OFDM传输技术与研究外文翻译.docx

OFDM传输技术与研究外文翻译.docx

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OFDM传输技术与研究外文翻译

RapidMIMO-OFDMSoftwareDefinedRadio

SystemPrototyping

AmitGupta,AntonioForenza,andRobertW.HeathJr.

WirelessNetworkingandCommunicationsGroup

DepartmentofElectricalandComputerEngineering,TheUniversityofTexasatAustin

1UniversityStationC0803,Austin,TX78712-0240USA

Phone:

+1-512-232-2014,Fax:

+1-512-471-6512

{agupta,forenza,rheath}@ece.utexas.edu

Abstract—Multipleinput-multipleoutput（MIMO）isanattractivetechnologyforfuturewirelesssystems.MIMOcommunication,enabledbytheuseofmultipletransmitandmultiplereceiveantennas,isknownforitshighspectralefficiencyas

wellasitsrobustnessagainstfadingandinterference.CombiningMIMOwithorthogonalfrequencydivisionmultiplexing（OFDM）,itispossibletosignificantlyreducereceivercomplexityasOFDMgreatlysimplifiesequalizationatthereceiver.MIMO-OFDMiscurrentlybeingconsideredforanumberofdevelopingwireless

standards;consequently,thestudyofMIMO-OFDMinrealisticenvironmentsisofgreatimportance.ThispaperdescribesanapproachforprototypingaMIMO-OFDMsystemusingaflexiblesoftwaredefinedradio（SDR）systemarchitectureinconjunctionwithcommerciallyavailablehardware.Anemphasisonsoftwarepermitsafocusonalgorithmandsystemdesignissuesratherthanimplementationandhardwareconfiguration.Thepenaltyofthisflexibility,however,isthattheeaseofusecomesattheexpenseofoverallthroughput.Toillustratethebenefitsoftheproposed

architecture,applicationstoMIMO-OFDMsystemprototypingandpreliminaryMIMOchannelmeasurementsarepresented.Adetaileddescriptionofthehardwareisprovidedalongwithdownloadablesoftwaretoreproducethesystem.

I.INTRODUCTION

Multiple-inputmultiple-output（MIMO）wirelesssystemsusemultipletransmitandmultiplereceiveantennastoincreasecapacityandproviderobustnesstofading[1].Toobtainthesebenefitsinbroadbandchannelswithextensivefrequencyselectivity,MIMOcommunicationlinksrequirecomplexspacetimeequalizers.ThecomplexityofMIMOsystemscanbereduced,however,throughorthogonalfrequencydivisionmultiplexing（OFDM）.OFDMisanattractivedigitalmodulationtechniquethatpermitsgreatlysimplifiedequalizationatthereceiver.WithOFDM,themodulatedsignaliseffectivelytransmittedinparalleloverNorthogonalfrequencytones.ThisconvertsawidebandfrequencyselectivechannelintoNnarrowbandflatfadingchannels.CurrentlyOFDMisusedinmanywirelessdigitalcommunicationsystems,suchastheIEEE802.11a/g[2],[3]standardsforwirelesslocalareanetworks（WLANs）.MIMO-OFDMtechnologyisintheprocessofbeingstandardizedbytheIEEETechnicalGroup802.11n[4]andpromisestobeastrongcandidateforfourthgeneration（4G）wirelesscommunicationsystems[5].

AsthetheorybehindMIMO-OFDMcommunicationcontinuestogrow,itbecomesincreasinglyimportanttodevelopprototypeswhichcanevaluatethesetheoriesinrealworldchannelconditions.Duringthepastfewyears,anumberofMIMO-OFDMprototypeshavebeendeveloped[6]–[12].TheseimplementationsmakeuseofFPGAsorDSPs,whichrequirealargeamountoflowlevelprogrammingandafixedpointimplementation.Thisisthepreferredsolutionwhendevelopinghigh-speedimplementations;however,ithinderstheflexibilityoftheplatformasthesesystemsarenoteasilyreconfigurable.Asaresultwhenexperimentingwithmanydifferentspace-timecodingschemesorreceiverdesigns,amoreflexiblesolutionmaybepreferred.

InthispaperweproposeaMIMO-OFDMsystemarchitecturebasedonthesoftwaredefinedradio（SDR）paradigm.Theadvantageofthisapproachliesinthefactthattheuserisnotrequiredtohaveindepthhardwareknowledgeandmayimplementanumberofdifferentschemesbysimplyreconfiguringthesoftware.TheplatformusesNationalInstrumentsradiofrequency（RF）hardwareinconjunctionwiththeLabVIEWgraphicalprogramminglanguage.Withthisarchitecture,itispossibletodefineandsimulateasysteminahighlevelprogramminglanguageandthenseamlesslyapplythatcodetowardsthehardwareimplementation–thisgreatlyreducesthetimeinvolvedinsystemprototyping.Comparedwith[6]–[12],ourprototypingplatformcaneasilybereduplicatedasitconsistsofcommercial-off-the-shelfhardwareandpubliclyavailablesoftware.AuserwhopurchasestheRFhardwarefromNationalInstrumentsanddownloadstheavailableMIMOsoftwaretoolkitalongwiththeprototypingcodedevelopedbytheauthors（availableat[13],[14]）canrealizethesamerapidprototypingbenefitswhichwediscussinthispaper.

