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An Overview of Space Division Multiple AccessTechniques in Cellular SystemsXichiel P. Lij t ter t and Pieter van Rooyen *

T Alcatel Altech Telecoms. P.O. Box 286. Boksburg, S ou th Africa 1460mlotterC?alcatel.altech.co.za

* Department of Electrical and Electronic Engineering, University of Preto ria, Pretor ia, South-4frica 0002

pvroopen@post ino.up.ac.zaAbstrac t - This paper provides an overview of th ebasic principles of Smart Antenna techniques as ap-plied t o the introduction of Space Division Multi-ple Access ( SD MA ) techniques in current mobile ra-dio networks, as well as in Personal CommunicationServices (P CS ), Personal Communications Networks(P CN) and the Universal Mobile TelecommunicationSystem ( UM TS ). It is shown qualitatively how SDMAtechniques can increase the capacity of cellular net-work. Furthermore, a new SDMA principle based onthe non homogeneous Quality of Service requirements

in mobile m ulti-med ia networks are introduced withapplication to systems proposed fo r UMTS.

I. INTRODUCTIONFaced with the rapid increase in mobile subscribers

in t,he last years, cellular network operators a nd ser-vice providers have been forced to optimize and up-grade their networks on a continuous basis in order toensure that subscribers are guaranteed adequate cov-erage and signal quality. For example, in Jap an usersof mobile communication systems: including cellularand cordless, have increased dramatically at the rateof more than one million new users per month [I].With spectrum being an extremely expensive com-modity, ope rators are looking to equipment manufac-turers t o supply innovative solutions to enable themto optimally use their allocated resources. In addi-tion to the rapid uptake of voice services: users ofmobile telephony are increasingly showing an inter-est in the availability of more advanced data andmulti-media services via mobile networks, sparkingsuch initiatives such as UMTS [ l ] .

This path of rapid evolution has brought with itsome of the biggest engineering challenges to date.,4 couple of years ago, the design of a communica-tion system capable of operating in multiple prop-agation environments: delivering a broad range ofservices each with i ts ow n quality requirements on aglobal scale t o a person with a single handheld ter-minal [a ] would have sounded far fetched. Todaythis is reality. The European TelecommunicationStandards Institute (ETSI) have recently decidedto adopt Wideband CDM-4 (W-CDh4A) and Time-Division CDhlA (TD-CDMA) as basis for the imple-mentation of UhlTS, opening the wa y for the stan-dardization process to be completed and for the in-

This work was sponsored by Alcatel CIT S.A. an d AlcatelAltech Telecoms (Pty.) Ltd. at the Alcatel Research Un i t forWireless Access at the University of Pretoria

troduction of advanced mobile service t o commence.What is of specific interest in the m'-CDMA and TD -CDMA proposals is that the use of smart antennaand SDMA [ 3 ] , 4], [5]concepts are fully supportedby the standa rd in order to increase the capacity ofthird generation mobile systems. Specifically, SDhfAtechniques will be implemented to yield [6]Increased cell coverage areas to reduce high basestation site cost in low traffic areas.Reduc t i on of i n t e r f e rence to improve service qualityand/or increase the frequency re-use factor. Thispoint is especially important in CDMA based sys-tems which are interference limited.E x t e n d s y s t e m trafic capacity.In the following section, the operating principle ofsmart antennas and S DM A systems will be discussedand it will be shown qua1itat)ively how these t.ech-niques increase the performance of cellular systems.Section I11 will highlight the aspects that influencethe efficiency of the smart antenna and SDMA tech-niques discussed in section I1 and present a n exampleof hoK smart antenna techniques may be applied toTD-CDM.4 systems. Finally. some conclusions willbe presented in section 11;.

