CDMA

Code Division Multiple Access

2. Outline:
Introduction.
Spread Spectrum Methods.
Technologies based on CDMA.
Performance analysis of IEEE 802.11 protocol.
Major Development in CDMA.

3. Introduction:
Codedivisionmultipleaccess(CDMA)isafixedassignmentaccess
techniquethatusesspreadspectrumandaspecialkindofcoding
schemetoallowthetransmitterstosharethemediaatthesame
time.
Whyitisconsideredasaformofspreadspectrumtechnique?
Itisconsideredasaformofthespread-spectrumtechniques,
becausethemodulatedcodedsignalhasahigherbandwidththan
thedatabeingtransmitted.

4. Introduction (Cont.):
Howdoesitsupportmorethanonegroupinthesamechannel?
Itallowsmultiplegroupstosharethesamechannelatthesametime
byassigningeachgrouptodifferentcode.
CDMAModulationSteps:
1.Generatealocalpseudo-randomcodewithahigherratethanthe
datatobetransmitted.
2.XORthedatawhichneedtobetransmittedwiththegenerated
code.

5. b
T c
T Data Signal
Pseudo-random
Code
Transmitted
Signal
Time
Introduction (Cont.):

6. Channel
Encoder
Modulator Channel Demodulator
Channel
Decoder
Pseudo-noise
Generator
Pseudo-noise
Generator
General Model of Spread Spectrum Digital Communication System
Input
data
Output
data
Introduction (Cont.):

7. Introduction (Cont.):
What can we gain from the spreading of the spectrum?
The immunity from the various types of noise.
The immunity from the multipath distortion.
It can be used as an encryptingtechnique.
Several users can use the same channel with little interference.

8. Introduction (Cont.):
Generating Pseudo-Random Codes:
The base station generates a unique pseudo-random code for every
connection and these codes should have the following properties:
Itshouldbedeterministic.
Itmustappearrandomtoalistenerwhodoesnothaveaprior
knowledgeofthecode.
Thecross-correlationbetweenanytwocodesmustbesmall.
Thecodeshouldhavealongperiod.

9. Introduction (Cont.):
Code Correlation:
Eachuserusesdifferentcodetomodulatehissignal.
Thebestperformancewillbewhentheusers’signalsareseparated
asmuchaspossible.
Theseparationcanbedonebycorrelatingthereceivedsignalwith
thelocalgeneratedcode.
Ifthecorrelationwashigh,thenweexcludethiscodeandwesearch
foranothercodewithlesscorrelation.

10. Introduction (Cont.):
Correlation Functions:
1.Cross-Correlation:Thecorrelationoftwodifferentcodes.“Usedto
separatethetransmitters’signalsfromeachother”.
2.Auto-Correlation:Thecorrelationofacodewithatime-delayed
versionofitself.“Usedtorejectmultipathinterference”.

11. Spread Spectrum Methods:
Direct Sequence Spread Spectrum (DSSS):
Eachbitfromthedatawhichneedtobesentisrepresentedby
multiplebitsinthetransmitteddata,byusingthespreadingcode.
Theoriginaldataisspreadoutacrossawiderfrequencybandby
usingthespreadingcode.
Thespreadingproportiondependsonthenumberofbitsusedinthe
code.

12. b
T c
T Original
Data
Pseudo-
random
Bit
Transmitted
Data
Time
0 0
00 0 00 0 0 011 1 11 1111 11 1
0011 001110 00
1 1 1
1 1 100 00 0
Example of Direct Sequence Spread Spectrum

13. Spread Spectrum Methods (Cont.):
Frequency Hopping Spread Spectrum (FHSS):
Atthetransmitter,theoriginalsignalisbroadcastedoveraseriesof
radiofrequencies,hoppingfromdifferentfrequenciesinafixed
pattern.
Thereceivershouldusethesamehoppingpatternsimultaneously
withthetransmitterinordertoreceivethedatacorrectly.
Thespreadingcodespecifiesthesequenceofchannelsandthe
receivershouldusethesamecodetotuneintothesequenceof
channelsthatareusedbythesender.

14. f1f2f3f4f5f6f7f8
f1
f2
f3
f4
f5
f6
f7
f8
1
2
3
4
5
6
7
8
Example of Frequency Hoping Spread Spectrum
(a) Channel assignment (b) Channel use
Frequency
Frequency
Time
Energy

15. Spread Spectrum Methods (Cont.):
Chirp Spread Spectrum (CSS):
ItisthefirstandsimplestapplicationofMultiDimensionalMultipleAccess
(MDMA)technology.
Itencodestheinformationbyusingwidebandlinearfrequencymodulatedchirp.
Itusestheentireallocatedbandwidthtobroadcastthesignal,whichhelpsmaking
itrobusttochannelnoise.
Also,itisimmunizedfromthemulti-pathfading,becauseitutilizesthespectrum
broadband.
Itfindsthenoisebyrelyingonthelinearnatureofthechirppulse.

