Multiple Antenna Technology and Its Application in WiMAX System

Zhao Qiang Wang Yanwen

（1. School of Electronic Engineering of Xidian University， Xi'an 710071， China；

2.Xi'an R&D Department of Mobile Research Institute， ZTE Corporation， Xi'an 710065， China）

Abstra c t:Worldw ide Interoperab ility for Microwave Access（WiMAX）technology has rap id ly developed in recent years.It is becom ing one of the hotspots am ong b roadband wireless access technologies.A WiMAX system can p rovide bandw id th efficiency higher than 2 bps/Hz.In order to imp rove coverage and re liab ility of the WiMAX system,the IEEE 802.16 standard supports multip le antenna techniques such as Alamouti Space-Time Cod ing（STC）,Adap tive Antenna System（AAS）and Multip le Input Multip le Output（M IMO）.The app lications of the mu ltip le antenna techniques im p rove system capac ity and bandwid th efficiency significantly,and enhance the competence of the WiMAX techno logy g reatly.

W orldw ide Interoperability for Microwave Access（WiMAX）is aw ireless b roadband access technology based on IEEE 802.16 standard fam ily.AWiMAX system can p rovide high-speed data,audio and video services in both fixed and mobile environments.

Demonstrating its unparalleled advantages in im p roving spec tral efficiency,supporting higher speed data transm ission,enhancing signalquality,expanding system coverage,and p roviding extra-large system capacity where itis needed,themultip le antenna technology has become the focus of current research and is w idely app lied to allkinds ofmobile telecommunications system s[1-5].

As the w ireless b roadband access technology to p rovide the best lastm ile accessmodel,WiMAXneeds to adop t themultip le antenna technology to enhance its own competence.

1 Advantages of Multip le Antenna Technology

With the rap id developmentofw ireless communication technologies,the g rave deficiency of radio spec trum resources is becom ing the bottleneck for further developmentofw ireless communications.How tofully develop and utilize the lim ited rad io spec trum resources and increase spec tral efficiency is one of the hottest research sub jects in the telecommunications field.

Themultip le antenna technology has won greatattention from the world for its ability to increase transm ission efficiency and spectralefficiency w ithoutadd itional bandw id th.

1.1 Advantages of Multiple Antennas over Single Antenna

（1）Array Gain

Multip le antennas can im p rove signals'coherence and generate array gain.

（2）Diversity Gain

Multip le antennas increase d iversity gain throughmulti-path,inwhich malperformance ofone path w illnot affect the entire system.In the w ireless fad ing channel,reliability ofdata transm ission can be im p roved by enhancing stability of the intensity of received signals.The d iversity gain can be ob tained in space,time and frequency domains.

（3）Elim ination ofCo-Channel Interference

By analyzing d ifferentchannel responses to interference,the app lication ofmultip le antennas can elim inate co-channelinterference signals.

1.2 Econom ical Multiple Antenna Technology

（1）The technology increases transm ission capacity and decreases network estab lishmentcosts in busy call areas.

With the unceasing expanding ofdata services,especially app lications like mobile phone TV and high-speed w ireless Internetservices,there has been increasing needs for such data services,however,services thatcan be offered are confined to the network capacity which lags farbehind users'needs,particularly in busy callareas like city centers or hotspotp laces.Besides,due to imbalance in data services,the network capacity is often circum scribed by the downlink speed.Take the hybrid network as an exam p le；a 2×2 spatiald iversity receiving Multip le InputMultip le Output（MIMO）system can raise the sing le Base Station（BS）capacity by nearly 20%.Provided that the area coverage and capacity need are unchanged,using the 2×2MIMO can save over15%of the numberofBS needed for the 1×2MIMO.This significantly reduces the network estab lishmentcosts in busy callareas.

（2）Itcuts cost for capacity expansion.

When using receivers thatare based on themultip le antenna technology,2×2 MIMO can raise the sector throughputby 40%to 60%.Particularly in city,downtown or indoorm icro-cellular cond itionswith comp licated multi-path environment,where there is higher demand on system capacity,sector throughputcan be inc reased by nearly 60%.

Therefore,w ithoutincreasing carrier frequency and the numberof BS,the multip le antenna technology can be used as one of the capacity expansion solutions thatcan notonly satisfy peop le's needs butalso do itw ith a low costso as to achieve a low-cost expansion.

