基于FPGA的chirp通信系统的调制与解调技术毕业设计.docx
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基于FPGA的chirp通信系统的调制与解调技术毕业设计
基于FPGA的chirp通信系统的调制与解调技术毕业设计
编号:
毕业设计(论文)外文翻译
(原文)
学院:
信息与通信学院
专业:
通信工程
学生姓名:
李忠斌
学号:
1000210219
指导教师单位:
信息与通信系
姓名:
樊孝明
职称:
讲师
2015年6月8日
InvestigationofmodulatorchirpandextinctionratioindifferentRZ-andNRZduo-binarytransmittermodulesforperformanceoptimization
Abstract-Inthisworkwepresentacomparativeinvestigationofmodulatorchirpandextinctionratioindifferenttransmittermodulesfor10Gbps,20Gbpsand40GbpsRZ-andNRZduo-binarytransmission.Forcomparativeanalysisthreetypesoftransmissionmoduleshavebeenconsideredviz.push–pullconfigurationbasedondualarmMZIM,delay-and-addcircuitbasedsinglearmMZIMandaduo-binaryfilterfollowedbysinglearmMZIM.Foreachcase,themodulatorchirphasbeenoptimizedwithanextinctionratioof20dB.InvestigationhasbeencarriedouttofindextinctionratioofsinglearmMZIMusedforRZ-andNRZduo-binarytransmissionthatofferssystemperformancecomparabletodualarmMZIMat10,20and40Gbps.Theresultshelpchoosingthebestsuitabletransmittermodulewithoptimizedmodulatorchirpandextinctionratio.
I.INTRODUCTION
Inhighspeedopticalcommunicationsystemsduo-binarymodulationisaneffectivesolutionthatprovidesabetterspectralefficiencyandreducestheperformancedegradationowingtodispersionandnonlineareffects[1–3].Inthe1980sandtheearly1990s,directmodulationofsemiconductorlaserwasthemostlyusedtechnique.However,directmodulationhasseverallimitingfactorslikeitinducesunwantedchirpswhichresultsinspectrabroadeningofthesignal,hencecausingseveredispersionpenalties[4].Directly-modulatedopticalsignalsexperiencefluctuationsinintensityduetoRelativeIntensityNoise(RIN)ofthesemiconductorlaser.Nonzerolinewidthoflasersourcesintroduceslaserphasenoisethusforhighspeedtransmissiondirectmodulationisusuallynotpreferred.Hence,externalmodulationhasbeenanessentialchoiceforthehighspeedlong-haulcommunication[5,6].Externalmodulatorsremovethelargeamountofwavelengthchirpingwhichwillotherwisebeincludediflaserdiodeisdirectlymodulated.Mainlytwotypesofsemiconductorexternalmodulatorsareavailableviz.Electro-OpticModulator(EOM)andElectro-AbsorptionModulators(EAM)[6].Ofthesetwomodulators,EOMthatchangesthebandgapenergywithappliedelectricfieldismostlypreferredbecauseofvariousadvantageslikelinearresponsecharacteristic,highextinctionratio,abilitytocontrolphase,frequencyoramplitudeofthelightwavecarrierowingtothepropertiesofelectroopticmaterial.EvenforverylowvalueofdrivingvoltageEOMisabletoachievehighspeedmodulation[7,8].
II.Theory
Inopticaldomain,datamodulationisachievedusingtwotypesofmodulators:
opticalphasemodulatorandopticalintensitymodulator.AnElectro-OpticPhaseModulator(EOPM)usesonlyoneelectrode.Whenadrivingvoltageisappliedtotheelectrode,therefractiveindexoftheelectro-opticwaveguidechangesaccordingly,thusslowingdownthelightwaveandhenceinducingadelayontheopticalsignal.Theinduceddelaycorrespondstothephasechange,thusanEOPMisabletomanipulatethephaseofthelightwavecarrier[9].
whereVπisthedrivingvoltagerequiredtocreateaphaseshift,V(t)isatime-varyingdrivingsignalvoltageandVbiasisdcbiasvoltage.OpticalfieldEoattheoutputoftheEOPMisgivenas:
Eo(t)=Ei(t)ejϕ(t)
(2)
OpticalintensitymodulatorusestwoEOPMsinaparallelstructuretoformaMach–ZehnderinterferometercommonlyknownastheMach–Zehnderintensitymodulator(MZIM)[9].InputopticalsignalsplitsequallyinthetwoarmsoftheMZIMwhichareactuallyEOPMsformodulatingthephaseoftheopticalcarrier.Attheoutput,thetwoarmsarecoupledeitherconstructivelyordestructivelytoprovideintensitymodulatedopticalpulses.
Fig.1.Duobinarytransmittermodulewithdual-armMZIM(T#1).
Fig.2.Duobinarytransmitterwithsingle-armMZIMdelay-and-addcircuit(T#2).
Fig.3.Duo-binarytransmitterwithsingle-armtransmitterandfiltercircuit(T#3).
MZIMcanbeoftwotypes:
single-armMZIManddualarmMZIM.Insingle-armMZIMonlyonesingledrivingvoltageisappliedtotheeitherarmofMZIM[9]andtheoutputtransmittedopticalfieldEo(t)isgivenas:
ExistenceofthephaseterminEq.(3)showsthatthechirpingeffectispresent,thuswecansaythatthesingle-armMZIMgeneratedsignalsarenotchirp-free.ParticularstructureoftheMZIMcanonlyminimizethechirping(x-cutMZIM).Ithasbeenfoundthatasmallamountofchirpisusefulfortransmission[9].Dual-armMZIMhaspush–pullarrangementwherethedualdrivevoltagesV1(t)andV2(t)areinversetoeachotherandthus,abletocompletelyeliminatethechirpingeffectinthemodulation.Thetransmittedopticalfieldcanbewrittenas:
Fig.4.Comparisonof10GbpsRZduo-binarytransmittermodules
Fig.5.Comparisonof20GbpsRZduo-binarytransmittermodules.
