Lecture 2
OverviewofWirelessSystems(1)
1
+Wireless Systems are Different!
nPhysicallayerisnotreally“physical”nHigherrorratescomparedtocopper/fibre
nEnddevicesarenotfixednMobilitymakesprotocolsmorecomplicated
nPowerisabigissuenDevicesrunonbatterypower,havesmallsize,andlimitedcapabilities
2
+PHY Layer Issues
n Theradiochannelisharshn Cablesandwireshave“predictable”andtime-invarianttransmissioncharacteristics
n Theradiochannelisdynamicandharsh(Higherrorratesneedmitigation)
n Examplesofproblemsn Fadingn Multipathdispersionn Signalattenuationduetorainorsnown Interference(again!)
n Coverage
n Effectonprotocols
3 +Spectrum Regulation (1)
n SpectrumRegulationn Themediumoftransmissionisairn ThemediumcannotbeeasilyduplicatedanditmustbesharedbyALLapplicationsn Communications,broadcast,emergencyservices,television,military,etc.
n Sharingisachievedbyallocatingseparate“bands”ofspectrumtousersofdifferentapplications
n Broadcastradio:520-1605.5kHz– AMRadion Broadcastradio:87.5– 108MHz– FMRadio
n Abandofspectrumreferstoarangeofelectromagneticfrequencies
n TheFCCregulatesthespectrumallocatedtoapplications
4
Showspectrumallocationchart
+Spectrum Regulation (2)
n ThereisLIMITEDspectrumfordifferentapplications
n Thefrequencybandsarenot“contained”asinthecaseofwiredtransmissionsn Thereissomeinterferencebetweensignalstransmittedinonefrequencybandandanother
n Samethingistrueifyouchoosetosplitthebandforanapplication(thinkAM)
n Capacityislimitedandweneednovelmethodstoimprovecapacity
n SUMMARYn Spectrumandhencebandwidthislimitedn Radiotransmissionscancauseinterference
5 +MAC layer Issues + Network Design & DeploymentnMAClayerissues
n Shared“broadcast”mediumn Needforasimplemediumaccessmechanism
n Performancen Throughput,delayandQoS
nNetworkdesignanddeploymentn Nosingletypeofwirelessaccessisavailableeverywheren Spectrumisscarce
n Coexistence,interference,planningn Frequencyreuseandcellulartopology
6
+Radio Resource Management & Power Managementn Resourcelimitations
n Radioresources– link“quality”variessubstantiallyovertimeandspacen Power:
n Amobiledevicereliesonbatterypowern Transmissionsconsumeenergy!
n ThetransmissionschemeMUSTbeefficientintermsofenergyconsumption
n Computation:Thedevicecannotperformcomplexoperationslikeawireddevice
n Radioresourceandpowermanagementn Assignmentofradiochannelsandtransmitpowern Admissioncontrol,powercontrolandhandoffdecision
7 +Mobility Management
n Wirelessdevicesarepopularbecausetheydonotneedtobetetheredtoaplacelikewireddevices
n Wirelessdevicesarecontinuouslychanginglocationsn Theconnectivitychangesn Devicesmaymoveoutof
coverageofaservicen Someoneshouldkeeptrackof
wherethedeviceistodeliverinformationtoit
n Someoneshouldmakesurethattheconnectionisnotbrokenasawirelessdevicemoves
n Inwiredcommunicationsthe“address”ofthedeviceidentifiesitslocation– thisisnolongertruewithwirelessdevices
n Amovingdevicewill“see”aharsherchannel!
n Mobilitymanagementn Locationmanagement
n TrackingwhereaMSisn Handoffmanagement
n Routingcalls/packetsasaMSmoves
n Routinginadhocnetworksn Databaseissues
8
+Operations and Security
n ManagementandSecurityn Mobileendhostisnolonger
confinedtothehomenetworkn Wirelesslinkscanbeeasily
“tapped”n Fraud
n Accountingandbillingn Conflictswithotherissues
n Networkoperationsandmanagementn Accountingandbillingtocharge
subscriberscorrectlyn Accesstoresourcesandservices
onthenetwork
n Servicediscoveryanddatamanagement
n SensorsandRF-IDs
n Howisdatamaintained?
n Whereshoulddatareside?
n Howcanitbeefficientlyaccessed?