（3）Itincrease costs of transceiver and antenna-feeder system.

The introduc tion of themultip le antenna technologymakes transceiver's p rocessing more com p licated:the BS mustnotonly supportmore than two independent transm itting antennas（each encoding,modulating,expand ing frequency and transm itting independently）,butalso p rocess the up link feedback signaling of two spatial data flows（such as CQIand ACK/NACK）.All these w illincrease the costof BS and end equipmentata certain deg ree.Besides,the installation of multip le antennas and the antenna feeder system ismore com p lex than thatof normalantennas.Therefore,the costs of BS and antenna-feeder system w ill rise w ith the introduction of themultip le antenna technology.

Undoubted ly,themultip le antenna technology,on one hand,can increase transm ission capacity of the system,and can according ly b ring g reatp rofit potential fornetwork operators who charge theirservices by data flow；on the otherhand,itw illmake the BS and term inalequipmentmore com p lex.Excep t forsignaling and measurement information,themultip le antenna technology actually affects the network slightly.

With regard to operation cost,the app lication ofmultip le antenna technology needs to be carried outafter fullconsideration of the influence ofmore com p licated system and equipment.

IEEE 802.16 standards supportsuch multip le antenna technologies as Alamoutispace-time code,Adap tive Antenna System（AAS）and MIMO.IEEE 802.16e,being ab le to effec tively imp rove system's anti-fading performance,takes transm itting d iversity,which is realized through space-time coding,as one op tionalitem.

2 Application of Multip le Antenna Technology in W iMAX System

2.1 AAS

AAS can realize automatic adap tation of system parameters,ob tain Signal-to-Noise Ratio（SNR）gains and reduce co-channelinterference.Ituses a certain d igitalsignalp rocessing technology to generate d irectional rad io beam in spacewhich can aim themain radio beam of the antenna atexpected d irection ofarrivaland supp ress interference by nullso as to achieve the best receiving ofexpected signals.The design and app lication of AAS technology in WiMAX is based on the Time Division Dup lex（TDD）mode.Since the up link and downlink share the same frequency band resources in the TDD mode,the downlink/up link channel information can be inferred from the up link/downlink so thatat the BSor term inal,the weighing value of radio beam can be easily calculated.Under the Frequency Division Dup lex（FDD）mode,the up link and the downlink channels are usually d ifferentand cannot recognize one from the other.In this case,the weighing value can only be calculated by feedback,which increases the overhead of the whole system.

In the WiMAX system,AAS is an op tionaltechnology thatcan be chosen forboth the up link and the downlink.Adop tion of AAS can increase system capacity,expand scope of coverage,enhance communication reliability and cutoperation costs.AAS can be fulfilled by eithermultip le rad io beam selection or adap tation.

2.2 MIMO

MIMO technology was firstput forward by Marconiin 1908.Itequips the BSand term inals w ithmultip le antennas to refrain from channel fad ing so as to increase channelcapacity,coverage and spectral efficiency by a largemargin.According to the numberof transm itting and receiving antennas,MIMO inc ludes Sing le InputMultip le Output（SIMO）and Multip le InputSing le Output（MISO）.

Themostimportantpartin MIMO is space-time signalp rocessing,or in otherwords,using multip le antennas to p rocess signalsw ith combined time and space domain information.Therefore,MIMO technology can be regarded as an expansion ofsmartantenna.In its broad sense,MIMO technology inc ludes transm itdiversity technology and space multip lexing technology.Transm it diversity technologymeans send ing signalsw ith the samemessage through differentantennas（although the form of the signalsmay notbe the same）so as to achieve the spatiald iversity,increase channel reliability and reduce BitError Ratio（BER）.Different from transm it d iversity technology,spacemultip lexing technology sends d ifferentmessages through differentantennas,thus ob taining space gains and accord ing ly increasing system capacity and spec tral efficiency substantially.Therefore,the combined app lication ofspace-time coding and spatialmultip lexing in WiMAX system can greatly increase system capacity and spectralefficiency.

Manifested in IEEE 802.16e,MIMO has become one of the op tions for multip le antenna technology in IEEE 802.16,which supports three MIMO modes:space-time transm itd iversity,spacemultip lexing,and the combination of these twomodes.

2.2.1 Space-time Transm itdiversity

The biggestadvantage of transm it diversity lies in the fact thatmultip le antennas can be used at the BS instead of receivers to avoid p ressure on the term inalequipments,thus reducing the markethind rance to IEEE 802.16.

