Simulation of OFDM System over Frequency Selective Channel文档格式.docx
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Simulation of OFDM System over Frequency Selective Channel文档格式.docx
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Withtheequivalentbasebandsignaldescription,OFDMsymbolscanbeexpressedas:
WhereTisthedurationofthe0FDMsymbol;
isthestartingtime,Nisthenumberofcarriers.
isthedatasymbolassignedtoeachchannel
Thesubcarrierspectrumandbasebandtransmittedsignalisshownfigure.1,figure.2,respectively.
Figure.1subcarrierspectrum
Figure.2basebandsignal
二、DemodulationtheOFDMsignalatreceiver
三、IFFTImplementationofOFDM
Weassumethat
and
sowecangetthefollowresult
SoOFDMcomplexequivalentbasebandsignalcanbeimplementedusingdiscreteinverseFouriertransform(IDFT).wecangetthetransmittedsymbolusingdiscreteFouriertransform(DFT)atreceiver.
Fromtheaboveanalysis,wecangetthattheOFDMsystemcantransformthesymboltobetransmitedatfrequencydomainintodatasymbolatthetimedomainusingNpointsIDFT.Afterthecarriermodulation,theOFDMsymbolissenttothechannel.Thereceivedsignalissubjectedtocoherentdemodulationatreceiver.Inpractice,OFDMmodulationanddemodulationcanberealizedbyfastFouriertransform(FFT/IFFT).
TheOFDMimplementationofmulticarriermodulationisshowninFigure.3.
Figure.3OFDMwithIFFT/FFTImplementation
三、TheCyclicPrefix
Eachsub-datastreamratedecreases,reducingtheinter-symbolinterference(ISI).Inaddition,inter-symbolinterferencecanbecompletelyeliminatedbyaddingaguardinterval.
Figure.4TheCyclicPrefix
四、MATLABsimulation
(1).Firstly,thesourcegeneratedthebitstream,andthentheinformationsequenceismodulatedbyQAMandmappedintofrequencydomainsignal,thefrequencydomainsignalissubjectedtoserial-to-parallelconversion.Theparalleldataisallocatedto1075sub-carriers,weaddsomezeroelementstothedatainordertobecalculatedbyIFFT.
(2).thedataistransformedtimedomainbyIFFT,andthein-phaseandquadraturecomponentsshowninFigure.5tomakethegraphicsclear,weplot20points.
Figure.5in-phaseandquadraturecomponents
(3).ThelatterpartofeachOFDMsymboliscopiedtoitsfronttoformanOFDMsignalhavingacyclicprefix.ShowninFigure.4.
(4).thetimedomaindataisprocessedbyparallel-to-serialconversionandD/Afilter,andthenismodulatedbythecarrierstobesend.AndwecangetthesignalwaveformandspectrumofdatatobesendshowninFigure.6andFigure.7.
Figure.6thesignalwaveformtobesend
Figure.7spectrumofsignaltobesend
(5).thetransmitteddataaretransimittedoverthewirelesschannelwithadditiveGaussiannoise,SNRissettoSNR=10dB.
(6).Thereceivingmoduleemulationprocedureisoppositetothesendingmoduleprocedure.theresultsareshownasfollow.
Figure.8receivedatacomponent
Figure.9constellationofQAM
五、Resultofsimulationanalysis
Thesystemsimulationshowsthatthemulti-carrierOFDMtechnologyhasthefollowingcharacteristicscomparedwiththesingle-carriermodulationtechnique:
(1).multi-carriersystemcaneffectivelycombattheinter-symbolinterferencecausedbymultipathpropagationandgreatlysimplifythesystemstructuresothatthereceivercannotuseacomplexequalizeronlybyinsertingthecyclicprefixmethodtoeliminateinter-symbolinterference.
(2).OFDMsystemscanmaximizetheuseofspectralresourcesduetotheorthogonalpermutationoftheinter-carrier-to-sub-carrierfrequencyspectrumsoverlappingeachother,whichisofgreatimportanceintheutilizationofincreasinglytensespectrumresources.
(3).OFDMsystemcanbeimplementedusingdiscreteFouriertransformalgorithmwiththedigitalsignalprocessingtechnologyandlarge-scaleintegratedcircuittechnologytomeetthevariousreal-timerequirements.
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