9 +Summary – I
nSpectrumisscarcenWeneedtosqueezeasmanydatabitsaspossibleinagivenbandwidth
n Themoredatabitsyousqueezeinthemorestringentarethesystemrequirementsn Example:Squeezinginmoredata=>largersignaltonoiseratiorequirement=>largertransmitpower=>lowerbatterylife
n Example:multipathdispersionisnotaproblematlowdatarates
n Example:complexprocessingcanresultinlargeformfactor
10(ClassFocus)
+Summary - II
nPhysicallayermakeswirelesscommunicationsunreliableanderroneousnContributesgreatlytothecomplexityofthesystem
n Impactsallotheraspectsofawirelesssystem
nFundamentallydifferentfromwirednetworksnResourceissues
nMobilityissuesnNetworkdesignissues
11(ClassFocus)
CircuitSwitched
MostlyCircuitSwitched1980s 1990s 2000s 2010s
1G Analog FM/FDMA
2G Digital TDMA/CDMA
3G Cellular CDMA
4G LTE-Advanced OFDMA/MIMO
Independent Mobile Data
Cellular Overlay CDPD
Wid
e Ar
ea N
etwo
rks
Loca
l and
Per
sona
l Are
a Ne
twor
ks
Analog Cordless Phone FM/FDMA
Digital Cordless TDMA
DSS/FHSSDECT - TDMA
IEEE 802.11DSSS/FHSS
IEEE 802.11bCCK/DSSS
IEEE 802.15.1Bluetooth/FHSS
IEEE 802.11a/gOFDM
IEEE 802.11nOFDM/MIMO
IEEE 802.15.3UWB/OFDM/DSSS
IEEE 802.15.4Zigbee/DSSS
60 GHz GigabitUWB
IEEE 802.15.6BAN
WLAN
WPAN
PSTN Access
Internet Access
4G LTEOFDMA/MIMO
4G WiMaxOFDMA/MIMO
12
+Circuit switched voice call (simplified)
……CentralOffice
Alice
CentralOffice
TandemOffice
IXC– A’sPOP
IXC– B’sPOP
IXC-A’sNetwork
IXC-B’sNetwork
TandemOffice
IXC– A’sPOP
IXC– B’sPOP
CentralOffice
POP=PointofPresence
Circuit
13
Makethiswireless&mobile
+First Generation Cellular Systems
n Goal:Providebasicvoiceservicetomobileusersoverlargearea
n 1GSystemsdevelopedlate70’searly80’s,deployedin80’s
n AdvancedMobilePhoneSystem(AMPS)- USA
n TotalAccessCommunicationsSystems(TACS)- UKn NordicMobileTelephone(NMT)System– ScandinavianPTTs
n C450- W.Germanyn NTTSystem- NipponTelephone&Telegraph(NTT)– Japan
n Incompatiblesystemsusingdifferentfrequencies!n Havesimilarcharacteristicsthough
14
OR
+1G Cellular (voice) Standards
Standard Downlink(MHz)
Uplink(MHz)
ChannelSpacing
MultipleAccess
Region Comments
AMPS 869-894 824-849
30kHz FDMA USA FM&FSKModulation
TACS 935-960 890-915 25kHz FDMA EC Later,bandsallocatedtoGSM
ETACS 917-950 872-905 25kHz FDMA UK
NMT-450 463-467.5 453-457.5 25kHz FDMA EC
NMT-900 935-960 890-915 12.5kHz FDMA EC
JTACS 860-870&others
915-925+ 25/12.5kHz
FDMA Japan
15
ShowRFSpectrumAllocationChart
+Duplexing Modes
n Simplex– onewaycommunication(e.g.,broadcastAM)
n Duplex– twowaycommunicationn TDD– timedivisionduplex
n Userstaketurnsonthefrequencychanneln FDD– frequencydivisionduplex
n Usersgettwochannels– oneforeachdirectionofcommunicationn Forexampleonefrequencychannelforuplink(mobiletobasestation)anotherfrequencychannelfordownlink(basestationtomobile)
n Half-duplexn Asin802.11,adevicecannotsimultaneouslybetransmittingandreceiving
16
+Uplink and Downlink
