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