In MIMO,the space-time transm it diversitymode ismainly im p lemented by space-time coding.Themain idea of space-time coding is to achieve a certain space diversity and time d iversity by adop ting cod ing w ith space and time domain information,thus reducing BER.When app lying the space-time code,MIMO system can realize the biggest d iversity gain even in a casewhere the senderhas no idea about the channel condition.The commonly seen space-time codes inc lude Space-Time Block Code（STBC）and Space-Time Trellis Code（STTC）.STBC has won w ider app lication for its relatively easier cod ing p rocess.

Based on AlamoutiSpace-Time Code（STC）,IEEE 802.16d standard adop ts transm itd iversity of two antennas to countervaildeep fad ing caused by Obstructed Line ofSight（OLOS）and Non-Line ofSight（NLOS）.Themost im portant feature of this scheme is the orthogonalsequences of the two transm itting antennas.Alamouticod ing can ob tain the biggestd iversity gain from the two transm itting antennas system.Besides,speaking from the history of cod ing development,Alamoutiis a kind ofspace-time b lock code specially designed to offer fulltransm itdiversity gain fora system w ith two transm itting antennas,and itonly needs simp le p rocessing of the received signalswhen decoding,thus g reatly sim p lifying the calculation p rocess.

2.2.2 Space Multip lexing

Spacemultip lexing technology sends mutually independentsignals at the transm itters,and decode at the receivers w ith the interference restraintapp roach in order to increase transm ission rate of the system.Currently,the ways thatuse spacemultip lexing to increase channel capacity aremainly d ifferent layered space-time codes such as Bell Labs Layered Space-Time（BLAST）.The BLASTstructure does not introduce orthogonality by signalchanges like coding and modulation,butby fully utilizing themulti-path feature of channels to elim inate the correlations between signals.

The BLASTstruc ture ismainly d ivided into two categories:VerticalBellLabs Layered Space-Time（V-BLAST）and DiagonalBellLabs Layered Space-Time（D-BLAST）.V-BLASTsends M bit flows outbymutually independentantennas after they are encoded,mapped and interleaved,so that the d iversity gain is fully tapped,and eachmessage flow can be tested separately.D-BLASTgoes through the same p rocedure as V-BLAST,butcod ing b locks are assigned to differentantennas for sending,thus reducing the possibility of performance deg rad ing of the entire system due to the unsatisfactory transm ission effectofan independent channel.However,this im p lies that transm itting and receiving equipmentwill becomemore com p licated.

Though the BLASTstructure reaches higher spectralefficiency than other structures,itnormally requires that the numberof receiving antennas is bigger than orequal to thatof the transm itting ones,which can hard ly be realized in the downlink.Besides,since the BLAST structure sends d ifferentsignals in d ifferent routes,ifone route is damaged there w illbe an irrevocab lem istake.these two technologies can offerd iversity gain and system capacity increase at the same time,thus p roviding a nice comp rom ise between high spectral efficiency and good transm ission quality.However,this solution ismore comp licated than using d iversity and multip lexing separately.

2.2.3 Integ ration ofDiversity and Multip lexing Space-time transm itd iversity can ob tain add itionaldiversity gain and coding gain,butcannot increase data transm ission rate；spacemultip lexing canmaxim ize the average transm ission rate of the MIMO system,butcan only get lim ited d iversity gain.A solution thatcombines

2.2.4 IntelligentMIMO MobileWiMAX supports Adap tive MIMO Sw itching（AMS）between d ifferentMIMO modes.AMS,as shown in Figure 1,is also called intelligentMIMO.Intelligent MIMO can select themostsuitab le MIMO mode accord ing to the channel cond itions,and increase the spectral efficiency w ithout reducing the scope of coverage.The adop tion of intelligent MIMO can overcome uncertainty brought by d ifferentapp lication scenarios,making the MIMO technologyw idely accep ted.For instance,d ifferent term inals in one networkmay have differentnumberofantennas,and if just one MIMOmode is used in a cell,the network op tim ization can hard ly reach the design goal.Besides,differentusers experience d ifferentsignal fading.DifferentMIMO technologies are selected to adap tchannelchanges,which can op tim ize system performance.To support the AMS,the transm itting end needsmore feedback information about channels orweight.

▲Figure 1. AMS.

