Chapter2CellularSystems

Wirelesscommunicationslinksareespeciallyusefulformobileapplicationsandwirelesscommunicationssystemsareoftendesignedtocoversuchareasbysplittingthemintomanysmallercells.Thatcellularpropertyintroducesmanydifficultiessuchashowtohandover(orhandoff)fromonecelltoanother,whilemaintaininggoodservicequality.Coverage,capacity,interference,andspectrumreuseareimportantconcernsofcellularsystemsthischapterreviewstheseaspectsaswellasthetechnologies,tools,andstandardsusedtooptimizethem.

2.1CellularConcepts

Themanyfrequencyblocksdetailedearlierareusedforavarietyofcommunicationsservices.Higherfrequencies(sayabove6GHz)aremostlyusedforpointtopointservicessuchasdedicatedprivatelines.Lowerfrequenciesarebettersuitedforbroadercoverage,andaresplitintogeographicalcells.

2.1.1FrequencyReuse

Coveringalargegeographicareawithlimitedamountofspectrumleadstothereuseofthesamefrequencyinmultiplelocationsthisleadstocochannelinterferenceconsiderations,meaninginterferencefromdifferentareas(orcells)thatusethesamefrequencychannel.1Cochannelinterferenceconsiderationsareusuallyapproachedbyconsideringthefollowingparameters:

St:totalnumberofRFchannelsavailable(giventheamountofspectrumandchannelwidthdictatedbytechnologystandard),S0:numberofchannelspercell,whichreflectssystemcapacityatagivenlocation,K:thereusefactor,thenumberofcellsthatisrepeatedtoprovidecoverageoveralargearea.

Thethreequantitiesarelinkedbythestraightforwardrelation:

(2.1)

ThereusefactorKisthereforeanimportantparameterforcapacity.Thelowestreusefactor(K=1)maximizescapacitybutthishastobebalancedwithinterferenceconsiderations:indeedahigherreusefactor(K=3,4,7,orhigher)providesmoredistancebetweencellsusingthesamefrequency,whichlowersinterferences.

2.1.2InterferenceConsiderationsinReuse

Toquantifyinterferenceduetoreusewehavetoconsiderhowasignalpropagatesfromonecelltoanother.Wewillstudypropagationmodelslaterinchapter??,butweneedafewsimplenotionshere.Assumeapropagationmodelusingapowerpathlossexponentn,thatisamodelwherepowerdecaysin1Rn(Rbeingthedistanceseparatingtransmitstationfromreceiver).ThismeansthattheratioofreceivedpowertotransmitpowermaybeexpressedasPrPt=ARn,whereAissomeconstant.

Figure2.1:FrequencyreusepatternsK=3,4,and7,onhexagonalcells.Boldcontourshowsthepatternofcellsrepeatedtoprovidewideareacoverage.Dishowstheshortestdistancebetweencellsreusingthesamefrequency.

Withthismodel,signaltointerferenceratiosareestimatedas

(2.2)

wherei0isthenumberofcochannelcellsnearesttothecell(calledfirsttierortierone)thatnumberincreaseswithK.AndDiisthedistancetothetieronecellsreusingthesamefrequency(asshowninfigure2.1).Inthecaseofhexagonalcellapproximationtheexpressionsimplifiesto[1]:

(2.3)

Wellseemoredetailsonnfurther,itsvaluesvarytypicallybetween2and4withthetypesofterrain.Wellalsoseethatspecificwirelesstechnologiesrequireacertainsignaltonoiseandinterferenceratio(mostlybasedondatarates)soequation(2.3)leadstoaminimalacceptablevalueforK.

2.1.3MultipleAccess

Amajorrequirementofcellularnetworksistoprovideanefficienttechniqueformultipledevicesto

accessthewirelesssystem.Thesetechniquesinclude:

FDMA:frequencydivisionmultipleaccess,perhapsthemoststraightforward,inwhicheveryuserdeviceusesitsownfrequencychannel.Thismethodwasusedinthefirstgenerationanalogsystems.

TDMA:timedivisionmultipleaccess,inwhicharadiochannelisdividedintimeslots,andusedevicesusetheirallocatedtimeslots.InfactTDMAsystemsareoftenhybridFDMAaswellasmultiplechannelsareused,most2GsystemswereTDMA.

CDMA:codedivisionmultipleaccess,inwhichorthogonal(orpseudoorthogonal)codesareusedtodifferentiateuserdevices.CDMAisveryspectrumefficient,andwasusedby3Gstandards.ThereareseveralapproachestoachieveCDMA,suchasfrequencyhooping(FHCDMA)ordirectspreading(DSCDMA).

Thesearethemainmultipleaccesstechniques,butsubtleextensionsandcombinationscanbedevisedtoobtainmoreefficientschemes,whichwewillexamineinlaterchapters(includingorthogonalfrequencydivisionmultiplexingOFDMA).

2.2SystemCapacity

Wirelesscommunicationsdealwithatleasttwomainconcerns:coverageandcapacity.Wewilllookatcoveragepredictioninthenextchapters,andstartherewithafewwordsoncapacity.

2.2.1ChannelCapacity

Onefundamentalconceptofinformationtheoryisoneofchannelcapacity,orhowmuchinformationcanbetransmittedinacommunicationchannel.Inthe1940sClaudeShannoninventedformalcharacterizationofinformationtheoryandderivedthewellknownShanonscapacitytheorem(Theorem17in[13],p.628).Thattheoremappliestowirelesscommunications.Agreatpresentationofthisequationcanbefoundin[10]p.82itpresentsaconcisederivationoftheequation,andincludesagoodintroductiontoimportantinformationtheoryconceptssuchasinformationandentropy.2

TheShannoncapacityequationgivesanupperboundforthecapacityinanonfadedchannelwithaddedwhiteGaussiannoise:

(2.4)

whereC=capacity(bits/s),W=bandwidth(Hz),SN=signaltonoise(andinterference)ratio.

Thatcapacityequationassumesonetransmitterandonereceiver,thoughmultipleantennascanbeusedindiversityschemeonthereceivingside.Theformulawillberevisitedformultiantennasystemsin9.1.3.Theequationsinglesouttwofundamentallyimportantaspects:bandwidthandSNR.Bandwidthreflectshowmuchspectrumawirelesssystemuses,andexplainswhythespectrumconsiderationsseenin1.2aresoimportant:theyhaveadirectimpactonsystemcapacity.SNRofcoursereflectsthequalityofthepropagationchannel,andwillbedealtwithinnumerousways:modulation,coding,errorcorrection,andimportantdesignchoicessuchascellsizesandreusepatterns.

