Simulation of OFDM System over Frequency Selective Channel.docx

Simulation of OFDM System over Frequency Selective Channel.docx

《Simulation of OFDM System over Frequency Selective Channel.docx》由会员分享，可在线阅读，更多相关《Simulation of OFDM System over Frequency Selective Channel.docx（7页珍藏版）》请在冰豆网上搜索。

SimulationofOFDMSystemoverFrequencySelectiveChannel

SimulationofOFDMSystemoverFrequencySelectiveChannel

一、OrthogonalFrequencyDivisionMultiplexing（OFDM）

ThesimplestformofOFDMdividesthedatastreamintomultiplesubstreamstobetransmittedoverdifferentorthogonalsubchannelscenteredatdifferentsubcarrierfrequencies.Thenumberofsubstreamsischosentomakethesymboltimeoneachsubstreammuchgreaterthanthedelayspreadofthechannelor,equivalently,tomakethesubstreambandwidthlessthanthechannelcoherencebandwidth.ThisinsuresthatthesubstreamswillnotexperiencesignificantISI.

Themodulatedsignalsassociatedwithallthesubchannelsaresummedtogethertoformthetransmittedsignal,givenas

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.