One of themajorquestions and research focuses on futuremobile communication system s is how to guarantee the Quality of Service（QoS）in the case ofNLOS and bad channels.For a Sing le InputSing le Output（SISO）system,QoS guarantee requires quite much spectralresources and comp licated cod ing and modulating technologies,which restrict the developmentof SISO.Therefore,MIMO is a key technology for futuremobile communications.MIMO technology is im p lemented by either space multip lexing orspace-time cod ing,which have allbeen app lied in the WiMAX system.WiMAXalso allows anotherMIMO op tion,thatis,to use spacemultip lexing and space-time cod ing simultaneously.

2.3 Com parison between AAS and MIMO

AAS and MIMO are bothmultip le antenna technologiesw ritten in the 802.16 standards,but theirworking theory and app lication scenarios are different.Hereafter is the comparison between the two technologies.

（1）System Capacity

Both the AASand MIMO technologies usemultip le antennas to streng then signals and acquire additionalsystem capacity.AASgenerates sing le highly-energy-concentrated rad io beam.Theoretically speaking,the system's capacity is geometrical to the strength of the radio beam.In the WIMAX system,the BS throughputis 25Mb/s when equipped w ith the basic SISO configuration.An AASw ith fourarray elements can raise the throughputby 50%and reach 33Mb/s,and an AAS w ith eightarray elements can reach 38Mb/s.Different from AAS,MIMO system's capacity is linear to the number ofantennas,so 2×2MIMO system's capacity is 2 times SISO and 4×4MIMO system's capacity is 2 times 2×2MIMO.Therefore,MIMO can getmore system capacity than AAS.

（2）Mobility

Com pared w ith IEEE 802.16d,the biggestim p rovement that IEEE 802.16e hasmade is its ability to supportmobile environment.AAS depends heavily on accurate and reliab le estimation about the channel to get the weighing value of the rad io beam.However,inmobile environments,the channelestimation w ill become very comp licated.Forexamp le,at30 km/h and at3.5 GHz,channel estimation would need to occurevery m illisecond tomaintain best performance,which w illcause quite a large system overhead.Therefore,in a high speed mobile environment,AAS can hard lywork,while in the MIMOmode,the channelinformation is notnecessary at the transm itting end,making MIMO a better fit formobile environments.

（3）Others

Since AAS needs channel information,it is,in some deg ree,confined to work in the TDDmode,while MIMO can work normally in both the TDD and FDDmodes.MIMO can also be integ rated w ith OFDM technology to reach better performance—one of the development trends formobile communication technologies in the future.

MIMO technologymeets the requirements forw ireless signal p rocessing in urbanmulti-path cond itions verywell.Itcan p rovide space diversity and paralleltransm ission in multip le channels.Itis a well-designed technology to increase coverage and throughputof theWiMAX system.AAS is good at raising the d irectionalgains of the BSand themoving ob ject,aswellas atd irectionalrestraint to interference signals.However,in the urban multi-path environment,MIMO ismore likely to be welcomed by equipment developers.In otherwords,MIMO technology suitsWiMAX better.Only when WiMAX supports the basic MIMO working mechanism can itbe com patib le w ith other technologies and become more competitive.

3 Conclusions

WiMAX is the specialtechnology for mobile b roadband w ireless access.Itis developed w ith sound performance,high efficiency and low cost,and itcanmeet requirements ofvarious app lication scenarios by flexib le system configuration.

Faced w ith the highly intense com petition in the w ireless communicationmarket,WiMAX,in order to w in a solid footing,has to adop t the mostadvanced technologies and convert the technologicaladvantages to p roduct advantages to enlarge its commercial use scale,and in themeantime,to maintain its streng ths in big capacity,large coverage,NLOS and unified standard.

Ofall the key technologies forWiMAX system,multip le antenna technologies,such as AASand MIMO,have attracted g reatattention from themarketbecause they are ab le to substantially enhance the capacity ofw ireless communication system s.Differentmultip le antenna technologies fitdifferentscenarios well；so,a p roper technology should be selected to enab le the system to perform at its best.Meanwhile,more consideration should be given to the cost of the technology in realapp lication,as wellas its integration w ith other technologies,such as OFDM.WiMAX is expected to surpass Long Term Evolution（LTE）and Air Interface Evolution（AIE）and develop into the best w ireless b roadband access technology for lastm ile access.