2.2.2CellularCapacity

PracticalcapacityofmanywirelesssystemsarefarfromtheShannonslimit(althoughrecentstandardsarecomingclosetoit)andpracticalcapacityisheavilydependentonimplementationandstandardchoices.

Digitalstandardsdealintheirownwaywithhowtodeployandoptimizecapacity.Mostsystemsarelimitedbychannelwidth,timeslots,andvoicecodingcharacteristics.CDMAsystemsareinterferencelimited,andhavetradeoffsbetweencapacity,coverage,andotherperformancemetrics(suchasdroppedcallratesorvoicequality).

Cellularanalogcapacity:Fairlystraightforward,everyvoicechannelusesa30kHzfrequencychannel,thesefrequenciesmaybereusedaccordingtoareusepattern,thesystemisFDMA.Theoverallcapacitysimplycomesfromthetotalamountofspectrum,thechannelwidthandthereusepattern.

TDMA/FDMAcapacity:IndigitalFDMAsystems,capacityimprovementsmainlycomefromthevoicecodingandelaborateschemes(suchasfrequencyhopping)todecreasereusefactor.Thefrequencyreusefactorhidesalotofcomplexityitsvaluedependsgreatlyonthesignaltointerferencelevelsacceptabletoagivencellularsystem([1]ch.3.2,and9.7).TDMAsystemscombinemultipletimeslotsperchannels.

CDMAcapacity:ausualcapacityequationforCDMAsystemsmaybefairlyeasilyderivedasfollows(forthereverselink):firstexamineabasestationwithNmobiles,itsnoiseandinterferencepowerspectraldensityduestoallmobilesinthatsamecellisISC=(N1)S,whereSisthereceivedpowerdensityforeachmobile,andisthevoiceactivityfactor.OthercellinterferencesIOCareestimatedbyareusefractionofthesamecellinterferencelevel,suchthatIOC=ISC(usualvaluesofarearound12).ThetotalnoiseandinterferenceatthebaseisthereforeNt=ISC(1+).NextassumethemobilesignalpowerdensityreceivedatthebasestationisS=REbW.EliminatingISC,wederive:

(2.5)

where

Wisthechannelbandwidth(inHz),Ristheuserdatabitrate(symbolrateinsymbolpersecond),EbNtistheratioofenergyperbitbytotalnoise(usuallygivenindBEbNt7dB),isthevoiceactivityfactor(forthereverselink),typically0.5,andistheinterferencereusefraction,typicallyaround0.5,andrepresentstheratioofinterferencelevelfromthecellinconsiderationbyinterferencesduetoothercells.(Thenumber1+issometimescalledreusefactor,and1(1+)reuseefficiency)

Thissimpleequation(2.5)givesusanumberofvoicechannelsinaCDMAfrequencychannel3.

WecanalreadyseesomehintsofCDMAoptimizationandinvestigatecertainpossibleimprovementfora3Gsystem.Inparticular:improvingcanbeachievedwithdimandburstcapabilities,withinterferencemitigationandantennadowntiltconsiderations,Rwithvocoderrate,WwithwiderbandCDMA,EbNtwithbettercodingandinterferencemitigationtechniques.

Someaspectshoweverareomittedinthisequationandarerequiredtoquantifyothercapacityimprovementsmainlythoseduetopowercontrol,andsofter/softhandoffalgorithms.

Ofcourseotherlimitationscomeintoplayforwirelesssystems,suchasbasestation(andmobile)sensitivity,whichmaybeincorporatedintosimilarformulasandfurtherconsiderationscomeintoplaysuchas:forwardpowerlimitations,channelelementblocking,backhaulcapacity,mobility,andhandoff.

Afinalnoteoncapacity:voicecapacityisoftengiveninErlang,andreferstotrunkingefficiencygivenacertainblockingprobability.([1]3.6,[2]p.350.)

2.3ModulationandCoding

Modulationtechniquesareanecessarypartofanywirelesssystem,withoutthem,nousefulinformationcanbetransmitted.Codingtechniquesarealmostasimportant,andcombinetwoimportantaspects:firsttotransmitinformationefficiently,andsecondtodealwitherrorcorrection(toavoidretransmissions).

2.3.1Modulation

Acontinuouswavesignal(atacarrierfrequencyfc)initselfencodesandtransmitsnoinformation.Thebitsofinformationareencodedinthevariationsofthatsignal(inphase,amplitude,oracombinationthereof).Thesevariationscausetheoccupiedspectrumtoincrease,thusoccupyingabandwidtharoundfcandtheoptimaluseofthatbandwidthisanimportantpartofawirelesssystem.Variousmodulationschemesandcodingschemesareusedtomaximizetheuseofthatspectrumfordifferentapplications(voiceorhighspeeddata),andinvariousconditionsofnoise,interference,andRFchannelresourcesingeneral.

Classicmodulationtechniquesarewellcoveredinseveraltexts[1][10],andwesimplyrecallhereafewimportantaspectsofdigitalmodulations(thatwillbeimportantinlinkbudgets).Themaindigitalmodulationsusedinmodernwirelesssystemsareoutlinedintable2.1.

Modulation Bitsencodedby: ExamplesAmplitudeShiftKeying Discreteamplitudelevels On/offkeyingFrequencyShiftKeing MultiplediscretefrequenciesPhaseShiftKeying Multiplediscretephases BPSK,QPSK,8PSKQuadratureAmpl.Mod. Bothphaseandamplitude 16,64,256QAM

Table2.1:Digitalmodulations

Modulationisapowerfulandefficienttoolusedtoencodeinformationafewsimpledefinitionsarecommonlyused:

Symboldenotesthephysicalencodingofinformation,overaspecificsymboltime(orperiod)Ts,duringwhichthesystemtransmitsamodulatedsignalcontainingdigitalinformation.

Bitdenotesalogicalbit(0or1)ofinformationoneormorebitsareencodedbyamodulationschemeinasymbol.

Higherordermodulationscanencodemultiplebitsinasymbol,andrequirehigherSNRtodecodeerrorfree.Figure2.2illustrateshowmultiplephasesandamplitudesareusedtocombinemultiplebitsintoonesymboltransmission.ThetradeoffbetweenbitsencodedpersymbolisoftenreferredtoasameasureinbitsperHertz(b/Hz),itsrelationtoSNRisboundedbyShannonstheoremseenearlier

(2.2.1).

Figure2.2:Digitalmodulationsencodemultiplebitsofinformationoverthetransmittedsignal.Thesimplestmodulation(BPSK)simplyencodesonebitofinformationinthesignofthewave.Higherordermodulationscombineorthogonalsignals(sineandcosine)andmultipleamplitudestoencodemultiplebits:2inQPSK,4in16QAM,and6in64QAM.

