ScienTECH ST2115 User manual

CDMATrainer
ST2115
LearningMaterial
Ver.1.2
AnISO9001:2008 company
94, ElectronicComplex, Pardesipura
Indore-452 010India
Tel:[phone]
Fax:[phone]
email: [e-mail]
Websites: www.caddo.bz
www.scientech.bz
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CDMATrainer
ST2115
TableofContents
1. SafetyInstructions4
2. Introduction5
3. Features7
4. TechnicalSpecifications8
5. Theory
•MultipleAccess Techniques9
•TypesofCodeDivisionMultiplexing14
•AdvantagesofAsynchronousCDMA17
•CDMAfeatures19
•Spread-SpectrumCommunicationSystems19
•Direct-Sequence Spread-SpectrumSystems22
•PNSequencesandGenerators25
•ST2115 CDMATrainer- Briefdescription28
6. Experiments
•Experiment131
StudyofDS-SS modulation/DemodulationProcess
•Experiment234
StudyofBPSKModulationandDemodulation
•Experiment337
StudyofPulseWidthModulationandDemodulation
•Experiment440
StudyofCDMA(DS-SS)technique
•Experiment544
StudyofDS-SS usinganalogsignalasaninputsignal
7. FrequentlyAskedQuestions47
8. Warranty50
9. ListofAccessories50
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SafetyInstructions
Readthefollowing safetyinstructionscarefullybeforeoperatingtheinstrument.To
avoidanypersonalinjuryordamagetotheinstrumentoranyproductconnectedtoit.
Donotoperatetheinstrumentifsuspectanydamagetoit.
Theinstrumentshouldbeservicedbyqualifiedpersonnelonly.
Foryoursafety:
UseproperMainscord:Useonlythemainscorddesignedforthisinstrument.
Ensurethatthemainscordissuitableforyour
country.
Ground theInstrument:Thisinstrumentisgroundedthroughtheprotective
earthconductorofthemainscord.Toavoidelectric
shockthegroundingconductormustbeconnectedto
theearthground.Beforemaking connectionstothe
inputterminals,ensurethattheinstrumentisproperly
grounded.
ObserveTerminalRatings: Toavoidfireorshockhazards, observeall ratingsand
marksontheinstrument.
UseonlytheproperFuse:Usethefusetypeandratingspecifiedforthis
instrument.
UseinproperAtmosphere: Pleaserefertooperatingconditionsgiveninthe
manual.
1. Donotoperateinwet/dampconditions.
2. Donotoperateinanexplosiveatmosphere.
3. Keeptheproductdustfree, cleanand dry.
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Introduction
ST2115,CDMATrainer isdesignedtoassiststudentstounderstandthebasic
workingprincipalofCDMAtechnique,specifically,DirectSequence Spread
Spectrumandtounderstanditsusageintoday’scellularsystems.
ST2115,CDMATrainercomprisesoffollowing majorblocks:
•AudioSignalGenerator(0-3.4KHz)
•BinaryDataGenerator
•PNSequence Generator
•DirectSequence spreadspectrumGeneratorandreceiver
•BPSKModulatorandDemodulator
•PulseWidthModulatorandDemodulator
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RoHS Compliance
ScientechProductsareRoHS Complied.
RoHS DirectiveconcernswiththerestrictiveuseofHazardoussubstances(Pb, Cd, Cr, Hg,
Brcompounds)inelectricandelectronicequipments.
Scientechproductsare“LeadFree”and“EnvironmentFriendly”.
Itismandatorythatserviceengineersuseleadfree solderwireand usethesolderingirons
upto(25 W)thatreachatemperatureof450°Catthetipasthemeltingtemperatureofthe
unleadedsolderishigherthantheleadedsolder.
Features
•Selfcontainedandeasytouse
•Functionalblocksindicatedonboardmimic
•Directsequence spreadspectrum(DSSS)generatorand decoder
•Analog Modulators:
BinaryPhaseShiftKeying(BPSK)Modulator
PulseWidthModulator
•Analog Demodulators:
BinaryPhaseShiftKeying(BPSK)Demodulator
PulseWidthDemodulator
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TechnicalSpecifications
•Powersupplyrequirement:230VAC, 50Hz.
•InbuiltDCregulatedpowersupply.
•OnboardDigitalDatasignalgeneratortogenerateanybinaryinput.
-Wordlength:8bit
-DataRate:15Kbits/s.
-DataformatNRZ(NonReturntoZero)
•OnboardPseudoRandomBitSignalgeneratortogeneratepseudorandombit
sequence signal.
Bitlength:8bitsperDatabit
•Onboardanalogsignalgeneratorvariableup to3.4KHz.
•Carriergenerator:1.44MHz.
