实施无线通信OFDM调制英文.docx
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实施无线通信OFDM调制英文.docx
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实施无线通信OFDM调制英文
ImplementingOFDMmodulationforwirelesscommunications
HighFFTthroughputisessentialinbroadbandsystemsespeciallywhenFFTissharedbetweenmultipledatapaths.
ShortintroductiontoOFDM
Inthetraditionalmulti-carriercommunicationsystem,theentiresystembandisdividedintoanumberoftheseparationofsubchannels(carriers).Certainprotectiontothecarrierspacingbetweenthereceivingendthroughthefilteraftertheseparationofthevarioussub-channeltoreceivetherequiredinformation.Althoughsuchadifferentchanneltoavoidinterferencewitheachother,butthecostofsacrificingthefrequencyofutilization.Andwhenagreatnumbersubchannelswhenalargenumberofthesub-channelsignalseparationfiltersettingsbecomesalmostimpossible.
Middleoflastcentury,havebeenproposedbandoverlappingmulti-carriercommunicationscheme,choosebetweenthecarrierfrequencyfororthogonalsubcarriers,whichiswhatwecallOFDM.This"orthogonal"indicatesthatthecarrierfrequencybetweentheprecisemathematicalrelationships.Accordingtothisidea,OFDMcannotonlymakefulluseofchannelbandwidth,butalsoavoidtheuseofhigh-speedbalanceandanti-burstnoiseerrors.OFDMisaspecialmulti-carriercommunicationscheme,asingleuser'sinformationflowbyseries/paralleltransformationintomultiplelowerratestreams,eachstreamissentwithasub-carrier.OFDMwithoutband-passfiltertoseparatesub-carriers,butbyfastFouriertransform(FFT)toselectthosewhoareabletomaintainevenmixingorthogonalwaveforms.
OFDMisawirelesshigh-speedtransmissiontechnologyenvironment.Wirelesschannel'sfrequencyresponsecurveofmostnon-flat,whilethemainideaofOFDMtechnologyisgiveninthefrequencydomainchannelintomanyorthogonalsubchannels,eachsubchannelusingasub-carriermodulation,andtheparalleltransmissionofallsub-carriers.Thus,althoughthegeneralnon-flatchannelwithfrequencyselective,eachsubchannelisrelativelyflatineachsubchannelisnarrow-bandtransmissiononthesignalbandwidthissmallerthanthecorrespondingbandwidthofthechannel,itcangreatlyeliminatethesignalinterferencebetweenwaveforms.Sinceeachsub-channelOFDMsystemeachorthogonalcarrier,theirspectrumisoverlapping,itwillnotonlyreduceinterferencebetweensubcarriers,atthesametimeimprovetheefficiencyofspectrum.
OFDMtechnologyismulti-carriermodulation(Multi-CarrierModulation,MCM)technology.OFDMandMCMwillbesomemixofliterature,infact,closeenough.CommonlyusedinMCMandOFDMwirelesschannel,theirdifferenceis:
OFDMtechnologyTezhithechannelisdividedintoorthogonalsubchannels,channelutilizationhigh;andMCM,maybemorekindsofchanneldivisionmethod.
OFDMtechnologyisintroducedtoimprovecarrierspectrumefficiency,ortoimprovethemulti-carriermodulation,whichischaracterizedbymutuallyorthogonalsub-carriersothatthespectrumcanbemodulatedspreadspectrumoverlap,thusreducinginterferencebetweensubcarriers.Inthemodulationofeachcarrierafterthecompletion,inordertoincreasedatathroughput,improvedatatransmissionspeed,itusesatechnologycalledHomePlugtreatmenttoallthedatawillbesenttomergethecarriersignalbitprocessingthelargenumberofindividualsignalsintoasingleindependenttransmissionsignaltosend.AnotherreasonforconcernOFDM,oneimportantreasonisthatitcanusethediscreteFourierinversetransform/discreteFouriertransform(IDFT/DFT)insteadofmulti-carriermodulationanddemodulation.
ODFMmodulationbasics
OFDMisatechniqueusedinmodernbroadbandwirelesscommunicationssystems.TomitigatetheeffectofdispersivechanneldistortioninhighdatarateOFDMsystems,CPisintroducedtoeliminateinter-symbolinterference(ISI).ItcopiestheendsectionofanIFFTpackettothebeginningofanOFDMsymbol.Typically,thelengthoftheCPmustbelongerthanthelengthofthedispersivechanneltocompletelyremoveISI.OFDMmodulationinatransmitterincludesinversefastFouriertransform(IFFT)operationandCPinsertion.InanOFDMreceiver,theCPisremovedbeforethepacketdataissenttoFFTfordemodulation.Nextgenerationwirelesssystemsfeaturehighlydynamicconfigurations,wheretheCPlengthchangesaccordingtothetransmissionmode,framestructure(showninFigures1and2),andhigherlevelprotocol.Forinstance,theCPconfigurationfor3GPPLTEchangeswithineachslot.
TheCPlengthisbasedonanOFDMsymbolwith2048timeintervals.SimilarvariableCPstructuresinWiMAXsystemscanbeobserved.
Figure1:
3GPPLTEFrameStructure1,ApplicabletoTDDandFDD
Figure2:
3GPPLTEFrameStructure2,applicabletoTDD.
ImplementingOFDMmodulation
ThefollowingsectionsdiscussimplementingcyclicprefixinsertionandremovalforOFDMmodulationanddemodulation.
