Multichannel Detection for Wideband Underwater Aco

MultichannelDetectionforWidebandUnderwaterAcousticCDMACommunicationsMilicaStojanovic,Member,IEEEandLeeFreitag,Member,IEEEAbstractDirect-sequenceCDMAisconsideredforfuturewidebandmobileunderwateracousticnetworks,whereatypicalconfigurationmayincludeseveralautonomousunderwatervehiclesoperatingwithinafewkilometersofacentralreceiver.Tworeceiversthatutilizemultichannel(array)processingofasynchronousmultiusersignalsareproposed:thesymboldecisionfeedback(SDF)receiverandthechiphypothesisfeedback(CHF)receiver.Bothreceiversuseachip-resolutionadaptivefrontendconsistingofamany-to-fewcombinerandabankoffractionally-spacedfeedforwardequalizers.IntheSDFreceiver,feedbackequalizationisimplementedatsymbolresolution,andreceiverfilters,includingadecision-directedphase-lockedloop,areadaptedatthesymbolrate.Thislimitsitsapplicabilitytothechannelswhosetimevariationisslowcomparedtothesymbolrate.Inawidebandacousticsystem,whichtransmitsatmaximalchiprate,thesymbolrateisdown-scaledbythespreadingfactor,andaninverseeffectmayoccurbywhichincreasingthespreadingfactorresultsinperformancedegradation.Toeliminatethiseffect,feedbackequalization,whichisnecessaryforthemajorityofacousticchannels,isperformedintheCHFreceiveratchipresolutionandreceiverparametersareadjustedatthechiprate.Atthepriceofincreasedcomputationalcomplexity(thereareasmanyadaptivefiltersastherearesymbolvalues)thisreceiverprovidesimprovedperformanceforsystemswheretimevariationcannotbeneglectedwithrespecttothesymbolrate(e.g.LPDacousticsystems).Performanceofthetworeceiverswasdemonstratedinafour-userscenario,usingexperimentaldataobtainedovera2kmshallowwaterchannel.Atthechiprateof19.2kilochipspersecondwithQPSKmodulation,excellentresultswereachievedatanaggregatedatarateofupto10kbps.IndexTermsUnderwateracousticcommunications,decision-feedbackequalization,widebandCDMA,direct-sequencespread-spectrum,adaptivealgorithms,multichannelcombining,space-timeprocessing.ManuscriptreceivedNovember1,2004;revisedNovember8,2005;acceptedMarch21,2006.ThisworkwassupportedbytheOfficeofNavalResearch,throughtheprogram“DemonstrationofUnderseaAutonomousOperationCapabilitiesandRelatedTechnologyDevelopment.”AssociateEditor:J.Preisig.M.StojanoviciswiththeMassachusettsInstituteofTechnology.L.FreitagiswiththeWoodsHoleOceanographicInstitution.IEEEJOURNALOFOCEANICENGINEERING,VOL.X,NO.Y,APRIL20061I.INTRODUCTIONDirectsequence(DS)code-divisionmultiple-access(CDMA)isconsideredasacandidatetechniqueforthefuturemobileunderwaterwirelesscommunicationnetworks.Itsadvantagesincludeefficientuseofbandwidth,relaxedsynchronizationrequirementsascomparedtoTDMA,inherentmultipathdiversityandasoftcapacitylimit[1].Autonomousunderwatervehicles(AUVs),crawlersandgliders,aswellasfixedinstrumentsequippedwithacousticmodemswillbetheusersofthesenetworks.Inacentralizedconfiguration,afixedormovablebasestation,mountedonthebottomoftheoceanoronasurfacebuoy,employsanarrayofhydrophones.Amobilecanalsohaveanarrayofreceiveelements,possiblyasmallerone.Atthistime,itispracticaltoconceiveofanetworkconsistingoffewerthantenuserswithinaradiusofseveralkilometers.Thispaperdealswiththedesignofcommunicationmethodssuitableformultichannel(array)detectionofmultiusersignalsinsuchanetwork.Themajorobstaclesencounteredonanunderwaterchannelareextendedmultipathpropagationandrapidtimevariation,whichcauseseverefrequency-selectivefadinginawidebandacousticsystem.Duetothelimitedbandwidth,whichcanbeontheorderof10kHzaroundacenterfrequencyofseveraltensofkHzforasystemoperatingoverfewkilometers,asystemisdesignedfortransmissionatamaximalchiprate,whiletheachievablebitrateisdeterminedbythespreadingfactor.Thespreadingfactorisselectedtosupportthedesirednumberofusersandprovidethenecessaryqualityofperformance.Atthesametime,itisdesiredthatashighabitrateaspossiblebeachieved.Tosupportseveralusers,sayfewerthanten,withoutexcessivebandwidthexpansionwhichwouldresultinthereductionoftheinformationthroughput,thespreadingfactorontheorderof10shouldbechosen.Ahigherspreadingfactorisjustifiedinsystemsthatseektotransmitatlowpowerthusprovidinglowprobabilityofdetection(LPD)inadditiontosupportingmultipleusers.Becauseoftheseverefrequency-selectivedistortioncausedbymultipathpropagation,itisbeneficial,ifnotimperative,touseadecision-feedbackequalizeroverunderwateracousticchannelsofinterest.Inparticular,multichanneldecision-feedbackequalizationisconsideredtobethemosteffectivemethodfordetectionofnon-spread,bandwidthefficientsignalsinunderwateracousticchannels[2]-[4].Themultipathinterference,seenasintersymbolinterference(ISI)inanon-spreadsystem,appearsasinterchipinterferenceinaDSspread-spectrumsystem.Ifthemultipathspreadonashallowwaterchannelisontheorderof10ms(see[2]andreferencesthereinforcharacteristicsofacousticcommunicationchannels),itwillcausemultipathinterferencethatcanspanahundredchipintervals.Thus,notonlyistheinterchipinterferencesevere,buttheintersymbolinterferencecannotbeneglected.Incontrasttothissituation,ISIinmostoftheoperatingDSCDMAradiosystemscanbeneglected[5].However,aswidebandradiosystemscomeintouse,ISIemergesasaprobl