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Theory
MultipleAccess Techniques:
Alimitedamountofbandwidthis allocatedforwireless services.Awirelesssystemis
requiredtoaccommodateasmanyusersaspossiblebyeffectivelysharingthelimited
bandwidth.Therefore,inthefieldofcommunications,theterm multipleaccess could
bedefinedasameansofallowing multipleuserstosimultaneouslysharethefinite
bandwidthwithleastpossibledegradationintheperformanceofthesystem. Thereare
severaltechniquesthroughwhichmultipleaccessingcanbeachieved.Therearethree
basicschemes:
1. frequencyDivisionMultipleAccess (FDMA)
2. TimeDivisionMultipleAccess (TDMA)
3. CodeDivisionMultipleAccess (CDMA)
1. FrequencyDivisionMultipleAccesses(FDMA):
FDMAisoneoftheearliestmultiple-accesstechniquesforcellularsystemswhen
continuoustransmissionisrequiredforanalogservices.Inthistechniquethe
bandwidthisdividedintoanumberofchannelsanddistributedamonguserswitha
finiteportionofbandwidthforpermanentuseasillustratedinfigure1theverticalaxis
thatrepresentsthecodeisshownherejusttomakeaclearcomparisonwithCDMA.
Thechannelsareassignedonlywhendemandedbytheusers.Thereforewhena
channelisnotinuseitbecomesawastedresource.FDMAchannelshavenarrow
bandwidth(30 KHz)andthereforetheyareusuallyimplementedinnarrowband
systems.Since theuserhashisportionofthebandwidthallthetime,FDMAdoesnot
requiresynchronizationortiming control, whichmakesitalgorithmicallysimple.
ChannelUsesbyFDMAFigure 1
Eventhough notwousersusethesamefrequencybandatthesametime, guardbands
areintroducedbetweenfrequencybandstominimize adjacentchannelinterference.
Guardbandsareunusedfrequencyslotsthatseparateneighboringchannels.This
leadstoawasteofbandwidth.Whencontinuoustransmissionisnotrequired,
bandwidthgoeswastedsince itisnotbeingutilizedforaportionofthetime.In
wirelesscommunications,FDMAachievessimultaneoustransmissionandreception
byusing Frequencydivisionduplexing (FDD).Inorderforboththetransmitterand
thereceivertooperateatthesametime, FDD requiresduplexers.
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2. TimeDivisionMultipleAccess (TDMA):
Indigitalsystems,continuoustransmissionisnotrequiredbecauseusersdonotuse
theallottedbandwidthallthetime.Insuchsystems,TDMAisacomplimentary
accesstechniquetoFDMA.GlobalSystemsforMobilecommunications(GSM)uses
theTDMAtechnique. InTDMA, theentirebandwidthisavailabletotheuserbutonly
forafiniteperiodoftime. Inmostcasestheavailablebandwidthis dividedintofewer
channelscomparedtoFDMAandtheusersareallottedtimeslotsduringwhichthey
havetheentirechannelbandwidthattheirdisposal.Thisisillustratedinfigure2
TDMArequirescarefultimesynchronizationsinceuserssharethebandwidthinthe
frequencydomain.Sincethenumberofchannelsisless,interchannelinterference is
almostnegligible;hence theguardtimebetweenthechannelsisconsiderablysmaller.
Guardtimeis spacingintimebetweentheTDMAbursts. Incellularcommunications,
whenausermovesfromonecelltoanotherthereisachance thatusercould
experienceacalllossiftherearenofreetimeslotsavailable.TDMAusesdifferent
timeslotsfortransmissionandreception.Thistypeofduplexing isreferredtoas
Timedivisionduplexing (TDD).TDDdoesnotrequireduplexers.
ChannelusagebyTDMAFigure 2
3. CodeDivisionMultipleAccess:
Theworldisdemanding morefromwirelesscommunicationtechnologiesthanever
beforeasmorepeoplearoundtheworldaresubscribing towireless.Add inexciting
Third-Generation(3G)wireless dataservicesandapplications-suchaswireless
email, web, digitalpicturetaking/sending, assisted-GPS positionlocation
applications, videoandaudiostreaming andTVbroadcasting-andwirelessnetworks
aredoingmuchmorethanjustafewyearsago.
Thisis where CDMA technologyfitsin. CDMA consistentlyprovidesbettercapacity
forvoice anddatacommunicationsthanothercommercialmobiletechnologies,
allowing moresubscriberstoconnectatanygiventime,anditisthecommon
platformonwhich3Gtechnologiesarebuilt.
Code-Division Multiple Access,adigitalCellulartechnologythatuses spread-
spectrum techniques.UnlikecompetingsystemssuchasGSM,thatuse TDMA.
CDMA doesnotassignaspecificfrequencytoeachuser. Instead, everyChanneluses
thefullavailablespectrum.Individualconversationsareencodedwithapseudo
randomdigitalsequence,butgivetherighttousebothtoall userssimultaneously.To
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dothis,itusesatechniqueknownas SpreadSpectrum.Ineffect,eachuseris
assignedacodewhichspreadsitssignalbandwidthinsuchawaythatonlythesame
codecanrecoveritatthereceiverend.Thismethodhasthepropertythatthe
unwantedsignalswithdifferentcodesgetspreadevenmorebytheprocess,making
themlikenoisetothereceiver.
InCDMA,alltheusersoccupythesamebandwidth;howevertheyareall assigned
separatecodes,whichdifferentiatethemfromeachotherasshowninfigure3the
figureshowstypicalWalshcodesusedforthispurpose.CDMAsystemsutilize a
spreadspectrumtechniqueinwhichaspreading signal,whichisuncorrelatedtothe
signalandhasalargebandwidth,isusedtospreadthenarrowbandmessagesignal.
DirectSequence SpreadSpectrum(DS-SS)ismostcommonlyusedforCDMA.In
DS-SS, themessagesignalismultipliedbyaPseudoRandomNoiseCode(PNcode),
whichhasnoise-likeproperties.Eachuserhashisowncodewordwhichisorthogonal
tothecodesofotherusers. Inordertodetecttheuser, thereceiveris requiredtoknow
thecodewordusedbythetransmitter.UnlikeTDMA,CDMAdoesnotrequiretime
synchronizationbetweentheusers.ACDMAsystemexperiencesaproblemcalled
self-jamming whichariseswhenthespreading codesusedfordifferentusersarenot
exactlyorthogonal.Whiledespreading,thisleadstoasignificantcontributionfrom
otheruserstothereceiverdecisionstatistic.Ifthepowerofthemultipleusersina
CDMAsystemisunequal,thentheuserwiththestrongestsignalpowerwill be
demodulatedatthereceiver.Thestrengthofthereceivedsignalraisesthenoisefloor
fortheweakersignalsatthedemodulators.Thisreducestheprobabilitythatweaker
signalswillbereceived.Thisproblem,knownasthe near-farproblem canbetaken
careofbyusing powercontrol. Thisensuresthatallthesignalswithinthecoverageof
thebasestationarrivewithsamepoweratthereceiver.
ChannelusagebyCDMAFigure 3
Cellularsystemsdivideageographicregionintocellswhereamobileunitineachcell
communicateswithabasestation.Thegoalinthedesignofcellularsystemsistobe
abletohandleasmanycallsaspossible(thisiscalledcapacityincellular
terminology)inagivenbandwidthwithsomereliability.Allthethreetechniques
describedabovecanbeseentogetherforacomparativestudy.
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ComparisonofdifferentMultiplexingTechniquesFigure 4
CodeDivisionMultipleAccess (CDMA)–AnOverview:
Cellularsystemsdivideageographicregionintocellswhereamobileunitineachcell
communicateswithabasestation.Thegoalinthedesignofcellularsystemsistobe
abletohandleasmanycallsaspossible(thisiscalledcapacityincellular
terminology)inagivenbandwidthwithsomereliability.Forthecellularsystem,
Codedivisionmultiple-accesstechniquesallowmanyuserstosimultaneouslyaccess
agivenfrequencyallocation.
Codedivisionmultipleaccess(CDMA)isaformofmultiplexing (notamodulation
scheme)andamethodofmultipleaccess thatdoesnotdivideup thechannelbytime
(asinTDMA),orfrequency(asinFDMA),butinsteadencodesdatawithaspecial
codeassociatedwitheachchannelandusestheconstructiveinterferencepropertiesof
thespecialcodestoperformthemultiplexing.CDMAalsoreferstodigitalcellular
telephonysystemsthatmakeuseofthismultipleaccess scheme,suchasthose
pioneeredbyQualcomm,andW-CDMAbytheInternationalTelecommunication
UnionorITU.
AnumberofdifferenttermsareusedtorefertoCDMAimplementations. Theoriginal
U.S.standarddefinedbyQUALCOMM wasknownasIS-95,theISreferringtoan
Interim StandardoftheTelecommunicationsIndustryAssociation(TIA).IS-95 is
oftenreferredtoas2Gorsecondgenerationcellular.TheCDMAhasbeensubmitted
forapprovalasamobileairinterface standardtotheITUInternational
TelecommunicationUnion.