2.3.2Coding

Efficientcodingschemesarethepowerfulenginesbehindthegrowthofthewirelessindustry.Theyhaveallowedwirelesssystemstobebothspectrallyefficientandrobustintermsoferrorcorrections.

Blockcodingaretheclassicalapproach:blocksofdataareusedasinputtoproduceusuallylargeroutputblockscontainingaddedredundancy.

SecondgenerationwirelesssystemslikecdmaOneintroducedtheuseofconvolutionalcoding.Thecodingschemeprovidesanefficientredundantanderrorcorrectingscheme.Thisisparticularlyusefulforvoicetransmissionwheretheneedforretransmissioncausesdelaysanddegradesvoicequality.

Figure2.3:Convolutionalcodingconsistsinsendingadatastreamofbitsintoanencoderthatproducesmultipleoutputstreams.

Wirelessdatasystemsofhigherratesoftenuseturbocoding,whichareacombinationoftwoconvolutionalcodersreadingeachother(thenamecomesfromtheturbochargedengine,whichusessomeofitsoutputpowertocompresssomeairfedtotheintake,andissomewhatreminiscentoftheturbocodingdiagramoffigure2.4).

Figure2.4:Turbocodingconsistsinsplittingadatastream,andsendingitandaninterleavedreplicaintoconvolutionalencoders.

Convolutionalcodingandturbocodingareexampleofcontinuouscodingschemes,whereabitstreamisencodedintoanotherbitstream,usuallyofgreaterspeed(withamultiplierof2,3,4ormore).Theaddednumberofbitscanbeseenasspreadingthespectrum,andtheinformation,whichrequiresmoredatatotransmit,butinherentlycontainsusefulredundancyproperties(aformoftimediversity).Thedecodingofsuchschemeswashistoricallydifficultandhasbecomepossibleonlywithrecentprocessingpower(seeforinstanceViterbialgorithms[102]).

2.3.3CombinedModulationandCoding

Thecombinationofmodulationandcodingprovidesgreatflexibilitybetweenredundancyand

throughput.Highermodulationincreasesspectralefficiencyingoodpropagationconditionwhenconditionsworsen,lowermodulationhelps,butincreasedredundancyissometimesanefficientalternative.Combined,thetwoschemescanreachimpressiveefficiencies,closetoShannonslimit(2.2).

2.4StandardAirInterfaces

Wefirstbrieflyreviewcurrentmobiledigitaltechnologies,howtheywereinitiallyintroduced,andhowandtheyevolved.4

FirstGenerationAnalogcellularphones:Advancedmobilephoneservice(AMPS)wasdevelopedbyBellLaboratoriesinthe1970s,andstartedintheUSafterFCCallocationin1983of40MHzpairedspectruminthe800MHzfrequencyrange.Thesystemusedafrequencydividedmodulationaccess(FDMA),duplexfrequenciesforupanddownlink(frequencydivisionduplexingFDD),with30kHzchannels,oneuserperchannel,analogvoicemodulation(FM),blankandbursttransmission.

RFchannel 30kHzReusepattern typically7Duplex FDDMultipleaccess FDMAMultiplex 1trafficchannelperRFchannelVoice FMmodulation

SecondGenerationDigitalwirelesssystems:Secondgenerationcellularsystemsarecharacterizedbytheintroductionofvoicedigitizinganddigitalencoding,thusopeninganumberofDSPpossibilitiessuchasforwarderrorcorrectionschemes.Frequencyortimedivisionmultipleaccesstechniquesareused(FDMAorTDMA).Codedivisionmultipleaccess(CDMA)isintroducedbyQualcomm(TIAEIAIS95,orANSI95)andbecomesthebasisforthemain3Gsystems.Overallcapacityisincreased,signalingcapabilitiesandsystemintelligenceisconsiderablyenriched.

RFchannel 30kHz,200kHzinGSM,1.25MHzforCDMAReusepattern 7(lesswithfrequencyhopping),1forCDMADuplex mostlyFDD(emergenceofTDD)Multipleaccess FDMA,TDMA(8fullratetimeslotsforGSM),orCDMAVoice Digitalencoded:GSMfullrate13.4kbps,CDMA13kbpsQCELPor

8kbpsEVRC

Thirdgenerationsystems:Digitalsystemswerefurtherimprovedupon,mostlyforhighervoicecapacityandhigherdataratestheyevolvedintothirdgenerationstandards.

RFchannel 1.25,5,10,15MHzReusepattern 1(CDMA)Duplex mostlyFDD,someTDDMultipleaccess CDMAVoice Digitalencoded:bitrates8kbpsandbelowData UptoseveralMbps(3.1MbpsforEVDO,15MbpsforHSDPA)

Fourthgenerationsystems:Fourthgenerationstandardsdealwithhigherthroughput,lowlatency,IPnetworkarchitecture.AirinterfacesfocusonmulticarriertechniqueslikeOFDM,andadvancedantennasystemssuchasmultipleinputmultipleoutput(MIMO)systems.

RFchannel generallywider:10,20MHz,more

Reusepattern 11.5(OFDMAsee8.3.3)Duplex FDDorTDDdependingonspectrumMultipleaccess OFDMAVoice basedonVoIPData IPbased,flatarchitecture,convergence

2.5SpeechCoding

TheintroductionofdigitalwirelesssystemsmeansthattheacousticvoicewavefrontisnotsimplyconvertedtoanelectricalsignaldirectlytransmittedoverRFchannel.Voiceisnowdigitized,encoded,andtheresultingbitstreamistransmittedandofcoursedecodedonthereceivingside.Althoughthisprocessrequiresadditionaldigitalsignalprocessing(DSP),itopensthedoortomanyoptimizationalgorithmsandismuchmoreefficientthanusualanalogvoicetransmission.

2.5.1BasicVocoderTheory

Digitalvoicecoding(vocoding)isveryimportantyetverysubjective.Voicecodingtheoryisadomainofstudyofitsownintroductoryoverviewsarepresentedforinstancein[1]ch.8or[2]ch.15.

