无线通信网络第5章-天线与传播

IntroductionAnantennaisanelectricalconductororsystemofconductorsTransmission-radiateselectromagneticenergyintospaceReception-collectselectromagneticenergyfromspaceIntwo-waycommunication,thesameantennacanbeusedfortransmissionandreceptionRadiationPatternsRadiationpatternGraphicalrepresentationofradiationpropertiesofanantennaDepictedastwo-dimensionalcrosssectionBeamwidth(orhalf-powerbeamwidth)MeasureofdirectivityofantennaReceptionpatternReceivingantenna’sequivalenttoradiationpatternTypesofAntennasIsotropicantenna(idealized)RadiatespowerequallyinalldirectionsDipoleantennasHalf-wavedipoleantenna(orHertzantenna)Quarter-waveverticalantenna(orMarconiantenna)ParabolicReflectiveAntennaAntennaGainAntennagainPoweroutput,inaparticulardirection,comparedtothatproducedinanydirectionbyaperfectomnidirectionalantenna(isotropicantenna)EffectiveareaRelatedtophysicalsizeandshapeofantennaAntennaGainRelationshipbetweenantennagainandeffectiveareaG=antennagainAe=effectiveareaf=carrierfrequencyc=speedoflight(»3´108m/s)=carrierwavelength22244cAfAGeePropagationModesGround-wavepropagationSky-wavepropagationLine-of-sightpropagationGroundWavePropagationGroundWavePropagationFollowscontouroftheearthCanPropagateconsiderabledistancesFrequenciesupto2MHzExampleAMradioSkyWavePropagationSkyWavePropagationSignalreflectedfromionizedlayerofatmospherebackdowntoearthSignalcantravelanumberofhops,backandforthbetweenionosphereandearth’ssurfaceReflectioneffectcausedbyrefractionExamplesAmateurradioCBradioLine-of-SightPropagationLine-of-SightPropagationTransmittingandreceivingantennasmustbewithinlineofsightSatellitecommunication–signalabove30MHznotreflectedbyionosphereGroundcommunication–antennaswithineffectivelineofsiteduetorefractionRefraction–bendingofmicrowavesbytheatmosphereVelocityofelectromagneticwaveisafunctionofthedensityofthemediumWhenwavechangesmedium,speedchangesWavebendsattheboundarybetweenmediumsLine-of-SightEquationsOpticallineofsightEffective,orradio,lineofsightd=distancebetweenantennaandhorizon(km)h=antennaheight(m)K=adjustmentfactortoaccountforrefraction,ruleofthumbK=4/3hd57.3hd57.3Line-of-SightEquationsMaximumdistancebetweentwoantennasforLOSpropagation:h1=heightofantennaoneh2=heightofantennatwo2157.3hhLOSWirelessTransmissionImpairmentsAttenuationandattenuationdistortionFreespacelossNoiseAtmosphericabsorptionMultipathRefractionThermalnoiseAttenuationStrengthofsignalfallsoffwithdistanceovertransmissionmediumAttenuationfactorsforunguidedmedia:ReceivedsignalmusthavesufficientstrengthsothatcircuitryinthereceivercaninterpretthesignalSignalmustmaintainalevelsufficientlyhigherthannoisetobereceivedwithouterrorAttenuationisgreaterathigherfrequencies,causingdistortionFreeSpaceLossFreespaceloss,idealisotropicantennaPt=signalpowerattransmittingantennaPr=signalpoweratreceivingantenna=carrierwavelengthd=propagationdistancebetweenantennasc=speedoflight(»3´108m/s)wheredandareinthesameunits(e.g.,meters)222244cfddPPrtFreeSpaceLossFreespacelossequationcanberecast:dPPLrtdB4log20log10dB98.21log20log20ddB56.147log20log204log20dfcfdFreeSpaceLossFreespacelossaccountingforgainofotherantennasGt=gainoftransmittingantennaGr=gainofreceivingantennaAt=effectiveareaoftransmittingantennaAr=effectiveareaofreceivingantennatrtrtrrtAAfcdAAdGGdPP2222224FreeSpaceLossFreespacelossaccountingforgainofotherantennascanberecastasrtdBAAdLlog10log20log20dB54.169log10log20log20rtAAdfCategoriesofNoiseThermalNoiseIntermodulationnoiseCrosstalkImpulseNoiseThermalNoiseThermalnoiseduetoagitationofelectronsPresentinallelectronicdevicesandtransmissionmediaCannotbeeliminatedFunctionoftemperatureParticularlysignificantforsatellitecommunicationThermalNoiseAmountofthermalnoisetobefoundinabandwidthof1Hzinanydeviceorconductoris:N0=noisepowerdensityinwattsper1Hzofbandwidthk=Boltzmann'sconstant=1.3803´10-23J/KT=temperature,inkelvins(absolutetemperature)W/Hzk0TNThermalNoiseNoiseisassumedtobeindependentoffrequencyThermalnoisepresentinabandwidthofBHertz(inwatts):or,indecibel-wattsTBNkBTNlog10log10klog10BTlog10log10dBW6.228NoiseTerminologyIntermodulationnoise–occursifsignalswithdifferentfrequenciessharethesamemediumInterferencecausedbyasignalproducedatafrequencythatisthesumordifferenceoforiginalfrequenciesCrosstalk–unwantedcouplingbetweensignalpathsImpulsenoise–irregularpulsesornoisespikesShortdurationandofrelativelyhighamplitudeCausedbyexternalelectromagneticdisturbances,orfaultsandflawsinthecommunicationssystemExpressionEb/N0RatioofsignalenergyperbittonoisepowerdensityperHertzThebiterrorratefordigitaldataisafunctionofEb/N0GivenavalueforEb/N0toachieveadesirederrorrate,parametersofthisformulacanbeselectedAsbitrateRincreases,transmittedsignalpowermustincreasetomaintainrequiredEb/N0TRSNRSNEbk/00OtherImpairmentsAtmosphericabsorption–watervaporandoxygencontributetoattenuationMultipath–obstaclesrefle