Lecture 6

NetworkDeployment(2)

BasicsofTransmissionSchemes(1)

+Capacity Expansion

nMaininvestmentindeployingacellularnetworkisthecostofinfrastructure,land,basestationequipment,switchesinstallation,interconnection,etc.

n Incomeisproportionaltosubscriberbase

n Initialinstallmentmaynotbeabletosupportincreasingsubscriberdemand

nHowcancapacitybeincreasedwithoutreplicatingdeployment?

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+Techniques to expand capacity

n Additionalspectrumn Veryhardtoobtain– alsoexpensiven 1900MHzbandsforPCS;700MHzbandsfromTV

n Architecturalapproachesn Cellsplittingn Cellsectorizationn Reusepartitioningn Lee’smicrocellzonetechnique

n Changingtodigital– TDMAorCDMA

n Dynamicchannelallocation

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+Cell Splitting

n Hotspotsarecreatedincertainareas

n Introduceasmallercellofhalfthesizemidwaybetweentwoco-channelcells

n Interferenceproblems

n Channelsmustbesplitbetweenthelargerandsmallercells

a A-a

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+The Overlaid Cell Concept

n Channelsaredividedbetweenalargermacro-cellthatco-existswithasmallermicro-cellthatiscompletelycontainedwithinthemacro-cell

n D2/R2 islargerthanD1/R1

n Split-bandanalogsystems

n Reusepartitioningn UsedinLTE(Revisit)

R1

DL,(1)

DL,(2)

R2

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+Cell Sectoring

n Usedirectionalantennastoreduceinterference

n Radiopropagationisfocusedincertaindirectionsn Antennacoverageisrestrictedtopartofacellcalledasector

n Byreducinginterference,theclustersizecanbereduced(Js isreduced,andsowecanreduceNc)

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+Three-sector cells and a cluster size of Nc= 4

• 120o directionalantennasareemployed• Channelsallocatedtoacellarefurtherdividedintothreeparts• Withoutdirectionalantennas,Sr =13.8dBwhichisinadequate• Withdirectionalantennas,Sr =18.5dB

Sr ≈R−4

JsD−4 =

R−4

2D−4 =12DR

#

$%

&

'(4

=92Nc2

Cell Sector UnderConsideration

Interfering Sectors

Non-Interfering Sectors

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+ Sectored Frequency Planningn Example:AllocatefrequenciesforanAMPSoperatorincellularB-blockwhousesa7cellfrequencyreusepatternwith3sectorspercell

n UseaFrequencyChart– availablefromFCCwebsiten Groupsfrequenciesinto21categoriesCells1-7andsectorsA-B-Cineachcell

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+ Sectored Frequency Planning

n Example:AllocatefrequenciesforaGSMoperatorinU.S.PCSB-blockwhousesa7cellfrequencyreusepatternwith3sectorspercell

n UseaFrequencyChart– availablefromFCCwebsite

n Groupsfrequenciesinto21categoriesCellsA-Gandsectors1-3ineachcell

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+Summary: Cell sectoring

n Theclustersizecanbereducedbyemployingdirectionalantennas

n Thecapacityincreaseis1.67timesforN=4and2.3timesforN=3comparedtoN=7

n Sectoringisbetterthansplittingn Nonewbasestationhastobesetupn Nonewplanningeffortsareneededtomaintaininterferencelevels

n Sectoringleadstohandoffbetweensectorswhichincreasessignalingloadandsomelossofcallquality

n Acellcannotbeideallysectoredandthesignaltointerferencevaluesobtainedhereareoptimistic

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+Channel Allocation Techniques

n Idea:n Duringthedayonweekdays,downtownareashavealotof

demandforwirelesschannelsn Inweekendsandevenings,suburbanareashavealarger

demandanddowntownareashaveverylittledemandn Insteadofallocatingchannelsstaticallytocells,allocate

channels ondemand whilemaintainingsignal-to-interferenceratiorequirements

n The(voice)userdoesnotcarehowthechannelsareallocatedaslongas

n He/shegetsaccesstothechannelwheneverrequiredn Thequalityofthesignalisacceptable

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+Channel Allocation Techniques (2)

nFixedchannelallocation(FCA)nChannelborrowing

nDynamicchannelallocation(DCA)nCentralizedDCAnDistributedDCAnCell-basednMeasurement-based

nHybridchannelallocation(HCA)