2.5.2ClassicCellularVocoders

AnalogvocodershaveemergedatBellLaboratoriesinthelate1920s,andhavebecomemoreelaborateandefficientindealingwithharmonicsimportanttoagoodunderstandingofvoice(500Hzto3400Hz)whileminimizingbandwidth.Thedigitalareabroughtsignificantchanges.Initialdigitalsystemssampledthatrange,whichattheNyquistrateleadstoa64kilobitspersecond(kbps,kbit/s,orkb/s)bandwidth.Thisisreferredtoaspulsecodemodulation(PCM).Moreelaboratealgorithmshowevercanachievereasonablygoodvoicetransmissionbytransmittingacodebook(setofparametersforagivenvoicecodingalgorithm)withaslittleas2.4kbpsrate:a26foldimprovement.Usuallythesealgorithmsprovideacceptablevoicequality,butmayprovidepoorperformanceinspecificsituationssuchasinanoisyenvironment,withbackgroundmusic,orwhencombinedwithdifferentvoicecodingsystems(suchasPCMorvoicemailsystems).Severalvocodersystemsexistandhavebeenchosenin2Gand3Gstandards:

CELP:CodeExcitedLinearPrediction,2.4and4.8kbps,FederalStandard1016,usedinSTUIII.

QCELP:QualcommCodeExcitedLinearPrediction,developedin1994,wasusedininitialIS95CDMAnetworks.Twobitratesavailable:QCELP8andQCELP13using8and13kbpsrespectively,whichiswelladaptedforthisstandards9.6kbpsand14.4kbpsframes.ItwaslaterimproveduponbyEVRC.

RCELP:RelaxedCodeExcitedLinearPrediction,amoreadvancedadvancedalgorithmthatdoesnotattempttomatchtheoriginalsignalexactlybutasimplifiedpitchcontour.

EVRC:EnhancedVariableRateCODECisaspeechcodecusedinCDMAnetworks,itusesRCELP8kbpsandimprovesqualityover8QCELP.HalfrateEVRCwerealsodevelopedtofurtherlowerbitrateatthecostofsomequality.

CVSD:ContinuouslyVariableSlopeDeltamodulation,16kbps,usedinwidebandencryptorssuchastheKY57.

MELP:MixedExcitationLinearPrediction,MILSTD3005,2.4kbps.

ADPCM:AdaptiveDifferentialPulseCodeModulation(G.721,G.726).

Comparingthequalitydifferencesbetweenvocoderisusuallydonebytestinganumberofstandardphrases,andassessingthequalityofthetransmittedresultundervariousconditions.ThatassessmentissubjectiveandisusuallygivenagradecalledMeanOpinionScore(MOS)between0(completelyunintelligible)and4(perfectquality).Initialtestsreliedonactualopinionsurveys,buttestdevicesnowofferalgorithmsprovidingaMOSandareregularlyusedbywirelessnetworkoperatorstobenchmarknetworkquality.

2.6Migrationto3G

Secondgenerationcellularsystemscertainlyachievedmajorcapacityimprovementsandcontributedtothefastadoptionofwirelesshandsetsthroughouttheworld.Andthegrowthcontinues.

Thirdgenerationsystemsfocusedonincreasingcapacityyetagain,andonintroducingefficienthighspeedmobiledatasystems.Givenpastheavyinvestmentsindifferent2Gnetworks,adoptionofacommon3Gstandardhadtremendouscostimplicationsandcompetitiveadvantages.

Theseeffortsfromthewirelessindustryfocusedonimprovingwidelydeployedsystems,andmigratethemtowardsathirdgeneration.Allmajordigitaltechnologiesproposedanevolutionpathtoanextgeneration,typicallybroaderband(inthroughputandspectrum).

Severalproposals:Initially10newproposalsweresubmittedtotheITUbodyresponsibleforstandardizingnextgenerationsystems:2TDMA,8CDMA.(SeedetailsinaUScontributiontotheITU:US8F0116,February2001.)

Harmonizationprocess:Adifficultharmonizationeffortwasundertakenfrom1998to2001bytheITU.Manytechnicalcomparisonsanddiscussionsensued,resultinginsomeharmonization,butfallingshortofselectingoneuniqueworldwidestandard.

Successes:TDMAsolutionsdisappeared.CDMAsolutionswerenarroweddowntotwo.Otherissuessuchasspectrumplansandemissionlevelswerealsodiscussedandapprovedwithrelativesuccess.

Failures:Onemajorissueremained:tomergethelasttwoCDMAcamps:the3Gpartnershipproject(3GPP)proposedUMTS(WCDMA),and3GPP2proposedcdma2000.Theformerwasveryreluctanttotreadonintellectualpropertyofthelatter,andthelatterwasadamantaboutconservingsmoothevolutionandbackwardcompatibilitywithcdmaOne.Anddiscussionsstalleditseemedobviousthatneithercamphadanyincentiveingivingin,hencetwocompetingstandards:UMTSWCDMAandcdma2000.

Figure2.5:ExistingCDMAcarrieruse(left)isconvenientformigrationtomulticarrierstandard,butmaybelessefficientthanfullspreadingonsamefrequencyblock(right).

Inshorttwomajor3Gstandardsremainincompetition,andthechoiceofanycarrierisclear:GSMoperatorsclearlyoptforamigrationtoUMTS(3GPP),andcdmaOneoperatorstocdma2000(3GPP2).Thelatteriscertainlyinitiallycheaper,hasadvantagesinequipmentavailability,andhaswellknownperformancesbuttheformermaybenefitfromlargereconomiesofscalesasGSMcarriersmigratetoUMTSservices.

In2002,CDMAAmericasCongress(SanDiego,December2002)estimatedthatcdmaOneoperatorsbenefitedfromasmoothtransitionandawellknownstandard,thusgivingthemaoneortwoyearadvanceoverGSMeffortstowardsUMTS.Indeedcdma2000(3G1X)systemshavebeenavailablesince2002,IS856(3G1XEVDO)havebeenwidelyavailableintheUSandAsiasince2004.GPRSandUMTSarefinallycatchingupin2006.Highspeeddataservices(HSPA)stilllagincoveragebehindEVDOin2008,butmostdenseareasintheUSarewellcoveredbybothtechnologies.

Choosingamigrationpathisonlythefirststepupgradingthenetworkisofcourseverycostly.Initiallyserviceprovidershadtodecidehowlongtodelaynetworkupgrade:voicecapacityandtimetomarketforhighspeeddataserviceswerethedrivingfactors.Nowserviceprovidershavetodecidehowmuchresourcestodedicatetovoiceversusdata.

2.7Anothermigrationto4G

Secondgenerationcellularsystemsachieveddigitalvoiceefficiency,thirdgenerationsystemsfocusedonincreasingcapacityanddatarates,whatmorecanafourthgenerationstandardachieve?

Accordingtomostdefinitions(fromtheITUinparticular),4Gsystemsarerequiredtoachievethroughputratesaround100Mbpsformobilityand1Gbpsforfixedwirelessaccesssotheairinterfacehastobeincrediblyefficient.Therearecertainlyadditionalrequirements(mostlyonthenetworkinfrastructure)suchaslowlatency,flatIParchitecture,andtheuseofsmallcells,heterogeneousnetworks,andmore(whichwellreviewinlaterchapters).