Thesize ofagivencelldependsonthepowerofthesignaltransmittedbythehandset,
theterrain,andtheradiofrequencybeing used.Variousalgorithmscanreducethe
noiseintroducedbyvariationsinterrain,butrequireextrainformationbesentto
validatethetransfer.Hence,theradiofrequencyandpowerofthehandseteffectively
determinethecell size.Longwavelengthsneedless energytotravelagivendistance
vs.shortwavelengths,solowerfrequenciesgenerallyresultingreatercoveragewhile
higherfrequenciesresultinshortercoverage. Thesecharacteristicsareusedbymobile
networkplannersindetermining thesizeandplacementofthecellsinthenetwork. In
cities,manysmallcellsareneeded;theuseofhighfrequenciesallowssitestobe
placedmore-closelytogether,withmoresubscribersprovidedservice.Inruralareas
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withalowerdensityofsubscribers,useoflowerfrequenciesallowseachsiteto
providebroadercoverage.(See alsothe Marketsituation section of GSM.)
VariouscompaniesusedifferentvariantsofCDMAtoprovidefixed-linenetworks
usingWirelesslocalloop(WLL)technology.Sincetheycanplanwithaspecific
numberofsubscriberspercellinmind,andtheseareallstationary,thisapplicationof
CDMAcanbefoundinmostparts oftheworld.
CDMAissuitedfordatatransferwithburstybehaviourandwheredelayscanbe
accepted.ItisthereforeusedinWirelessLANapplications;thecell sizehereis500
feetbecauseofthehighfrequency(2.4GHz)andlowpower.Thesuitabilityfordata
transferisthereasonforwhyW-CDMAseemstobe"winningtechnology"forthe
dataportionofthird-generation(3G)mobilecellularnetworks.
Codedivisionmultiple-accesstechniquesallowmanyuserstosimultaneouslyaccess
agivenfrequencyallocation.Userseparationatthereceiverispossiblebecauseeach
userspreadsthemodulatedwaveformoverawidebandwidthusing uniquespreading
codes.TherearetwobasictypesofCDMA.Direct-sequence CDMA(DS-CDMA)
spreadsthesignaldirectlybymultiplying thedatawaveformwithauser-uniquehigh
bandwidthpseudo-noisebinarysequence.Theresultingsignalisthenmixedup toa
carrierfrequencyandtransmitted.Thereceivermixesdowntobasebandandthenre-
multiplieswiththebinary{±1} pseudo-noisesequence.Thiseffectively(assuming
perfectsynchronization)removesthepseudo-noisesignalandwhatremains(ofthe
desiredsignal)isjustthetransmitteddatawaveform.Afterremoving thepseudo-noise
signal,afilterwithbandwidthproportionaltothedatarateisappliedtothesignal.
Becauseotherusersdonotusecompletelyorthogonalspreading codes,thereis
residualmultiple-accessinterference presentatthefilteroutput.
Thismultiple-accessinterference canpresentasignificantproblemifthepowerlevel
ofthedesiredsignalissignificantlylower(duetodistance)thanthepowerlevelof
theinterfering user.Thisiscalledthenear-farproblem.Overthelast15 yearsthere
hasbeenconsiderabletheoreticalresearchonsolutionstothenear-farproblem
beginning withthederivationoftheoptimalmulti-userreceiverandnowwithmany
companies(e.g.,Fujitsu,NTT DoComo,andNEC)building suboptimalreduced
complexitymulti-userreceivers.Theapproachbeing consideredbycompaniesis
eithersuccessiveinterference cancellationorparallelinterference cancellation.One
advantageofthesetechniquesisthattheygenerallydonotrequirespreading codes
withperiodequaltothebitduration.Anotheradvantageisthattheydonotrequire
significantcomplexity(comparedtoaminimummeansquareerror-MMSE-detector
oradecorrelatingdetector).Theseinterferencecancellationdetectorscanalsoeasily
beimprovedbycascading severalstagestogether.
Asatypicalexample,Fujitsuhasamultistageparallelinterference cancellerwithfull
parallelstructurethatallowsforshortprocessingdelay.Accuratechannelestimation
ispossibleusing pilotanddatasymbols.Softdecisioninformationispassedbetween
stages, whichimprovestheperformance.Fujitsu'ssystemuses1-2stagesgiving fairly
lowcomplexity.Fujitsuclaimsthatthenumberofuserspercell increasesbyabouta
factorof2(100%) comparedtoconventionalreceiversand1.3timesifintercell
interferenceisconsidered.
