无线通信原理与应用-第二章传输技术基础课件

TransmissionFundamentalsChapter2TransmissionFundamentalsChapt1ElectromagneticSignalFunctionoftimeCanalsobeexpressedasafunctionoffrequencySignalconsistsofcomponentsofdifferentfrequenciesElectromagneticSignalFunctionTime-DomainConceptsAnalogsignal-signalintensityvariesinasmoothfashionovertimeNobreaksordiscontinuitiesinthesignalDigitalsignal-signalintensitymaintainsaconstantlevelforsomeperiodoftimeandthenchangestoanotherconstantlevelPeriodicsignal-analogordigitalsignalpatternthatrepeatsovertime

s(t+T)=s(t) -¥<t<+¥whereTistheperiodofthesignalTime-DomainConceptsAnalogsigTime-DomainConceptsAperiodicsignal-analogordigitalsignalpatternthatdoesn'trepeatovertimePeakamplitude(A)-maximumvalueorstrengthofthesignalovertime;typicallymeasuredinvoltsFrequency(f)Rate,incyclespersecond,orHertz(Hz)atwhichthesignalrepeatsTime-DomainConceptsAperiodicTime-DomainConceptsPeriod(T)-amountoftimeittakesforonerepetitionofthesignalT=1/fPhase()-measureoftherelativepositionintimewithinasingleperiodofasignalWavelength()-distanceoccupiedbyasinglecycleofthesignalOr,thedistancebetweentwopointsofcorrespondingphaseoftwoconsecutivecyclesTime-DomainConceptsPeriod(TSineWaveParametersGeneralsinewaves(t)=Asin(2ft+)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:2radians=360°=1periodSineWaveParametersGeneralsiSineWaveParametersSineWaveParametersTimevs.DistanceWhenthehorizontalaxisistime,asinFigure2.3,graphsdisplaythevalueofasignalatagivenpointinspaceasafunctionoftimeWiththehorizontalaxisinspace,graphsdisplaythevalueofasignalatagivenpointintimeasafunctionofdistanceAtaparticularinstantoftime,theintensityofthesignalvariesasafunctionofdistancefromthesourceTimevs.DistanceWhenthehoriFrequency-DomainConceptsFundamentalfrequency-whenallfrequencycomponentsofasignalareintegermultiplesofonefrequency,it’sreferredtoasthefundamentalfrequencySpectrum-rangeoffrequenciesthatasignalcontainsAbsolutebandwidth-widthofthespectrumofasignalEffectivebandwidth(orjustbandwidth)-narrowbandoffrequenciesthatmostofthesignal’senergyiscontainedinFrequency-DomainConceptsFundaFrequency-DomainConceptsAnyelectromagneticsignalcanbeshowntoconsistofacollectionofperiodicanalogsignals(sinewaves)atdifferentamplitudes,frequencies,andphasesTheperiodofthetotalsignalisequaltotheperiodofthefundamentalfrequencyFrequency-DomainConceptsAnyeRelationshipbetweenDataRateandBandwidthThegreaterthebandwidth,thehighertheinformation-carryingcapacityConclusionsAnydigitalwaveformwillhaveinfinitebandwidthBUTthetransmissionsystemwilllimitthebandwidththatcanbetransmittedAND,foranygivenmedium,thegreaterthebandwidthtransmitted,thegreaterthecostHOWEVER,limitingthebandwidthcreatesdistortionsRelationshipbetweenDataRateDataCommunicationTermsData-entitiesthatconveymeaning,orinformationSignals-electricorelectromagneticrepresentationsofdataTransmission-communicationofdatabythepropagationandprocessingofsignalsDataCommunicationTermsData-ExamplesofAnalogandDigitalDataAnalogVideoAudioDigitalTextIntegersExamplesofAnalogandDigitalAnalogSignalsAcontinuouslyvaryingelectromagneticwavethatmaybepropagatedoveravarietyofmedia,dependingonfrequencyExamplesofmedia:Copperwiremedia(twistedpairandcoaxialcable)FiberopticcableAtmosphereorspacepropagationAnalogsignalscanpropagateanaloganddigitaldataAnalogSignalsAcontinuouslyvDigitalSignalsAsequenceofvoltagepulsesthatmaybetransmittedoveracopperwiremediumGenerallycheaperthananalogsignalingLesssusceptibletonoiseinterferenceSuffermorefromattenuationDigitalsignalscanpropagateanaloganddigitaldataDigitalSignalsAsequenceofvAnalogSignalingAnalogSignalingDigitalSignalingDigitalSignalingReasonsforChoosingDataandSignalCombinationsDigitaldata,digitalsignalEquipmentforencodingislessexpensivethandigital-to-analogequipmentAnalogdata,digitalsignalConversionpermitsuseofmoderndigitaltransmissionandswitchingequipmentDigitaldata,analogsignalSometransmissionmediawillonlypropagateanalogsignalsExamplesincludeopticalfiberandsatelliteAnalogdata,analogsignalAnalogdataeasilyconvertedtoanalogsignalReasonsforChoosingDataandAnalogTransmissionTransmitanalogsignalswithoutregardtocontentAttenuationlimitslengthoftransmissionlinkCascadedamplifiersboostsignal’senergyforlongerdistancesbutcausedistortionAnalogdatacantoleratedistortionIntroduceserrorsindigitaldataAnalogTransmissionTransmitanDigitalTransmissionConcernedwiththecontentofthesignalAttenuationendangersintegrityofdataDigitalSignalRepeatersachievegreaterdistanceRepeatersrecoverthesignalandretransmitAnalogsignalcarryingdigitaldataRetransmissiondevicerecoversthedigitaldatafromanalogsignalGeneratesnew,cleananalogsignalDigitalTransmissionConcernedAboutChannelCapacityImpairments,suchasnoise,limitdataratethatcanbeachievedFordigitaldata,towhatextentdoimpairmentslimitdatarate?ChannelCapacity–themaximumrateatwhichdatacanbetransmittedoveragivencommunicationpath,orchannel,undergivenconditionsAboutChannelCapacityImpairmeConceptsRelatedtoChannelCapacityDatarate-rateatwhichdatacanbecommunicated(bps)Bandwidth-thebandwidthofthetransmittedsignalasconstrainedbythetransmitterandthenatureofthetransmissionmedium(Hertz)Noise-averagelevelofnoiseoverthecommunicationspathErrorrate-rateatwhicherrorsoccurError=transmit1andreceive0;transmit0andreceive1ConceptsRelatedtoChannelCaNyquistBandwidthForbinarysignals(twovoltagelevels)C=2BWithmultilevelsignalingC=2Blog2