Themain3Gstandardshaveanevolutiontowardsa4Gstandard,evenifnotallaspectsareetinitsearlyiteration,these4Gstandardshaveevolutionlinestowardstrue4Grequirements.Theyhaveanumberofcommonalities:

LTE:LongTermEvolutionofthecurrentGSM/UMTS/3GPPsetofstandardisOFDMAontheforwardlink,andSCFDMA(asinglecarrierOFDMAscheme)onreverselink.Interestingly,GSMcarriersmigratedoncetoCDMA,andnowproposetoabandonitforOFDMA.LTEpromisestocarrymuchoftheinternationalcrowdofoperatorsandcreateeconomiesofscale,allowforinternationalroaming,etc.

WiMAX:WiMAXisawirelessstandardbasedonIEEE802.16e(anditsevolution802.16m).Itsstrengthisthat(unlikeother4Gstandards)itsevolutionpathpreservesbackwardcompatibilitywithcurrent802.16esystems.

Oddlyenoughtwodifferentcampsseemtoemergeagain:LTEandWiMAX,eachbackedupbydifferentsuppliers,anddifferentoperators,bothusingverysimilartechnologies(basedonOFDMA),andwithveryfewtechnicalreasonswhytheyshouldnotharmonizetoauniquestandard.

Animportantargumenttoconsideristhatofspectrum:thevastmajorityofmobileoperatorsoperateinFDDspectrum(seesections1.2.3and1.3)LTEprovidedanevolutionfirstinthatmode.WiMAXontheotherhandchosetofocusfirstonTDDbandsandistheobviouschoiceforTDDspectrumowners.Theoveralltimelineforevolutionisalsoimportant:somecellularprovidershavemadesignificantinvestmentsinEVDOorinHSPA.Newcomersontheotherhandwhoneedhigh

dataratestodaywithsmoothevolutiontowards4GlatermaybemorelikelytochoseWiMAX.Practicallyhowever,since2010thevastmajorityofthemobileindustryisfollowingLTEplans,andthatstandardisbecomingthedefactostandardforthe4Gmobilewirelessworld.

2.8TechnologyAdvances

Recenttechnologyadvancesaimatincreasingcapacityfurther.Technologyimprovementsaresometimestheresultofamajorstandardmodification,butsometimessimpleschemesthatcanbeaddedtoexistingstandardsandallowforadditionalimprovementswithminimalinfrastructurechanges.

2.8.1SpeechCodingImprovement

VoicecodingalgorithmsandDSPcapabilitieshaveimproved,andcurrentvoicecodecsoperateonlesspower,andwithgreaterprocessingefficiencies.(Referto[2]ch.15,or[1]ch.8forspeechcodingdetails).GSMforinstanceisimprovingvoicedigitizationandquantizingfromRPELPTtoaseriesofAMRstandards.IS95systemshaveaparallelevolution,withEVRC,andhalfrateEVRC.

Anotherstandardforselectablemodevocoder(SMV)wasintheworkbutneversawanysuccessintheindustryitbasedrequirementson:operationinpresenceofframeerasures,noisesuppressionrecommendedforbackgroundnoises,reasonableperformancewithmusicforonholdsituations,equivalentperformanceswithdifferentlanguages,multiplequalitymodesandmultiplebitrates,seamlesstransitionfrommodetomode.SMVwasdesigntoofferfourmodesofoperations:

Mode0isdesignedtoimprovevoicequalityoverEVRCwiththesamecapacityrequirementsasEVRC.Mode1isdesignedtomaintainthequalityprovidedbyEVRCwhilerealizingacapacitybenefit.Mode2isforthesystemoperatorwhoiswillingtosacrificesomevoicequalityrobustnessinordertorealizeasignificantcapacitygain.Similarly,Mode3ofSMVprovidesevenmorecapacitygains.Butthevoicequalityis,bytollgradestandards,poor.

Theresultingcapacityvs.qualitytradeoffsseemusefulandattractivetoserviceproviders,yetthisstandardnevertookoff,whichmayillustratethatsomestandardevolutions(evenwhenbasedonsoundrequirementsandgoodimprovements)maymisstheirwindowofopportunity.

2.8.2EfficientCodingandModulation

Forsystemsprimarilydesignedforvoice,latencywasamainconcern,andmodulationswerechosentobereliableandoperatingwellatfairlylowSNR(likeQPSK).Fordatasystemsitisadvantageoustotakeadvantageofhighermodulationschemessuchas16QAMand64QAMwhentheradiolinkallowsit.Highermodulationsaremorespectralefficientbutpronetomorebiterrorratesandmaycausemoreretransmissions,latency,orjitter.

Databursts:whenlowSNRallowsforit,usehighermodulationandcodingratesforbetterspectralefficiency.

Adaptivemodulation:fastmodulationchangesframebyframeallowforefficientschedulingofhighspeeddataburstswhentheradiochanneliscapableofit.

ForwardErrorCorrection:averyimportantaspectofwirelesscommunication:errorcorrectingcodingvariesfromvoiceto

databurstsblockcoding,convolutionalcoding,andturbocodingcanbeusedoptimizeefficiency.

ARQ:automaticretransmitrequestsareusedtolowermodulationwhennecessaryandretransmitfadeddata.

2.8.3InterferenceMitigation

Interferencesmaybecancelledormitigatedbychangingantennapatternsasrequired.Suchsystemsaresometimesreferredtoassmartantennas,andareinessenceanelaborateextensionofsectoring.Theaimmaybetobalancetheload,orsteeramainlobetowardauser,orcreateanullinthedirectionofaninterferer.Somesystemsarestatic,othersaredynamicandchangewithcellload.Somesystemsarepassiveothersincludeactiveamplificationdevices.Themaintypesofsmartantennasystemsmaybedescribedasfollows:

Activeantennas:Anarrayofpassiveandactiveelementsusingmultiplepoweramplifiersonthetransmitside,andalownoiseamplifieronthereceiveside.

Switchedbeams:Afixedarrayofnarrowbeams,combinedtoformvarioussizesectors.

Adaptivearrays:Anarrayofelementsofferingseveraldegreesoffreedomtosteerabeaminacertaindirection,orcreatenulls.Arrayelementaresometimesamplified,orattenuated,orarepurelypassiveandutilizephaseshifttocreatethewantedpatterns.

SpatialDivisionMultipleAccess(SDMA):Asophisticatedcombinationofmanyadaptiveelements.