Theuseofopticalduo-binarytransmitterwiththedual-armMZIMistheusualchoiceintransmitterdesignathighdatarate,however,dualarmconfigurationdemandsmorestringentrequirementofsymmetrytobemet[10].Single-armMZIMwithduo-binaryfiltercanalsobeusedwhereduo-binaryfiltercanbeapproximatedbyalowpassfilterwithhalf-powercutoffatapproximatelyonefourththedatarate.Atthiscutofffrequency,thespectraloccupancyofthemodulatedopticalfieldisrestrictedto[f0±(Bitrate)/2],wheref0isanominalcontinuouswave(CW)frequency[11].Thefrequencyspectrumofanidealduo-binarysignalexhibitsthefirstnullsat[f0±(Bitrate)/2].So,wecanband-limitthesignalat[f0±(Bitrate)/2]whilepreservingatthesametime,sufficientinformationforlaterreconstructionofthesignal[11].Theduo-binaryfilterusedforsimulationisafifthorderlow-passBesselfilter.Duo-binarysignalgenerationcanalsobeachievedbydirectlyapplyingtheRZ/NRZsignaltothedelay-and-addcircuitfollowedbybandlimitingfilterofbandwidth(Bitrate)/2.
III.Performancemeasure
IV.Systemdescriptionandresults
Aperformancemeasurecriterionprovidesustheplatformforinvestigationandanalysis.Inthisworkwehaveconsideredthesemi-analyticBERevaluationtechniquefortheestimationoftheBitErrorRate(BER)andBERequivalentQ-factorisconsideredasperformancemeasurecriterion.
Inthepresentanalysis,weareconsideringthreetypesofduo-binarytransmittermodulesviz.
(1)duo-binarytransmitterbasedonpush-pullconfigurationofdual-armMZIM(hereafterdenotedbyT#1),
(2)single-armMZIMwithdelay-and-addcircuit(hereafterdenotedbyT#2)and(3)thesingle-armMZIMfollowedbyaduo-binaryfilter(hereafterdenotedbyT#3).SimulativeanalysishasbeenperformedusingcommercialpackageOptSimTM.Wecarriedoutouranalysisatthreedifferentbitratesof10,20and40Gbps.Thesimulationsetuptypicallycomprisesofthreesections:
duo-binarygeneration,i.e.transmittersection,opticalfilterorchannelandthereceiversection.Receiversectionwhichiscommoninthelinkdesignforallthreeabove-mentionedtransmittermodulesconsistsofaPINphotodetectorhavingtheresponsivityof0.8A/WfollowedbyafourthorderelectricalBessellowpassfilterof30GHzbandwidthcenteredat1550nmwhichprovidesfiltereddatatotheBERestimatorforperformancemeasurement.WeoptimizethemodulatorchirpCforallthreetransmittermodulesatanExtinctionratioof20dB.TofindthevalueofCthatoptimizessystemperformance,wevarytheCvaluefrom-4to+2.Fig.1showsthesimulationsetupofNRZduo-binarytransmissionusingT#1module.OpticallinkconsistingofSingleModeFiber(SMF)of80km,20kmand4kmlengthshavebeenconsideredforduo-binarytransmissionat10,20and40Gbpsrespectively.SMFhasthefollowingspecifications:
attenuationlossof0.2dB/km,dispersionanddispersionslopeof16ps/km-nmand0.07ps/nm2-kmat1550nm,respectively,effectiveareaof80m2andrefractiveindexof2.5×10-20m2/W.SMFisfollowedbyanEDFAthatprovidesafixedpowerof3dBmandhasanoisefigureof4.5dB.Inthissimulativework,anopticallaserhaving10MHzlinewidthhasbeenconsidered.
Tocarryoutsimulation,firstRZduo-binarytransmissionhasbeenconsideredandthentheRZpulsegeneratorinFigs.1–3hasbeenreplacedwithNRZpulsegeneratortoobtainNRZduo-binarytransmission.ForaparticulardatarateoptimalvalueofChasbeenfiguredoutforeachtypeoftransmittermodule.Q-factorofT#1atoptimizedCforextinctionratioof20dBhasbeenconsideredasreferencetocomparewithT#2andT#3.ExtinctionratioatwhichT#2andT#3offerscomparableperformancetoT#1isinvestigated.
Fig.4reportsthatat10GbpsRZduo-binarytransmissions,T#1isoptimizedatC=0.4whileT#2andT#3optimizeat−0.8and−1.4Cvalues,respectively.Itisobservedthatat20dBextinctionratioT#2performsbetterthanT#1andT#3.ConsideringT#1with20dBextinctionratioasreference,itcanbeinterpretedthatT#2achievestheperformanceofT#1atarelativelylowervalueofextinctionratio.
Fig.5recordsoptimumCvalueof0.2,−1.0and−1.6forT#1,T#2,T#3respectivelyusedin20GbpsRZduo-binarydatatransmission.Comparisonofdifferenttransmittermodulesat20dBextinctionratioshowssimilarqualitativebehaviorasseeninFig.4.
Fig.6showsthatCvalueof0.2,−0.6and−1.4ensureshighestQfactorforthethreetransmittermodulesrespectively.At40GbpsRZduo-binarytransmittedfromT#1providesbetterperformancethanT#2andT#3.TheperformanceofT#1at20dB