Noniterative Detection Receiving Method for LACO-OFDM in Wireless Optical Communications
Chinese Journal of Lasers（2022）
Abstract
ObjectiveTheorthogonalfrequencydivisionmultiplexingOFDMiscurrentlywidelystudiedforwirelessopticalcommunications AsymmetricallyclippedopticalOFDMACO-OFDMisaunipolartechniquewithhighenergyefficiencybutonlyoddsubcarrierscarrydata resultinginlowspectralefficiencyLayeredACO-OFDMLACO-OFDMisproposedtoimprovethespectralefficiencyofACO-OFDM AmongmanyunipolarOFDMtechnologies LACO-OFDMhasthehighestsignal-to-noiseratioSNRgainatthesamespectralefficiencyThemaximumlikelihooddetectionofthecurrentlayersignalmustbeconductedinthefrequencydomainintheconventionalLACO-OFDMreceiver Thedetectionresultisthenfedbacktothereceivedsignal andtheupperACO-OFDMisrestored whichiscallediterativedetection However thenonlinearnoiseintheiterativedetectionreceiverinnonlinearchannelscanbetransmittedfromthelowertotheupperlayerofLACO-OFDM TheperformanceoftheLACO-OFDMsystemdeterioratesduetoiterativedetection AnoniterativedetectionreceivingmethodisproposedinthispapertomitigatethenonlinearityforLACO-OFDM MethodsThemaximumlikelihooddetectioninthefrequencydomainoftheproposedreceiverisremoved whichisusedintheconventionaliterativedetectionreceiver First fastFouriertransformFFTisperformedforthereceivedLACO-OFDMsignalFig1Then LayerlofLACO-OFDMisdirectlydemodulated TheinversefastFouriertransformIFFTasymmetricclippingandFFTareperformedforLayerltorestoreLayerl+1 LayerlcanthenberemovedforthereceivedLACO-OFDMsignaltoobtaintheLayerl+1 ThenoniterativedetectionreceiverhassomeperformancelossintheadditivewhiteGaussiannoisechannel However thisreceiverdoesnotintroducenonlinearnoiseintotheupperACO-OFDMlayerinthenonlinearchannel thusavoidingthenonlinearnoisepropagationbetweenACO-OFDMlayers ThecomplexityoftheconventionaliterativedetectionreceiverdependsonFFTandIFFT Comparedwiththeconventionalreceiver onlyiterativedetectionisnotusedintheproposedscheme whileotherstepsremainthesame Therefore thecomplexityoftheproposedreceiverisconsistentwiththeiterativedetectionreceiver ResultsandDiscussionsSimulationresultsoftheproposedanditerativedetectionreceiversforLACO-OFDMarepresentedinthispaper ThebiterrorrateBERperformancesareprovidedconsideringEboptN0ThetotalnumberofsubcarriersNissetto1024 First thepeak-to-averagepowerratioPAPRanalysisofeachlayerinLACO-OFDMsystemsisrequired TheRlPAPRofLayerlishigherthantheRl+1PAPRofLayerl+1 Thenumberofdata-carryingsubcarriersinthefrequencydomainofLayer1is512 whilethatofLayers2 3 and4are256 128 and64 respectivelyFig2AdditionalsubcarriersinthefrequencydomainleadtohighPAPRinthetimedomain Layer1hasseriousnonlineardistortionbecausetheLACO-OFDMofLayer1hasthehighestPAPR Thenonlineardistortiondecreaseswiththeincreasingnumberoflayers SimulationresultsofLACO-OFDMinthenonlinea amplitudeclippingchannelarepresentedtoanalyzethenonlineardistortionofeachlayerFigs 3 4 and5Thenonlinearnoiseistransmittedfromthelowertotheupperlayerwithiterativedetectioninthefrequencydomain whichleadstopoorperformanceFig3aLayer1ismostaffectedbynonlinearityinthenoniterativedetectionreceiver whileLayers2 3 and4areonlyslightlyaffectedbynonlinearityFig3bThisfindingisduetotheabsenceofiterativedetectionandtransmissionfailureofnonlinearnoiseofthelowertotheupperlayer TheBERperformancesforLACO-OFDMwiththreelayersanddifferentM-quadratureamplitudemodulationsM-QAMsintheamplitudeclippingchannelarepresentedFig4Comparedwiththeiterativedetectionreceiver theproposedreceiverhas1 5dB--2 0dBSNRgainatthe7%forwarderrorcorrectionFEClimit TheBERperformancesforLACO-OFDMwith16-QAManddifferentnumbersoflayersintheamplitudeclippingchannelareshowninFig5 Theproposedreceiverhas1 0dB--2 5dBSNRgaincomparedwiththeiterativedetectionreceiver Thenonlinearnoisecannotbetransmittedtotheupperlayerinthenoniterativedetectionreceiver whichhassomeSNRgain TheadvantageofnoniterativedetectionwillbeachievedasthenumberoflayersinLACO-OFDMincreases ConclusionsAnoniterativedetectionmethodisproposedinthispapertoreducethenonlinearinterferenceoftheLACO-OFDMsysteminnonlinearchannels Maximumlikelihooddetectioninthefrequencydomainoftheproposednoniterativereceiverisnotrequired whichcanavoidthepropagationofnonlinearnoisefromthelowertotheupperlayerofLACO-OFDM SimulationresultsshowthatthePAPRofLACO-OFDMsignalsofLayer1isthehighestinthenonlinearamplitudeclippingchannel whichhasthehighestlevelofnonlinearinterference Eachlayersignalisindependentlydemodulatedinthenoniterativedetectionreceiver andthefrequencydomaindetectionofthefirstlayersignalisnotperformed Therefore thenonlinearnoiseofLayer1isnottransmittedtothesignaloftheupperlayer andtheACO-OFDMsignalofLayers2 3 and4isalmostunaffectedbythenonlinearityComparedwiththeiterativedetectionreceiver theperformanceoftheproposedreceiverexceedsthatoftheiterativedetectionmethodwiththeenhancementofnonlinearityTheSNRgainoftheproposedreceiveris1dB--3dBatthe7%FEClimit
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Key words
optical communications,wireless optical communication,asymmetrically clipped optical orthogonal frequency division multiplexing,noniterative detection,nonlinearity channel
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