基于快速MIMO-OFDM无线电定义系统的原型设计

沈阳航空航天大学电子信息工程学院毕业设计（外文翻译）M.Nettles,“AJointMIMOOFDMTransceiverandMACDesignforMobileAdHocNetworking,”inProceedingsoftheInternationalWorkshoponWirelessAdHocNetworks,Oulu,Finland,May31-June32004.[21]J.W.Wallace,M.A.Jensen,A.L.Swindlehurst,andB.D.Jeffs,“ExperimentalcharacterizationoftheMIMOwirelesschannel:dataacquisitionandanalysis,”IEEETransactionsonWirelessCommunications,vol.2,pp.335–343,March2003.RapidMIMO-OFDMSoftwareDefinedRadioSystemPrototyping（Received:AmitGupta,AntonioForenza,andRobertW.HeathJr.WirelessNetworkingandCommunicationsGroupDepartmentofElectricalandComputerEngineering,TheUniversityofTexasatAustin1UniversityStationC0803,Austin,TX78712-0240USAPhone:+1-512-232-2014,Fax:+1-512-471-6512{agupta,forenza,rheath}@)AbstractMultipleinput-multipleoutput(MIMO)isanattractivetechnologyforfuturewirelesssystems.MIMOcommunication,enabledbytheuseofmultipletransmitandmultiplereceiveantennas,isknownforitshighspectralefficiencyaswellasitsrobustnessagainstfadingandinterference.CombiningMIMOwithorthogonalfrequencydivisionmultiplexing(OFDM),itispossibletosignificantlyreducereceivercomplexityasOFDMgreatlysimplifiesequalizationatthereceiver.MIMO-OFDMiscurrentlybeingconsideredforanumberofdevelopingwirelessstandards;consequently,thestudyofMIMO-OFDMinrealisticenvironmentsisofgreatimportance.ThispaperdescribesanapproachforprototypingaMIMO-OFDMsystemusingaflexiblesoftwaredefinedradio(SDR)systemarchitectureinconjunctionwithcommerciallyavailablehardware.Anemphasisonsoftwarepermitsafocusonalgorithmandsystemdesignissuesratherthanimplementationandhardwareconfiguration.Thepenaltyofthisflexibility,however,isthattheeaseofusecomesattheexpenseofoverallthroughput.Toillustratethebenefitsoftheproposedarchitecture,applicationstoMIMO-OFDMsystemprototypingandpreliminaryMIMOchannelmeasurementsarepresented.Adetaileddescriptionofthehardwareisprovidedalongwithdownloadablesoftwaretoreproducethesystem.INTRODUCTIONMultiple-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-OFDMplatformdesignedforsystemvalidationandOFDMmodulatorRec0mbineBitStreamsOFDMEqualizerOFDMdemodOFDMdomodOFDMmodulatorSpatialMultiplexingOFDMmodulatorRec0mbineBitStreamsOFDMEqualizerOFDMdemodOFDMdomodOFDMmodulatorSpatialMultiplexingFig.1.A2×2MIMO-OFDMspatialmultiplexingsystemchannelmeasurements.Moreworkneedstobeinvestigatedtoovercomethelimitationsandexpandthecapabilitiesofourinitialdesign.Thispaperisorganizedasfollows.SectionIIexploresthesignalmodelforaMIMO-OFDMsystemandourspecificMIMO-OFDMimplementation.SectionIIIdiscussesthespecificsofthehardwareandsoftwareplatform.SectionIVshowspreliminaryresultsfromoursystemimplementationaswellaschannelmeasurementsinindoorenvironments.II.MIMO-OFDMIMPLEMENTATIONInthissectionwereviewtheMIMO-OFDMsignalmodelandthendescribeourspecificMIMO-OFDMsystemimplementation.A.MIMO-OFDMSignalModelInaMIMO-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.SystemImplementationandSpecificationsThefirstimplementationfeaturesspatialmultiplexingwithtwotransmitandtworeceiveantennas,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.TABLEISPECIFICATIONSOFOURMIMO-OFDMIMPLEMENTATIONNo.ofTransmitAntennas2No.ofReceiveAntennas2CarrierFreq.2.4GHzBandwidth16MHzNo.ofTones64SubcarrierSpacing25kHzOFDMSymbolDuration5usGuardIntervalDuration1usOFDMdataduration4usLengthofCyclicPrefix16samplesMIMOSchemespatialmultiplexingPacketDuration200msFig.2.PictureoftheNationalInstrumentsRFhardwareWearecurrentlyavoidingcarriersynchronizationissuesbydirectlywiringtheclocksofthetransmitterandreceivertogether.Additionally,inordertoavoidtimingissueswearesendingatriggerfromthetransmittertothereceiverwhendatatransmissionbegins.Softwaresynchronizationisunderdevelopmentandwillbeincludedinfuturework.AswearefollowingaSDRapproachtoprototyping,therearemanyparametersofthesystemwhichcanbeadjustedprogrammatically.TheflexibilityenabledbyaSDRMIMOOFDMprototypebecomesclearinthefollowingsectionwherewepresentadetaileddescriptionoftheprototypingplatform.III.PROTOTYPINGPLATFORMHardwareDescriptionNationalInstruments’RFhardwareisthefoundationofourprototype,asillustratedinFigure2.ThehardwarecomesinthePCIextensionsforinstrumentation(PXI)formfactor(whichissimilartoPCIexceptdesignedforindustrialapplications).EachpairoftransmittersandreceiversishousedinseparatePXIchassis.EachPXIchassisisconnectedtoaPCthroughaPCIbridgewhichconnectsthePXIhardwaretothePCIbus.EachPCisequippedwithdual2.8GHzprocessorsand2Gbofmemory.ThespecificationsofthehardwareislistedinTableIIandthecorrespondingblockdiagramisdepictedinFigure3.TABLEIIHARDWARESPECIFICATIONSTransmitterArbitraryWaveformGeneratorPXI－5421100millionsamplespersec.16bitresolution43MHzbandwidthUpconverterPXI－5610250KHz-2.7GHzCarrier+13dBrangeDigitizerPXI－5620Receiver64millionsamplespersec.14bitresolution30MHzbandwidthDownconverterPXI－56009kHz-2.7GHzCarrier20MHzreal-timebandwidthEachtransmitunitiscalledaRFsignalgeneratoranditconsistsoftwoparts.Thefirstisthearbitrarywaveformgenerator(ARB).TheARBactsasthedigitaltoanalogconverter(DAC)anditoperatesatamaximumof100Msamples/secwitha16-bitresolution.TheARBhasa256MBbuffer,althoughthehardwareisenabledtocyclethroughthebuffertoprovideforcontinuoustransmission.Whentransmittingcomplexdata,theARBitselfupconvertsthesignaltoanintermediatefrequency(IF)of25MHz(thisIFcanbeprogrammaticallychanged)beforethesignalissenttotheRFupconverter.Theupconvertercanmodulateasignaluptoacarrierfrequencyof2.7GHzwithbandwidthsupto20MHzandiscapableofamaximumof13dBmofpower.TheARBhasatriggerlineavailablefortimingsynchronization,whiletheupconverterhasaninputavailableforclocksynchronization.ToextendtheusablerangeoftheprototypeweusedaMinicircuitsZQL-2700MLNWLNA(lownoiseamplifier)atthereceiver.WederivedthegainoftheLNAfromthelink-budgetequation[15]Additionally,fortimingandclocksynchronization,wefoundthatweneededa3waypowersplittertosendeachsignalouttoeachofthetransmitandreceivecomponents.Thissplitterwillbereplacedoncefullopenloopsynchronizationisimplemented.SoftwareDescriptionTheRFHardwareisdesignedtobeeasilyconfiguredandprogrammedthroughNationalInstruments’LabVIEWprogrammingenvironment.LabVIEWisadataflowbasedgraphicalprogramminglanguage.Thehardwarecanbeprogrammedinotherlanguages,however,LabVIEWprovidestheuserwithsimpleprogrammingandrapidprototypingcapabilities.WebegantheprototypingprocessbycreatingasimulationoftheMIMO-OFDMsysteminLabVIEW.ForthispurposeourresearchgrouphascreatedapubliclyavailableMIMOtoolkitforLabVIEWwhichcanbedownloadedat[14].ThistoolkitincludesthebuildingblockstosimulatevariousMIMOschemesaswellasthefunctionswhicharenecessarytosimulatethesystemcompletelyfrombasebanduptothemodulationanddecoders.TheMIMOschemesthatarecurrentlyincludedinthetoolkitarespatialmultiplexing,Alamoutiencoding