无线通信基础(双语)-教学课件-(6).ppt

无线通信基础(双语)-教学课件-(6).ppt

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ReviewofLastClass,2、Timedispersion,3、ImpairmentsoftheWirelessChannel,1、FourBasicPropagationMechanism,Fading,Large-scalepathlossandshadowing,Small-scalefading,ReviewofLastClass,RadioPropagationModel“.characterizationofradiowavepropagationasafunctionoffrequency,distanceandotherconditions.”,ReviewofLastClass,Generalray-tracingmethod（simulation）Multipleray-tracingwithpropagationeffects（reflection,diffraction,LOSpath,scattering,etc）Mightconsiderbuildingmaterial（steel,concrete,brick,etc）EmpiricalmethodOn-sitemeasurementCurve-fittingStatisticalmethod,ReviewofLastClass,Chapter2-Lecture2,Large-scalepathlossandshadowing

（1）,2.4Large-scalepathlossandshadowing,ThepathlossisdependentonthedistancebetweenthetransmitterandreceiverItalsodependsonthepropagationenvironmentTerraincharacteristicsWavelengthTransmittingandreceivingantennaheightsApathmodelisimportantfordeterminationofthebasestation（cell）coveragearea,Large-ScaleFading,2.4Large-scalepathlossandshadowing,2.4.2PropagationOverSmoothPlane,2.4.4Log-DistancePathLosswithShadowing,2.4.1FreeSpacePropagation,2.4.5Indoor/OutdoorPathLossModel,2.4.3DiffractionandFresnelZones,2.4.6RadioCellCoverage,TheFriisEquationFree-SpacePathLoss,1、FreeSpacepropagation,2.4Large-scalepathlossandshadowing,Nointerference,noobstructionsThereisnolossofenergyinfreespace,butthereisattenuationduetothespreadingofthewaves,1、FreeSpacepropagation,TheFriisEquation,Where:

Pt=totalpowerradiatedbyanisotropicsourceGt=transmittingantennagainGr=receivingantennagaind=distancebetweentransmittingandreceivingantennas=wavelengthofthecarriersignal=c/fcfc=carrierfrequency,1、FreeSpacepropagation,TheFriisEquation,Whattroublessomepeoplewhentheyseethisquestionisthatthepathlossalsoincreasesasthesquareofthefrequency.WhileitistruethatabsorptionofRFbyvariousmaterials（buildings,trees,watervapor,etc.）tendstoincreasewithfrequency,rememberwearetalkingabout“freespace”here.,1、FreeSpacepropagation,TheFriisEquation,Effectiveareaofanisotropicantenna,However,fromelectromagnetictheory,wenotethattheeffectiveareaofanisotropicantennainanydirectionisgivenby:

A=2/（4）,2.4Large-scalepathlossandshadowing,1.Freespacepowerfluxdensity（W/m2）powerradiatedoversurfaceareaofsphereSPt/（4d2）Bycoveringsomeofthisarea,receiversantenna“catches”someofthisflux.,1、FreeSpacepropagation,TheFriisEquation,2.EffectiveareaofanisotropicantennaThepowerreceivedbyanantennaofeffectiveareaAisgivenby:

PR=SAHowever,fromelectromagnetictheory,wenotethattheeffectiveareaofanisotropicantennainanydirectionisgivenby:

A=2/（4）ThusPR=SA=Pt2/（4d）2,1、FreeSpacepropagation,TheFriisEquation,3.DirectionalRadiationWhiletheisotropicantennaisausefulillustrativedevice,mostantennasarenotisotropic.Instead,theyhavegainthatisafunctionoftheazimuthangleandtheelevationangle.Whennonisotropicantennasareused,weget,1、FreeSpacepropagation,TheFriisEquation,dDandd,wheredisthedistancebetweenthetransmittingantennaandreceivingantennaDisthelargestlineardimensionofantennaisthecarrierwavelengthddf,Assumesfar-field,1、FreeSpacepropagation,1、FreeSpacepropagation,TheFriisEquation,Theterm（4d/）2isknownasthefree-spacepathlossdenotedbyLp（d）,whichis,1、FreeSpacepropagation,Free-SpacePathLoss,Whatdoes“dB”mean?

dBstandsfordeciBelor1/10ofaBelTheBelisadimensionlessunitforexpressingratiosandgainsonalogscaleGainsaddratherthanmultiplyEasiertohandlelargedynamicranges,1、FreeSpacepropagation,1、FreeSpacepropagation,1、FreeSpacepropagation,Whatdoes“dB”mean?

Ex:

AttenuationfromtransmittertoreceiverPT=100,PR=10attenuationisratioofPTtoPRPT/PRdB=10log（PT/PR）=10log（10）=10dBUsefulnumbers:

1/2dB-3dB1/1000dB=-30dB,1、FreeSpacepropagation,Whatdoes“dB”mean?

dBcanexpressratios,butwhataboutabsolutequantities?

