通信原理论文中英文版

.

.

OptimizedPulseShapingforIntra-channelNonlinearitiesMitigationina10GbaudDual-Polarization16-QAMSystem

BenoîtChâtelain1,CharlesLaperle2,KimRoberts2,XianXu1,MathieuChagnon1,AndrzejBorowiec2,FrançoisGagnon3,JohnC.Cartledge4,andDavidV.Plant1

1McGillUniversity,Montreal,Quebec,Canada,H3A2A72CienaCorporation,Ottawa,Ontario,Canada,K2H8E9

3Écoledetechnologiesupérieure,Montreal,Quebec,Canada,H3C1K34Queen’sUniversity,Kingston,Ontario,Canada,K7L3N6Email:

benoit.chatelainmail.mcgill.ca

Abstract:Anoptimizedpulseshapeisshowntoreduceintra-channelnonlineareffectsina10Gbauddual-polarization16-QAMEDFA-amplifiedsystemwithoutopticaldispersionpensation.

OCIScodes:(060.2330)Fiberopticsmunications;(060.4080)Modulation

1.Introduction

Intra-channelnonlineardistortionisanimportantsourceofsignaldegradationinopticalmunicationsystemsusingadvancedmodulationformatssuchasquadratureamplitudemodulation(QAM).Itsmainimpactistoreducethemaximalpowerthatcanbelaunchedintothefiber,therebylimitingopticalsignal-to-noiseratio(OSNR)levelsatthereceiver,reducingsystemmarginsandreducingthemaximumpropagationdistancethatcanbeachieved.Inthispaper,thenonlineartoleranceofanoptimizedpulseandaroot-raisedcosine(RRC)pulseisparedintermsofbiterrorrate(BER)performance,forvaryingpropagationdistancesandlaunchpowers.Experimentalresultsshowthattheoptimizedpulseconsiderablyreducesintra-channelnonlineareffectsinalonghaul,10Gbauddual-polarization(DP)16-QAMsystemrelyingonerbium-dopedfiberamplifiers(EDFAs)andG.652fiber(noopticaldispersionpensation).

Theimprovednonlineartoleranceofthereturn-to-zero(RZ)pulseshapewasrecentlyhighlightedin[1],fora112Gb/sDP-quadraturephaseshiftkeying(QPSK)systemwithoutopticaldispersionpensation.paringtheRZpulsetothenonreturn-to-zero(NRZ)pulse,theauthorsreportedanincreaseinsinglechannelmaximumpropagationdistanceof18%.However,theimprovednonlineartoleranceachievedusingtheRZpulseesattheexpenseofincreasedspectralcontent.Forinstance,thebandwidthofa50%RZpulseistwicethebandwidthoftheNRZpulseortheRRCpulsewitharoll-offfactor(α)of1.TheuseofaRZpulsethusreducesspectralefficiencybyafactoroftwo,andforasystemusingdigitalsignalprocessingatthetransmitterandreceiver,attheNyquistrate,itdoublestherequiredbandwidthandsamplingfrequencyofthedigital-to-analogconverters(DACs)andanalog-to-digitalconverters(ADCs).

〔a〕

(b)

Figure1.Impulse(a)andfrequency(b)responsesoftheRRCandoptimizedpulses.

InparisontotheRRCpulse,theoptimizedpulseshapepresentedin[2]wasshowntoimprovethenonlinearperformanceofDP-QPSKsystemswithoutopticaldispersionpensationandtoincreasethemaximumtransmissiondistancebyasmuchas22%.Thespecializedpulsewasobtainedbynumericaloptimization,withtheprimaryobjectiveformulatedtoreduceitswidth,butconstrainedtohaveabandwidthequaltothebandwidthofaNRZorRRCpulse(α=1).Therefore,theoptimizedpulsecanbeusedwithoutpenalizingspectralefficiencyandwithoutusinghigherspeedDACsandADCs.Fig.1exhibitsthetimeandfrequencycharacteristicsoftheoptimizedandRRCpulses.ItcanbeseenthattheoptimizedpulseisnarrowerthantheRRCpulsewithα=1,andthatitsfirst-nullbandwidthisequaltothebaudrate.Itisalsoapparentthatthereducedwidthoftheoptimizedpulsetranslatesinhigherenergycontentforfrequenciesbetween6GHzand10GHz.Throughdispersion,thehigherenergyfrequencyponentsproduceapropagatingpulseandapropagatingwaveformthatismorebroadened,withlowerpeakpowerexcursions.Sinceintra-channelnonlineardistortionisproportionaltopeakpower,theoptimizedpulsemakesthesystemmoretoleranttononlineareffects.Inthisworkandasin[2−5],theRRCpulseisusedasthereferencepulseforitsabilitytominimizepulse-inducedintersymbolinterference(ISI)andtominimizeout-of-bandpower.