MM=numberofdiscretesignalorvoltagelevelsNyquistBandwidthForbinarysiSignal-to-NoiseRatioRatioofthepowerinasignaltothepowercontainedinthenoisethat’spresentataparticularpointinthetransmissionTypicallymeasuredatareceiverSignal-to-noiseratio(SNR,orS/N)AhighSNRmeansahigh-qualitysignal,lownumberofrequiredintermediaterepeatersSNRsetsupperboundonachievabledatarateSignal-to-NoiseRatioRatioofShannonCapacityFormulaEquation:RepresentstheoreticalmaximumthatcanbeachievedInpractice,onlymuchlowerratesachievedFormulaassumeswhitenoise(thermalnoise)ImpulsenoiseisnotaccountedforAttenuationdistortionordelaydistortionnotaccountedforShannonCapacityFormulaEquatiExampleofNyquistandShannonFormulationsSpectrumofachannelbetween3MHzand4MHz;SNRdB=24dBUsingShannon’sformulaExampleofNyquistandShannonExampleofNyquistandShannonFormulationsHowmanysignalinglevelsarerequired?ExampleofNyquistandShannonClassificationsofTransmissionMediaTransmissionMediumPhysicalpathbetweentransmitterandreceiverGuidedMediaWavesareguidedalongasolidmediumE.g.,coppertwistedpair,coppercoaxialcable,opticalfiberUnguidedMediaProvidesmeansoftransmissionbutdoesnotguideelectromagneticsignalsUsuallyreferredtoaswirelesstransmissionE.g.,atmosphere,outerspaceClassificationsofTransmissioUnguidedMediaTransmissionandreceptionareachievedbymeansofanantennaConfigurationsforwirelesstransmissionDirectionalOmnidirectional

UnguidedMediaTransmissionandGeneralFrequencyRangesMicrowavefrequencyrange1GHzto40GHzDirectionalbeamspossibleSuitableforpoint-to-pointtransmissionUsedforsatellitecommunicationsRadiofrequencyrange30MHzto1GHzSuitableforomnidirectionalapplicationsInfraredfrequencyrangeRoughly,3x1011to2x1014HzUsefulinlocalpoint-to-pointmultipointapplicationswithinconfinedareasGeneralFrequencyRangesMicrowTerrestrialMicrowaveDescriptionofcommonmicrowaveantennaParabolic"dish",3mindiameterFixedrigidlyandfocusesanarrowbeamAchievesline-of-sighttransmissiontoreceivingantennaLocatedatsubstantialheightsabovegroundlevelApplicationsLonghaultelecommunicationsserviceShortpoint-to-pointlinksbetweenbuildingsTerrestrialMicrowaveDescriptiSatelliteMicrowaveDescriptionofcommunicationsatelliteMicrowaverelaystationUsedtolinktwoormoreground-basedmicrowavetransmitter/receiversReceivestransmissionsononefrequencyband(uplink),amplifiesorrepeatsthesignal,andtransmitsitonanotherfrequency(downlink)ApplicationsTelevisiondistributionLong-distancetelephonetransmissionPrivatebusinessnetworksSatelliteMicrowaveDescriptionBroadcastRadioDescriptionofbroadcastradioantennasOmnidirectionalAntennasnotrequiredtobedish-shapedAntennasneednotberigidlymountedtoaprecisealignmentApplicationsBroadcastradioVHFandpartoftheUHFband;30MHZto1GHzCoversFMradioandUHFandVHFtelevisionBroadcastRadioDescriptionofMulti