无线通信与网络chap2

TransmissionFundamentalsChapter2ElectromagneticSignalFunctionoftimeCanalsobeexpressedasafunctionoffrequencySignalconsistsofcomponentsofdifferentfrequenciesTime-DomainConceptsAnalogsignal-signalintensityvariesinasmoothfashionovertimeNobreaksordiscontinuitiesinthesignalDigitalsignal-signalintensitymaintainsaconstantlevelforsomeperiodoftimeandthenchangestoanotherconstantlevelPeriodicsignal-analogordigitalsignalpatternthatrepeatsovertimes(t+T)=s(t)-¥t+¥whereTistheperiodofthesignalTime-DomainConceptsAperiodicsignal-analogordigitalsignalpatternthatdoesn'trepeatovertimePeakamplitude(A)-maximumvalueorstrengthofthesignalovertime;typicallymeasuredinvoltsFrequency(f)Rate,incyclespersecond,orHertz(Hz)atwhichthesignalrepeatsTime-DomainConceptsPeriod(T)-amountoftimeittakesforonerepetitionofthesignalT=1/fPhase()-measureoftherelativepositionintimewithinasingleperiodofasignalWavelength()-distanceoccupiedbyasinglecycleofthesignalOr,thedistancebetweentwopointsofcorrespondingphaseoftwoconsecutivecyclesSineWaveParametersGeneralsinewaves(t)=Asin(2ft+)Figure2.3showstheeffectofvaryingeachofthethreeparameters(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.DistanceWhenthehorizontalaxisistime,asinFigure2.3,graphsdisplaythevalueofasignalatagivenpointinspaceasafunctionoftimeWiththehorizontalaxisinspace,graphsdisplaythevalueofasignalatagivenpointintimeasafunctionofdistanceAtaparticularinstantoftime,theintensityofthesignalvariesasafunctionofdistancefromthesourceFrequency-DomainConceptsFundamentalfrequency-whenallfrequencycomponentsofasignalareintegermultiplesofonefrequency,it’sreferredtoasthefundamentalfrequencySpectrum-rangeoffrequenciesthatasignalcontainsAbsolutebandwidth-widthofthespectrumofasignalEffectivebandwidth(orjustbandwidth)-narrowbandoffrequenciesthatmostofthesignal’senergyiscontainedinFrequency-DomainConceptsAnyelectromagneticsignalcanbeshowntoconsistofacollectionofperiodicanalogsignals(sinewaves)atdifferentamplitudes,frequencies,andphasesTheperiodofthetotalsignalisequaltotheperiodofthefundamentalfrequencyRelationshipbetweenDataRateandBandwidthThegreaterthebandwidth,thehighertheinformation-carryingcapacityConclusionsAnydigitalwaveformwillhaveinfinitebandwidthBUTthetransmissionsystemwilllimitthebandwidththatcanbetransmittedAND,foranygivenmedium,thegreaterthebandwidthtransmitted,thegreaterthecostHOWEVER,limitingthebandwidthcreatesdistortionsDataCommunicationTermsData-entitiesthatconveymeaning,orinformationSignals-electricorelectromagneticrepresentationsofdataTransmission-communicationofdatabythepropagationandprocessingofsignalsExamplesofAnalogandDigitalDataAnalogVideoAudioDigitalTextIntegersAnalogSignalsAcontinuouslyvaryingelectromagneticwavethatmaybepropagatedoveravarietyofmedia,dependingonfrequencyExamplesofmedia:Copperwiremedia(twistedpairandcoaxialcable)FiberopticcableAtmosphereorspacepropagationAnalogsignalscanpropagateanaloganddigitaldataDigitalSignalsAsequenceofvoltagepulsesthatmaybetransmittedoveracopperwiremediumGenerallycheaperthananalogsignalingLesssusceptibletonoiseinterferenceSuffermorefromattenuationDigitalsignalscanpropagateanaloganddigitaldataAnalogSignalingDigitalSignalingReasonsforChoosingDataandSignalCombinationsDigitaldata,digitalsignalEquipmentforencodingislessexpensivethandigital-to-analogequipmentAnalogdata,digitalsignalConversionpermitsuseofmoderndigitaltransmissionandswitchingequipmentDigitaldata,analogsignalSometransmissionmediawillonlypropagateanalogsignalsExamplesincludeopticalfiberandsatelliteAnalogdata,analogsignalAnalogdataeasilyconvertedtoanalogsignalAnalogTransmissionTransmitanalogsignalswithoutregardtocontentAttenuationlimitslengthoftransmissionlinkCascadedamplifiersboostsignal’senergyforlongerdistancesbutcausedistortionAnalogdatacantoleratedistortionIntroduceserrorsindigitaldataDigitalTransmissionConcernedwiththecontentofthesignalAttenuationendangersintegrityofdataDigitalSignalRepeatersachievegreaterdistanceRepeatersrecoverthesignalandretransmitAnalogsignalcarryingdigitaldataRetransmissiondevicerecoversthedigitaldatafromanalogsignalGeneratesnew,cleananalogsignalAboutChannelCapacityImpairments,suchasnoise,limitdataratethatcanbeachievedFordigitaldata,towhatextentdoimpairmentslimitdatarate?ChannelCapacity–themaximumrateatwhichdatacanbetransmittedoveragivencommunicationpath,orchannel,undergivenconditionsConceptsRelatedtoChannelCapacityDatarate-rateatwhichdatacanbecommunicated(bps)Bandwidth-thebandwidthofthetransmittedsignalasconstrainedbythetransmitterandthenatureofthetransmissionmedium(Hertz)Noise-averagelevelofnoiseoverthecommunicationspathErrorrate-rateatwhicherrorsoccurError=transmit1andreceive0;transmit0andreceive1NyquistBandwidthForbinarysignals(twovoltagelevels)C=2BWithmultilevelsignalingC=2Blog2MM=numberofdiscretesignalorvoltagelevelsSignal-to-NoiseRatioRatiooft