无线通信原理与应用

Chapter3:

MobileRadioPropagation:

Large-ScalePathLoss4/28/20231

3.1IntroductiontoRadiowavePropagationSmall-scaleandlarge-scalefading4/28/202323.2FreeSpacePropagationModelInfreespace,thereceivedpowerispredictedbyFiriisEqu.Pr(d):ReceivedpowerwithadistancedbetweenTxandRxPt:TransmittedpowerGt:TransmittingantennagainGr:Receiveantennagain:Thewavelengthinmeters.d:distanceinmetersL:ThemiscellaneouslossesL(L>=1)areusuallyduetotransmissionlineattenuation,filterlosses,andantennalossesinthecommunicationsystem.L=1indicatesnolossinthesystemhardware.4/28/20233Reflection:occurfromthesurfaceoftheearthandfrombuildingsandwalls.Diffraction:occurswhentheradiopathbetweenthetransmitterandreceiverisobstructedbyasurfacethathassharpirregularities(edges).

Scattering:occurswhenthemediumthroughwhichthewavetravelsconsistsofobjectswithdimensionsthataresmallcomparedtothewavelength,andwherethenumberofobstaclesperunitvolumeislarge.3.3ThethreeBasicPropagationMechanisms4/28/20234reflection（反射）atlargeobstacles,Scattering(散射)atsmallobstacles,diffraction(衍射)atedges4/28/20235EIRP&ERPEIRP:EffectiveIsotropicRadiatedPowerRepresentsthemaximumradiatedpoweravailablefromatransmitterinthedirectionofmaximumantennagain,ascomparedtoanisotropicradiator.ERP:EffectiveRadiatedPowerERPisusedinsteadofEIRPtodenotethemaximumradiatedpowerascomparedtoahalf-wavedipoleantenna(insteadofanisotropicantenna).Inpractice,antennagainsaregiveninunitsofdBi(dBgainwithrespecttoanisotropicsourse)ordBd(dBgainwithrespecttoahalf-wavedipole)2.15dB4/28/202369dBiantenna&3dBiantenna4/28/20237PathLossThepathloss,whichrepresentssignalattenuationasapositivedifference(indB)betweentheeffectivetransmittedpowerandthereceivedpower.ThepathlossforthefreespacemodelwhenantennagainsareincludedisgivenbyquantitymeasuredindB,isdefinedastheWhenantennagainsareexcluded,theantennasareassumedtohaveunitygain,andpathlossisgivenby(f:MHz,d:km)4/28/20238TheFriisfreespacemodelisonlyavalidpredictorforPrforvaluesofd,whichareinthefar-fieldofthetransmittingantenna.Thefar-fieldofatransmittingantennaisdefinedastheregionbeyondthefar-fielddistancedf,whichisrelatedtothelargestlineardimensionofthetransmitterantennaapertureandthecarrierwavelength.Thefar-fielddistanceisgivenbyTobeinthefar-fieldregion,dmustsatisfyThefar-fieldregionofatransmittingantenna4/28/20239TheReferenceDistanceItisclearthatequationdoesnotholdford=0.Forthisreason,large-scalepropagationmodelsuseaknownreceivedpowerreferencepoint.Thereceivedpower,Pr(d),atanydistanced>d0,mayberelatedtoPratd0.IfPrisinunitsofdBmordBW,thereceivedpowerisgivenby4/28/2023103.4LinkbudgedesignusingpathlossmodelLog-distancepathlossmodelBoththeoreticalandmeasurement-basedpropagationmodelsindicatethataveragereceivedsignalpowerdecreaseslogarithmicallywithdistance,whetherinoutdoororindoorchannels.Theaveragelarge-scalepathlossforanarbitraryT-Rseparationisexpressedasafunctionofdistancebyusingpathlossexponentn.nisthepathlossexponentwhichindicatestherateatwhichthepathlossincreaseswithdistanced0istheclose-inreferencedistancewhichisdetermineddistheT-Rseparationdistance4/28/202311Path-lossexponents4/28/202312Ifatransmitterproducespower:Pt=50w,receivesensitivity(minimumusablesignallevel)is-100dbm.Assumed0=100m,witha900MHzcarrierfrequency,n=4,Gt=Gr=1;findthecoveragedistanced.TransmitPower:Pt=50W=47dBmPr(d0)=-24.5dBmPL(dB)=40log(d/d0)=-24.5-(-100)=75.5dbIfn=4,log(d/d0)=75.5/40=1.8875,d=7718mExample14/28/202313ThemodelinEquation(3.11)doesnotconsiderthefactthatthesurroundingenvironmentalcluttermaybevastlydifferentattwodifferentlocationshavingthesameT-Rseparation.ThisleadstomeasuredsignalswhicharevastlydifferentthantheaveragevaluepredictedbyEquation(3.11).

