无线通信原理与.ppt
Chapter 3:Mobile Radio Propagation:Large-Scale Path Loss12/11/20221 3.1 Introduction to Radio wave Propagation Small-scale and large-scale fading12/11/202223.2 Free Space Propagation ModelIn free space,the received power is predicted by Firiis Equ.Pr(d):ReceivedpowerwithadistancedbetweenTxandRxPt:TransmittedpowerGt:TransmittingantennagainGr:Receiveantennagain:Thewavelengthinmeters.d:distanceinmetersL:ThemiscellaneouslossesL(L=1)areusuallyduetotransmissionlineattenuation,filterlosses,andantennalossesinthecommunicationsystem.L=1indicatesnolossinthesystemhardware.12/11/20223Reflection:occur from the surface of the earth and from buildings and walls.Diffraction:occurs when the radio path between the transmitter and receiver is obstructed by a surface that has sharp irregularities(edges).Scattering:occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength,and where the number of obstacles per unit volume is large.3.3 The three Basic Propagation Mechanisms12/11/20224reflection（反射）atlargeobstacles,Scattering(散射)atsmallobstacles,diffraction(衍射)atedges12/11/20225EIRP&ERPEIRP:EffectiveIsotropicRadiatedPowerRepresentsthemaximumradiatedpoweravailablefromatransmitterinthedirectionofmaximumantennagain,ascomparedtoanisotropicradiator.ERP:EffectiveRadiatedPowerERPisusedinsteadofEIRPtodenotethemaximumradiatedpowerascomparedtoahalf-wavedipoleantenna(insteadofanisotropicantenna).Inpractice,antennagainsaregiveninunitsofdBi(dBgainwithrespecttoanisotropicsourse)ordBd(dBgainwithrespecttoahalf-wavedipole)2.15dB12/11/202269dBiantenna&3dBiantenna12/11/20227PathLossThepathloss,whichrepresentssignalattenuationasapositivedifference(indB)betweentheeffectivetransmittedpowerandthereceivedpower.ThepathlossforthefreespacemodelwhenantennagainsareincludedisgivenbyquantitymeasuredindB,isdefinedastheWhenantennagainsareexcluded,theantennasareassumedtohaveunitygain,andpathlossisgivenby(f:MHz,d:km)12/11/20228TheFriisfreespacemodelisonlyavalidpredictorforPrforvaluesofd,whichareinthefar-fieldofthetransmittingantenna.Thefar-fieldofatransmittingantennaisdefinedastheregionbeyondthefar-fielddistancedf,whichisrelatedtothelargestlineardimensionofthetransmitterantennaapertureandthecarrierwavelength.Thefar-fielddistanceisgivenbyTobeinthefar-fieldregion,dmustsatisfyThefar-fieldregionofatransmittingantenna12/11/20229The Reference DistanceItisclearthatequationdoesnotholdford=0.Forthisreason,large-scalepropagationmodelsuseaknownreceivedpowerreferencepoint.Thereceivedpower,Pr(d),atanydistancedd0,mayberelatedtoPratd0.IfPrisinunitsofdBmordBW,thereceivedpowerisgivenby12/11/2022103.4LinkbudgedesignusingpathlossmodelLog-distancepathlossmodelBoth theoretical and measurement-based propagation models indicate that average received signal power decreases logarithmically with distance,whether in outdoor or indoor channels.The average large-scale path loss for an arbitrary T-R separation is expressed as a function of distance by using path loss exponent n.n is the path loss exponent which indicates the rate at which the path loss increases with distanced0 is the close-in reference distance which is determined d is the T-R separation distance12/11/202211Path-lossexponents12/11/202212If a transmitter produces power:Pt=50w,receive sensitivity(minimum usable signal level)is-100dbm.Assume d0=100m,with a 900MHz carrier frequency,n=4,Gt=Gr=1;find the coverage distance d.Transmit Power:Pt=50W=47dBmPr(d0)=-24.5dBmPL(dB)=40log(d/d0)=-24.5-(-100)=75.5dbIf n=4,log(d/d0)=75.5/40=1.8875,d=7718mExample112/11/202213ThemodelinEquation(3.11)doesnotconsiderthefactthatthesurroundingenvironmentalcluttermaybevastlydifferentattwodifferentlocationshavingthesameT-Rseparation.Thisleadstomeasuredsignalswhicharevastlydifferentthantheaverage valuepredictedbyEquation(3.11).Log-normalShadowing12/11/202214Log-normal Shadowing 12/11/202215Determination of Percentage of Coverage Area12/11/202216U(r)asafunctionofprobabilityofsignalabovethresholdonthecellboundary.12/11/202217Example2Alocalaveragesignalstrengthfieldmeasurements,themeasureddatafitadistant-dependentmeanpowerlawmodelhavingalog-normaldistributionaboutthemean.Assumethemeanpowerlawwasfoundtobe.Ifasignalof1mWwasreceivedatd0=1mfromthetransmitter,andatadistanceof10m,10%ofthemeasurementswerestrongerthan-25dBm,definethestandarddeviation,forthepathlossmodelatd=10m.12/11/202218Fourreceivedpowermeasurementsweretakenatdistancesof100m,200m,1km,and 3 kmfroma transmitter.These measuredvaluesaregiveninthefollowingtable.ItisassumedthatthepathlossforthesemeasurementsfollowsthemodelinEquation(3.12.a),whered0=100m:(a)findtheminimummeansquareerror(MMSE)estimateforthepathlossexponent,n;(b)calculatethestandarddeviationaboutthemeanvalue;(c)estimatethereceivedpoweratd=2 kmusingtheresultingmodel;(d)predictthelikelihoodthatthereceivedsignal level at 2 km will be greater than-60 dBm;and(e)predict thepercentageofareawithina2kmradiuscellthatreceivessignalsgreaterthan-60dBm,giventheresultin(d).Example312/11/202219The value of n which minimizes the mean square error can be obtained byequatingthederivativeofJ(n)tozero,andthensolvingforn.