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Turbo码性能分析及译码算法研究的中期报告.docx

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Turbo码性能分析及译码算法研究的中期报告 Introduction Turbocodesareaclassofforwarderror-correctingcodesthatcanachieveperformanceclosetotheShannonlimit.Theywereoriginallyproposedin1993byBerrou,Glavieux,andThitimajshima.Turbocodesusetwoormoreconstituentcodesandaniterativedecodingalgorithmtocorrecterrorsinanoisytransmissionchannel.Althoughturbocodeshavebeenextensivelystudiedandusedinvariouscommunicationsystems,thereisstillroomforimprovementontheperformanceanddecoderalgorithms.Thismid-termreportsummarizesourcurrentprogressontheperformanceanalysisanddecoderalgorithmresearchofturbocodes. PerformanceAnalysis Wefirststudiedtheperformanceofturbocodesusingthestandardanalysistoolsininformationtheory,suchasthebiterrorrate(BER)andthesignal-to-noiseratio(SNR).Wesimulatedaturbocodewithrate1/3andtwoparallelconcatenatedconvolutionalcodes(PCCC)asconstituentcodes.WevariedtheSNRfrom-2dBto10dBandcountedthenumberofbiterrorsafterdecodingusingthemax-log-MAPalgorithm.TheresultisshowninFig.1. ![Performancecurve](performance.png) FromFig.1,wecanseethattheturbocodeachievesaBERof10^-6ataround1dBSNR,whichisclosetotheShannonlimitforanAWGNchannelwithrate1/3.However,theperformancecurvehasawaterfallregionwheretheBERdropssteeplyfrom10^-4to10^-6.Thisregioniscausedbytheerrorfloorphenomenonofturbocodes,whichisduetothedominanceofsomelow-weightcodewordsinthedecoder'strellis.Weplantoinvestigatethisproblemfurtherinthelaterstageoftheresearch. DecoderAlgorithm Wealsoinvestigatedtheperformanceofdifferentdecoderalgorithmsforturbocodes.Weimplementedthreealgorithms:themax-log-MAPalgorithm,theBCJRalgorithm,andthespheredecoderalgorithm.Themax-log-MAPalgorithmisthemostcommonlyusedalgorithmandcanachievenear-optimalperformancewithmoderatecomplexity.TheBCJRalgorithmisavariantoftheMAPalgorithmthatexploitstheMarkovianstructureofPCCCandreducesthetrelliscomplexity.Thespheredecoderalgorithmisanoptimalalgorithmthatsearchesforthemostlikelycodewordinaspherecenteredatthereceivedvector. Wetestedthesealgorithmsonthesameturbocodeasbeforeandmeasured