Chapter5.MobileRadioPropagationSmall-ScalefadingandmultiPath„Small-ScaleMultipathƒImpulseResponseModelofaMultipathChannelƒSmall-ScaleMultipathMeasurementsƒParametersofMobileMultipathChannelƒTypesofSmall-ScaleFadingƒRayleighandRiceanDistributionsƒStatisticalModelsforMultipathFadingChannelPCN&SSLab.USTC5.1Small-ScaleMultipathPropagationƒSmall-scalefading,orsimplyfading,isusedtodescribetherapidfluctuationsoftheamplitudes,phases,ormultipathdelaysofaradiosignaloverashortperiodoftimeortraveldistance,sothatlarge-scalepathlosseffectsmaybeignored.ƒFadingiscausedbyinterferencebetweentwoormoreversionsofthetransmittedsignalwhicharriveatthereceiveratslightlydifferenttimes.PCN&SSLab.USTCPCN&SSLab.USTCƒMultipathintheradiochannelcreatessmall-scalefadingeffects.ƒMostimportanteffects：（1）Rapidchangesinsignalstrengthoverasmalltraveldistanceortimeinterval;（2）RandomfrequencymodulationduetovaryingDopplershiftsondifferentmultipathsignals;（3）Timedispersion(echoes)causedbymultipathpropagationdelays.PCN&SSLab.USTC•FactorsInfluencingSmall-ScaleFading：（1）Multipathpropagation－signalsmearingduetoISI.（2)Speedofthemobile－randomfrequencymodulationduetodifferentDopplershiftsoneachofthemultipathcomponents.（3）Speedofsurroundingobjects－timevaryingDopplershiftonmultipathcomponents.（4）Thetransmissionbandwidthofthesignal－transmittedradiosignalbandwidthisgreaterthanthe“bandwidth”ofthemultipathchannel,thereceivedsignalwillbedistorted.PCN&SSLab.USTCƒDopplerShift22π∆lvπ∆t由路程差造成的接收信号相位变化为：∆=ϕ＝cosθλλ1∆ϕv频率变化，即多普勒频移ff为：=⋅=⋅cosθdd2πλ∆tS∆lθθXdYvPCN&SSLab.USTC5.2ImpulseResponseModelofaMultipathChan.ƒThesmall-scalevariationscandirectlyrelatedtotheimpulseresponseofmobilechannel;ƒTheimpulseresponseisawidebandchannelcharacterizationandcontainsallinformationnecessarytosimulateoranalyzeanytypeofradiotransmissionthroughthechannel;ƒAmobilechannelmaybemodeledasalinearfilterwithatimevaryingimpulseresponse.PCN&SSLab.USTCƒImpulseResponseMod