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无线通信网络第5章天线与传播RadiationPatternsRadiationpatternGraphicalrepresentationofradiationpropertiesofanantennaDepictedastwo-dimensionalcrosssectionBeamwidth(orhalf-powerbeamwidth)
MeasureofdirectivityofantennaReceptionpatternReceivingantenna’sequivalenttoradiationpatternTypesofAntennasIsotropicantenna(idealized)RadiatespowerequallyinalldirectionsDipoleantennasHalf-wavedipoleantenna(orHertzantenna)Quarter-waveverticalantenna(orMarconiantenna)ParabolicReflectiveAntennaAntennaGain AntennagainPoweroutput,inaparticulardirection,comparedtothatproducedinanydirectionbyaperfectomnidirectionalantenna(isotropicantenna)EffectiveareaRelatedtophysicalsizeandshapeofantennaAntennaGainRelationshipbetweenantennagainandeffectiveareaG=antennagainAe
=effectiveareaf=carrierfrequencyc=speedoflight(»3´108m/s)=carrierwavelengthPropagationModesGround-wavepropagationSky-wavepropagationLine-of-sightpropagationGroundWavePropagationGroundWavePropagationFollowscontouroftheearthCanPropagateconsiderabledistancesFrequenciesupto2MHzExampleAMradioSkyWavePropagationSkyWavePropagationSignalreflectedfromionizedlayerofatmospherebackdowntoearthSignalcantravelanumberofhops,backandforthbetweenionosphereandearth’ssurfaceReflectioneffectcausedbyrefractionExamplesAmateurradioCBradioLine-of-SightPropagationLine-of-SightPropagationTransmittingandreceivingantennasmustbewithinlineofsightSatellitecommunication–signalabove30MHznotreflectedbyionosphereGroundcommunication–antennaswithineffectivelineofsiteduetorefractionRefraction–bendingofmicrowavesbytheatmosphereVelocityofelectromagneticwaveisafunctionofthedensityofthemediumWhenwavechangesmedium,speedchangesWavebendsattheboundarybetweenmediumsLine-of-SightEquationsOpticallineofsightEffective,orradio,lineofsightd=distancebetweenantennaandhorizon(km)h=antennaheight(m)K=adjustmentfactortoaccountforrefraction,ruleofthumbK=4/3Line-of-SightEquationsMaximumdistancebetweentwoantennasforLOSpropagation:h1=heightofantennaoneh2=heightofantennatwoLOSWirelessTransmissionImpairmentsAttenuationandattenuationdistortionFreespacelossNoiseAtmosphericabsorptionMultipathRefractionThermalnoiseAttenuationStrengthofsignalfallsoffwithdistanceovertransmissionmediumAttenuationfactorsforunguidedmedia:ReceivedsignalmusthavesufficientstrengthsothatcircuitryinthereceivercaninterpretthesignalSignalmustmaintainalevelsufficientlyhigherthannoisetobereceivedwithouterrorAttenuationisgreaterathigherfrequencies,causingdistortionFreeSpaceLossFreespaceloss,idealisotropicantennaPt=signalpowerattransmittingantennaPr=signalpoweratreceivingantenna=carrierwavelengthd=propagationdistancebetweenantennasc=speedoflight(»3´108m/s)wheredandareinthesameunits(e.g.,meters)FreeSpaceLossFreespacelossequationcanberecast:FreeSpaceLossFreespacelossaccountingforgainofotherantennasGt=gainoftransmittingantennaGr=gainofreceivingantennaAt=effectiveareaoftransmittingantennaAr=effectiveareaofreceivingantennaFreeSpaceLossFreespacelossaccountingforgainofotherantennascanberecastasCategoriesofNoiseThermalNoiseIntermodulationnoiseCrosstalkImpulseNoiseThermalNoiseThermalnoiseduetoagitationofelectronsPresentinallelectronicdevicesandtransmissionmediaCannotbeeliminatedFunctionoftemperatureParticularlysignificantforsatellitecommunicationThermalNoiseAmountofthermalnoisetobefoundinabandwidthof1Hzinanydeviceorconductoris:N0=noisepowerdensityinwattsper1Hzofbandwidthk=Boltzmann'sconstant=1.3803´10-23J/KT=temperature,inkelvins(absolutetemperature)ThermalNoiseNoiseisassumedtobeindependentoffrequencyThermalnoisepresentinabandwidthofBHertz(inwatts):or,indecibel-wattsNoiseTerminologyIntermodulationnoise–occursifsignalswithdifferentfrequenciessharethesamemediumInterferencecausedbyasignalproducedatafrequencythatisthesumordifferenceoforiginalfrequenciesCrosstalk–unwantedcouplingbetweensignalpathsImpulsenoise–irregularpulsesornoisespikesShortdurationandofrelativelyhighamplitudeCausedbyexternalelectromagneticdisturbances,orfaultsandflawsinthecommunicationssystemExpressionEb/N0RatioofsignalenergyperbittonoisepowerdensityperHertzThebiterrorratefordigitaldataisafunctionofEb/N0GivenavalueforEb/N0toachieveadesirederrorrate,parametersofthisformulacanbeselectedAsbitrateRincreases,transmittedsignalpowermustincreasetomaintainrequiredEb/N0OtherImpairmentsAtmosphericabsorption–watervaporandoxygencontributetoattenuationMultipath–obstaclesreflectsignalssothatmultiplecopieswithvaryingdelaysarereceivedRefraction–bendingofradiowavesastheypropagatethroughtheatmosphereMultipathPropagationMultipathPropagationReflection-occurswhensignalencountersasurfacethatislargerelativetothewavelengthofthesignalDiffraction-occursattheedgeofanimpenetrablebodythatislargecomparedtowavelengthofradiowaveScattering–occurswhenincomingsignalhitsanobjectwhosesizeintheorderofthewavelengthofthesignalorlessTheEffectsofMultipathPropagationMultiplecopiesofasignalmayarriveatdifferentphasesIfphasesadddestructively,thesignallevelrelativetonoisedeclines,makingdetectionmoredifficultIntersymbolinte
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