,lineardispersionencoding,trelliscoding,alongwithothercommonMIMOschemesandfunctionstosupportsimulation.Aftercompletingsimulations,thenextstageinourprototypingprocesswastoactuallyprogramthehardware.WiththeLabVIEWsimulationsalreadycompleted,thetransitiontohardwareprogrammingwasverysimpleasthecodewrittenforthesystemsimulationcouldthenbeappliedwiththehardware.ManyofthelowlevelhardwareissueswereavoidedbyusingtheLabVIEWhardwaredevicedrivers.IV.RESULTSInthissectionwedescribeasimplesystemimplementationaswellasabasicchannelmeasurementsetup.Bothimple-mentationsareavailablefordownloadat[13]alongFig.4.TheLabVIEWtransmittersoftwareinterfacewithvariousprogramablesystemparametersMIMO-OFDMSystemImplementationOneoftheobjectivesofhardwareimplementationwastotestvariouscandidatecodingandreceiverstrategiesfortheMIMO-OFDMphysicallayerunderconsiderationintheIEEE802.11nstandard,whichiscurrentlyunderdevelopment.ThesystemfollowedthespecificationsoutlinedinSectionIIIwitha2×2spatialmultiplexingMIMOsystemcombinedwith64OFDMtones.QPSKmodulationwascombinedwithachannelestimationschemethatsendsatrainingsymboleveryfifthOFDMsymbol.Aspreviouslydiscussed,duetomemoryconstraints,only200msworthofpacketsofdataaretransmittedinagiventimeinterval.Inthisperiodweareconsistentlyabletoachieveadatarateof40.96Mbits/s(8.192Mbitsaretransmittedinthis200mstimeframe)withourgivenhardware.Duetoourcurrenthardwarelimitations,eachacquisitionrequiresapproximately4secondsofprocessingbeforeanewacquisitioncanoccur.Thustheoverallthroughputweachieveisabout2Mbitswhenoperatedusingthisdutycycle.Futurehardwareupgradeswillallowrealtimesystemimplementations.Fordemonstrationpurposeswetransmittedanimageasdisplayedinthegraphicaluserinterface(GUI)depictedinFigure4.ThedecodedimageisreportedinFigure5alongwiththeGUIatthereceiverside.Thedifferenceindownloadspeedoftheimageinnon-MIMOandMIMOconfigurationsprovidesanintuitivemotivationforMIMO,e.g.theimagetransferstwiceasfastwithMIMOinourconfiguration.WewillcontinuetobuildonourcurrentsystemimplementationasweimplementanumberofotheravailableMIMOschemeswhichareavailablethroughourMIMOtoolkit[14].Wearealsoinvestigatingaddingamediumaccesscontrol(MAC)protocolaswellasafeedbackchannel.B.ChannelMeasurementsAlongwiththesystemimplementation,weareusingtheprototypetoconductchannelmeasurements.Weonlyhavepreliminaryresultsfromthehardware,buttheyrevealthepotentialofthisapproachforconductingmeaningfulchannelmeasurements.Weperformedthechannelmeasurementsinthewirelessnetworkingandcommunicationsgroup’s(WNCG)workspaceintheEngineeringSciencebuildingatTheUniversityofTexasatAustin.Theenvironmentisatypicalcubicleofficeenvironment.Wecollecteddataatacarrierfrequencyof2.4GHzovera16MHzbandwidthwith64tones.Weemployedhalf-wavelengthomnidirectionaldipoleantennasforourchannelmeasurementstoradiateisotropicallyalongmultiplepropagationpaths,distributedaroundtransmitterandreceiveraccordingtothemodelin[16].Thetransmitandreceiveantennaswereplacedapproximately8metersapartinbetweencubiclessothattherewasnolineofsightpropagationpaths,withtwowallsbetweenthetransmitterandreceiver.Thetwotransmitantennaswerespacedfivewavelengthsapartfromeachother(approximately60centimeters)toreducethespatialcorrelationacrossdifferentMIMOchannels[17]andtheeffectofmutualcoupling[18].Thetworeceiveantennaswereseparatedbythesamedistance.Fig.5.TheLabVIEWreceiversoftwareinterfacewithvariousprogramablesystemparameters.preliminaryresultsfromthehardware,buttheyrevealthepotentialofthisapproachforconductingmeaningfulchannelmeasurements.Weperformedthechannelmeasurementsinthewirelessnetworkingandcommunicationsgroup’s(WNCG)workspaceintheEngineeringSciencebuildingatTheUniversityofTexasatAustin.Theenvironmentisatypicalcubicleofficeenvironment.Wecollecteddataatacarrierfrequencyof2.4GHzovera16MHzbandwidthwith64tones.Weemployedhalf-wavelengthomnidirectionaldipoleantennasforourchannelmeasurementstoradiateisotropicallyalongmultiplepropagationpaths,distributedaroundtransmitterandreceiveraccordingtothemodelin[16].Thetransmitandreceiveantennaswereplacedapproximately8metersapartinbetweencubiclessothattherewasnolineofsightpropagationpaths,withtwowallsbetweenthetransmitterandreceiver.Thetwotransmitantennaswerespacedfivewavelengthsapartfromeachother(approximately60centimeters)toreducethespatialcorrelationacrossdifferentMIMOchannels[17]andtheeffectofmutualcoupling[18].Thetworeceiveantennaswereseparatedbythesamedistance.WemeasuredtheMIMOchannelsoverthetimeandfrequencydomainsandtheresultsareshowninFigure6.ThetemporalevolutionofthechannelisflatduetothelowDopplereffectinfixedwirelessscenarios,whereasthefluctuationsinthefrequencydomainareduetothemultiplepropagationpaths.InFigure7wedisplaytheaveragepowerdelayprofile(PDP)ofchannelH22.WefoundagoodfitofthisPDPwithwellknownmodelsandmeasurementsresultsforindoorpropagationenvironments[12],[16],[19].NotethatthemodestselectivityofthechannelH22isduetothesmallspreadinthedelayprofile.V.CONCLUSIONSANDFUTUREWORKInthispaper,wepresentedaMIMO-OFDMprototypingarchitecturewhichemphasizesaSDRparadigm.Theplatformfreestheuserfromlow-levelhardwareimplementationissuesandallowsmoreintensivestudyofalgorithmandsystemdesignissues.Weillustratedapplicationsofthisapproachtosystemimplementationandchannelmeasurements.InfutureworkweplantoapplythistostudyMACprotocoldesignsforMIMO-OFDMadhocnetworks[22]aswellastomorecomprehensivelyanalyzeMIMOchannelsasin[21].ACKNOWLEDGEMENTSTheauthorswouldliketothankNationalInstrumentsfortheirhardwaredonationaswellasAndyHindeatNationalInstrumentsforhisassistancewiththehardwareandUTAustinstudentsBrettWesterveltandVeynuNarasimanfortheircontributionstothesystemprototype.REFERENCES[1]A.Paulraj,R.Nabar,andD.Gore,IntroductiontoSpace-TimeWirelessCommunications,CambridgeUniversityPress,2003.[2]“Part11:WirelessLANMediumAccessControl(MAC)andPhysicalLayer(PHY)Specifications:High-SpeedPhysicalLayerinthe5GHzBand,”IEEEStandard802.11a1999.[3]“Part11:WirelessLANMediumAccessControl(MAC)andPhysicalLayer(PHY)specificationsAmendment4:FurtherHigher-SpeedPhysicalLayerExtensioninthe2.4GHzBand,”IEEEStandard802.11g2003.[4]“IEEE802.11nTaskGroup,”/groups/802/11/Reports/tgnupdate.htm.[5]H.Sampath,S.Talwar,J.Tellado,V.Erceg,andA.Paulraj,“AFourth-GenerationMIMO-OFDMBroadbandWirelessSystem:Design,Performance,andFieldTrialResults,”IEEECommunicationsMagazine,vol.40,no.9,pp.143–149,September2002.[6]A.Adjoudaniet.al.,“PrototypeExperienceforMIMOBLASToverThird-GenerationWirelessSystem,”IEEEJournalonSelectedAreasinComm.,vol.21,no.3,pp.440–451,April2003.[7]P.Murphy,F.Lou,andP.Frantz,“AHardwareTestbedfortheImplementationandEvaluationofMIMOAlgorithms,”inIEEEInternationalConferenceonMobileandWirelessCommunicationsNetworks,October2003.[8]G.L.Stuber,J.R.Barry,S.W.Mclaughlin,Y.Li,M.A