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MeasuringACLRPerformanceinLTETransmittersApplicationNote2IntroductionAswirelessserviceproviderspushformorebandwidthtodeliverIPservicestomoreusers,LTEhasemergedasanext-generationcellulartechnologywithgreatpotentialtoenhancecurrentdeploymentsof3GPPnetworksandtoenablesig-nificantnewserviceopportunities.However,LTE’scomplex,evolvedarchitectureintroducesnewchallengesindesigningandtestingnetworkanduserequipment.ThecommercialsuccessofLTEwilldependinpartontheabilityofalldevicestoworkasspecified.Oneoftheparticularchallengesattheairinterfacewillbepowermanagementduringsignaltransmission.InadigitalcommunicationsystemsuchasLTE,thepowerthatleaksfromatransmittedsignalintoadjacentchannelscaninterferewithtransmissionsintheneighboringchannelsandimpairsystemperformance.Theadjacentchannelleakage-powerratio(ACLR)testverifiesthatsystemtransmittersareperformingwithinspecifiedlimits.ThiscriticalyetcomplextransmittertestcanbemadequicklyandaccuratelyusingmodernsignalanalyzerssuchastheAgilentX-Series(PXA/MXA/EXA)signalanalyzerswithLTEmeasurementsoftwareandsignalgeneratorssuchastheAgilentMXGsignalgeneratorwithLTEsignalcreationsoftware.LTEproductdevelopmentisunderway,andRFengineersaretacklingthemanydesignandmeasurementchallengesthiscomplextechnologypresents.LTErequiressupportforsixchannelbandwidths(from1.4to20MHz),differenttransmissionschemesforthedownlinkandtheuplink(OFDMAandSC-FDMA),twotransmissionmodes(FDDandTDD),andmultipleantennatechniques(MIMOspatialmultiplexing,diversity,beamsteering).AsaresultofLTE’sflexibletransmissionschemes,thephysicalchannelconfigurationhasalargeimpactonRFperformance—muchgreaterthanincurrentCDMA-basedsystems.Withper-formancetargetssetexceptionallyhighforLTE,engineershavetomakecarefuldesigntradeoffstocovereachcriticalpartoftheradiotransmitterchain.Oneimportantaspectoftransmitterdesignistheneedtominimizeunwantedemissions.BecauseLTEwillbedeployedinthesamefrequencybandsasW-CDMAandotherlegacycellulartechnologies,the3GPPspecificationsregu-lateemissionstominimizeinterferenceandensurecompatibilitybetweenthedifferentradiosystems.Theprimaryconcerniscontrolofspuriousemissions,whichcanoccuratanyfrequency.InthisrespectLTEissimilartootherradiosystems.However,newchallengesariseatthebandedges,wherethetransmit-tedsignalmustcomplywithrigorouspowerleakagerequirements.WithLTEsupportingchannelbandwidthsupto20MHz,andwithmanybandstoonarrowtosupportmorethanafewchannels,alargeproportionoftheLTEchannelswillbeattheedgeoftheband.Controllingtransmitterperformanceattheedgeofthebandrequiresadesignwithfilteringtoattenuateout-of-bandemissionswithoutaffectingin-channelperformance.Factorssuchascost,powerefficiencies,physicalsize,andlocationinthetransmitterblockdiagramarealsoimportantconsiderations.UltimatelytheLTEtransmittermustmeetallspecifiedlimitsforunwantedemissions,includinglimitsontheamountofpowerthatleaksintoadjacentchannels,asdefinedbyACLR.ChallengesofLTEtransmitterdesign3ACLRtestrequirementsACLRisakeytransmittercharacteristicincludedintheLTERFtransmitterconformancetests(Table1).Thesetestsverifythatminimumrequirementsarebeingmetinthebasestation(eNB)anduserequipment(UE).MostoftheLTEconformancetestsforout-of-bandemissionsaresimilarinscopeandpurposetothoseforW-CDMAandshouldlookfamiliar.However,whileW-CDMAspeci-fiesaroot-raisedcosine(RRC)filterformakingtransmittermeasurements,noequivalentfilterisdefinedforLTE.ThusdifferentfilterimplementationscanbeusedforLTEtransmittertestingtooptimizeeitherin-channelperformance,resultinginimprovederrorvectormagnitude,orout-of-channelperformance,resultinginbetteradjacentchannelpowercharacteristics.Table1.ConformancetestsforRFtransmitters(from3GPPTS36.141[1]and36.521-1[2])Giventheextensivenumberofcomplextransmitterconfigurationsthatcanbeusedtotesttransmitterperformance,LTEspecifiesaseriesofdownlinksignalconfigurationsknownasE-UTRAtestmodels(E-TM)fortestingtheeNB.Thesetestmodelsaregroupedintothreeclasses:E-TM1,E-TM2,andE-TM3.ThefirstandthirdclassesarefurthersubdividedintoE-TM1.1,E-TM1.2,E-TM3.1,E-TM3.2,andE-TM3.3(Table2).Notethatthe“E”inE-UTRAstandsfor“enhanced”anddesignatesLTEUMTSterrestrialradioaccess,whereasUTRAwithoutthe“E”referstoW-CDMA.BasestationRFtransmittercharacteristicstests36.141subclauseTestcase6.2Basestationoutputpower6.3.1Resourceelement(RE)powercontroldynamicrange6.3.2Totalpowerdynamicrange6.4.1TransmitterOFFpower6.4.2Transmittertransientperiod6.5.1Frequencyerror6.5.2Errorvectormagnitude(EVM)6.5.3Timealignmentbetweentransmitterbranches6.5.4Downlinkreferencesignalpower6.6.1Occupiedbandwidth6.6.2Adjacentchannelleakagepowerratio(ACLR)6.6.3Operatingbandunwantedemissions6.6.4Transmitterspuriousemissions6.7TransmitterintermodulationUEtransmittertestcases36.521-1subclauseTestcase6.2.2UEmaximumoutputpower6.2.3Maximumpowerreduction(MPR)6.2.4Additionalmaximumpowerreduction(A-MPR)6.2.5ConfiguredUEtransmittedoutputpower6.3.2Minimumoutputpower6.3.3TransmitOFFpower6.3.4.1GeneralON/OFFtimemask6.3.4.2PRACHandSRStimemask6.3.5.1Powercontrolabsolutepowertolerance6.3.5.2Powercontrolrelativepowertolerance6.3.5.