11. ShlART A N T E N N A S A N D SDM.4Cellular access systems rely on the fact that users

of a single resource - the Base Station (BS) - will beseparable in one or more domain: that is frequency(viz FDMA), time (viz TDMA) or code (viz CDMA).Thus, in a FDMA system (for example AMPS [7 ]and CT-2 [SI), simultaneous transmissions to a BSwill have different carrier frequencies and will there-fore not interfere with one moth er . Similarly, in aT D M A system (for example the access method em -ployed in one frequency allocation in an IS-54 sys-te m [SI), ransmissions to the BS are separated intime to prevent interference. These multiple ac-cess techniques can also be combined to form, forexample, TDMA/FDM A (IS-54) or FDMA/CDMA(IS-95 [SI) systems or any combination thereof. Allof the above mentioned multiple access techniquesdo however share one common trait, being the non-homogeneous geographical distribution of their sub-scribers. This means tha t all of the mentioned multi-ple access systems can exploit another dimension, vizthe spatial dimension, of the cellular problem to in-crease system capacity or cellular spectral efficiency.

1610-7803-5054-5.0161 $10.00 01998 IEEE

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In [9]: [ lo] . Ill]: cellular spectral efficiency is de-fined as a basis to rate the performarice of a cellularsyskrn. Many definitions for cellular spectral effi-ciency have been proposed. including b i t / s / H z [SI.[11](with the dat a rate measured as some predefinedBit Error Rate (BER,)):E r l u n y / M H ~ / k m ' , quiva-lent, telephone Erlangs per square kilometer [12] andeven Mbit/s-per-floor for indoor environments [la].Because SDMA and smart ant,enna systems rely onspatial para meters , a spatial parameter is included inthe definition of spectral efficiency in order to evalu-ate cellular system performance:Definition 1: Cellular Spectral Efficiency ( q )The cel lu lar spectral e f ic iency of a sys tem i s definedU S t he su m o f t he m ax im um da ta ra t e s t ha t can , bedelivered to subscribers afil iate d to all base stati onsin a r e u s e c l u st e r of cel ls j occupj ing a def ined phys-ical area.h,Iathematically, cellular spectral efficiency, rl is de-fined as

where r denotes the number of cells in a re-use clus-ter, Rij denotes the data rate measured in b i t s l s atsome predefined BER available t o subscriber i in cellj of the re-use cluster, B denotes the total bandwidthrneasured in H z allocated t,o all cells in the re-usecluster and -&luster denotes the physical area, mea-sured in km'. occupied by the re-use cluster. Clearly,the concept, of the re-use clus ter is fundamental in thedetermination of q . Definitions for a re-use clustercan be found in [ l l ] . 13]. In this paper, a re-usecluster will be defined as follows:Definition 2: Re-use Cluster ( r )A se t o f cells which have access t o the to talT ime /Frequency and Code ( T / F / C ) resources auail-able an the ce l lu lar sys tem.Figure 1 shows this scenario. for the case wherer = 3 . Thus, each set of cells forming a re-use patternexists tot,ally independent (as far as T/F/C resourcesgo) of th e other cells in the are a.

I a st Starion .Mobile Station

I Re-useclusterFig. 1. Cellular re-use concept

IVith the above definitions in mind. the task of asmart antenna or SDMA system is clear - optimizethe cellular spectral efficiency of the network, or in

other words, increase the re-use of the available cellu-lar resources. Examining i:i) t is clear ohat reducingthe size of the re-use clustlir will increase the cellularspectral efiiciency. However, a reduced cell size willsignificantly increase the interference presence in thecell limiting the capacity of the network in the caseof a CDM.4 system. The :increased amount of inter-ference, specifically in the uplink? can be overcomeusing a smart antenna technique called High Sensi-tivit,y Reception (HSR) [6].Definition 3: High Sensitivity ReceptionHigh Sensi t iv i ty Recept ion re fers to the use of adap-t i ve an t enna ar rays in the upl ink of a ce l lu lar net -work t o f ocus t he an t ennu beam ' on a spec if ic u se r ;t he reby i nc reas ing t he an t enna ga in in the direct ionof the user ard suppresszng transmiss ions receivedf r o m i n t e rf e r in g u s e r s.