nThelinkfromthemobiletothetoweriscalledthe“uplink”nSometimescalledthe“reverse”channel
nThelinkfromthetowertothemobileiscalledthe“downlink”nSometimescalledthe“forward”channel
17
Downlink or Forward Channel
Uplink or Reverse Channel
+Example: AMPS
n Voicechannelsoccupy30kHzandusefrequencymodulation(FM)
n 25MHzisallocatedtotheuplinkand25MHzforthedownlink
n 12.5MHzisallocatedtonon-traditionaltelephoneserviceproviders(BlockA)
n 12.5MHz/30kHz=416channels
n 395arededicatedforvoiceand21forcontrol
18
824 MHz
849 MHz
869 MHz
894 MHz
45 MHz
Uplink Band Downlink Band
Block A Block B
21 Control & 395 Voice Channels
30 kHz
+What does “Channel Spacing” mean?nAfrequencycarrier(sinusoid)withmodulationhasacertain“bandwidth”n Thisbandwidthvariesacrosstechnologiesandsystemsn Logically,youcanthinkofitasasliceorchunkofspectrumoccupiedbyasignal
n Physically,itismorecomplicated
nMultiplecarriersareusedinallwirelesssystemsn The“separation”betweenthefrequenciesofthecarriers=“channelspacing
n Thisisalsoroughly equaltothebandwidth ofthesignal
19 +Modulation and demodulation: Pictorial View
20
SynchronizationDecision
AnalogordigitaldataAnalog
Demodulation
RadioCarrier
101101001 RadioReceiver
SignalGeneration
AnalogorDigitalData Analog
Modulation
RadioCarrier
BasebandSignal
101101001 RadioTransmitter
BasebandSignal
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2x 104
−0.4
−0.2
0
0.2
0.4
0.6
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2x 104
−0.4
−0.2
0
0.2
0.4
0.6
+What is Modulation?
n BasicIdean Convertdigitaloranaloginformationtoawaveformn Waveformissuitablefortransmissionoveragivenmedium
n Processn Involvesvaryingsomeparameterofacarrierwaveasafunctionofthe“information”
n Carrier=sinusoidalwaveformatagivenfrequencyn Aftermodulation,signalhasa“bandwidth”
n Informationn Usuallycontainedinamessagesignalatbaseband thatiseitheranalogordigital
n Generalsinusoid
Amplitude FrequencyPhase
Acos(2pfCt +j)
21 +Terminology - Sinusoid
n Period(T)- amountoftimeittakesforonerepetitionofthesignaln T =1/f
n Phase(f)- measureoftherelativepositionintimewithinasingleperiodofthesignal
nWavelength(l)- physicaldistanceoccupiedbyasinglecycleofthesignaln Or,thedistancebetweentwopointsofcorrespondingphaseoftwoconsecutivecycles
n Forelectromagneticwavesinairorfreespace,l =cT =c/f wherec isthespeedoflight
22
+The sinusoid – A cos(2pft +f)
-1 0 1 2 3 4
-2
-1
0
1
2
-1 0 1 2 3 4
-2
-1
0
1
2
-1 0 1 2 3 4-2
-1
0
1
2
-1 0 1 2 3 4-2
-1
0
1
2
cos(2⇡t)
2 cos(2⇡t)
cos(2⇡ ⇥ 2⇥ t)
cos(2⇡t+ ⇡/4)
time
Amplitude
Morecycles
Shiftfromorigin
23 +The sinusoid continued
nGeneralsinusoidwaven s(t)=A cos(2pft +f)
nPreviousslideshowstheeffectofvaryingeachofthethreeparametersn A =1,f =1Hz,f =0=>T =1sn Increasedpeakamplitude;A=2n Increasedfrequency;f =2=>T =1/2n Phaseshift;f =p/4radians(45°)
nNote:2p radians=360° =1period
24
+Why Modulate?
