An Adaptive Channel Estimation Technique in MIMOOFDM Systems.docx

An Adaptive Channel Estimation Technique in MIMOOFDM Systems.docx

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AnAdaptiveChannelEstimationTechniqueinMIMOOFDMSystems

AnAdaptiveChannelEstimationTechniqueinMIMOOFDMSystems

JOURNALOFELECTRONICSCIENCEANDTECHNOLOGYOFCHINA,VOL.6,NO.3,SEPTEMBERQ08313AnAdaptiveChannelEstimationTechniqueinMIMOOFDMSystemsPei-ShengPanandBao-YuZhengAbstrac手_—Inthispape~anadapt~echannelestimationforMⅡooFDMisproposed.AsetofpilottonesfirstareplacedineachOFDMblockthenthechannelfrequencyresponseofthesepilottonesareadaptivelyestimatedbyrecursiveleastsquares（RLS）directlyinfrequencydomainnotintimedomain.Thenaftertheestimationofthechannelfrequencyresponseofpilottones,toobtainthechannelfrequencyresponseofdatatones,anewinterpolationmethodbasedonDFTdifferentfromtraditionallinearinterpolationmethodaccordingtoadjaeentpilottonesisproposed.Simulationresultsshowgoodperformanceofthetechnique./ndexTerms--Adaptive,channelestimation,frequencydomain,M1MOOFDM,recursiveleastsquares?

1.IntroductionOrthogonalfrequencydivisionmultiplexing（OFDM）technologyturnsafrequency-selectivechannelintoaparallelcollectionoffrequencyflatsub-channelstomitigatetheeffectsofintersymbolinterference（ISI）.Ontheotherhand,multi-inputmulti-output（MIMO）systemscangainmuchmorespectralefflciency（inbitspersecondperhert~byadoptingmulti-antennaonbothendsofthewirelesslinkthansingle-inputsingle-output（SISO）.Hence.thecombinationOf~ⅡM0systemswithOFDMtechnologyprovidesapromisingcandidatefornextgenerationfixedandmobilewirelesssystemsHoweveLaccuratechannelstateinformationsoisrequiredforspace-timedecodenThechannelstateinformationcanbeobtainedthroughtwotypesofmethods.Oneiscalledblindchannelestimation,whichexploresthestatisticalinformationofthechannelandcertainpropertiesofthetransmittedsignals.Theotheriscalledpilot-basedchannelestimation,whichisbasedonthepilotdatasentattheManuscriptreceivedOctobor16,2007;revisedNovember16,2007.ThisworkwassupposedbytheNafionaINatureScienceFoundationofChinaunderGrantNo.60372107.P..S.PanandB.-Y.ZhengarewithSchoolofCommunication&InformationEngineering,UniversityofNanjhagUniversityofPostsandTelecommunications,N眦jhag,210003,China（e-mail:

panps@,zby@}皿J）.transmitterandknownaprioratthereceivenThoughtheformerhasitsadvantageofnooverheadcost,itneedsalongdatarecordandcomplexalgorithm.Asforthelattersomepilot-basedchannelestimationschemeshavebeenproposed[41’[61.Buttheseschemeshavehighcomputationalcomplexityandmemoryrequirementsandpriorknowledgeofthesecondorderstatisticsofchanne1.UsingpilottonestoobtaintheCS1wasdiscussedin【7】,whereanoptimalplacementofthepilottoneswasproposedforMIMOOFDMsystems.In【81,anadaptivecharmelestimationschemewasproposedfor～ⅡMOOFDMsystems.Thisschemeadaptivelyestimatethechannelimpulseresponseintimedomainbasedontheleast-mean-square（LMS）algorithm..Inthispaper,weproposeanadaptivecharmelestimationtechniquewhichadaptivelyestimatethechannelfrequencyresponsedirectlyinfrequencydomainbasedontherecursiveleastsquare（RLS）algorithmoffrequencydomain[.Wefirstgiveorthogonalstructureofpilottones.ThenthechannelfrequencyresponseofthesepilottonesisadaptivelyestimatedbyRLSalgorithminfrequencydomain.Inordertoobtainthechannelfrequencyresponseofdatatones.aninterpolationmethoddifferentfromlinearinterpolationmethodaccordingtoaacentpilottonesisproposed.Here,theestimationofchannelfrequencyresponseisobtainedthroughthechannelimpulseresponseintimedomain,whilethechannelimpulseresponseintimedomainisobtainedthroughthechannelfrequencyresp0nseofpilottones.Simulationresultshaveshowngoodperformanceforthetechnique.2.SystemModelAsimplifiedblockdiagramofMIMOOFDMsystemswithtwotransmitantennasandtworeceiverantennasisshowninFig.1.Attime,adatablockb[n,尼】,k=0,l,…,Ⅳ一listransformedintotwodifferentsignals:

In,k】:

k=0,l,…,N—l;l,2atthespacetimeencoder,whereMandfisthenttmberoftonesoftheOFDMsystems,toneindex.andantennaindexrespectivelyEachofthesesignalsformsanOFDMblock.Then,thetwotransmittingantennassimultaneouslytransmit0FDMblocksafteranN-lengthIFFToperation.314JOURNALOFELECTRONICSCIENCEANDTECHNOLOGYOFCHINA,VOL.6,NO.3,SEPTEMBER2008Fig.1.MIMOOFDMsystem:

（a）transmitterand0o）receiverAtthereceiver,thediscreteFouriertransform（DFT）ofthereceivedsignalateachreceiverantennaisthesuperpositionoftwodistortedtransmittedsignals.TheoutputoftheFFTcanbeexpressedas即,】=∑In,】五[即,】+NIn,纠

（1）whereH即,】denotesthechannelfrequencyresponseatthekthtoneofthenthOFDMblock,correspondingtotheithtransmitandthejthreceiveantenna,N,[即,k】denotestheadditivecomplexGaussiannoiseonthejthreceivingantennaandisassumedtobezeromeanwithvariance.Wecanexpress

（1）inmatrixformas:

∑xH--lj+N

（2）whereViii/=[[即,0】,,1】,…,[即,N一111N=[Nj[n,othereceiveantenna,FdenotestheNxNDFTmatrix.ItiseasilyseenthatLtonescanbeusedfortrainingtorecoverthechanne1.LetK={白,,…,}bemesetofLtonesusedaspilottones.Wehave旦=（4）where望=[[】,[L¨?

[即,】F=略…时FisaVandermondematrix,soitisnon-singular,andtherefore,hcanbefoundbyinvertingit.Oncehisfound,wecanobtainthechannelfrequencyresponserequiredforspace—timedecoder.ThatmeansweonlyneedLpilottonesineachOFDMblocktorecoverythewholechannel~equencyresponse.LpilottonesareplacedequallyspacedwithN/Lin~equencydomain.Pilottonesaredividedinto2groupsfortwotransmitantennasrespectivelytomakethemorthogonal:

=,…,}=小?

+}.Forthe胁transmitantenna,sendingpilottoneson,sendingzerosignalson（≠f）andsendingdataonothertonesareshowninFig.2.Time（a）●PilotoZerosignaloDataTime（b）Fig.2.Pilotarrangement:

（a）antenna1andCo）antenna2.Onpilottonekoftheithtransmitantenna,onlythetoneitselfcansendsignals,then

（1）canbesimplifiedasfollowsaccordinglyL[n,k]:

In,k]x,In,k]+Ⅳ』In,k]（5）HereAlamoutitl~lspacetimeencoderisadopted,sowhenm∈wehave,Y（）=X（）H（）+N（）（6）where,,......................._II口h.Tg叮2.TPANet口:

AnAdaptiveChannelEstimationTechniqueinMIMOOFDMSystems315Y（）=[Yj[-,],Yj[n+1,]]xcH（）:

[H1In,],H2+l,]]N（）=[NjIn,],+1,]].ThecorrespondingadaptiveestimationofthechannelfrequencyofpilottonesisshowninFig.3.Fig.3.Adaptivechannelestimationofpilottones（）CaDbeupdatedbyusingRLSalgorithmtinfrequencydomainasfollows:

E十2（）:

Y（）一X（）（）（7）（）:

（）+G（）E（）（8）GP（:

