Peak-Cancellation-Crest-Factor-Reduction-Reference

XAPP1033(v1.0)December5,2007©2007Xilinx,Inc.Allrightsreserved.XILINX,theXilinxlogo,andotherdesignatedbrandsincludedhereinaretrademarksofXilinx,Inc.Allothertrademarksarethepropertyoftheirrespectiveowners.SummaryThisapplicationnoteprovidesdesignerswithahighlyoptimizedsolutionforCrestFactorReduction(CFR)thatcanbeadaptedtomeettheneedsofmultipleairinterfaceswithminimumeffort.Thesystem-levelperformanceofthePeakCancellationmethodofCFRisshowntobebetterthanothermethodssuchasPeakWindowingandNoiseShaping.Inaddition,thePeakCancellationmethodcanbeimplementedmoreefficientlythantheothermethods,resultinginreducedoverallcost.AccompanyingthisapplicationnotearedesignfilesandtestvectorsforquicklyevaluatingtheperformanceofthereferencedesignwithinMATLAB®.Instructionsonhowtointegratethereferencedesignintoalargersystemdesignareincluded.DesignfilesareavailableforbothVirtex™-4andVirtex-5devicearchitectures.IntroductionThewirelessindustryiscurrentlyfollowinganaggressivedrivetoreduceCapitalExpenditure(CapEx)andOperatingExpenditure(OpEx).Differentdynamicscanaffectbothofthesetoalesserorgreaterextent.Ifatypicalbasestationisbrokendownintoitsconstituentcomponents,itisestimatedthatanaverageof40to60percentoftheoverallCapExcostisincurredwiththeradiocards.Sincetheradioshelfcontainsthepoweramplifiers,theradioportionofthedesignisalsoresponsibleformuchoftheOpExincurredduringthelifetimeofthesite.Thisislargelyduetothelowefficiencyofthepoweramplifierswhenoperatinginahighlylinearregion.TheOpExcostisdirectlyrelatedtothepoweramplifierefficiencyinthebasestation.Currently,averysmallproportionoftheDCpowerconsumedbythebasestationisconvertedtoradiatedenergy.Theefficiencyatwhichapoweramplifiermaybeoperatedisafunctionofthetransmittedsignal.3GsignalshaveahighPeaktoAveragePowerRatio(PAPR)orCrestFactor.Thisimposessignificantoperatingrestrictionsonthepoweramplifier.Inordertohandlethepeaks,itisheavilybackedofffromitsmostefficientoperatingpoint.Toincreaseefficiency,CFRalgorithmscanbeusedtodecreasethePAPRofthetransmittedsignalpriortoitenteringthepoweramplifier.Bydoingso,thepoweramplifiercanoperatewithlessbackoff,andthusincreasedefficiency.AnothermethodofimprovingtheefficiencyofthepoweramplifieristouseDigitalPre-Distortion(DPD).Ratherthanusedigitalsignalprocessingtoreducethedynamicrangeofthetransmittedsignal(CFR),DPDisusedtolinearizethepoweramplifieritself.DPDisoutsidethescopeofthisdocument,butitsreferenceisincludedasawidelyusedmethodofamplifierefficiencyimprovement.Inmulti-carriersystems,suchasWCDMA,TD-SCDMAandCDMA2000,thePAPRofthesignalcanbehigherthaninsinglecarriersystems.Inaddition,theimplementationofsomeCFRmethods,suchasNoiseShaping,arecostlyformulti-carriersystems.ThepeakcancellationCFR(PC-CFR)techniqueoutlinedinthisapplicationnoteisverywellsuitedtomulti-carriersystems,andcanevenbeappliedtoradioswheremultiplestandardsmayberequiredinthesameradiotransmissionspectrum.ThisapplicationnotealsoillustratesthedramaticreductionindynamicpowerbetweengenerationsofFPGAs.ThisallowsdesignerstheabilitytodeterminehowmuchadditionalcostsavingscanbemadewhenevaluatingbothPowerSupplyandHeatsinkingneedsoftraditionalchassis-mountedequipmentandRemoteRadioHead(RRH)applications.ApplicationNote:Virtex-5andVirtex-4FamilyXAPP1033(v1.0)December5,2007PeakCancellationCrestFactorReductionReferenceDesignAuthors:EdHemphill,SteveSummerfield,GeorgeWang,andDaveHawkeRDescriptionofAlgorithmXAPP1033(v1.0)December5,2007[Ref1].AlgorithmOverviewThepeakcancellationmethodofCFRreducesthepeaktoaveragepowerratio(PAPR)ofasignalbysubtractingspectrallyshapedpulsesfromsignalpeaksthatexceedaspecifiedthreshold.ThecancellationpulsesaredesignedtohaveaspectrumthatmatchesthatoftheCFRinputsignalandthereforeintroducenegligibleout-of-bandinterference.Ingeneral,theCFRinputsignalandcancellationpulsesarecomplex,andthepeaksearch(describedin“PeakDetection,”page4)iscarriedoutonthesignalmagnitude.Becausethesignalsarecomplex,eachcancellationpulsemustberotatedtomatchthephaseofthecorrespondingsignalpeak.Thepeakmagnitudeofagivencancellationpulseissetequaltothedifferencebetweenthecorrespondingsignalpeakmagnitudeandthedesiredclippingthreshold.Thismethodreducesthesignalpeakmagnitudestothethresholdvaluewhilepreservingthesignalphase.Figure1illustratesthepeakcancellationprocessinthetimedomain.Thetopplotshowsasectionoftheinputsignalmagnitude.Thehorizontallineoverlaidontheplotindicatestheclippingthreshold.Anypeakthatexceedsthisthresholdisacandidateforcancellation.Themiddleplotshowsthemagnitudeofthecancellationpulsethatistobesubtractedfromtheinputsignal.Thebottomplotshowsthemagnitudeoftheoutputsignalaftersubtractingthecancellationpulsefromtheinputsignal.Figure2illustratesthecharacteristicsofthepeakcancellationmethodinthefrequencydomainforatypicalmulti-carrierconfiguration.Thepowerspec