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+Channel borrowing

n Idea:Borrowchannelsfromlowloadedcellsandreturnthemwheneverrequiredn Temporarychannelborrowingn Returnchannelaftercalliscompleted

n Lockschannelinco-channelcells

n Staticchannelborrowingn Distributechannelsnon-uniformlybutchangetheminapredictableway

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Affected Cells(Locked Channels)

Borrow Channels

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+Dynamic Channel Allocation

n Allchannelsareplacedinapooln Whenanewcallcomesin,achannelisselectedbasedontheoverallSIRinthecell

n Selectionofthechannelinthiswayiscostlyn NeedsasearchandcomputationofSIRvalues

n Centralizedn Acentralentityselectschannelsforuseandreturnsittothepoolaftercompletionofcalls

n Distributedn Basestationslocallycomputethechannelsthatcanbeused

n Cell-based– BSs communicatewitheachotheronthewiredbackbonetodeterminethebestwaytoselectchannels

n Measurement-based– BSs measureRSSorreceiveRSSreportsfromMSsthattheyuseintheirdecisions

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+Comparison of FCA and DCA

Attribute Fixed Channel Allocation Dynamic Channel AllocationTraffic Load Better under heavy traffic load Better under light/moderate traffic load

Flexibility in channel allocation Low HighReusability of channels Maximum possible LimitedTemporal and spatial changes Very sensitive InsensitiveGrade of service Fluctuating StableForced Call Termination Large probability Low/moderate probabilitySuitability of cell size Macro-cellular Micro-cellularRadio equipment Covers only the channels

allocated to the cellHas to cover all possible channels that could be assigned to the cell

Computational effort Low HighCall set up delay Low Moderate/HighImplementation complexity Low Moderate/HighFrequency planning Laborious and complex NoneSignaling load Low Moderate/HighControl Centralized Centralized, decentralized or distributed

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+Interference Management in LTE-OFDMAn BorrowsideasfromReusePartitioningandDynamicChannelAllocation

n Aimsforafrequencyreuseof1n Sub-carriersand“resourceblocks”(RBs)maynotbeuniversallyreused

n BasestationstalkwitheachothertomanageinterferenceandalsoschedulingRBs tousers

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+Strict and Soft Fractional Frequency Reuse in LTE

(a) Strict FFR with reuse of 3+1

Pow

er

frequency

(b) Soft FFR with reuse of 3

f1 f2 f3 f4

f2

+f3

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+f3

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+f4Po

wer

frequencyf2 f3 f4

Power distributionin this cell

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+Femtocells

n InitialIdean CoveragechallengedareaswithgoodInternetconnectivity

n ProgressiveBenefitsn Highspectrumefficiencies

n Typicallyindoor!n Highdataratesarepossible

n Reducingsubscriberchurnn ReducingCAPEXandOPEXcostsforserviceproviders

n Backhaulcapacityandcapitalexpendituresarereduced

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+Issues with Femtocell deployment

n Femtocell basestationcannottransmitathighpowernoratlowpowern Shouldnotswampusersthatdonotbelongtofemtocelln Shouldnotdenycoveragetosomeonewhoinstallsthefemtocell

n Femtocell basestationreceptionhastobedynamicn Amobilethatisnearshouldnotswampitbecauseofitsminimumtransmitpower

n Amobilefarawayshouldnotbeforcedtotransmitathighpowertoreachthefemtocelln Thismayinterferewithtransmissionsinamacrocell

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+ Design Issues in Local Area Wireless Data

n IEEE802.11n Initialdeploymentswerebasedonthe915MHzbands

n Therewasonlyonechanneln Inthe2.4GHzbands

n Therearethreenon-overlappingchannelsà frequencyreuseispossible

n Thresholds!n Inthe5GHzbands,thereareelevennon-overlappingchannels

n Threedimensionalplanningisrequiredn Antennapatternsandbuildingarchitecture

n TherearethreelevelsoftransmitpowerattheAPn NotclearwhatcanbedoneattheMS

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+ Overlapping channels in the 802.11 specifications