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Types ofCodeDivisionMultiplexing
SynchronousCDMA:
SynchronousCDMA,alsoknownasCodeDivisionMultiplexing (CDM),exploitsat
itscoremathematicalpropertiesoforthogonally. Supposewerepresentdatasignals as
vectors.Forexample,thebinarystring"1011"wouldberepresentedbythevector(1,
0,1,1).Wemaywishtogiveavectoraname,wemaydosobyusing boldface
letters, e.g.a.Wealsouseanoperationonvectors, knownasthedotproduct,to
"multiply"vectors,bysummingtheproductofthecomponents.Theoperationis
denotedwithadotbetweenthevectors.Forexample,thedotproductofa=(1,0,1,
1)andb=(1,-1,-1,0),writtenasa.b,wouldbe(1) ×(1)+(0) ×(-1)+(1) ×(-1)+
(1) ×(0)=1+(-1)=0.Forthespecialcasewhenthedotproductoftwovectorsis
identically0,thetwovectorsaresaidtobe orthogonal toeachother.Thedotproduct
hasanumberofproperties,whichwillaidinunderstandinghowCDMworks.For
vectors a, b, c:
a.(b+c)=a.b+a.c,and
a.kb=k(a.b), where kisanarbitraryconstant, andnotavector
Thesquarerootofa.aisarealnumber,andiscalledthemagnitudeofthevectora.It
isdenotedas
.
aaa
=
Supposevectors aand bareorthogonal. Then:
22
22
22
22
.()since a.a+a.b=0
.()since -a.a+a.b=-0,
.()since b.a+b.b=0+
.()since b.a-b.b=0-
aabaa
aabaa
babbb
aabbb
+=+
−+=−+
+=
−=−
Example:
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Anexampleof4orthogonaldigitalsignalsFigure 5
Supposenowwehaveasetofvectorsthataremutuallyorthogonaltoeachother.
Usuallythesevectorsarespeciallyconstructedforeaseofdecoding-theyarecolumns
orrowsfromWalshmatricesthatareconstructedfromWalshfunctions-butstrictly
mathematicallytheonlyrestrictiononthesevectorsisthattheyareorthogonal.An
exampleoforthogonalfunctionsisshowninthepictureontheright.Now,associate
withonesenderavectorfromthisset,sayv,whichiscalledthechipcode.Associate
azerodigitwiththevector-v, andaonedigitwiththevectorv. Forexample,ifv=(1,-
1),thenthebinaryvector(1,0,1,1)wouldcorrespondto((1,-1),(-1,1),(1,-1),(1,-1)).
Forthepurposesofthisarticle, wecall thisconstructedvectorthetransmittedvector.
Eachsenderhasadifferent,uniquevectorchosenfromthatset,buttheconstruction
ofthetransmittedvectorisidentical.
Now,thephysicalpropertiesofinterference saythatiftwosignalsatapointarein
phase, theywill"add up"togivetwice theamplitudeofeachsignal, butiftheyareout
ofphase, theywill"subtract"andgiveasignalthatisthedifference oftheamplitudes.
Digitally,thisbehaviourcanbemodelledsimplybytheadditionofthetransmission
vectors, componentbycomponent.So,ifwehavetwosenders,bothsending
simultaneously,onewiththechipcode(1, -1)anddatavector(1, 0, 1, 1), andanother
withthechipcode(1,1),anddatavector(0,0,1,1),therawsignalreceivedwouldbe
thesumofthetransmissionvectors(1,-1,-1,1,1,-1,1,-1)+(-1,-1,-1,-1,1,1,1,1)=(0,-2,-
2,0,2,0,2,0).
Supposeareceivergetssuchasignal,andwantstodetectwhatthetransmitterwith
chipcode(1,-1)issending.Thereceiverwillmakeuseofthepropertydescribedin
theabovefoundationsection,andtakethedotproducttothereceivedvectorinparts.
Takethefirsttwocomponentsofthereceivedvectorthatis,(0,-2).Now,(0,-2).(1,-
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1)=(0)(1)+(-2)(-1)=2.Sincethisispositive,wecandeduce thataonedigitwas
sent.Takingthenexttwocomponents,(-2,0),(-2,0).(1,-1)=(-2)(1)+(0)(-1)=-2.Since
thisisnegative,wecandeduce thatazerodigitwassent.Continuing inthisfashion,
wecansuccessfullydecodewhatthetransmitterwithchipcode(1,-1)wassending:
(1, 0, 1, 1).
Likewise,applying thesameprocess withchipcode(1,1):(1,1).(0,-2)=-2gives
digit0,(1,1).(-2,0)=(1)(-2)+(1)(0)=-2givesdigit0,andsoon,togiveusthedata
vectorsentbythetransmitterwithchipcode(1,1):(0,0,1,1).