n Supportmediumcharacteristics
n Antennasizen Usuallythesizeofanantennaisproportionaltothewavelengthn Typicalantennasize=l/4
n Smallerwavelengths=>smallerantennas
n Example:n 3000Hzbasebandsignaln l =3´ 108/3´ 103 =105 mn Antennais100km/4=25km=15.6mileslong
n Example:n 900MHzRFsignaln l =3´ 108/9´ 108 =1/3m=33.3
cmn Antennais33.3/4=8.3cmlong
n Interferencerejectionn Somemodulationschemesare
morerobustthanothersinthepresenceofinterference
n CompareAMandFM
n Multiplexingn Signalscanbemovedtoa
differentpartofthespectrum
n Simplifycircuitry
25 +Analog Modulation
nMessagesignal(baseband)isanalog:wehavethreetypesofmodulationnAmplitudemodulation(AM)nVarytheamplitudewiththeanalogbasebandsignal
nFrequencymodulation(FM)nVarythefrequencywiththeanalogbasebandsignal
nPhasemodulation(PM)nVarythephasewiththeanalogbasebandsignal
26
+FM in Wireless Systems
n Frequencymodulationwasemployedin1Ganalogcellulartelephonesystemsn AMPS(30kHzchannels)n JTACS(25kHzchannels)n NMT
n Onechannelcarriedonevoicecalln Uplinkordownlink
n ReasonforselectingFMn MorerobusttointerferenceandfadingcomparedtoAMn Captureeffectwasusefulandformsthebasisfortheoriginalfrequencyreusedesign
27 +Centralized Multiple Access Techniquesn FDMA(frequencydivisionmultipleaccess)
n Separatespectrumintonon-overlappingfrequencybands
n Assignacertainfrequencytoatransmissionchannelbetweenasenderandareceiver
n Differentusersshareuseofthemediumbytransmittingonnon-overlappingfrequencybandsatthesametime
n TDMA(timedivisionmultipleaccess):n Assignafixedfrequencytoatransmissionchannelbetweenasenderandareceiverfora
certainamountoftime(usersshareafrequencychannelintimeslices)
n CDMA(codedivisionmultipleaccess):n Assignauserauniquecodefortransmissionbetweensenderandreceiver,userstransmiton
thesamefrequencyatthesametime
n OFDMA(orthogonalfrequencydivisionmultipleaccess):n AdvancedformofFDMA/TDMA
28
+Multiple Access
29
time
freq
uency
user1
user2
user3
guardband
guardband
FDMA
time
freq
uency
user1
guardtim
e
TDMA
time
freq
uency
3
2
CDMA
user2
user3
guardtim
e
1
31,2,3
Wirelesssystemsoftenuseacombinationofschemes;GSM– FDD/FDMA/TDMA
+
MS 1MS 2MS 3MS 4MS 5MS 6MS 7MS 8
MS 1
MS 2MS 3MS 7
time
frequ
ency
subcarrier
Slot
MS 8
MS 6
MS 2
MS 3
OFDMA
nFlexibleresourceallocationintimeandfrequency
nUnitofallocation:“PhysicalResourceBlocks”
nLTEDownlink,WiMax uplinkanddownlink
30
+FDMA
n FDMA– simplestandoldestmethod
n BandofwidthF isdividedinto Tnon-overlappingfrequencychannelsn Guardbandsminimizeinterferencebetweenchannelsn Eachstationisassignedadifferentfrequency
n CanbeinefficientifmorethanT stationswanttotransmitortrafficisburstyn Resultsinunusedbandwidthanddelays
n Receiverrequireshighqualityfiltersforadjacentchannelrejection
