无线通信与网络课件.ppt

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1、TransmissionFundamentalsChapter2ElectromagneticSignalFunctionoftimeCanalsobeexpressedasafunctionoffrequencySignalconsistsofcomponentsofdifferentfrequenciesTime-DomainConceptsAnalogsignal-signalintensityvariesinasmoothfashionovertimeNobreaksordiscontinuitiesinthesignalDigitalsi

2、gnal-signalintensitymaintainsaconstantlevelforsomeperiodoftimeandthenchangestoanotherconstantlevelPeriodicsignal-analogordigitalsignalpatternthatrepeatsovertimes(t+T)=s(t)-¥

3、n'trepeatovertimePeakamplitude(A)-maximumvalueorstrengthofthesignalovertime;typicallymeasuredinvoltsFrequency(f)Rate,incyclespersecond,orHertz(Hz)atwhichthesignalrepeatsTime-DomainConceptsPeriod(T)-amountoftimeittakesforonerepetitionofthesignalT=1/fPhase()-measureoftherelativ

4、epositionintimewithinasingleperiodofasignalWavelength()-distanceoccupiedbyasinglecycleofthesignalOr,thedistancebetweentwopointsofcorrespondingphaseoftwoconsecutivecyclesSineWaveParametersGeneralsinewaves(t)=Asin(2ft+)Figure2.3showstheeffectofvaryingeachofthethreeparameters(

5、a)A=1,f=1Hz,=0;thusT=1s(b)Reducedpeakamplitude;A=0.5(c)Increasedfrequency;f=2,thusT=½(d)Phaseshift;=/4radians(45degrees)note:2radians=360°=1periodSineWaveParametersTimevs.DistanceWhenthehorizontalaxisistime,asinFigure2.3,graphsdisplaythevalueofasignalatagivenpointinspaceas

6、afunctionoftimeWiththehorizontalaxisinspace,graphsdisplaythevalueofasignalatagivenpointintimeasafunctionofdistanceAtaparticularinstantoftime,theintensityofthesignalvariesasafunctionofdistancefromthesourceFrequency-DomainConceptsFundamentalfrequency-whenallfrequencycomponentsof

7、asignalareintegermultiplesofonefrequency,it’sreferredtoasthefundamentalfrequencySpectrum-rangeoffrequenciesthatasignalcontainsAbsolutebandwidth-widthofthespectrumofasignalEffectivebandwidth(orjustbandwidth)-narrowbandoffrequenciesthatmostofthesignal’senergyiscontainedinFrequen

8、cy-DomainConceptsAnyelectromagneticsignalcanbeshowntoconsisto