Asynchronous CDMA:
ThepreviousexampleoforthogonalWalshsequencesdescribeshow2userscanbe
multiplexedtogetherinasynchronoussystem,atechniquethatiscommonlyreferred
toasCodeDivisionMultiplexing (CDM).Thesetof4Walshsequencesshowninthe
Figure5will affordup to4users, andingeneral, anNxNWalshmatrixcanbeusedto
multiplexNusers. Multiplexing requiresall oftheuserstobecoordinatedsothateach
transmitstheirassignedsequence v(orthecomplement,-v)startingatexactlythe
sametime.Thus,thistechniquefindsuseinbase-to-mobilelinks,whereallofthe
transmissionsoriginatefromthesametransmitterandcanbeperfectlycoordinated.
Ontheotherhand,themobile-to-baselinkscannotbepreciselycoordinated,
particularlyduetothemobilityofthehandsets, andrequireasomewhatdifferent
approach.Sinceitisnotmathematicallypossibletocreatesignaturesequencesthat
areorthogonalforarbitrarilyrandomstartingpoints,unique"pseudo-random"or
"pseudo-noise"(PN)sequencesareusedin Asynchronous CDMAsystems.ThesePN
sequencesarestatisticallyuncorrelated,andthesumofalargenumberofPN
sequencesresultsinMultipleAccess Interference(MAI) thatisapproximatedbya
Gaussiannoiseprocess (viathetheoremofthe"lawoflargenumbers"instatistics). If
all oftheusersarereceivedwiththesamepowerlevel,thenthevariance (e.g., the
noisepower)oftheMAIincreasesindirectproportiontothenumberofusers.
AllformsofCDMAusespreadspectrumprocessgaintoallowreceiverstopartially
discriminateagainstunwantedsignals.Signalswiththedesiredchipcodeandtiming
arereceived, whilesignalswithdifferentchipcodes(orthesamespreading codebuta
differenttiming offset)appearaswidebandnoisereducedbytheprocess gain.
Since eachusergeneratesMAI,controlling thesignalstrengthisanimportantissue
withCDMAtransmitters.ACDM(SynchronousCDMA),TDMAorFDMAreceiver
canintheorycompletelyrejectarbitrarilystrongsignalsusing differentcodes,time
slotsorfrequencychannelsduetotheorthogonalityofthesesystems.Thisisnottrue
forAsynchronousCDMA;rejectionofunwantedsignalsisonlypartial.Ifanyorall
oftheunwantedsignalsaremuchstrongerthanthedesiredsignal,theywill
overwhelmit.ThisleadstoageneralrequirementinanyAsynchronousCDMA
systemtoapproximatelymatchthevarioussignalpowerlevels asseenatthereceiver.
InCDMAcellular,thebasestationusesafastclosed-looppowercontrolschemeto
tightlycontroleachmobile'stransmitpower.
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Advantages ofAsynchronous CDMAoverother techniques
AsynchronousCDMA'smainadvantageoverCDM(Synchronous CDMA),TDMA
andFDMAisthatitcanusethespectrummoreefficientlyinmobiletelephony
applications.TDMAsystemsmustcarefullysynchronize thetransmissiontimesofall
theuserstoensurethattheyarereceivedinthecorrecttimeslotanddonotcause
interference.Sincethiscannotbeperfectlycontrolledinamobileenvironment,each
timeslotmusthaveaguard-time,whichreducestheprobabilitythatuserswill
interfere,butdecreasesthespectralefficiency.Similarly,FDMAsystemsmustusea
guard-bandbetweenadjacentchannels,duetotherandomdopplershiftofthesignal
spectrumwhichoccursduetotheuser'smobility.Theguard-bandswillreduce the
probabilitythatadjacentchannelswillinterfere,butdecreasetheutilizationofthe
spectrum.
Mostimportantly,AsynchronousCDMAoffersakeyadvantageintheflexible
allocationofresources.Thereareafixednumberoforthogonalcodes,timeslotsor
frequencybandsthatcanbeallocatedforCDM,TDMAandFDMAsystems,which
remainunderutilizedduetotheburstynatureoftelephonyandpacketizeddata
transmissions.Thereisnostrictlimittothenumberofusersthatcanbesupportedin
an Asynchronous CDMAsystem,onlyapracticallimitgovernedbythedesiredbit
errorprobability,since theSIR(SignaltoInterferenceRatio)variesinverselywiththe
numberofusers.Inaburstytrafficenvironmentlikemobiletelephony,theadvantage
affordedbyAsynchronousCDMAisthattheperformance (biterrorrate)isallowed
tofluctuaterandomly,withanaveragevaluedeterminedbythenumberofuserstimes
thepercentageofutilization.Supposethereare2Nusersthatonlytalkhalfofthe
time,then2Nuserscanbeaccommodatedwiththesame average biterrorprobability
asNusersthattalkallofthetime.Thekeydifferencehereisthatthebiterror
probabilityforNuserstalkingallofthetimeisconstant,whereasitisa random
quantity(withthesamemean)for2Nusers talkinghalfofthetime.