n UsedinFirstGenerationCellular(AMPS,NMT,TACS)
31 +
f(c)=825,000+30×(channelnumber) kHz<- uplinkf(c)=f uplink+45,000kHz<- downlink
IngeneralallsystemsusesomeformofFDMA
FDD/FDMA - general scheme, example AMPS (B block)
32
f
t
799
355
799
355
20MHz
30kHz
835.65MHz
880.65MHz
849.97MHz
893.97MHz
+2G Cellular Systems (voice)
n Motivationfor2GDigitalCellularn IncreaseSystemCapacityn Addadditionalservices/features
n SMS,callerID,etc.n ReduceCostn ImproveSecurityn Interoperabilityamongcomponents/systems
n GSMonly
n Main2GSystemsn NorthAmericanTDMA(NA-TDMA)(IS-136)n GlobalSystemforMobile(GSM)n IS-95(cellularCDMA)
33 +GSM: Global System of Mobile Communicationsn AheterogeneousanalogcellularimplementationwasobservedinEuropeinthe1980sn UnitedKingdom,Italy,Spain,Austria:TACS(900MHz)n Scandinavia,Germany,TheNetherlands,Spain:NMT(450MHz,900MHz)
n France:Radiocom
n 1987:12MembercountriessignMOUforacommonstandard
n ETSI:EuropeanTelecommunicationsStandardsInstitutein1989tookoverthestandardizationofallcellulartelephonyinEuropen StronglyinfluencedbyISDNn SignalingSystem7
34
+2G Standards
System/Parameter IS-54/IS-136 JDC GSM IS-95
Region USA Japan Europe,Asiaetc. USA/Asia
Multipleaccess/duplex
TDMA/FDMA/FDD TDMA/FDMA/FDD TDMA/FDMA/FDD CDMA/FDMA/FDD
Channels/carrier 3 3 8 Variable
Channelspacing 30kHz 25kHz 200kHz 1.25MHz
Uplink 824-849MHz 810-826MHz 890-915MHz 824-849MHz
Downlink 869-894MHz 940-956MHz 935-960MHz 869-894MHz
Channelbitrate 48.6kbps 42kbps 270.83kbps 1.288 Mchips/s
ModulationScheme p/4- DQPSK p/4- DQPSK GMSK QPSK/OQPSK
Spectralefficiency 1.62bps/Hz 1.68bps/Hz 1.35bps/Hz Variable
Frameduration 40ms 20ms 4.615ms 20ms
Peakpower 0.6W 1W
DataService 9.6kbps 9.6-14.4kbps 9.6-14.4kbps 9.6-115kbps
35 +Digital Modulation (1)
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)
36
+Digital Modulation (2)
nBinaryn 2symbols– 0and1n 1bitpersymbol
nM-arynMsymbols-- a1,a2,a3,a4,…,aM
nWehavek =log2M bits/symboln UsuallyM =2k
nUseamplitude,frequencyorphasetocarrythediscreteinformation
37
RecapNRZ
‘0’->positivepulse‘1’->negativepulse
+Line Codes
n NonReturn-to-Zero(NRZ)n Eachdigitalvalueisrepresentedbyavoltagepulsen Thevoltagevalueisconstantfortheentiresymbolduration
n Return-to-Zero(RZ)n Voltagepulseshavedutycycles
n Theydonotlastfortheentiresymboldurationn Ifthedutycycleis100%,itbecomesNRZ
n Unipolarn Signalvaluesare0Vorsomepositivevoltage
n Antipodal(orbipolar)n Bothpositiveandnegativevoltagevalues(usuallyidenticalinmagnitude)exist
38
+Comparison of Four Combinations
n Combinationsn UnipolarRZ,NRZandBipolarRZ,NRZn AssumeRZhas50%dutycycle
n SignalPowern Peakandaverage
n Bandwidthn RZneedsmorebandwidth(why?)
