无线通信 Lecture 5 Path Loss

Lecture5:Large-ScalePathLossChapter4–MobileRadioPropagation:Large-ScalePathLoss2Lasttwolectures:PropertiesofcellularradiosystemsFrequencyreusebyusingcellsClusteringandsystemcapacitySystemcomponents-Mobileswitchingcenters,basestations,mobiles,PSTNHandoffstrategiesHandoffmargin,guardchannelsMobileAssistedHandoffUmbrellacellsHardandsofthandoffsCo-ChannelInterferenceAdjacentChannelInterferenceTrunkingandgradeofservice(GOS)CellsplittingSectoring3Thislecture:Electromagneticpropagationpropertiesandhindrances.Whatarereasonswhywirelesssignalsarehardtosendandreceive?4I.ProblemsUniquetoWireless(notwired)systems:Pathscanvaryfromsimpleline-of-sighttoonesthatareseverelyobstructedbybuildings,mountains,andfoliage.Radiochannelsareextremelyrandomanddifficulttoanalyze.Interferencefromotherserviceprovidersout-of-bandnon-linearTxemissions5Interferencefromotherusers(samenetwork)CCIduetofrequencyreuseACIduetoTx/Rxdesignlimitations&large#userssharingfiniteBWShadowingObstructionstoline-of-sightpathscauseareasofweakreceivedsignalstrength6FadingWhennoclearline-of-sightpathexists,signalsarereceivedthatarereflectionsoffobstructionsanddiffractionsaroundobstructionsMultipathsignalscanbereceivedthatinterferewitheachotherFixedWirelessChannel→random&unpredictablemustbecharacterizedinastatisticalfashionfieldmeasurementsoftenneededtocharacterizeradiochannelperformance7**TheMobileRadioChannel(MRC)hasuniqueproblemsthatlimitperformance**AmobileRxinmotioninfluencesratesoffadingthefasteramobilemoves,themorequicklycharacteristicschange8II.RadioSignalPropagation9Thesmoothedlineistheaveragesignalstrength.Theactualisthemorejaggedline.Actualreceivedsignalstrengthcanvarybymorethan20dBoverafewcentimeters.Theaveragesignalstrengthdecayswithdistancefromthetransmitter,anddependsonterrainandobstructions.10Twobasicgoalsofpropagationmodeling:1)Predictmagnitudeandrate(speed)ofreceivedsignalstrengthfluctuationsovershortdistances/timedurations“short”→typicallyafewwavelengths(λ)orsecondsat1Ghz,λ=c/f=3x108/1x109=0.3metersreceivedsignalstrengthcanvarydrasticallyby30to40dB11small-scalefluctuations→called_____(Chapter5)causedbyreceivedsignalcomingfromasumofmanysignalscomingtogetheratareceivermultiplesignalscomefromreflectionsandscatteringthesesignalscandestructivelyaddtogetherbybeingout-of-phase122)PredictaveragereceivedsignalstrengthforgivenTx/Rxseparationcharacterizereceivedsignalstrengthoverdistancesfrom20mto20kmLarge-scaleradiowavepropagationmodelmodelsneededtoestimatecoverageareaofbasestationingeneral,largescalepathlossdecaysgraduallywithdistancefromthetransmitterwillalsobeaffectedbygeographicalfeatureslikehillsandbuildings13Free-SpaceSignalPropagationclear,unobstructedline-of-sightpath→satelliteandfixedmicrowaveFriistransmissionformula→Rxpower(Pr)vs.T-Rseparation(d)14wherePt=Txpower(W)G=TxorRxantennagain(unitless)relativetoisotropicsource(idealantennawhichradiatespoweruniformlyinalldirections)inthe__________ofanantenna(beyondafewmeters)EffectiveIsotropicRadiatedPower(EIRP)EIRP=PtGtRepresentsthemax.radiatedpoweravailablefromaTxinthedirectionofmax.antennagain,ascomparetoanisotropicradiator15λ=wavelength=c/f(m).Atermisrelatedtoantennagain.So,asfrequencyincreases,whathappenstothepropagationcharacteristics?L=systemlosses(antennas,transmissionlinesbetweenequipmentandantennas,atmosphere,etc.)unitlessL=1forzerolossL1ingeneral16d=T-Rseparationdistance(m)Signalfadesinproportiontod2Wecanviewsignalstrengthasrelatedtothedensityofthesignalacrossalargesphere.Thisisthesurfaceareaofaspherewithradiusd.So,aterminthedenominatorisrelatedtodistanceanddensityofsurfaceareaacrossasphere.17⇒PathLoss(PL)indB:18d2→powerlawrelationshipPrdecreasesatrateofproportionaltod2Prdecreasesatrateof20dB/decade(forline-of-sight,evenworseforothercases)Forexample,pathloses20dBfrom100mto1kmComesfromthed2relationshipforsurfacearea.Note:Negative“loss”=“gain”19Example:Pathlosscanbecomputedintermsofalinkbudgetcalculation.ComputepathlossasasumofdBtermsforthefollowing:Unitygaintransmissionantenna.Unitygainreceivingantenna.NosystemlossesCarrierfrequencyof3GHzDistance=2000meters20Closeinreferencepoint(do)isusedinlarge-scalemodelsdo:knownreceivedpowerreferencepoint-typically100mor1kmforoutdoorsystemsand1mforindoorsystemsdf:far-fielddistanceofantenna,wewillalwaysworkproblemsinthefar-fieldD:thelargestphysicallineardimensionofantenna22fffDddDd21ReferencePointExample:Giventhefollowingsystemcharacteristicsforlarge-scalepropagation,findthereferencedistancedo.Receivedpoweratdo=20WReceivedpowerat5km=13dBmUsingWatts:UsingdBm:22III.ReflectionsTherearethreebasicpropagationmechanismsinadditiontoline-of-sightpathsReflection-WavesbouncingoffofobjectsoflargedimensionsDiffraction-WavesbendingaroundsharpedgesofobjectsScattering-Wavestravelingthroughamediumwithsmallobjectsinit(foliage,streetsigns,lampposts,etc.)orreflectingoffroughsurfaces23ReflectionoccurswhenRFenergyisincidentuponaboundarybetweentwomaterials(e.gair/ground)with