Smartantennasystemsareefficientindenseareas.Theircostofequipmenthowever(sometimesduetothecomplextransmitaspect)andlargeantennasizesaremajordrawbacks[11].SmartantennasarenowreplacedbyMIMOsystemscoveredinchapter9.

2.8.4Diversity

Antennadiversityisawonderfultechniquetoimprovelinkbudgetsreceivingdiversitysimplyconsistsinhavingmorethanoneantennaatthereceivingsite.Giventhepowerlimitationsofamobilehandset,receivingdiversityhasbeenimplementedatcellsitefromtheearlydaysofcellularsystems.Gooddiversityschemescanadd8to11dBontheuplinkbudget,thussignificantlyimprovingcoverage,qualityandcapacityonthatlink.Thegoalofantennadiversityistoprovidetwouncorrelatedpathsandcombinethetwosignals,thusreducingtheprobabilityofdeepfades.Ageneralguidelineistomeasureorcalculatethecorrelationcoefficient,,andtrytoachievethelowestpossiblecorrelationbetweenthetwopaths.

Diversityimprovementsareoftwokinds:improvementsonexistingreceivediversityintheuplink,andintroductionoftransmitdiversityfortheforwardlink.

Figure2.6:TestsetuptomeasureseveralantennaspacingforhorizontalspacediversityforaPCSsystem:antennasareplaced2,5,and10apart.

Figure2.7:Cellularnetworksutilizemanytypesoftowersandpoles,andevensomedisguisedependingontheareastocover.Differentantennasmakeuseofdifferentdiversityschemes(spaceforthelefttwo,polarizationforthefarright).Andsomeantennasareslightlydowntilted(right)toreduceinterferencestoneighboringcells.

Receivediversityhasbeenusedfromtheearlydaysofcellular,andisaspopularasever.Classicdiversityschemesusetwoantennasatthebasestationandsomealgorithmstocombinesignals5

Spatialdiversity:Usedateverysector,wellknowncombiningtechniques,probablythemostefficienttypeofdiversity.

Angulardiversity:Typicallyoflittleuse,itsbenefitsareusuallyexploitedbysofterhandoff(withinasite)orsmartantennas.

Timediversity:HeavilyusedinmodernstandardslikeCDMA:interleaving,halfchipoffsetinIandQ

transmission,rakereceivers.Polarizationdiversity:

Widelyused,convenientforsmallbasestationsiteswhereantennascannotbeseparated.

Transmitdiversityisanimportantfeatureforforwardlinkcapacityimprovement.Sincehandsetsarerathersmall,theirreceivediversitycapabilitiesarelimitedandtheretransmitdiversityschemeswerelongignored,butarenowusedinmanystandards.

OrthogonalTransmitDiversity(OTD):Codedsymbolstreamsaresplitintotwodatastreams,eachcontaininghalfthenumberofsymbols,modulatedandspreadseparately(withtwodifferentcodes),andtransmittedontwodifferentantennasthusdoublingtransmitrate.

SpaceTimeSpreading(STS):Codedsymbolstreamsareduplicatedintotwoidenticalstreams,modulatedandspreadseparately(withtwodifferentcodes),andtransmittedontwodifferentantennas.ThekeydifferencewithOTDisthatinSTSallofthedataissentoutoneachantenna.Thisschemeprovidesredundancyratherthandatarateimprovement.

Multipleinput,multipleoutputsystems(MIMO):Thesesystemsarekeytorecentwirelessstandards,fromwirelessLANlike802.11ntocellularevolutionslikeLTE.MIMOsystemsusemultiplestreamsencodeddifferently,transmittedoverdifferentantennas,andreceivedbymultipleantennas.(SeemoreonMIMOinchapter9.)

2.8.5OtherOptimizationTechniques

Technologyadvancesandstandardimprovementstargetanincreaseincapacity,coverage,datarate,orsomeothersystemperformanceaspect.Inmanycaseshoweversomesimpleoptimizationtechniquescanbeusedtoincreaseperformance:

Antennaheight:higherforfurtherrange,orlowertoreduceinterference.Cellsplitting(intosmallercells:microcells,picocells,femtocells).Sectoring:often3to6sectors.ImprovingRFcomponents:duplexers,combiners,jumpercables,connectors.Rangeextensionbyrepeatersorlownoiseamplifiersincreasecoverage.Changingantennasaccordingtoneeds:diversity,gain,beamwidth,downtilt,etc.Andanumberofparameteradjustments(powerlevels,handoffparameters,etc.)

Thesetechniquesareveryimportanttoolsusedbyoperatorstooptimizecapacityandcoverage.Insomecasesoptimizationmaybeseasonalduetofoliageordifferentusagepatterns.InallcasesRFnetworkdemandconstanttweakingtoprovideoptimalperformance.Morerecentlyselfoptimizingnetworks(SON)havetheabilitytocontinuallyandautomaticallyoptimizetheseparameters.

2.9FixedWirelessAccess

Fixedwirelessaccessissometimesreferredtoaswirelesslocalloop(WLL),andisanalternativetoprovidePlainOldTelephoneServices(POTS)andhighspeeddataservicesinremoteareaswherewiredsolutionsareimpracticalforvariousreasons.Inmostcases,trenchinglongdistancestoplacecommunicationconduits(forfiberorcopper)isverycostly,suchasinmountainousareas.Cellularserviceisoftenscarcetooinremoteareas.

2.9.1ClassicArchitectures

Radiosolutionsforwirelesslocalloopswererolledoutextensivelysincethe1970s.Somesuchradioservicesarestillinplace,andinusetoday.Earlysystemsuseanalogradiostooffervoiceserviceover

fairlylongdistances.NewerWLLsystemneedtobecosteffective,reliable,adaptabletoawiderangeofsituations,andcompliantwithlocalexchangecarriertechnical,legal,andregulatorystandards.ButthedemandforWLLservicesaregenerallylow,andsuppliersconsequentlytreattheopportunityasafairlylowpriority.

InitiallyWLLfocusedonprovidingextensionsofthepublicswitchedtelephonenetwork(PSTN)toreachremotecustomers.AsthePSTNevolvedtodigitalvoice,digitalswitching,andClass5features(suchascallwaiting,callerID,3waycalling,andothers),WLLsystemsevolvedtoincludemanyofthesefeatures.WLLproductsthereforefocusedonprovidingfeatureparityfortheseclass5services.ConnectivitytoClass5switcheslikeLucent5ESSorNortelDMS100isspecifiedinTelcordiastandardssuchasGR303orGR008andWLLsystemsevolvedtousethesestandardinterfacestothePSTN.