Log-normalShadowing4/28/202314Log-normalShadowing

4/28/202315DeterminationofPercentageofCoverageArea4/28/202316

U(r)asafunctionofprobabilityofsignalabovethresholdonthecellboundary.4/28/202317Example2Alocalaveragesignalstrengthfieldmeasurements,themeasureddatafitadistant-dependentmeanpowerlawmodelhavingalog-normaldistributionaboutthemean.Assumethemeanpowerlawwasfoundtobe.Ifasignalof1mWwasreceivedatd0=1mfromthetransmitter,andatadistanceof10m,10%ofthemeasurementswerestrongerthan-25dBm,definethestandarddeviation,,forthepathlossmodelatd=10m.4/28/202318Fourreceivedpowermeasurementsweretakenatdistancesof100m,200m,1km,and3kmfromatransmitter.Thesemeasuredvaluesaregiveninthefollowingtable.ItisassumedthatthepathlossforthesemeasurementsfollowsthemodelinEquation(3.12.a),whered0=100m:(a)findtheminimummeansquareerror(MMSE)estimateforthepathlossexponent,n;(b)calculatethestandarddeviationaboutthemeanvalue;(c)estimatethereceivedpoweratd=2kmusingtheresultingmodel;(d)predictthelikelihoodthatthereceivedsignallevelat2kmwillbegreaterthan-60dBm;and(e)predictthepercentageofareawithina2kmradiuscellthatreceivessignalsgreaterthan-60dBm,giventheresultin(d).

Example34/28/202319ThevalueofnwhichminimizesthemeansquareerrorcanbeobtainedbyequatingthederivativeofJ(n)tozero,andthensolvingforn.(a)UsingEquation(3.11),wefind=pi(d0)-10nlog(di/100m).RecognizingthatP(d0)=0dBm,wefindthefollowingestimatesforp,indBm:

TheMMSEestimatemaybefoundusingthefollowingmethod.Letpibethereceivedpoweratadistancedi,andletbetheestimateforpiusingthepathlossmodelofEquation(3.10).Thesumofsquarederrorsbetweenthemeasuredandestimatedvaluesisgivenby

Settingthisequaltozero,thevalueofnisobtainedasn=4.4.4/28/202320(b)Thesamplevariance2=J(n)/4atn=4.4canbeobtainedasfollows.

therefore=6.17dB,whichisabiasedestimate.4/28/202321(c)Theestimateofthereceivedpoweratd=2kmis(d)Theprobabilitythatthereceivedsignallevelwillbegreaterthan-60dBmis(e)67.4%oftheusersontheboundaryreceivesignalsgreaterthan-60dBm,then92%ofthecellareareceivescoverageabove–60dbm4/28/2023223.5OutdoorPropagationModelsOkumuraModel(150-1920MHz,1km-100km)HataModel(150-1500MHz,1km-20km)EgliModel(40-400MHz,0-64km)4/28/202323notprovideanyanalyticalexplanation

itsslowresponsetorapidchangesinterrain

OkumuraModel4/28/202324Okumuramedianattenuationandcorrection4/28/202325FindthemedianpathlossusingOkumura'smodelford=50km,hte

=100m,hre

=10minasuburbanenvironment.IfthebasestationtransmitterradiatesanEIRPof1kWatacarrierfrequencyof900MHz,findthepoweratthereceiver(assumeaunitygainreceivingantenna).

Example44/28/202326HATAmodel&COST–231extension4/28/202327Example5Inthesuburbanofalargecity,d=10km,hte

=200m,hre

=2m,carrierfrequencyof900MHz,usingHATA’smodelfindthepathloss.

4/28/2023283.6Indoorpropagationmodels4/28/202329FeatureofIndoorRadioChannelThedistancescoveredaremuchsmaller,andthevariabilityoftheenvironmentismuchgreaterforamuchsmallerrangeofT-Rseparationdistances.Ithasbeenobservedthatpropagationwithinbuildingsisstronglyinfluencedbyspecificfeaturessuchasthelayoutofthebuilding,theconstructionmaterials,andthebuildingtype.Indoorradiopropagationisdominatedbythesamemechanismsasoutdoor:reflection,diffraction,andscattering.However,conditionsaremuchmorevariable.

4/28/202330PathattenuationfactorsPartitionLossesinthesamefloorPartitionLossesbetweenFloors(floorattenuationfactors,FAF)4/28/202331Log-distancePathLossModelIndoorpathlosshasbeenshownbymanyresearcherstoobeythedistancepowerlaw

Wherethevalueofndependsonthesurroundingsandbuildingtype,andXrepresentsanormalrandomvariableindBhavingastandarddeviationofsigma.Thisisidenticalinformtothelog-normalshadowingmodelofoutdoorpathattenuationmodel.4/28/202332AttenuationFactorModelWherenSFrepresentstheexponentvalueforthe“samefloor”measurement.ThepathlossonadifferentfloorcanbepredictedbyaddinganappropriatevalueofFAF4/28/202333SignalPenetrationintobuildingsRFpenetrationhasbeenfoundtobeafunctionoffrequencyaswellasheightwithinthebuildingMeasurementsshowedthatpenetrationlossdecreaseswithincreasingfrequency.Specifically,penetrationattenuationvaluesof16.4dB,11.6dB,and7.6dBweremeasuredonthegroundfloorofabuildingatfrequenciesof441MHz,896.5MHz,and1400Mhz,respectly.Resultsshowedthatbuildingpenetrationlossdecreasedatarateof1.9dBperfloorfromthegroundleveluptothefifteenthfloorandthenbeganincreasingabovethefifteenfloor.4/28/202334RayTracingandSiteSpecificModelingInrecentyears,thecomputationalandvisualizationcapabilitiesofcomputershaveacceleratedrapidly.NewmethodsforpredictingradiosignalcoverageinvolvetheuseofSiteSpecific(SISP)propagationmodelsandgraphicalinformationsystem(GIS)