Similarunitsreferenceanabsolutequantityagainstadefinedreference.nmWdBm=n/mWdBnWdBW=n/WdBEx:

1mWdBW=？

dBW,1、FreeSpacepropagation,Example2.2Transmitterpower:

50W.Antenna:

unitgain.Carrierfrequency:

900MHz.Distance:

d=100m.Find:

1）dBmvalueofthetransmitter,2）dBmvalueofthereceivedpower,3）Pr（d=10Km）=?

Relativedistance,1、FreeSpacepropagation,Example2.3IndoorWLANwithfc=900MHz.findtherequiredtransmitpowerinordertoguaranteeminimumreceivedpowerof10uWwithdistanceupto10m.Whatiffc=5GHz?

Assumeunitantennagains.,1、FreeSpacepropagation,Freespacepropagationdoesnotapplyinamobileenvironmentandthepropagationpathlossdependsnotonlyonthedistanceandwavelength,butalsoontheantennaheightsofthemobilestationandthebasestation,andthelocalterraincharacteristicssuchasbuildingsandhills.Asimpletwo-pathmodelcanbeusedtoillustratetheeffectoftransmittingandreceivingantennaheights.,2.4Large-scalepathlossandshadowing,2、PropagationOverSmoothPlane,FreeSpacePropagation,FriisEquationFree-SpacePathLossWhatdoes“dB”mean?

ReviewofLastClass,PropagationOverSmoothPlane,Two-pathmodel,ReviewofLastClass,2、PropagationOverSmoothPlane,Two-PathModel,Thepropagationdistanceofthedirectpathis：

Thepropagationdistanceofthereflectedpathis：

Two-PathModel,2、PropagationOverSmoothPlane,Giventhatdhthr,and,Itcanbederivedthat,Two-PathModel,2、PropagationOverSmoothPlane,Giventhatdhthr,wehaved1dandd2d.However,sincethecarrierwavelengthisverysmallcomparedwithd,aslightchangeinthepropagationdistancecancauseasignificantchangeinthereceivedsignalcarrierphase.,Two-PathModel,2、PropagationOverSmoothPlane,Howistwo-raymodelobtained?

Takenintoaccountthephasedifference,thereceivedsignalpoweris,Wherefandfaretheamplitudeattenuationandcarrierphaseshiftintroducedbythereflection.Iff1andf=for1,then,Ithadbeenderivedthat,Two-PathModel,2、PropagationOverSmoothPlane,Howistwo-raymodelobtained?

Andthecorrespondingpathlossis:

Two-PathModel,2、PropagationOverSmoothPlane,Iftheproductdismuchgreaterthantheproducthtandhr,wecanapproximatesinby,thus,Andthecorrespondingpathlossis:

Two-PathModel,2、PropagationOverSmoothPlane,Asaconsequenceoftheassumptionthatdht,hr,theangelissmall,andcancelsoutoftheequation,leavingittobeessentiallyfrequencyindependentItshowsaninversefourth-powerlaw,ratherthantheinverse-squarelawoffree-spacepropagation.Thispointstoafarmorerapidattenuationofthepowerreceived.Itshowstheeffectofthetransmitandreceiveantennaheightsonpropagationlosses.Thedependenceonantennaheightmakesintuitivesense.,Andthecorrespondingpathlossis:

Two-PathModel,2、PropagationOverSmoothPlane,Criticaldistance,DefineCriticaldistanceIfddc,thensignalpowerfallsoffwithd-4.Ifddc,thensignalpowerfallsoffwithd-2.,2、PropagationOverSmoothPlane,Anapproximationfordccanbeobtainedbysetting,Criticaldistance,2、PropagationOverSmoothPlane,Thecriticaldistancecanbeusedforsystemdesign.Forexample,ifpropagationinacellularsystemobeysthetwo-raymodelthenthecriticaldistancewouldbeanaturalsizeforthecellradius,sincethepathlossassociatedwithinterferenceoutsidethecellwouldbemuchlargerthanpathlossfordesiredsignalsinsidethecell.,Criticaldistance,2、PropagationOverSmoothPlane,f=900MHzht=50mhr=2mGt=1Gr=1transmitpowernormalizedhthr,Two-PathModel,2、PropagationOverSmoothPlane,Thisplotcanbeseparatedintothreesegments:

Forsmalldistance（ddcthesignalcomponentsonlycombinedestructively.,Two-PathModel,2、PropagationOverSmoothPlane,Example2.4Iftwo-raymodelissuitableforurbanmicro-cellwithht=10m,hr=3m,andfc=2GHz.Finddc.Whatifforindoormicrocellwithht=3m,hr=2m?

Findthepowerlossfordc.,Comparedwiththefreespacemodel?

-96dB-83dB,Howaboutd=3Km?

2、PropagationOverSmoothPlane,Thismodelisasimplebutverygoodray-tracingmodelAccurateinpredictinglarge-scalesignalstrengthoverlargedistancewithtalltowersUsedwidelyinpredictioncellularpropagationlossEffectivewhenasinglegroundreflectiondominatesthemultipathpropagationeffect,Two-PathModel,2、PropagationOverSmoothPlane,Free-SpacePathLossTwo-PathModel,2.4Large-scalepathlossandshadowing,Conclusions,Comparethefreespacepathlossmodelwiththetwopathmodel.,Problems,