Radiofrequencieswereallocatedforwirelesslocalloopapplications,andarereferredtoasLandMobileRadio(LMR).LMRradiolinksfortelephonyusefrequenciesintheUHF/VHFband(138512MHz),whichprovidegreatpropagationcharacteristicsevenindifficultterrainandheavytreedensity.Thesefrequencieshoweverarebecomingveryrare.Infact,theyareinsuchdemandthattheFCCrecentlymandatedradiosystemstoincreasetheirspectralefficiencies,anduseonlyanarrowbandofspectrum.ManylegacyLMRequipmentusing2025kHzRFchannelsmustmigratetonarrowbandLMR12.5kHzchannelsbyJanuary1,2013.Inaddition,theFCCordermentionsthegoaltoreach6.25kHzchannelizationsonewWLLsystemsareurgedtodeploythesenarrowRFchannels.6

Otherradiosolutionsworkinthe2.4GHzand5GHzunlicensedbands,buildingonthepopularityandthereforeeconomiesofscaleof802.11a/b/gradios.UnfortunatelythepopularityoftheseradiosforWiFiLANalsocreatesalotofinterferences,whichisaconcernwhenprovidingemergencyservice(911lifeline).Afewsystemsthereforehavea900MHzversionalthoughlessspectrumisavailableandlesspowerisallowed,thatfrequencycanbeaveryusefulalternative.Finally,newTVwhitespacesareawonderfulnewopportunitytoexplore.

2.9.2CellularWLL

Inadditiontofrequenciesmentionedabove,wirelesscarrierscanusetheirlicensedspectrumtoprovidefixedapplications.Fixedradiolinksusuallybehavedifferentlyfrommobileradiolinks,theyaretypicallylessvariableintime(thereforeeasiertopredictandequalize),andtheirfadingstatisticsaregenerallyeasiertodealwith.Consequentlyfixedpropagationisusuallyadvantageousforawirelesssystem.Severalimportantaspectsoffixedsystemshouldbeemphasized.

Propagation

Mobilecommunicationslinkaremorelikelytobeobstructedandhaveahighpathlossexponent(seechapter??fixedlinksontheotherhandcanuseelevatedantennasinordertoestablishnearlineofsightwiththebasestationandthereforeimprovepropagationcharacteristics.

Propagationmodelingofafixedradiolinkhasfundamentaldifferenceswiththatofamobilelink.Wirelesspropagationmodelsnearlyalwayscomefromextensivedrivetesting(hencemobile)collectingfixeddataforanempiricalmodelismoredifficult:inmanycasesexperimenterspresentmethodstolocallyaveragedata(overonehalfofawavelength)toremovesmallscalefadingduetomultipath.(Smallscalefadingisdifficulttoquantifyaccurately,andevenalargenumberoffixeddatapointswouldprovideinsufficientsamplingtobeabletoevaluateitsimpact.)Anotherimportantissueisthatofantennabeamwidth(ordirectivity).Mobiledatacollectionsareconductedusinganomnidirectionalantenna(isotropicwithrespecttoazimuth).Ithaslongbeenknownthattheantennabeamwidthandmorespecificallythedistributionofanglesofarrivalwithrespecttothedirectionofmotionofamobileareimportantparameterstoquantifythefadingofamobilelink[1].

Consequentlyfixeddatamodelsmaydifferinsomecasesfromtheusualempiricalmodels.Goodfixedmodelswouldbepreciousforfixedwirelessaccess,butthecurrentuseofmobilemodelsislikelytocontinueforanumberofreasons:first,theyprovideagoodestimateforinitialdesign(sitespecificmodelsandsimulationsareusedformoreprecisepredictions)second,sometimeisnecessarytorolloutlargefixedwirelesssystemsthatcanbeusedandanalyzedinordertoprovideawidemodelingrangelastly,thefocusofwirelessaccessmostlyremainsonmobility.

AdvantagesofFixedLinks

Fixedlinkshaveafewimportantdifferencesinpropagationcharacteristics,whichhaveasignificantimpactonreach,capacity,andthereforeoverallcostofafixedwirelesssystem.

Mobileradiolinksoftenincurfastchangingfadingconditions.Fixedlinksontheotherhandexperienceslowerfading,mostlyduetothechangesintheneighboringscatterers.AsaresulterrorratesaretypicallyimprovedforagivenSNR.InanIS95CDMAsystemforinstance,theindustryusuallyacceptsEb/Nolevelsof4forfixedcommunication,ratherthan7neededformobility.Allotherparametersbeingequal,areductionofEb/Notargetof3dBnearlydoublescapacity.(RefertoCDMAcapacityin2.2.)Fixedusersusingnarrowantennabeamwidthsorientedtowardagivenbasestationoffersmoreefficientspectrumreusepatternsthanwhatmobileomnidirectionalusersrequire.Fixedusageincreasessystemcapacityasitdoesnotrequiretheradioresourcesthatmobileusersneedtohandoverbetweenbasestations.Anotheradvantageofnarrowbeamwidthantennasisthatantennagainisimproved.Inaddition,repeaterscanbestrategicallyplacedatcustomerpremisetofurtherimprovethelink.Antennaheightscanbeincreasedtobenefitpropagationcharacteristics.Antennascanbeplacedoutdoorswithacablereachinganindoordevice.Anotherimportantaspectofthewirelesschannelisitsvariability:themobilechannelistypicallymuchmorevariable,afixedaccesschanneliseasiertopredictandcanthereforebemorespectralefficient.

Fixedwirelesslinkscanthereforeprovideincreasedreachandcapacitythanequivalentmobilelinks.Asaresult,someoftheseotherwisecostlycellularsystemshavebeenusedforfixeduse,sometimeswithminormodifications.Insomecases,wirelesslocalloopbasestationsbecamehandytodeployinruralareastoprovideextendedcoverage,andreachminimumservicemandatedbytheFCCforPCSspectrumauctionsforinstance.Morerecently3Gand4Gsystemsareadvertisingtheirfixedcapabilitiesagainandmaybetryingtocompetewithotherwiredbroadbandservices.

2.9.3VoiceIntegration

VoiceoverIP(VoIP)isanefficientandwidelyacceptedmethodofprovidingtelephony.Whenconsideringwirelesstransport,theefficientcompressionofVoIPisanespeciallyvaluableproperty.MostrecentWLLradiosolutionsthereforeuseVoIPtransportthisisespeciallyconvenientasmostconsumerandenterpriseradiosolutionsarebasedonIPandEthernet.ConsequentlyfairlycheapofftheshelfsystemscanbeadaptedtoWLLvoiceanddatadelivery.Theproblemremainshowevertointerfacethesesystemswiththenearesttelephonynetwork.SeveralarchitecturesarepossibleforWLL,dependingonthelocationofnetworkelementswithvoicefeatures.