2.ExperimentalSetup

Fig.2illustratestheexperimentalsetup.Thesymbolsourcesconsistofrandomsymbolsequencesoflength214andwerefollowedbypulseshapingfiniteimpulseresponse(FIR)filters.DACswith6-bitresolutiondrivethein-phase-quadrature(I-Q)modulatorsassociatedwiththetwopolarizations.Theemittingwavelengthwassetto1547.715nm.Apolarizationscrambler(PS)wasinsertedafterthepolarizationbeambiner(PBC)inordertorandomlyrotatethepolarizationstate.Afiberlinkconsistingof15spansof80kmG.652fiberwasused,togetherwithEDFAsateachspan.Atthereceiver,anoisesourcewasusedtoadjusttheOSNR.Theopticalspectrumanalyzer(OSA)servedthepurposesofmeasuringtheOSNR.Acoherentfront-endintegratespolarizationbeamsplitters,opticalhybrids,alocaloscillatorandphotodetectors[6].Itprovidesfoursignalscorrespondingtothein-phaseandquadratureponentsofthetwopolarizations.Thesebasebandsignalsweresampledusinga50GSa/soscilloscopewith8-bitADCsandpost-processedinapersonalputer.Thelinewidthofthetransmitterandreceiverlaserswas100kHz,andtheirfrequencyoffsetwasbelow200MHz.ThesameclockreferencewasusedfortheDACsandADCs.Thereceiversignalprocessingfunctionsincludematchedfilteringandchromaticdispersionpensation(CD-1)byFIRfilters,polarizationrecoverybya13-tapfractionallyspaced(T/2)butterflyequalizer(EQ),carrieroffsetremovalandcarrierphaserecoverybyasecondorderphaselockloop(PLL)andfinally,detection.Theconstantmodulusalgorithm(CMA)isusedfortheequalizercoefficientspre-convergence.Oncethesignalisrecovered,thesystemswitchestoadecision-directedmodulusalgorithm(DDMA)[7].Twosymbolpatternperiods,totaling215symbolsforeachpolarization,wereusedtoproduceBERstatistics.

Inwhatfollows,thesystemperformanceisstudiedusingeithertheRRCpulseortheoptimizedpulsepresentedinFig.1astransmitFIRfiltersandreceivematchedfilters.

Figure2.Experimentalsetup.

3.ResultsandDiscussion

Fig.3displaystheelectricaleyediagramsofthesignalin-phaseponenttakenatthetransmitter,aftertheFIRpulseshapingfilter.Theinnereyeopeningsoftheoptimized16-QAMsequenceeyediagramarereducedparetothoseintheeyediagramobtainedusingtheRRCpulse,indicating,asmentionedin[2],thattheoptimizedpulsemaybemoresensitivetotimingjitter.Fig.3(b)furthershowsthattheeyediagramobtainedwiththeoptimizedpulseisverysimilartotheeyediagramthatwouldbeobtainedusinganRZpulse,andthattheoptimizedpulsesequencereturnsclosetoazerolevelbetweeneachsymbolperiod.SincetheRRCandtheoptimizedpulsesaredesignedtobeusedasmatchedfilter,itisexpectedtovisualizeontheeyediagramsatthetransmitteracertainamountofISI.TheRRCandoptimizedpulsesproduceafilteredsequencewithzeropulse-inducedISIonlyatthereceiver,aftermatchedfiltering.

Fig.4(a)reportsthemeasuredBERinaback-to-backconfigurationforasystemusingRRCfiltersandforasystemusingoptimizedfilters.ThelinearperformanceoftheRRCandoptimizedpulsesisverysimilarandfortheconsideredrangeofOSNR,theminimalandmaximalpenaltyis2.5dBand6.5dB,respectively.Fig.4(b)and(c)showpropagationresultsfor800kmandfor1200km.At800kmandconsideringaBERthresholdof8×10-3,theoptimizedpulsereducestheOSNRpenaltyby0.6dBand2.8dB,forlaunchpowersof–2dBmand0dBm,respectively.At1200kmandforalaunchpowerof–2dBmanimprovementof1.2dBisobserved.At1200kmandforalaunchpowerof0dBm,thesystemwithoptimizedpulseshapingalmostreachestheBERthreshold,whilethesystemusingRRCpulsesislimitedtoaBERof1.8×10-2.Atthislevel,theoptimizedpulseoutperformstheRRCpulseby4.3dB.Itshouldbenotedthattheintroductionofsoft-decisionforwarderrorcorrection(FEC)[8]wouldberequiredtobeabletooperateattheconsideredBERthreshold.

(a)(b)

Fig.3.Measured16-QAMeye-diagramsatthetransmitterwhenusing:(a)theRRCfilter,and(b)theoptimizedfilter.

(a)

(b)

(c)

Figure4.BERvsOSNR(noisebandwidth=0.1nm)forback-to-back(a),800km(b)and1200km(c).

Forsystemsusingmulti-bitDACsoperatingattheNyquistrate,themainadvantageoftheoptimizedpulseapproachisthatitisimplementedatnoextrahardwareorputationalcost,sincetheoptimizedpulseshapingfunctionreplacestheexistingshapingfilters.Yetanotheroptionwouldbetosynthesisetheoptimizedpulsetransferfunctionbyanalogfilters,thusenablingtheuseofsimpler2-bitDACstructuresclockedatthebaudrate.

4.Conclusion

Thecharacteristicsofanoptimizedpulseshapespecificallydesignedtoincreasethenonlineartoleranceoflong-haulopticaltransportsystemsweredescribed.Thespecializedpulsewasusedina10GbaudDP-16-QAMtransmissionexperimentandwasdemonstratedtosignificantlyreducetheimpactofintra-channelnonlinearities.

TheauthorswouldliketoacknowledgetheNSERC/BellCanadaIndustrialResearchChairProgram.

5.References

[1]E.Torrengo,S.Makovejs,D.S.Millar,I.Fatadin,R.I.Killey,S.J.Savory,P.Bayvel,"Influenceofpulseshapein112Gbit/sWDMPDM-QPSKtransmission,〞acceptedtoIEEEPhoton.Technol.Lett.,(2010).

[2]B.Châtelain,C.Laperle,D.Krause,K.Roberts,M.Chagnon,X.Xu,A.Borowiec,F.Gagnon,J.C.Cartledge,D.V.Plant,"SPM-tolerantpulseshapingfor40Gb/sand100Gb/sdual-polarizationQPSKsystems,〞acceptedtoIEEEPhoton.Technol.Lett.,(2010).

[3]M.Nakazawa,S.Ok