n Samplingthresholdfordiscriminationn Antipodalhasazerothresholdforbinary
n Transitionsn RZhasmoretransitions,butnodifference
betweenunipolarandantipodal
Bandwidthofarectangularpulse
T 2T 3T 4T 5T 6T time
1
-1
1
-1timeT
PulsetrainPulse
frequency
(f)
1T
� 1T
Spectrum(PowerSpectralDensity)
39 +Example of 4-Ary digital transmission scheme
40
0 1 1 0 01
Data
Coder 0 1 1 0 00 1 1 1 0
Coded Data 0 1
1 0
1 1
00 s0(t) = 3s(t)
s2(t) = �s(t)
s1(t) = s(t)
s3(t) = �3s(t)
Transmitted Symbols
Ts
3
1
-1
-3
Coderate=Rc
Ru Rb
Rs
+Symbol Rate, Bit Rate, Code Rate
nRs isthesymbolrate(howmanypulsespersecond)nSymboldurationisTs =1/RsnOftenwewillsimplyuseR andT
n BitrateisRb whichisRs × (NumberofBits/Symbol)
n IftheschemeisM-ary,therearek =log2M bits/symbol
n“Coderate”isRc whichtellsyouwhatisthefractionof“actualdata”nCoderateisusuallylessthanorequalto1
BW≈1/Ts
Formostpracticalpurposes
41 +Example
n Consideran8-aryscheme(thereare8differentsymbols)n Eachsymbolcarries3bits
n Letthesymboldurationbe1μs - onesymbolistransmittedeverymicrosecondn Symbolrateis1Msps (Rs =1/Ts)n Rawbitrateis3Mbps(Rb =3× Rs)n WhatistheapproximateBWofthesignal?
n LetthecoderateRc be1/3n Thismeans,oneoutofevery3bitsistheactualdatan 2outof3bitsareredundantn Actual(useful)datarateis1Mbps(Ru =Rc × Rb)
42
+Why Digital Modulation?
n Analogcellphonesystemsmigratedtodigitalinthe1990s
n Somemainreasonsn IncreaseSystemCapacity
n Squeezemorechannelsinagivenbandwidth
n Example:EmployTDMA(3channelsin30kHzinNA-TDMA)
n Voicecompression
n Efficienttransmissionn Errorcontrolcoding,equalizers,
etc.=>lowerpowerneeded
n Moreresistanttointerference
n Morereasonsn Addadditionalservices/featuresn SMS,callerID,etc.
n Reducecostandsizeofmobiledevices
n ImproveSecurity(encryptionpossible)
n Dataserviceandvoicetreatedsame(4Gsystems)
n Note:Stillanalogsignalscarryingdigitaldata
43 +What is Spectral Efficiency?
n Itisdefinedastherawbitratesupportedin1Hzofbandwidth
η=bitrate/bandwidth
nExample:ComputethespectralefficiencyofGSM
nAnswer:n Thechannelspacingorbandwidthis200kHzandtherawdatarateis270.83kbps.Sothespectralefficiencyis270.83/200=1.35bps/Hz
nComputethespectralefficiencyonSlide35
44
+TDMA
n Userssharesamefrequencybandinnon-overlappingtimeintervals,n E.g.Roundrobin
n Receiverfiltersarejustwindowsinsteadofbandpass filters(asinFDMA)
n Guardtimecanbeassmallasthesynchronizationofthenetworkpermitsn Allusersmustbesynchronizedwithbasestationtowithinafractionofguardtime
n Guardtimeof30-50μs commoninTDMA
n UsedinGSM,NA-TDMA,(PDC)PacificDigitalCellular
45 +2G GSM - TDMA/FDMA/FDD
46
1 2 3 4 5 6 7 8
higherGSMframestructures
935-960MHz124channels (200kHz)downlink
890-915MHz124channels(200kHz)uplink
time
GSMTDMAframe
GSMtime-slot(normalburst)
4.615ms
546.5µs577µs
tail userdata TrainingSguardspace S userdata tail
guardspace
3bits 57bits 26bits 57bits1 1 3
+TDMA/FDMA/TDD – example of DECT
47
.
.