1 23 456 7 8910 11

2.412 2.462

5MHz

Usethreenon-overlappingchannels

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+Using overlapping channels

n ItispossibletouseChannels1,4,7and11insteadof1,6and11n Thereisadropinthroughputn Therearesomeresultsofactualperformancebuttheyareinconclusive

n Itisnotclearwhetherthedropinthroughputisduetobackoff orpacketloss

1 234 567 8910 11

2.412 2.462

5MHz

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+SIRs in 802.11 WLANs (@2Mbps)

nReportsofmeasurementsandmodelsof802.11RSSandthroughputsare vendorspecificn OnereportsaysthataminimumSIRof15dBisrequiredforgoodthroughput

n UsedUDPstreamsandestimatedtheSIRusingapathlossmodel

n Throughputfallsfrom1.8Mbpsto1MbpsastheSIRreducesfrom15dBto10dB

n Reuseissuesarethensimulatedn Unlikevoice,dataisbursty – so thedesignanddeploymentissuesaredifferentn MostrealdeploymentsdesignthenetworkforcoverageratherthanspecificQoS goals

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+WLAN Deployment Methods

nRandomdeploymentbyusers

nArrangeaccesspointsinagrid

nOptimallyplaceaccesspointsforcoverage/interference

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+Coexistence?

nInterferencenTwowirelesstechnologiesinterfereifco-locationcausessignificantperformancedegradation

nCoexistencenTwowirelesstechnologiescoexistifthereisnosignificantimpactontheperformance

nInteroperablenDevicesbelongingtotwodifferentwirelesstechnologiesareinteroperableiftheycancommunicateandexchangedatabetweenthem

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+ Coexistence between HomeRF and IEEE 802.11

nHomeRFusesveryslowfrequencyhoppingn50hops/s– frameis20mslongnComparewithBluetooth– 1600hops/sand625µsnAlsooperatesinthe2.4GHzbands

nExperimentsonstudyingtheimpactofHomeRFon802.11throughputnHomeRFisverydetrimentalto802.11throughputnHomeRFisan“interference”

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+Bluetooth and 802.11 (1)

n ImpactofBTon802.11n AtlargeRSS,thethroughputisfairlygood

n AstheRSSfalls,thethroughputfallsdrastically

n BTcausessubstantialinterference,butthereissomekindofcapturewhentheRSSisgood

Source:J.Lansfordetal.,IEEENetwork,September2001

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+Bluetooth and IEEE 802.11 (2)

n ImpactofIEEE802.11onBTn 802.11signalislikeawidebandjammer

n AstheRSSfromtheAPfalls,thethroughputimproves

n AstheRSSfromtheAPincreases,voicepacketsaredroppedrandomly

n Thetransitionoccurssuddenly

n ShortACKs arelesslikelytocauseerrorsthanlongframes

Source:J.Lansfordetal.,IEEENetwork,September2001

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+ Communication Issues and Radio Propagation

Fading Channels

Large Scale

Fading

Path-Loss &

Shadowing

Impacts Coverage

Small Scale

Fading

Time Variation

Time Dispersion

Angular Dispersion

Impactssignaldesign,receiverdesign,coding,BER

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+Before we get into small-scale fading…nWhatisthebestwecandowhenthereisNO fading?

nWhatarethetradeoffsbetweenbiterrors,power,noise,andbandwidth?

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+Digital Modulation (Revisited)

n Changingtheparametersofasinusoidiscalled“shiftkeying”ifinformationisdigital

n Typesn Amplitude-shiftkeying(ASK)

n Amplitudedifferenceofcarriern Frequency-shiftkeying(FSK)

n Frequencydifferencenearcarrierfrequencyn Phase-shiftkeying(PSK)

n Phaseofcarriersignalshiftedn Quadrature amplitudemodulation(QAM)

n Bothamplitudeandphaseofthecarriercarrydata

n Bits/Symboln Binary(onebitinonesymbol=>twosymbols)n M-ary (log2M bitsinonesymbol=>M symbols)

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+Binary Amplitude-Shift Keying

n Idean Onebinarydigitrepresentedbythepresenceofthecarrier,atconstantamplitude

n Theotherbinarydigitisrepresentedbytheabsenceofthecarrier

n Remarks:n ThecarriersignalisA cos(2πfct)n ThesymboldurationisTseconds

n AlsocalledOn-OffkeyingorOOK

AveragePowerinSignal=

A2/4

AverageEnergyperbitEb =A2T/4

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+Amplitude-Shift Keying

n Susceptibletosuddengainchanges

n Inefficientmodulationtechnique(whatdowemeanbythis?)