FFTandinverseFFToperations
ThemostcomputationallyintensiveoperationofOFDMmodulationisIFFT,andsimilarly,thecoreofOFDMdemodulationisFFT.HighFFTthroughputisessentialinbroadbandsystems,especiallywhenFFTissharedbetweenmultipledatapaths.
InmodernscalablewirelesssystemssuchasWiMAXand3GPPLTE,run-timereconfigurabilityisalsoanintegralpartofsystemrequirements.TheFFTMegaCorefunctioninvariablestreamingmodetargetsspecificallyreconfigurablewirelesscommunicationsandisasuitablecandidatefordesigningOFDMsystems.
TheFFTMegaCorefunctionisconfiguredinthevariablestreamingmode,whichallowsFFTsizeanddirectionchangeonapacket-by-packetbasis.TheFFTMegaCorefunctionalsotakesadvantageofthememory-efficientengine-onlymodeoftheFFTcore,whichoutputsbit-reversedsymbolsdirectlyfromtheFFTbutterflyengines.BitreversalwithcyclicprefixinsertionoutsidetheFFTcorecanbecombined.TheoverallOFDMmodulationsavesasinglebuffer.
FFTmodulereuse
Toreducelogicusage,theFFTmoduleisoftenclockedmuchfasterthantherestofthebasebandmodulesandreused.TheFFTmodulecanbesharedbydifferentsources;forinstance,bymultipleantennas,transmissionandreceptionintimedivisionduplex(TDD),andfrequencydivisionduplex(FDD)systems.
TheFFTmodulecanalsobesharedwithotherfunctionalmodules,suchascrestfactorreductionorchannelestimation.However,suchreusedependsontheuser-specificalgorithmsandmaynotbeageneraldesignconsideration.Thefocusofthisarticleisonthemostcommonwirelesscommunicationsscenarios:
MIMOtechniquesandTDDandFDD.
TDDoperations
InTDDbasestations,transmissionandreceptionoccursonnon-overlappingtimeslots.TheFFTmodulecanbeeasilysharedbetweenthetransmitterandthereceiver,withpropersignalmultiplexing.Figure3showsatypicalsingleantennaTDDOFDMmodulator.
Figure3:
SharedOFDMmodulationanddemodulationinasingleantennaTDDsystem.
Inthetransmittingdatapath,basebanddataisfeddirectlytotheIFFTmodule.ToinsertCPandimplementbitreversalafterIFFT,manydifferentconfigurationscanbeused.Figure4showsahighlyefficientimplementationusingtheAlteraAvalonStreamingInterface(Avalon-ST).
Onthetransmitterdatapath,IFFToutputinbitreversedorderiswrittenintoasinglebuffersequentially,wherenaturalorderedsamplesfromthepreviousOFDMsymbolarereadoutatthesametimefromthedualportRAM.Whengeneratingcyclicprefix,theFFTcoreisstalledviaAvalon-STbackpressure.Cyclic-prefix-appendedcontinuousOFDMsymbolsarethensenttothedigitalupconverter(DUC)fortransmission.
Figure4:
Amemoryefficientimplementationofcyclicprefixinsertionwithbackpressure.
Onthereceivingpath,afterdigitaldownconversion(DDC),cyclicprefixisstrippedofffromtheincomingOFDMsymbols.ThecyclicremovalmoduleinFigure3searchesthecorrectstartofanOFDMsymbolandfeedsthedatatoFFTfordemodulation.ThesinglebufferaftertheFFTmoduleonlyservesasabitreversalbufferonthereceivingpathandnobackpressureisasserted.
Toreusethecontrolunits,theCPremovalandCPinsertionmodulesinFigure3areabletodistinguishifthecurrentdatapacketisfortransmissionorreception,andtakeactionaccordingly.Inthismemoryefficientimplementation,theFFTcoreoperatesatthesymbolrate.Asinglebufferissufficientforcyclicprefixinsertionandbitreversal.
FDDoperations
InFDDoperation,transmissionandreceptionoccuratthesametime.FFTcoresharingrequiresittooperateatnolessthantwicethebasebandsymbolrate.Thetransmissionandreceptiondatapathsneedtohaveaspecificsetofdatabuffers.
Figure5showsapossibleconfigurationofFFTreuseinanFDDsystem.ThedatapathoperationsfortransmissionandreceptionaresimilartoaTDDsystem—thedifferenceisthattheseoperationsoccurconcurrently.
Asaresult,thepre-FFTdatamustbebufferedandratechangedtohigherclockfrequency.Asinglebufferissufficientforratechange,sincethebufferwriteclockisalwaysslowerthanorequaltothereadclock.
Whilethecurrentpacketofdataiswrittenatslowclocktothebuffer,thepreviouspacket'sdataisreadoutatahigherclockrate.Whenreadingandwritingintothesamememorylocation,youneedtoconfigurethetwo-portRAMtooutputoldmemorycontents.
AfterFFTprocessing,thehighdatarateisconvertedbacktoOFDMtransmissionrateviaatwo-portRAM.Thispost-FFTmemorybufferalsoactsasabitreversalbuffer.Duetothehigh-rate-to-low-rateconversion,ifstreamingoutputisneeded,thenadoublebufferisrequired.Thatis,whileoneFFTpacketiswrittenintothebuffer,thepreviouspacket'sdataisreadoutfromtheotherbuffer.
Figure5:
SharedFFT
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