Introduction to OFDM and the IEEE 80211a Standard

1IntroductiontoOFDMandtheIEEE802.11aStandard2Motivation•Highbit-ratewirelessapplicationsinamultipathradioenvironment.•OFDMcanenablesuchapplicationswithoutahighcomplexityreceiver.•OFDMispartofWLAN,DVB,andBWAstandardsandisastrongcandidateforsomeofthe4Gwirelesstechnologies.3MultipathTransmission•Fadingduetoconstructiveanddestructiveadditionofmultipathsignals.•ChanneldelayspreadcancauseISI.•Flatfadingoccurswhenthesymbolperiodislargecomparedtothedelayspread.•FrequencyselectivefadingandISIgotogether.4DelaySpread•Powerdelayprofileconveysthemultipathdelayspreadeffectsofthechannel.•RMSdelayspreadquantifiestheseverityoftheISIphenomenon.•TheratioofRMSdelayspreadtothedatasymbolperioddeterminestheseverityoftheISI.5ASolutionforISIchannels•Conversionofahigh-dataratestreamintoseverallow-ratestreams.•Parallelstreamsaremodulatedontoorthogonalcarriers.•Datasymbolsmodulatedonthesecarrierscanberecoveredwithoutmutualinterference.•Overlapofthemodulatedcarriersinthefrequencydomain-differentfromFDM.6OFDM•OFDMisamulticarrierblocktransmissionsystem.•Blockof‘N’symbolsaregroupedandsentparallely.•Nointerferenceamongthedatasymbolssentinablock.7OFDMMathematics120()kNjftkkstXetos]OrthogonalityCondition*120().()0TgtgtdtInourcase220.0pqTjftjfteedtForpqWherefk=k/T8TransmittedSpectrum9OFDMterminology•Orthogonalcarriersreferredtoassubcarriers{fi,i=0,....N-1}.•OFDMsymbolperiod{Tos=NxTs}.•SubcarrierspacingDf=1/Tos.10OFDMandFFT•SamplesofthemulticarriersignalcanbeobtainedusingtheIFFTofthedatasymbols-akeyissue.•FFTcanbeusedatthereceivertoobtainthedatasymbols.•Noneedfor‘N’oscillators,filtersetc.•PopularityofOFDMisduetotheuseofIFFT/FFTwhichhaveefficientimplementations.11OFDMSignal1,0()(())NnkkosnkstXgtnT2()0kjftkegttos]OtherwisekoskfTK=0,..........N-112BysamplingthelowpassequivalentsignalatarateNtimeshigherthantheOFDMsymbolrate1/Tos,OFDMframecanbeexpressedas:1,0()()()NnnkkososkmFmXgtnTtnTN12,,0().mNjkNnnknkkFmXeNIDFTXm=0....N-113InterpretationofIFFT&FFT•IFFTatthetransmitter&FFTatthereceiver•DatasymbolsmodulatethespectrumandthetimedomainsymbolsareobtainedusingtheIFFT.•Timedomainsymbolsarethensentonthechannel.•FFTatthereceivertoobtainthedata.14InterferencebetweenOFDMSymbols•TransmittedSignal•DuetodelayspreadISIoccursDelaySpreadIOSIOS1OS2OS3•SolutioncouldbeguardintervalbetweenOFDMsymbols15CyclicPrefix•ZerosusedintheguardtimecanalleviateinterferencebetweenOFDMsymbols(IOSIproblem).•Orthogonalityofcarriersislostwhenmultipathchannelsareinvolved.•Cyclicprefixcanrestoretheorthogonality.16CyclicPrefix•Convertalinearconvolutionchannelintoacircularconvolutionchannel.•Thisrestorestheorthogonalityatthereceiver.•Energyiswastedinthecyclicprefixsamples.17CyclicPrefixIllustrationTosTgCyclicPrefixOS1OS2OS1,OS2-OFDMSymbolsTg-GuardTimeIntervalTs-DataSymbolPeriodTos-OFDMSymbolPeriod-N*Ts18SerialtoParallelX0XN-1x0xN-1IFFTParalleltoSerialandaddCPAddCPWindowingDACRFSectionInputSymbolsOFDMTransmitter19ADCandRemoveCPSerialtoParallelFFTParalleltoSerialandDecoderX0XN-1x0xN-1OutputSymbolsOFDMReceiver20Synchronization•Timingandfrequencyoffsetcaninfluenceperformance.•Frequencyoffsetcaninfluenceorthogonalityofsubcarriers.•LossoforthogonalityleadstoInterCarrierInterference.21PeaktoAverageRatio•MulticarriersignalshavehighPARascomparedtosinglecarriersystems.•PARincreaseswiththenumberofsubcarriers.•Affectspoweramplifierdesignandusage.22PeaktoAveragePowerRatio23TheIEEE802.11aStandard•BelongstotheIEEE802.11systemofspecificationsforwirelessLANs.•802.11coversbothMACandPHYlayers.•FivedifferentPHYlayers.•802.11abelongstotheHighSpeedWLANcategorywithpeakdatarateof54Mbps•PHYLayerverysimilartoETSI’sHIPERLANType224KeyPhysicalLayerThings•UseofOFDMfortransmission.•Multipledataratemodessupportedusingmodulationandcoding/puncturing.25MultipleDataRates/Modes26•UsefulSymbolDuration-3.2s•GuardIntervalDuration-0.8s•FFTSize-64•NumberofDataSubcarriers-48•NumberofPilotSubcarriers-4•SubcarrierSpacing-312.5kHzOFDMParameters27ScramblerConvolutionEncoderInterleaverBPSK/QPSK/64QAM/16QAMConstellationMappingOFDMSymbolConstructionIFFTandAddCPInputBitsOFDMTransmitterDAC28TransmitterFeatures•1/2rateconvolutionencodercombinedwithpuncturingtoobtaindifferentcodingrates•InterleavingofbitswithinanOFDMsymbol.•VariablenumberofbitswithinanOFDMsymbol.•Samplingperiod-50ns-64datasamples,16samplesforthecyclicprefix.•Windowingoperationforpulseshaping.29DataSubcarriers•DCsubcarrier(0th)notusedsinceitcancauseproblemsintheDAC•-32to-27and28to32notused.(Guardbandonbothextremes)•Nullsubcarriershelpinreducingoutofbandpower30Receiver•Synchronization•ChannelEstimationandEqualization•FFT(OFDMdemodulation)•Demapping•De-Interleaver•ViterbiDecoder•De-Scrambling31Synchro-nizationChannelEstimationAndEqualizationFFTDemappingDeinterleaverViterbiDecoderDescramblerReceivedSamples802.11aReceiverData32Frequencyoffsetestimationcontinued….•Implementingtheselfcorrelationschemeforshortpreamblesequence,sothat;114422*400()()()4estMMfjcccnnMJynyneynMNumberofsamplesintheshortpream