n Usedonvoice-gradelinesupto1200bps

n UsedtotransmitdigitaldataoveropticalfiberandinIRsystems

ON OFF ON

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+Binary Frequency-Shift Keying (BFSK)nTwobinarydigitsrepresentedbytwodifferentfrequenciesnearthecarrierfrequency

nf1 andf2 areoffsetfromcarrierfrequencyfcbyequalbutoppositeamounts

AveragePowerinSignal=A2/2

Eb =A2T/2

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+Frequency-Shift Keying (FSK)

n LesssusceptibletoerrorthanASK

n Onvoice-gradelines,usedupto1200bps

n Usedforhigh-frequency(3to30MHz)radiotransmission

n CanbeusedathigherfrequenciesonLANsthatusecoaxialcable

0 01

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+Binary Phase-Shift Keying (PSK)

nUsestwophasestorepresentbinarydigits

nOR

nWerevisitBPSKlaterAveragePowerinSignal=A2/2

Eb =A2T/2

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+M-ary Modulation Schemes

nM-ary=>Msymbolsn Thesymbolsarea1,a2,…,aM

nEachsymbolcarriesk =log2M bitsn Example:M=4=>thesymbolsarea1,a2,a3,a4

n Leta1 =00,a2 =01,a3 =10,anda4 =11nWehavek =2bits/symbol

nThesymbolscanonceagainberepresentedbytheamplitude,phase,orfrequencyofthecarrier

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+Example: 4-ASK

nIn4-ASK,weneed4differentamplitudesofthecarriertorepresent4symbols

nLettheamplitudesbe0,1,2and3

nThesymbolswillbe:

AveragePowerinSignal=7A2/4

Eb =7A2T/8

s(t) =

8>>><

>>>:

↵0 ! 00 : 0, 0 t T

↵1 ! 01 : cos(2⇡fct), 0 t T

↵2 ! 11 : 2 cos(2⇡fct), 0 t T

↵3 ! 10 : 3 cos(2⇡fct), 0 t T

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+More on M-ary modulation

nM-ASK(alsocalledPAM)isnotcommonnMorecommonarenMPSK– ThereareM phasesofthecarriertorepresenttheM symbols

nMFSK– ThereareM frequenciesaroundfc torepresenttheM symbols

nQuadrature amplitudemodulation(QAM)nUsesacombinationofamplitudeandphasenM-QAM

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+Advanced Modulation Schemes

nVariationsonASK,FSKandPSKpossible

nAttempttoimproveperformancen IncreasedataforafixedbandwidthnRemoverequirementfor phasesynchronizationnDifferentialmodulationanddetection

n ImproveBERperformance

nMainschemesforwirelesssystemsarebasedonFSKandPSKbecausetheyaremorerobusttonoise

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+Orthogonal signaling with codes

Whatisorthogonality?

Showintimeandfrequency

Showinspace

Bandwidth

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+ Modulation schemes used in wireless networks

n GMSKn GSM,CDPD,Mobitex,GPRS,HIPERLAN/1

n p/4– DQPSKn Tetra,IS-136

nOQPSKn IS-95,cdma2000

n FSKn ARDIS,802.11FHSS,Bluetooth

n BPSK,QPSK,16-QAM,64-QAMn HIPERLAN/2,IEEE802.11a(withOFDM),LTE

n BPSK,QPSKn IS-95,IEEE802.11(withDSSS)

n PulsePositionModulationn IEEE802.11IR

n OrthogonalModulationn IS-95,cdma2000

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+Communication Issues

n Noise(unwantedinterferingsignals)isnotnecessarilyadditive,whiteorGaussiann Examples:Inter-symbolinterference(ISI),Adjacentchannelinterference(ACI),Co-channelinterference(CCI)

n InCDMAinterferencefromusersetc.

n NoiseaffectstheBitErrorRate(BER)n Fractionofbitsthatareinvertedatthereceiver

n Also,theradiochannelhasmultiplicativecomponentsthatdegradetheperformancen ThebehavioroftheradiochannelcanincreaseISI,reducethesignalstrength,andincreasethebiterrorrate

