Lecture 6Summary: Cell sectoring n The cluster size can be reduced by employing directional antennas...
Transcript of Lecture 6Summary: Cell sectoring n The cluster size can be reduced by employing directional antennas...
+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
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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
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frequency
(b) Soft FFR with reuse of 3
f1 f2 f3 f4
f2
+f3
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+f3
f2
+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 (2)
n Givenanymodulationscheme,itispossibletoobtainitssignalconstellation.n RepresenteachpossiblesignalasavectorinaEuclideanspace.
n Insymboldetection– decodeincomingsignalasclosestsymbolinthesignalconstellationspace
n Ifweknowthesignalconstellation,wecanestimatetheperformanceintermsoftheprobabilityofsymbolerrorgiventhenoiseparameters
n Probabilityoferrordependsontheminimumdistancebetweentheconstellationpoints
8-PSK
16-QAM
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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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+Tradeoffs between BER, power and bandwidthn (1)TradeBERperformanceforpower– fixeddatarate
n (2)Tradedatarateforpower–fixedBER
n (3)TradeBERfordatarate– fixedpower
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