626-633, May 2004.

[3] J. Borkowski, J. Niemel, J. Lempiinen, Applicabilityof repeaters for hotspots in UMTS , In Proc. IST Mobile& Wireless Communications Summit , Dresden, June2005.

[4] J. Laiho, A. Wacker, T. Novosad, Radio network

planning and optimisation for UMTS , CD-ROM, JohnWiley & Sons, Ltd., 2002.[5] Qualcomm white paper, Repeaters for Indoor

Coverage in CDMA Networks , [online]. Available:http://www.repeaterone.com.

Scenario 1HSDF0.001

HSDF1

HSDF6

HSDF10

HSDF20

Opt. G REP (dB) 72 72 72 73 72

DL capacitygain (%) 13 16 27 35 48

Scenario 2Opt. G REP

(dB) 68 68 67 69 66

DL capacitygain (%) 13 16 22 28 39

Table 2: Optimum repeater gain values and downlink capacity gain valuesfor the scenarios 1 and 2.