Figure2.8:Fixedwirelesslinks,orwirelesslocalloop(WLL)providefixedwirelessvoiceand/ordatalinks.VoiceservicesuseavoiceoverIPgatewayadditionaldataservicesareroutedtoabroadbanddatanetwork,andbypassthevoicegateway.

Inmostruralareas,alocalcentralofficehasTDMvoicecircuitsavailableratherthanaVoIPsystem,soaVoIPgatewayisrequiredforWLLpurposes.SuppliersofWLLsystemsoftenhaveaVoIPgatewayaspartofthesolutionuntilrecently,thesesolutionswerestilldifficulttorolloutbecauseoftheVoIPgatewaycost,anditsoperationsintegration.Todaysmallsizegatewaysareavailableatreasonablepriceswithgoodinterfacestandards.Interfacesfromthegatewaytotheswitchingfabrichavetorelyonlegacytelephonystandards.OnesolutionistoconnecttheVoIPgatewaytoatelephonyCLASS5switchviaGR008orGR303.TheseTelcordiastandardsallowforagatewaytoconnecttoaswitch(withoneortwoT1lines),andtoaccessclass5features(suchascallwaiting,callerID,3waycalling,etc.)AnalternativesolutionwhenGR008orGR303interfacesarenotsupportedaretosimplyinterfacewithanalogtipandringlines,butthatmethodhasthedisadvantageofofferingnoremotealarmingortroubleshootingcapability.

TheremainderofthevoicetransportbetweenthevoicegatewayandthecustomerendpointfollowstypicalIPtransportarchitectures.NetworkelementsusuallyinterfacewithEthernet(10/100sometimes1000bT).ManyradiosystemsuseasomewhatproprietaryphysicalandMAClayertoinsurereliablevoicetransport,butoftenthesesystemsarebasedonWiFiorWiMAXphysicallayers.AnumberofprotocolsareavailabletoestablishareliableIPsessionthatcanprovidevoicetransport,includingsessioninitiationprotocol(SIP),orandMediaGatewayControlProtocol(MGCP)ITUrecommendationH.323alsoprovidesinteroperabilitystandardsformultimediacommunicationsoverIPincludingvoicefeatures.

2.9.4DataServices

DatafeaturesarealsoavailableonmanyWLLradios,butaresomewhatdifferent.Featureslikefaxandlowdatarates(upto56kbps)arefairlysimpletoaddtomostWLL,butthetaskisslightlydifferentwhentryingtoaddhigherdatarates(inthemultipleMbpsrange).Indeed,higherdataratescannolongerinterfacewiththevoiceswitchandneedtobesplitintoadatanetworkofitsown.Ifahighspeedinternetnetworkisavailableinthearea,datasessionshavetoberoutedtothatnetworkwhilevoicetrafficneedstobeidentifiedassuch,androutedtowardstheVoIPgateway.

2.10Homework

1. Inatable,listallthewirelesstechnologiespopularinmodernwirelessservices(2G,3G,WiFi,WiMAX,HSPA,LTE).Researchandlisttheirmainparameterssuchas:(a)frequencyofoperation(b)RFchannelbandwidth(c)peakuplinkanddownlinkdatarates(d)standardbodyforairinterface(e)modulationtype(f)multipleaccess(g)andsomekindofcapacityestimatesuchasthroughputperMHz.

2. ExaminetheShannoncapacityequationandcommentonwhathappensintochannelcapacity

inthefollowingdifferentsituations.a. YouoperateinafixedbandwidthW0,andincreasethepower(S)inthechannel.How

doescapacitybehave?b. Youhavealimitedpowerradio(thereforeSisfixed)youincreasesystembandwidth,but

asyoudothatsystemnoisetypicallyincreasesaswell:N=N0W(whereN0isafixednoisedensity).Howdoescapacitybehaveasbandwidthincreasesindefinitely?(calculatelimitofCasW).

c. Younowfixyourpowerspectraldensity:S=S0W(S0isyourfixedtransmitpowerdensity).Howdoescapacityincreasewithbandwidth?

3. Calculatethecapacity(invoicechannelpercellperMHz)ofthefollowingstandards(see2.2and2.4).Ineachcase,simplyassumeK=7asthereusefactor.

a. VerifythatAMPSsystemcapacityinindependentoftheamountofspectrumavailable,andism=4.7ch./cell/MHz.

b. CalculateGSMfullratesystemcapacity.(Answer:m=5.7)4. CDMAcapacityimprovement:

a. WhatcapacitygaindoesaCDMAserviceproviderachievebychangingitshandsetfromQCELPvocoderstoEVRCvocoders?

b. Inaddition,thebetterspeechcodingallowstypicalEbNttobereducedfrom7dBto6.5dB.Whatisthetotalcapacitygain?

5. CDMAcapacity:a. Deriveindetailsthecapacityformula(2.5)forCDMAsystems.b. Computearadiosystemcapacity(mCDMA)forIS95halfrateEVRC(EbNt=6.5dB)

6. Differentradiostandardssystemcapacity:a. CompareradiosystemcapacityforaboveIS95halfrateEVRC,GSMhalfratevoice

frames,DECT,andPHS(searchonline,orreferforinstanceto[1]chapter11forthelast2).

b. WhatarethechancesofPHSorDECTtoevolveintoa3Gstandard?7. Youinventedanewvoicecoderthatallowsyoutocodevoicein4.8kbpsratherthan9.6kbps

withnosignificantvoicedegradation.a. Whatwillthelinkbudgetimprovementbe?b. Usingcapacityequations,quantifytheimpactonnetworkcapacity.

8. Asanoperator,youarefacedwiththedifficultdecisionsofhavingtoregularlyupgradeyournetworktobetterstandardsandnewerequipment.AssumeyouareoperatingaGSMnetworkandyouconsiderupgradingittoUMTS.Consider(a)priceandavailabilityofequipment,(b)timelinetoupgrade,(c)impactofothercarrierstimeline,(d)fieldexperienceandproventechnology,(e)otherconsiderations.

9. Similarlytotheaboveproblem,younowoperateaUMTSnetworkwithvoiceandhighspeedpacketdata.WriteaproposaltoupgradeittoafourthgenerationsystemusingLTE(withthesameaboveconsiderations).

10. Youoperateawirelessserviceinasmalltown.YouinstalledaCDMAsystemthatcantypicallysupport50mobilecallspersector,butyouchosetoofferfixedserviceonly.Refertosection2.9.2,estimateallthegainyoucanrealizeandassumethattheyhaveadirectimpactonthesystemEb/No.Howwouldyouestimateyourfixedsystemcapacity.