Inotherwords,AsynchronousCDMAisideallysuitedtoamobilenetworkwhere
largenumbersoftransmitterseachgeneratearelativelysmallamountoftrafficat
irregularintervals.CDM(Synchronous CDMA),TDMAandFDMAsystemscannot
recovertheunderutilizedresourcesinherenttoburstytrafficduetothefixednumber
oforthogonalcodes,timeslotsorfrequencychannelsthatcanbeassignedto
individualtransmitters.Forinstance,ifthereareNtimeslotsinaTDMAsystemand
2Nusersthattalkhalfofthetime,thenhalfofthetimetherewillbemorethanN
usersneeding tousemorethanNtimeslots.Furthermore,itwouldrequiresignificant
overheadtocontinuallyallocateanddeallocatetheorthogonalcode,time-slotor
frequencychannelresources.Bycomparison,AsynchronousCDMAtransmitters
simplysendwhentheyhavesomething tosay,andgooff theairwhentheydon't,
keepingthesamePNsignaturesequence aslongastheyareconnectedtothesystem.
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Softhandoff:
Softhandoff (orsofthandover) isaninnovationinmobility. Itreferstothetechnique
ofadding additionalbasestations(inIS-95 asmanyas5)toaconnectiontobecertain
thatthenextbaseisreadyasyoumovethroughtheterrain.However,itcanalsobe
usedtomoveacallfromonebasestationthatisapproachingcongestiontoanother
withbettercapacity.Asaresult,signalqualityandhandoffrobustness isimproved
comparedtoTDMAsystems.
InTDMAandanalogsystems,eachcelltransmitsonitsownfrequency,different
fromthoseofitsneighbouringcells.Ifamobiledevice reachestheedgeofthecell
currentlyserving itscall,itistoldtobreakitsradiolink andquicklytunetothe
frequencyofoneoftheneighbouringcellswherethecall hasbeenmovedbythe
networkduetothemobile'smovement.Ifthemobileisunabletotunetothenew
frequencyintimethecallisdropped.
InCDMA, asetofneighbouringcells all usethesamefrequencyfortransmissionand
distinguishcells(orbasestations)bymeansofanumbercalledthe"PNoffset",a
timeoffsetfromthebeginning ofthewell-knownpseudo-randomnoisesequencethat
isusedtospreadthesignalfromthebasestation.Becauseall ofthecellsareonthe
samefrequency, listeningtodifferentbasestationsisnowanexerciseindigitalsignal
processingbasedonoffsetsfromthePNsequence,notRFtransmissionandreception
basedonseparatefrequencies.
AstheCDMAphoneroamsthroughthenetwork,itdetectsthePNoffsetsofthe
neighbouringcellsandreportsthestrengthofeachsignalbacktothereference cell of
thecall (usuallythestrongestcell).Ifthesignalfromaneighbouringcellisstrong
enough,themobilewillbedirectedto"add aleg"toitscallandstarttransmittingand
receiving toandfromthenewcell inadditiontothecell (orcells)alreadyhostingthe
call.Likewise,ifacell'ssignalbecomestooweakthemobileisdirectedtodropthat
leg.Inthisway,themobilecanmovefromcell tocell andadd anddroplegsas
necessaryinordertokeepthecall up withouteverdroppingthelink.
Itshouldbenotedthatthis"softhandoff"doesnothappenviaCDMAfromcelltower
tocell tower.Agroup ofcellsitesarelinkedup withwireandthecall issyncedover
wire, overTDM, ATM,orevenIP.
Whentherearefrequencyboundariesbetweendifferentcarriersorsub-networks, a
CDMAphonebehavesinthesamewayasTDMAoranalogandperformsahard
handoff inwhichitbreakstheexistingconnectionandtriestopickup onthenew
frequencywhereitleftoff.
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CDMAFeatures
Narrowbandmessagesignalmultipliedbywidebandspreading signalorpseudonoise
code
Eachuserhashisownpseudonoise(PN)code
Softcapacitylimit:systemperformance degradesforall usersasnumberofusers
increases
Cell frequencyreuse:nofrequencyplanning needed
Softhandoffincreasescapacity
Near-farproblem
Interference limited:powercontrolisrequired
Widebandwidthinducesdiversity:rakereceiveris used
Extendedbatterylifebecauseofeffectivepowercontrol
Spread-Spectrum CommunicationSystems
InourtreatmentofsignaldesignfordigitalcommunicationoveranAWGN channel,
themajorobjectivehasbeentheefficientutilizationoftransmitterpowerandchannel
bandwidth.Channelcodingallowsustoreduce thetransmitterpowerbyincreasing
thetransmittedsignalbandwidththroughcoderedundancyand,thus,totradeoff
transmitterpowerwithchannelbandwidth.Thisisthebasicmethodologyforthe
designofdigitalcommunicationsystemsforAWGNchannels.