Theflexibilityofthecurrentimplementationoftheprototypeislimitedbysomehardwareconstraints,suchasthebandwidthofthePCIbus,whichpreventsfullyreal-timetransmissionoverthewirelesslink,andsoftwareconstraintslikeourlackofcompletesynchronizationalgorithmsinthesoftware,whichcausesustouseawiredsynchronizationchannel.ThespiritofthiscontributionistosummarizeourmethodanddescribeourfirsteffortstowardsthedevelopmentofacompleteMIMO-OFDMplatformdesignedforsystemvalidationand

Fig.1.A2×2MIMO-OFDMspatialmultiplexingsystem

channelmeasurements.Moreworkneedstobeinvestigatedtoovercomethelimitationsandexpandthecapabilitiesofourinitialdesign.

Thispaperisorganizedasfollows.SectionIIexploresthesignalmodelforaMIMO-OFDMsystemandourspecificMIMO-OFDMimplementation.SectionIIIdiscussesthespecificsofthehardwareandsoftwareplatform.SectionIVshowspreliminaryresultsfromoursystemimplementationaswellaschannelmeasurementsinindoorenvironments.

II.MIMO-OFDMIMPLEMENTATION

InthissectionwereviewtheMIMO-OFDMsignalmodelandthendescribeourspecificMIMO-OFDMsystemimplementation.

A.MIMO-OFDMSignalModel

InaMIMO-OFDMsystem（see[8]andthereferencestherein）MIMOspace-timecodesarecombinedwithOFDMmodulationatthetransmitterwhilecomplicatedspace-timefrequencyprocessingisemployedatthereceiver.Forsimplicityofexplanation,weconsiderspatialmultiplexingasillustratedinFig.1thoughitwillbeapparentthatothertransmissiontechniquescanbeimplementedintheproposedarchitecture.

InaMIMO-OFDMsystemwithMTtransmitantennasandMRreceiveantennas,thesampledsignalatthereceiver（aftertheFFTandremovingthecyclicprefix）ofaspatialmultiplexingMIMOsystemforOFDMsymbolperiodnandtonekcanbeexpressedbythefollowingequation（assumingperfectlinearity,timing,andsynchronization.）[1]

（1）

TheequalizationinMIMO-OFDMsystemsmaybeenabledthroughdifferentproceduressuchaszero-forcingequalizer,minimummean-squarederrorequalizer,V-BLASTsuccessivecancellingequalizer,spheredecoder,andmaximumlikelihooddecoder（see[1]foranoverview）.Inourprototypewecurrentlyimplementthezero-forcingequalizer;theflexibilityoftheproposedarchitecturethoughallowsustoprototypemoresophisticatedequalizationstrategies.

B.SystemImplementationandSpecifications

Thefirstimplementationfeaturesspatialmultiplexingwithtwotransmitandtworeceiveantennas,asillustratedinFig.1.OtherMIMOschemesarealreadyavailableintheLabVIEWMIMOToolkit[14],andweareplanningtousethistoimplementotherspace-frequencycodesinthefuture.

ThespecificationsofthesystemarelistedinTableI.InourMIMO-OFDMimplementation,OFDMwith64tonesisemployedovera16MHzbandwidth.Thecyclicprefixis16sampleslong.ThiscorrespondstoanOFDMsymboldurationof5μs,withaguardintervalof1μsandadataportionof4μs.WetransmitourOFDMsymbolsin200msdatapackets.This200mswasdeterminedbyourhardwareasmemoryconstraintsatthereceiverpreventedlongeracquisitionperiods.Thesystemisequippedwithanadjustablecarrierfrequency.Wechosetorunoursystemat2.4GHz,whichisthecarrierfrequencyusedforWLANs[2],[3].Variousmodulationschemesarepossible（BPSK,QPSK,16-QAM,64-QAM）alongwithoptionalconvolutionalcoding.

ChannelestimationiscarriedoutbyperiodicallytransmittinganOFDMtrainingsymbol.Thefrequencyatwhichtrainingsymbolsaresentcanbeprogrammaticallychangedinthesoftwareanddependsontheexpectedvariationofthechannel.Theestimationatthereceiverisenabledbythepilotsymbols,sentoutoverorthogonaltonesacrossthetransmitantennas.Wethenusealinearinterpolationacrossthetonestoestimatethechannel’sfullfrequencyresponse.Oncewehaveachannelestimate,thedataisdemodulatedbyaMIMOzero-forcinglinearreceiver.Duetospacelimitations,inthiscontributionwedonotprovideanalyticaldetailsofthechannelestimationalgorithmemployedintheprototype.

TABLEI

SPECIFICATIONSOFOURMIMO-OFDMIMPLEMENTATION

No.ofTransmitAntennas

2

No.ofReceiveAntennas

2

CarrierFreq.

2.4GHz

Bandwidth

16MHz

No.ofTones

64

SubcarrierSpacing

25kHz

OFDMSymbolDuration

5us

GuardIntervalDuration

1us

OFDMdataduration

4us

LengthofCyclicPrefix

16samples

MIMOScheme

spatialmultiplexing

PacketDuration

200ms

Fig.2.PictureoftheNationalInstrumentsRFhardware

Wearecurrentlyavoidingcarriersynchronizationissuesbydirectlywiringtheclocksofthetransmitterandreceivertogether.Additionally,inordertoavoidtimingissueswearesendingatriggerfromthetransmittertothereceiverwhendatatransmissionbegins.Softwaresynchronizationisunderdevelopmentandwillbeincludedinfuturework.

AswearefollowingaSDRapproachtoprototyping,therearemanyparametersofthesystemwhichcanbeadjustedprogrammatically.TheflexibilityenabledbyaSDRMIMOOFDMprototypebecomesclearinthefollowingsectionwhereweprese