均衡算法减少码间干扰

ImprovementofDateRatebyusingEqualizationinanIndoorVisibleLightCommunicationSystemLubinZeng,DominicO’BrienandHoaL[e-mail],DaekwangJungandYunjeOhTelecommunicationsR[e-mail]—Inthispaper,anopticalwirelesscommunicationsystemthatuseswhiteLEDsisdescribed.Equalizationisemployedatthereceivertoimprovethedatarate.SimulationshowsahighlyreliablecommunicationchannelwithanSNRofupto81dB.Moreover,thedataratecanbeimprovedfrom16Mb/sto32Mb/sNRZ-OOKataBERof10−6.I.INTRODUCTIONComparedwiththeconventionallightingmethods,whiteLEDsoffertheadvantageouspropertiesoflowerpowercon-sumption,andlonglifetime.WhiteLEDbasedindoorlightingsystemsarepredictedtobecomethepredominantmethodusedforilluminatingindoorspaceswithinthenextdecadeorso.LEDsarenotonlyutilizedforilluminationbutalsofordatatransmission.Abasicmodelofavisible-lightcommunication(VLC)systemusingwhiteLEDswasfirstlyintroducedbyToshihikoKomineandMasaoNakagawain[1].Inthispaper,simulationofasystemusingasimilargeometryisreported.Inpractice,theLEDmodulationbandwidthlimitstheavailabledatarate,andinthispaper,weinvestigateequalizationtech-niquestoimprovethis.Thefollowingareasareinvestigated:•Powerbudget•DatarateimprovementusingchannelequalizationII.AVISIBLELIGHTINDOORCOMMUNICATIONSYSTEMMODELAvisible-lightindooropticalwirelesssystemutilizingar-raysofLEDlampsisshowninFig.1[1].ThecommunicationmodelforanindividualLEDisdepictedinFig.2,includingaLEDsource,transmissionchannelsandaopticalreceiver.ThetypicalsystemmodelparametersareshowninTable.I.A.TransmitterItisassumedthatanLEDhasaLambertianradiantintensity,Ro(φ)=[(m+1)/2π]cosm(φ)[2][4](1)wheremistheorderofLambertianemission,andisrelatedtoφ1/2,thetransmittersemi-angle(athalfpower)asm=−ln2/ln(cosφ1/2).ThepoweremittedbytheLEDisPLED,andφandψaretheirradianceandincidenceangles.ThetransmittedpowerisPtx=PLED∗Ro(φ).Fig.1.Avisible-lightindooropticalwirelesssystemutilizingLEDlightsafter[1]B.ChannelLightbeamspropagatefromtheLEDtothereceiverviatwomainchannels:lightofsight(LOS)anddiffusechannels.Inthissection,bothchannelmodelswillbediscussed.1)LOSChannel:TheDCgaincanbeestimatedaccuratelybyconsideringonlytheLOSpropagationpath[2].Equ.2expressesthechanneltransferfunction:H(0)LOS=
Arxd2Ro(φ)cos(ψ)0≤ψ≤ψc0ψψc(2)WhereArxisthedetectorarea,disthedistancebetweenthetransmitterandthereceiver,Ro(φ)isthetransmitterradiantintensityandgivenbyEqu.1,ψistheangleofincidence,ψcistheFOVofthephotodiode.ThetotalpowerofiLEDsinthedirectedpath,seeFig.3iscalculatedinEqu.3:Prx,LOS=ΣLEDsi=1PtxHiLOS(0)(3)whereHiLOS(0)istheithLEDchannelDCgain.978-1-4244-1708-7/08/$25.00©2008IEEE678TABLEITHEPARAMETERSSOFTHESYSTEMMODELRoomsize5m*5m*3mDeskheightfromtheceiling2.15mAmplifierBandwidth50MHzSingleLEDpowerPLED20mWLEDresponsetime150nsSemi-angleathalfpower70deg.NumberofLEDsarrays4NumberofLEDsperarray3600(60*60)LEDpitch0.01mFloorreflectivity0.15Ceilingreflectivity0.8Wallreflectivity0.7FOVatthereciver120deg.DetectorphysicalareaofPD1.0cm2Transmissioncoefficientofopticalfilter1.0RefractiveindexoflensatPD1.5Photodioderesponsivity(R)0.4TurningParameter(P)2Amplifiernoisedensity5pA/√Hz[3]Ambientlightphotocurrent5840µA[5]Noise-bandwidthfactor(I2)0.5622)DiffuseChannel:Aintegratingspheremodelfortheopticalwirelessdiffusesignalwasintroducedin[7]andthisisusedhere.Inaroom,thefirstdiffusereflectionofawide-beamopticalsourceemitsaintensityI1overthewholeroomsurfaceAroomisgivenbyI1=ρ1PtotalLEDAroom(4)whereρ1isthereflectivityofthesurfaceandPtotalLEDisthetotalpowerofalltheLEDs.Theaveragereflectivityρisdefinedasρ=1AroomiAiρi(5)Fig.2.SystemmodelwithoneLEDFig.3.SystemmodelwithLEDarrayswheretheindividualreflectivitiesρofwalls,windowsandotherobjectsintheroomareweightedbytheirindividualareasAi.Therefore,thetotalintensityisgivenbysummingupageometricalseriesI=I1∞j=1ρj−1=I11−ρ(6)wheretheindexjisthenumberofreflections.Thereceiverisassumedasasmallpartoftheroomsurface,sothereceiveddiffusedpowerPdiffwiththereceivingareaArxisPdiff=ArxI(7)Therefore,thediffusechannellossisηdiff=PdiffPtotalLED=ArxAroomρ11−ρ(8)C.ReceivingPowerBudgetAtthereceiver,lightpassesthroughtheopticalfilterandconcentrator,sothereceivedpowerisPrx=(PLOS+Pdiff)∗Tf(ψ)∗g(ψ)(9)WhereTf(ψ)isthetransmissioncoefficientoftheopticalfilter,andgψistheconcentratorgain.Thephotodiodeisusedtoconvertthereceivedopticalpowerintotheelectricalcurrent,andtheoutputcurrentis:i=Prx∗R(10)whereRisthephotodioderesponsivity(A/W).TheSNRisgivenby:SNR=(RPrx)2σ2total(11)whereσ2totalistotalnoisevarianceanditisgivenby:σ2total=σ2shot+σ2amplifier(12)wheretheshot-noisevarianceσ2shotisgivenby:σ2shot=2qR(Prx+Pn)Bn(13)679Fig.4.RoomSNRdistribution’stestforfullopticalbandwidth,datarate=10Mb/swhereBnisthenoise-bandwidthandPnisthenoisepoweroftheambientlight.Bn=I2Rb,whereRbisdatarateandI2isthenoise-bandwidthfactor[6].Theamplifiernoisevarianceisgivenby:σ2amplifier=i2amplifierBa(14)whereBaistheamplifierbandwidth.Thetotalnoiseisthereforeσ2total=2qR(Pr+Pn)I2Rb+i2amplifierBa(