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OFDMtransmissionstep-by-step
ThisintroductionillustratesanOFDMdatatransmissionusingexamplevalues.
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1.Channelestimation
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Initiallyapilotsequenceissentforchannelestimationandequalizerinitialization.
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d(i):Thepilotsequenceisaseriesofrealconstantsymbols.Inthefrequencydomain(beforeIFFT)itcontainsallfrequencieswithamplitude1V.
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u(i):ItIFFT-transformstoanOFDMsymbol,inthiscaseanimpulsesymbol.
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s(i):Thecyclicprefix(CP)isadded.TheOFDMsymbolisextendedbyaprefixcontainingthelastsinglesymbols.
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r(i):Thereceivedsymbolisdegeneratedbythechannel.Theechooftheexamplechannelhasarelativepowerof0.9.
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v(i):TheOFDMsymbolisobtainedbyremovingtheguardinterval,i.e.thefirsttwosinglesymbols.
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w(i):ItFFT-transformstothespectralrepresentationoftheOFDMsymbolw(i).
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w(i)isthefrequencyresponsetothepilotsequencecontainingallfrequencieswithamplitude1V.Hence,w(i)isthechanneltransferfunctionHC,i.
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Toinvertthechanneltransferfunction,theequalizercoefficientsHEaresettothereciprocalofHC(zero-forcing).
reciprocal
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2.End-to-enddatatransmission
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d(i):Ablockcontaining8datasymbols(representing16bits)isgeneratedandserial-to-parallelconverted.
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u(i):ItIFFT-transformstoanOFDMsymbol.
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s(i):Theguardinterval(GI)isadded,i.e.theOFDMsymbolisextendedbyacyclicprefixcontainingthelastsinglesymbols.
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r(i):Thereceivedsymbolisdegeneratedbythechannel.
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v(i):TheOFDMsymbolisobtainedbyremovingtheguardinterval,i.e.thefirsttwosinglesymbols.
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w(i):ItFFT-transformstothespectralrepresentationoftheOFDMsymbol.
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d’(i):Theequalizedsignalrepresentsthe8datasymbols.
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Thedatasymbolsaredetectederrorfree:d’(i)=d(i).Inthisscenariononoiseispresent.
=
OFDMtransmissionimplementsISIfreedetection.