(a)UsingEquation(3.11),wefind=pi(d0)-10nlog(di/100m).RecognizingthatP(d0)=0 dBm,wefindthefollowingestimatesforp,indBm:TheMMSEestimatemaybefoundusingthefollowingmethod.Letpibethereceivedpoweratadistancedi,andletbetheestimateforpiusingthepathlossmodel of Equation(3.10).The sum of squared errors between the measured andestimatedvaluesisgivenbySettingthisequaltozero,thevalueofn isobtainedasn=4.4.12/11/202220(b)The samplevariance 2=J(n)/4 atn=4.4 can beobtainedasfollows.therefore=6.17dB,whichisabiasedestimate.12/11/202221(c)The estimate of the received power at d=2 km is(d)The probability that the received signal level will be greater than-60 dBm is(e)67.4%of the users on the boundary receive signals greater than-60 dBm,then 92%of the cell area receives coverage above 60dbm 12/11/2022223.5 Outdoor Propagation ModelsOkumura Model(150-1920MHz,1km-100km)Hata Model(150-1500MHz,1km-20km)Egli Model(40-400MHz,0-64km)12/11/202223notprovideanyanalyticalexplanationitsslowresponsetorapidchangesinterrainOkumuraModel12/11/202224Okumuramedianattenuationandcorrection12/11/202225Find the median path loss using Okumuras model for d=50 km,hte=100 m,hre=10 m in a suburban environment.If the base station transmitter radiates an EIRP of 1 kW at a carrier frequency of 900 MHz,find the power at the receiver(assume a unity gain receiving antenna).Example 412/11/202226HATAmodel&COST231extension12/11/202227Example 5In the suburban of a large city,d=10 km,hte=200 m,hre=2 m,carrier frequency of 900 MHz,using HATA s model find the path loss.12/11/2022283.6Indoorpropagationmodels12/11/202229FeatureofIndoorRadioChannelThedistancescoveredaremuchsmaller,andthevariabilityoftheenvironmentismuchgreaterforamuchsmallerrangeofT-Rseparationdistances.Ithasbeenobservedthatpropagationwithinbuildingsisstronglyinfluencedbyspecificfeaturessuchasthelayoutofthebuilding,theconstructionmaterials,andthebuildingtype.Indoorradiopropagationisdominatedbythesamemechanismsasoutdoor:reflection,diffraction,andscattering.However,conditionsaremuchmorevariable.12/11/202230PathattenuationfactorsPartitionLossesinthesamefloorPartitionLossesbetweenFloors(floorattenuationfactors,FAF)12/11/202231Log-distancePathLossModelIndoorpathlosshasbeenshownbymanyresearcherstoobeythedistancepowerlawWherethevalueofndependsonthesurroundingsandbuildingtype,andXrepresentsanormalrandomvariableindBhavingastandarddeviationofsigma.Thisisidenticalinformtothelog-normalshadowingmodelofoutdoorpathattenuationmodel.12/11/202232AttenuationFactorModelWhere nSF represents the exponent value for the“same floor”measurement.The path loss on a different floor can be predicted by adding an appropriate value of FAF12/11/202233SignalPenetrationintobuildingsRF penetration has been found to be a function of frequency as well as height within the buildingMeasurements showed that penetration loss decreases with increasing frequency.Specifically,penetration attenuation values of 16.4dB,11.6dB,and 7.6dB were measured on the ground floor of a building at frequencies of 441MHz,896.5MHz,and 1400Mhz,respectly.Results showed that building penetration loss decreased at a rate of 1.9dB per floor from the ground level up to the fifteenth floor and then began increasing above the fifteen floor.12/11/202234RayTracingandSiteSpecificModeling In recent years,the computational and visualization capabilities of computers have accelerated rapidly.New methods for predicting radio signal coverage involve the use of Site Specific(SISP)propagation models and graphical information system(GIS)database.SISP models support ray tracing as a means of deterministically modeling any indoor or outdoor propagation environment.Through the use of building databases,which may be drawn or digitized using standard graphical software packages,wireless system designers are able to include accurate representations of building and terrain features.12/11/202235Exercises1.If a transmitter produces 50W of power,express the transmit power in units of(a)dBm,and(b)dBW.If 50 W is applied to a unity gain antenna with a 900 MHz carrier frequency,find the received power in dBm at a free space distance of 100 m from the antenna.What is Pr(10 km)?Assume unity gain for the receiver antenna.2.If the base stations use 20 W transmitter powers and 10 dBi gain omnidirectional antennas,determine the cell coverage distance d.Let n=4 and the standard deviation of 8 dB hold as the path loss model for each cell in the city.Also assume that a required signal level of-90 dBm must be provided for 90%of the coverage area in each cell.Assume d0=1 km.(Q(0.7)=0.24,f=900MHz)12/11/202236
