移动通信课件2

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1、Chapter2:WirelessTransmission•Frequencies•Signals•Antenna•Signalpropagation•Multiplexing•Modulation•Spreadspectrum•CellularsystemsFrequenciesforradiotransmissiontwistedcoaxcableFrequencyandwaveopticaltransmissionpairlength:λ=c/f1Mm10km100m1m10mm100μm1μm300Hz30kHz3MHz300MHz30GHz3

2、THz300THzVLFLFMFHFVHFUHFSHFEHFinfraredvisiblelightUVVLF=VeryLowFrequencyLF=LowFrequencyMF=MediumFrequencyHF=HighFrequencyVHF=VeryHighFrequencyUHF=UltraHighFrequencySHF=SuperHighFrequencyEHF=ExtremelyHighFrequencyFrequencyandwavelength:λ=c/fwavelengthλ,speedoflightc≅3x108m/s,freq

3、uencyfSignalsA[V]t[s]•physicalrepresentationofdata•functionoftimeandlocationϕ•signalparameters:parametersrepresentingthevalueofdata•signalparametersofperiodicsignals:periodT,frequencyf=1/T,amplitudeA,phaseshiftϕ•sinewaveasspecialperiodicsignalforacarrier:A[V]s(t)=Asin(2πft+ϕ)ttt

4、∞∞f[Hz]1g(t)=c+∑asin(2πnft)+∑bcos(2πnft)Q=Msinϕnn2n=1n=1ϕI=McosϕAntennas:isotropicradiator•Radiationandreceptionofelectromagneticwaves,couplingofwirestospaceforradiotransmission•Isotropicradiator(全向辐射器):equalradiationpowerinalldirections(threedimensional)-onlyatheoreticalreferen

5、ceantenna•Radiationpattern:measurementofradiationzaroundanantennayzyxx•Realantennasalwayshavedirectiveeffects(vertically/horizontally)，notisotropicradiatorsAntennas------dipole（偶极子天线）λ/2λ/4Length=λ/2(Hertziandipole)L=λ/4oncarroof(Marconiantenna)•Example:Radiationpatternofasimple

6、Hertziandipoleyyzxzxsideview(xy-plane)sideview(yz-plane)topview(xz-plane)•Gain:maximumpowerinthedirectionofthemainlobecomparedtothepowerofanisotropicradiator(withthesameaveragepower)Antennas:directedandsectorized（扇形的）yyzdirectedxzxantennasideview(xy-plane)sideview(yz-plane)topvi

7、ew(xz-plane)zzsectorizedxxantennatopview,3sectortopview,6sectorAntennas:diversity（分集）•Groupingoftwoormoreantennas•multi-elementantennaarrays•Antennadiversity•switcheddiversity,selectiondiversity–receiverchoosesantennawithlargestoutput•diversitycombining–combinationofthepowerofal

8、lsignalstoproducegain–Cophasing（同相）toavoidcance