Log-likelihood ratio (LLR) conversion schemes in orthogonal code hop

104IEEECOMMUNICATIONSLETTERS,VOL.7,NO.3,MARCH2003Log-LikelihoodRatio(LLR)ConversionSchemesinOrthogonalCodeHoppingMultiplexingJaeKyunKwon,StudentMember,IEEE,SuwonPark,StudentMember,IEEE,andDanKeunSung,SeniorMember,IEEEAbstract—Log-likelihoodratio(LLR)conversionschemesareproposedtoreducetheeffectofperforationsthatoccurinorthog-onalcodehoppingmultiplexing(OCHM),whichwaspreviouslyproposedtoacposedLLRconversionschemesgreatlyreducetherequiredsignal-to-noiseratio(SNR)formanceoftheproposedschemesisevaluatedbysimulationintermsoftherequired0fora1%erms—Demapping,log-likelihoodratio(LLR),orthog-onalcodehoppingmultiplexing(OCHM),UCTIONIdownlinnnelactivityduetoburstinessresultsininefficieogonalcodehoppingmultiplexing(OCHM)schemehasbeenproposed[1],[2]toaccommodatemorelow-ncauseperforationsamongsymbols,whichdegraosenewlog-likelihoodratio(LLR)conversionscperisorganizedasfollows:WebconversionschemesarepresentedinSectionIIIandtheperformanceoftheproposedschemesisevaluatedbysimu-lationintermsoftherequired-inconventionalBPSK/QPSK,theLLRcanbeexpressedas[3]

KWONetal.:LLRCONVERSIONSCHEMESINOCHM(a)(b)mapping(BPSK).(a)Perforation.(b)VERSIONFORBPSK/QPSKWhenOCHM-baseddownlinkexperiencescollisionsamongsymbols,transmittedsymbolscanbelocatedatplacesdifferentfromtheoriginallocationsinthe2andreceive(2)r,theequatiatively,weapprox-imate(2)toasimplepiecewise-linearfunctiontoreducethecomputationalcomplexityusingthefollowingapproximation:is1/.4illustratestherequired105RegionII

106IEEECOMMUNICATIONSLETTERS,VOL.7,NO.3,MARCH2003ever,erencecanincreaseasmoreusersarebeamantennawith(a)(b)edEfora1%blockerrorrate.(a)AWGNchannels.(b)lsandindependentRayleighfadingchannels,annelsexhibitslowlyvaryingfadingchanneleeschemesyieldsimila-3OCHMcanaccommodatesignificantlymoredownlinkchannelsthanthenumberoforthogonalcodewords,64[1].4Aninterference(power)-limitedsituationmeansthatthemaximumallowablenumberofusersislese-limitedsystemhereindicatesthatusersfullyutilizethenumberofcodechannelsandthenumberofuserscanexceedthenumberofcodechannelsifasuitablescheme(OCHM)setherequired,whichwill