川（一G也（x（P川（）（1o）where（）isfiltergainmatrix.Initializing,（）=o,P[0]（）=81.（11）Aftertheestimationofthechannelfrequencyresponseofpilottones,wecaDobtainthechannelfrequencyresponseofdatatones.By（4）thechannelimpulseresponseintimedomainwithmaximum1engthofchannelhcaD—befound,ther~byf3）thechannelfrequencyresponseneededbyspacetimedecodercaDbeobtained.Wenoticethefactthatthelengthofwirelesschannelist-miteaDdusuallyfarsmallerthanthenumberoftonesinaOFDMblock,soweaddawindowtothechannelimpulseresponseintimedomain,thenbyDFToperationtoacquirethechannelfrequencyresponse.4.SimulationInoursimulationsystem,twotransmitantennasusinga2x2Alamoutispacetimecodeareutilizedfordiversity.TheOFDMblocksizeischosenasN=512addacyctcprefix（CP）withlengthof16isinsertedattheheadofeachOFDMblock.Thefoursub.channelsareassumedtobeindependenttoeachotheraddtohavemaximum1engthofchannelL=16individually,whicharesimulatedbytheJakes’model[“.Dopplerrespectivelysimulatedasshiftaddcarrierfrequencyare{a148HzaDd{c3.2GHz.QPSKisapplied.Fig.4showsthattheBERperformanceofthereceiverwithadaptiveRLSchannelestimationisclosetOthatofthereceiverknowingthechanne1.InFig.5.weshowtheconvergencebehaviorofourRLSalgorithmsat15dBinthepilottone1.TheMSEresultswereObtainedbvaveragingover1OOsimulationrtms.Bycomparison.theconvergencebehaviorofLMSalgorithmsproposedin【8]isalsoshownwiththeadaptiveconstant1.Forconveniencetocomparethechannelfrequencyresponseatpilottone1ObtainedbyourRLSalgorithmswithLMSalgorithms,thechannelimpulseresponseintimedomainobtainedbyLMSalgorithmsineveryiteration（oneOFDMsymbo1duration）are仃aDSformedintothefrequencydomain.Sonot1osm’gjustness,weshowtheconvergencebehaviorofbothRLSaddLMS.ThereasonthattheproposedILSalgorithmconvergesmuchfasterthanLMSliesinitsprocessingdoneinfrequencydomainnotintimedomain.024681Ol214161820SNR（dB）Fig.4.Biterrorratev.s.SNRwimDopplershiftis148Hz.050100150200250300350400450500OFDMsymbolFig.5.ExperimentallearningcurvesofRLSandLMS.5000OFDMsymbolsaredealttodemonstratethetrackingforthepilottone1at15dBinFig.6aDdFig.7.Therealcomponentofthefiequencyresponseinthepilottone1aDdtheimaginarycomponentofthefrequencyresponseinpilot1are打ackedinFig.6aDdFig.7respectivelybyRLSalgorithm.Theresultshowsthegoodtrackingforthepilottone.ThisgoodtrackingisduetofasterconvergenceofRLSdoneinfrequencydomainthan●Lr【2●∞∞50505050505一pJ∞∞苫316JOURNALOFELECTRONICSCⅢNCEANDTECHN0【DGYOFCHrNA.VOL.6,NO.3,SEPTEMBER2008LMSdoneintimedomain.ItisclearthatthecomputationalcomplexityorderoftheRLSalgorithmiso（ZM~）andthatoftheLMSalgorithmisOLMr,）,whereMrdenotesthenumberofreceiveantennas.OFDMsymbolOFDMsymbolFig.6.Comparisonoftherealcomponentofthefrequencyresponseofpilottone1betweentheestimationandrealchannelat15dB.OFDMsymbolOFDMsymbolFig.7.Comparisonoftheimaginarycomponentofthefrequencyresponseofpilottone1betweentheestimationandrealchannelat15dB.5.ConclusionsWeproposeanefficientadaptivechannelestimationtechniqueforMⅡMOOFDMsystems.TwosetsofLpilot.tonesaretransmittedattwoantennassimultaneouslyandthechannelcanbeestimatedemciently.Theadaptivechannelestimationtechniqueisbased0nISalgorithmanddonedirectlyinfrequencydomain.ThemainadvantageisrootedinitsabilitytotrackthechannelfrequencyresponseofpilottonesquicklyandtheacquirementofchannelfrequencyresponseindatatonesbasedonDFTnotlinearinterpolation.References【1】GJ.Foschini,”Layeredspace—timearchitectureforwirelesscommunicationsinafadingenvironmentwhenusingmulti—elementantennas,’’BellLabs.Tech.,vo1.1,no.2,pp.41.59,1996.【2】GJ.FoschiniandM.J.Gans,”Onlimitsofwirelesscommunicationsinafadingenvironmentwhenusingmulti.elementantennas,”FlessPersonalCommun.,vo1.6,no.3,pp.311-335,Mar.1998.【3】H.Sampath,S.Talwar,J.Tellado,VErceg,andA.Panlraj,‘Afourth-generationMM0.OFDMbroadbandwirelesssystem:

design.performanceandfieldtrialresults,”衄CommunicationsMagazine,vo1.40,no.9,PP.143—149,Sep.2002.【4]YLi,”Channelestimationfor0FDMsystemswimtransmiaerdiversityinmobilewirelesschannel,IEEEJSAC.vo1.17,no.3,PP.461—471,1999.【5】YLi,H.W.mters,andN.R.Sollenberger,”MIM0一OFDMforwirelesscommunications:

signaldetectionwithenhancedchannelestimation,”衄Trans.Comm.,vo1.50,no.9,PP.1471—1477,2002.【6】YLi,”Sim