Crest Factor Reductionin Digital Front End Hardware for Wireless Base Station Transmitter

Abstract

ManyofthemodulationmethodscurrentlyinusesufferfromahighPeak-to-AveragepowerRatio(PAR),alsoknownastheCrestFactor(CF).TheGlobalSystemforMobilecommunication(GSM)isawidespreadsecond-generation(2G)systemthatusesconstantenvelopeGaussianminimumshiftkeyingmodulation.Theadvantageachievedbyconstantenvelopemodulationisthepossibilityofusingpower-efficientpoweramplifiers(PAs).However,itmightbebeneficialtocombinethecarriersinadigitalintermediatefrequencyinordertoreducethenumberofanaloguecomponents.Thedrawbackwiththisisthatthesignalisnomoreaconstantenvelopesignal,butithasastronglyfluctuatingenvelopewithahighCF.EnhancedDataratesforGSMEvolution(EDGE)isanenhancementtotheGSMsystemwiththeprimaryobjectiveoftriplingtheon-airdataratewhilemeetingessentiallythesamebandwidthoccupancyoftheoriginalGSMsignal.AlsointhecaseofEDGE,ifthecarriersarecombinedpriortoamplificationwewouldendupwiththesamehighCFproblem.WidebandCodeDivisionMultipleAccess(WCDMA)hasbeenselectedbytheEuropeanTelecommunicationsStandardsInstituteforwidebandwirelessaccesstosupportthird-generation(3G)services.OrthogonalFrequencyDivisionMultiplexing(OFDM)iscommonlyconsideredtobeatechnicalsolutionforfourth-generation(4G)services.Inbothcases,thetransmittedsignalisgeneratedbyaddingtogetheralargenumberofstatisticallyindependentsignals,whichleadstoasignalwithahighCF.ThehighCF,setsstrictrequirementsforthelinearityofthePA.Inordertolimittheadjacentchannelleakage,itisdesirableforthePAtooperateinitslinearregion.HighlinearityrequirementsforthePAleadstolowpowerefficiencyandthereforetohighpowerconsumption.Analternativetotheexpenseofawide-dynamic-rangePAistheuseofdeliberateclippingtodigitallydistortthesignalsothatthesignalqualityisstillmaintainedatasufficientlevel.Asanextraadvantage,thedecreasedCFgivesrisetothepossibilityofutilizingthedynamicrangeofthedigitalcircuitryanddigital-to-analogconverterefficiently.Thisthesisdiscussesdigitalmodulatordesign,concentratingonCFreductionalgorithms.