This concept is depicted in Figure 2. In the case ofTDMA and FDMA systems the HSR system ma>-usepencil an tenna beams [14] tJo ocus on the active userswhereas in CDM.4 systems, the HSR system can in-crease the signal-to-noise ratio (SKR) in the uplinkby introducing nulls in the antenna pattern in thedirection of strong interfering signals. T he antennagain of HSR with an M-element antenna is equal to10 og M ? where M s the riumber of elements in thearray.

..

Fig. 2. S D M A system implemented using adaptive antennaarrays.

In a manner similar to HSR, Spatial Filtering forInterference Reduction (SFIR) [6] can be used in thedownlink of a cellular system to focus all the energyradiated by the base station onto a, single user orcluster of users. Therefore, SFIR is defined as:Definition 4: Spatial Filtering for InterferenceReductionSFIR reduces the in ter ference experienced by mobi lec o m m u n i c a t i o n s y s t e m s in the downl ink by concen-trat ing al l radiated e lec tromagnet ic energy in the di -rect ion of a user or group of users , avoiding geo-graphical areas where no w e r s are act ive .

Because the uplink of a cellular network is in gen-eral the capacity limiting factor, it might seem tha tHSR. systems will yield greater capacity advantagesthan SFIR systems. However, th e increased down-link quality afforded by SFIR techniques may lead toless dropped calls during handovers (because of th ebetter signal quality estimates available to the mo-

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bile), increasing the overall quality of service. =Ilso. A . Angu lar dastrabutzon of subscrzbrrsdue to the dynamic nature of an adaptive antennaarray: a SFIR system can facilitate the tracking of auser across cell boundaries. increasing the chances ofa successful handover T,O the next cell.

Khereas HSR and SFIR techniques iricrcase thecellular spectral efficiency b y decreasing the totalco-channel interference levels in a cell, SDMA tech-niques increase cellular spect ral efficiency by decreas-ing . 4C17L s fp rith the consequent decreasing in thephysical size of the re-use cluster. In other words.the same physical cellular network resources can bere-used more often. \-arious definitions have beenproposed t o define SDXlX techniques [ 3 ] , SI3[4].yesummaries these as follows:Definition 5 : Space Division Multiple AccessA S D M A s y s t e m i s a multiple access technique whichenables tula or more subscr ibers , a f i l i a t e d t o th es u m e bnsc: s t a t i o n . t o u s c th,e sunie Time an d Fre-quency a n d Code (T/F/C) resources on the grotindsof their pl i ys i cnl locat ion or s p a t i d s e p " z .

SDMAcluiinciA,n>, 0 DM A Chmncl Bi ' \ E DMA Channel C/ / MS

r ig 3 Space DiLisiori hiultiple Access ( S D M A ) Aliowmguser$ In th e same cell to share time/frequenc\ and codem o u r c e s

This scenario is depicted in Figure 3 where MobileStations (AIS) MS1, %IS2 and YIS3 share a same setof T/F/C resources with hfS4. 5IS5, MS6 and LIST,XISS arid MS9 For example, AIS4 and AIS1 mayboth be allocated carrier frequency fl. time slot TIand Lode c1 although the\ are affiliated to the sameBS, because of their spatial separation In [G ] it isshown analytically and by measurements that gainsin the order of 5-9 dB can be obtained using antenndarravs with 8 elements

111. S Y S T E M ASPECTS I N F L U E N C I N G SDM21\PERFORMA4NCE

It should be clear from the discussion above thatth e gain offered by a SD51L4 ystem will rely or1 manyparamet ers. some of which are beyond the control ofthe design engineer. Specifically tw o main areas ofinfluence can be identified namely, (i ) angular distri-but ion of subscribers arid (ii) the Quality of servicerequired required by each subscriber. In this section,it is shown how each of these aspects influence thegains of a SDhL4 network.

In [ l z ] it 17 slionn that the distribution of theangle-of-arrival of signals in d cellular sxstem is de-pendent on both the distribution of subscribers in acell. as a e l l as the dist ribution of scattering elementsaround each wbsrriber Specihcall), the manner inwhich subscribers axe clustered together in angle (a swould be the case on a road). significantl! influence