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+Performance in General

n Whatdetermineshowsuccessfulareceiverwillbeininterpretinganincomingsignal?n Signal-to-noiseratio=>powern Dataraten Bandwidth

n Typicaltrendsn Anincreaseindatarateincreasesbiterrorraten AnincreaseinSNRdecreasesbiterrorraten Anincreaseinbandwidthallowsanincreaseindatarate

n Inmobilewirelesssystemsbothbandwidthandpowerareinshortsupply

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+Wireless Performance Considerations

n Inwirelesscommunications,theprimaryissuesaren Spectrumn Powern Effectsoftheradiochannel

n Whenwelookatmodulationschemes,weareinterestedinthefollowingn PerformanceinAWGNchannels

n Providesabaselineperformancen Performanceinmultipathfadingchannels

n Expectedperformanceinrealisticchannelsn Bandwidthefficiencyn Costandcomplexity

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+Performance in AWGN Channels

n SupposethecommunicationschannelisonlyaffectedbyAWGN(thermalnoise)n Thisisthemostidealconditionsyoumaygetn Similartoawire

n Providesabenchmarkorbaselineperformancen Cangetsomeinsightintowhetheronemodulationschemeisbetterthananother

n Ideallywewantn Verylowbiterrorratesatsmallsignal-to-noiseration Ensureswecanconservebatterypowerbytransmittingatlowpowers

n Yettheinformationcanberecoveredreliably

BitErrorRateorBERisafunctionof

Eb

N0= �b

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+Performance in AWGN channels (2)

n AWGN=AdditiveWhiteGaussianNoise

n Thishasa“flat”noisespectrumwithaveragepowerspectraldensityofN0

n Theprobabilityofbiterror(biterrorrate)ismeasuredasafunctionofratioofthe“energyperbit”– Eb totheaveragenoisePSDvalue

n BERorPe variationwithEb/N0

n Eb/N0isameasureofthe“powerrequirements”

n Tradeoffs!

BinaryModulationSchemes

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+Signal Constellation (1)

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+Signal Constellation (2)

n Givenanymodulationscheme,itispossibletoobtainitssignalconstellation.n RepresenteachpossiblesignalasavectorinaEuclideanspace.

n Insymboldetection– decodeincomingsignalasclosestsymbolinthesignalconstellationspace

n Ifweknowthesignalconstellation,wecanestimatetheperformanceintermsoftheprobabilityofsymbolerrorgiventhenoiseparameters

n Probabilityoferrordependsontheminimumdistancebetweentheconstellationpoints

8-PSK

16-QAM

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+Probability of Error

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+Performance in AWGN(2)

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+M-ary modulation schemes

n Bitspersymbol=log2M

n Phaseshiftkeyingn Signalpointsareonacirclen Morebits/sec/HzbutlargerPeforgiven Eb/N0

n Orthogonalkeyingn M-dimensionalconstellation

n FSKn Pulsepositionmodulationn Orthogonalkeying/signaling

n Lessbits/sec/HzbutmuchsmallerPe forgiven Eb/N0

n QAMn WorksmostlylikePSK

increasingM

phaseshiftkeyingorQAM

increasingM

orthogonalkeying

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+Bandwidth Efficiency and Complexity

n Bandwidthefficiency(relatedtospectralefficiency)

n Foragivenbiterrorratewhatistherequiredbandwidthforaspecifieddatarate?

n Recalldiscussionofcapacity

n Example– AtaBERof10-5,BPSKrequires2MHzforadatarateof2Mbps

n Ideallyourgoalistostuffasmanybitsaspossibleinagivenbandwidth

n Bandwidth(spectrum)efficiencyismeasuredintermsofthedataratesupportedoveragivenbandwidth

n Units:bits/sec/Hz.

n Cost/Complexity

n Inachievinggoodperformanceandbandwidthefficiency,themodulationschemeshouldnotbetooexpensiveorcomplextoimplement

n Circuitryshouldbesimpletoimplementandinexpensive(e.g.detection,amplifiers)

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+Bandwidth of modulation schemes

GMSK

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+Tradeoffs between BER, power and bandwidthn (1)TradeBERperformanceforpower– fixeddatarate

n (2)Tradedatarateforpower–fixedBER

n (3)TradeBERfordatarate– fixedpower

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+Next Week

nSmallScaleFading

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