Inpractice,oneencountersotherfactorsthatinfluence thedesignofanefficient
digitalcommunicationsystem.Forexample,inmultiple-access communicationwhen
twoormoretransmittersusethesamecommonchanneltotransmitinformation,the
interferencecreatedbytheusersofthechannellimitstheperformanceachievedby
thesystem.Thesystemdesignermusttakeintoaccounttheexistence ofsuch
interferenceinthedesignofareliabledigitalcommunicationsystem.
Eveninthiscomplexdesignproblem,thebasicsystemdesignparametersare
transmitterpowerandchannelbandwidth. Toovercometheproblemsofintentionalor
unintentionalinterference,wemayfurtherincreasethebandwidthofthetransmitted
signal,asdescribedbelow,sothatthebandwidthexpansionfactor Be= W/Rismuch
greaterthanunity.Thisisonecharacteristicofa spread-spectrumsignal.Asecond
importantcharacteristicisthattheinformationsignalatthemodulatorisspreadin
bandwidthbymeansofacodethatisindependentoftheinformationsequence.This
codehasthepropertyofbeing pseudorandom;i.e., itappearstoberandomto
receiversotherthantheintendedreceiverthatusestheknowledgeofthecodeto
demodulatethesignal.Itisthissecondcharacteristicpropertythatdistinguishesa
spread-spectrumcommunicationsystemfromtheconventionalcommunication
systemthatexpandsthetransmittedsignalbandwidthbymeansofchannelcode
redundancy.However,weshouldemphasize thatchannelcodingisanimportant
elementinthedesignofanefficientspread-spectrumcommunicationsystem.
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Spread-spectrumsignalsfordigitalcommunicationswereoriginallydevelopedand
usedformilitarycommunications either(1)toprovideresistancetojamming(antijam
protection),or(2)tohidethesignalbytransmitting itatlowpowerand, thus,making
itdifficultforanunintendedlistenertodetectitspresence innoise(lowprobabilityof
intercept).However,spread-spectrumsignalsarenowusedtoprovidereliable
communicationsinavarietyofcivilianapplications,including digitalcellular
communicationsandinterofficecommunications.
Inthissectionwepresentthebasiccharacteristicsofspread-spectrumsignalsand
assess theirperformance intermsofprobabilityoferror.Weconcentrateour
discussionontwomethodsforspreadingthesignalbandwidth,namely,bydirect
sequence modulationandbyfrequencyhopping.Bothmethodsrequiretheuseof
pseudorandomcodesequenceswhosegenerationisalsodescribed.Several
applicationsofspread-spectrumsignalsarepresented.
ModelofaSpread-SpectrumDigitalCommunicationSystem:
Thebasicelementsofaspreadspectrumdigitalcommunicationsystemareillustrated
inthefigure5weobservethatthechannelencoderanddecoderandthemodulator
anddemodulatorarethebasicelementsofaconventionaldigitalcommunication
system.Inadditiontotheseelements,aspread-spectrumsystememploystwo
identicalpseudorandomsequencegenerators,onewhichinterfaceswiththemodulator
atthetransmittingendandthesecondwhichinterfaceswiththedemodulatoratthe
receiving end.Thesetwogeneratorsproduce apseudorandomorpseudonoise(PN)
binary-valuedsequence,whichisusedtospreadthetransmittedsignalatthe
modulatorandtodispreadthereceivedsignalatthedemodulator.
TimesynchronizationofthePNsequencegeneratedatthereceiverwiththePN
sequence containedinthereceivedsignalisrequiredinordertoproperlydispreadthe
receivedspread-spectrumsignal.Inapracticalsystem,synchronizationisestablished
priortothetransmissionofinformationbytransmittingafixedPNbitpatternwhichis
designedsothatthereceiverwill detectitwithhighprobabilityinthepresence of
interference.AftertimesynchronizationofthePNsequence generatorsisestablished,
thetransmissionofinformationcommences.Inthedatamode,thecommunication
systemusuallytracksthetiming oftheincomingreceivedsignalandkeepsthePN
sequence generatorinsynchronism.
Interferenceisintroducedinthetransmissionofthespread-spectrumsignalthrough
thechannel.Thecharacteristicsoftheinterference dependtoalargeextentonits
origin.Theinterferencemaybegenerallycategorizedasbeingeitherbroadbandor
narrowband(partialband)relativetothebandwidthoftheinformation-bearing signal,
andeithercontinuousintimeorpulsed(discontinuous)intime. Forexample, an
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