.10MHz 0 1 … 8 9 11
1.728MHz
Timeslotsallocatedtousersononecarrier
Carrier1
Carrier2
Carrier5
10 12 … 21 2313 2220Downlink5ms Uplink5ms
32bitpreamble
388bitdata
60bitguard
64controlbits
320bitchannel
4paritybits
+CDMA
n Narrowbandmessagesignalismultipliedbyverylargebandwidthspreadingsignalusingdirectsequencespreadspectrum
n Alluserscanusesamecarrierfrequencyandmaytransmitsimultaneously
n Eachuserhasownuniqueaccessspreadingcodewordwhichisapproximatelyorthogonaltootheruserscodewords
n Receiverperformstimecorrelationoperationtodetectonlyspecificcodeword,otheruserscodewords appearasnoiseduetodecorrelation
n Cocktailpartyexample
48
+Direct Sequence Spread Spectrum
49
n Theoriginaldatastreamis“chipped”upintoapatternofpulsesofsmallerduration
n Goodautocorrelationproperties
n Goodcross-correlationpropertieswithotherpatterns
n Eachpatterniscalledaspreadspectrumcodeorspreadspectrumsequencen E.g.WalshCode
-10 -8 -6 -4 -2 0 2 4 6 8 100
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1original spectrumspread spectrum
Data Bit
“Spread” Bits
chip
1 2 3 4 5 6 7 8 9 10 11
PeriodicSpreadingCode
DataIn
SpreadingCodeIn
normalizedfrequency
PNCodes(Later)
+Orthogonality
s(t)
u(t)
s(t)⇥ u(t)
t
t
t
T
T
T
Tc
AreaundertheproductisZERO!
Vectornotation:[1-11-11-11-1]
×[1-1-111-1-11]
=[11-1-111-1-1]
Sumtheelementsintheproduct
=>0
multiply
50
+Simple example illustrating CDMAn Traditional
n Tosenda0,send+1VforTseconds
n Tosenda1,send-1VforTseconds
n Useseparatetimeslotsorfrequencybandstoseparatesignals
n SimpleCDMA
n Tosenda0,Bobsends+1VforTseconds;Alicesends+1VforT/2secondsand-1VforT/2seconds
n Tosenda1,Bobsends-1VforTseconds;Alicesends-1VforT/2secondsand+1VforT/2
51
1 0 1
[1,1] [-1,-1] [1,-1] [-1,1]Code
chip
T T T T
11
time
V
Data
0
+ Simple CDMA Transmitter
52
Spread
Spread
User1datain
User2datain
0011
1010
=[1,1,1,1,-1,-1,-1,-1]
=[-1,1,1,-1,-1,1,1,-1]
Transmittedsignal
t
V
V
t
t
V
T 2T 3T 4T
T 2T 3T 4T
T 2T 3T 4T
1
1
2
+Simple CDMA Receiver
53
Despread
Despread
User1dataout
User2dataout
Receivedsignal t
V
correlatewith[1,1]
correlatewith[1,-1]
t
VAlice’sCode =
ò =-2xT/2=-T
-ThasanegativesignÞ Alicesenta1asthefirstbit
T 2T 3T 4T
T 2T 3T 4T
2
1
+Simple CDMA continued
n Proceedinginthisfashionforeach“bit”,theinformationtransmittedbyAlicecanberecovered
n TorecovertheinformationtransmittedbyBob,thereceivedsignaliscorrelatedbit-by-bitwithBob’scode[1,1]
n Suchcodesare“orthogonal”n Multiplythecodeselement-wise
n [1,1]× [1,-1]=[1,-1]n Addtheelementsoftheresultingproduct
n 1+(-1)=0=>thecodesareorthogonal
n CDMAusedinIS-95standardandboth3Gstandards:UMTS,cdma2000
n CDMAhasbigcapacityadvantageasfrequencyreuseclustersize=1
54
+Old Wide Area Data Systems
System ARDIS Mobitex CDPD GPRS Tetra
Frequencyband(MHz)
Bandsaround800MHz
935-940896-901
869-894824-849
890-915935-960
380-383390-393
Channelbitrate(kbps)
4.8- 19.2 8.0– 19.2 19.2 160 36
RFchannelspacing
25kHz 12.5kHz 30kHz 200kHz 25kHz
Channel&Multi-userAccess
FDMA/DSMA FDMA/Dyn.SlottedALOHA
FDMA/DSMA FDMA/TDMA/Reservation
FDMA/DSMA
ModulationTechnique
4-FSK GMSK GMSK GMSK p/4-DQPSK
55
Canbeconsidered1GMobileData,GPRSis2GMobileData