16. Spread Spectrum Methods (Cont.):
Time Hopped Spread Spectrum (THSS):
Theperiodanddutycycleofapulsedradiofrequencycarrierare
variedpseudo-randomlyunderthecontrolofcodedsequencesimilar
toFHSS.
Itconsumeslowpowerduetotheverylowdutycycles.
InTHSS,thepseudorandomnumberdeterminesthesequenceof
howtheburstwillbetransmitted.

17. Time-Hopping Spread Spectrum
Time
Framef T f 2T f 3T f 4T 0
Transmitted time slots (k bits)
t

18. Technologies based on CDMA:
WiFi(IEEE 802.11):
802.11band802.11gworkinginthe2.4frequencyband
(Industry,ScienceandMedicinebandorISM).
DSSSandFHSSphysicallayeroptionshavebeendesignedspecifically
toovercometheinterferingwithotherdevicesinthisband.

19. Three Non-Overlapping DSSS Channels in the ISM Band
Channel 1
2.4120
Channel 6
2.4370
Channel 11
2.4620
2.4000
GHz
2.4835
GHz
22 MHz

20. IEEE 802.11
2.4 GHz
FHSSS
2 Mbps
4 GFSK
1 Mbps
2 GFSK
DSSS
1 Mbps
DBPSK
2 Mbps
DQPSK
IEEE 802.11 b
Extension
5.5 Mbps
DQPSK –CCK
BPSK -PBCC
11 Mbps
DQPSK –CCK
QPSK -PBCC
850 to 950 nm
Diffuse IR

21. Technologies based on CDMA (Cont.):
Bluetooth:
ItoperatesintheunlicensedISMbandanditcanconnectuptoeight
devicesinthesametime.
ItusesFrequencyHopingSpreadSpectrum(FHSS)tomakeitrarefor
morethanonedevicetotransmitonthesametimeusingthesame
frequencyandtoavoidanyinterference.
Eachdevicewilluse79individual,randomlychosenfrequencies
withinadesignatedrange,changingfromonetoanotherona
regularbasis.

22. Technologies based on CDMA (Cont.):
Bluetooth (Cont.):
ABluetoothchannelisdividedintotimeslotseachhasalengthof
625microsecondandeachchannelhasawideof1MHz.
Thedeviceshopthroughthetimeslotsmaking1600hopspersecond.

23. Technologies based on CDMA (Cont.):
Global Positioning System (GPS):
AllGPSsatellitesbroadcastattwofrequencies,1.57542GHz(L1
signal)and1.2276GHz(L2signal).
Thereceivercandistinguishthesourceofthesignalbyusingdirect
sequencespreadspectrum(DSSS)modulationtechniquewherethe
low-bitratemessagedataisencodedwithahigh-ratepseudo-
randomnoise(PRN)sequence.

24. Technologies based on CDMA (Cont.):
Global Positioning System (GPS) (Cont.):
DSSSprovidesthestructureforthetransmissionofrangingsignal
andessentialdatasuchassatelliteco-ordinatesandhealth.
Thesatellitecarrierfrequenciesaremodulatedusingtheranging
signalwhichisPRNcodewhichisdifferentbetweensatellites.
ThereceiverusesthesatellitePRNcodetoreconstructtheactual
messagedata.
Thedataistransmittedatrateof50bitspersecond.

25. Technologies based on CDMA (Cont.):
Global Positioning System (GPS) (Cont.):
There are two types of PRN ranging distinct codes used:
1.The short coarse/acquisition (C/A) code (also called Gold code) at
1.023 million chips per second.
2.The long precise (P) code at 10.23 million chips per second.

26. Performance analysis of IEEE 802.11 protocol:
There are several assumptions have been considered in presenting
this analytical framework:
1.Theeffectofbiterrorsintroducedbychannelnoisehasbeen
ignored.Theonlyconsiderederrorsaretheonewhichcausedby
collisionsduetoothersimultaneoustransmissions.
2.There are no hidden stations.
3.The propagation delays are not considered.
4.There are n stations in the network.

27. Performance analysis of IEEE 802.11 protocol (Cont.):
5.Everystationalwayshasapackettosend(saturatedconditions).
6.Thecollisionprobabilityofatransmittedframeisconstantand
independentofthenumberofretransmissionsthatthisframehas
experiencedinthepast.
7.RTS/CTSaccessmechanism.

28. Thesequenceofeventsinasuccessfulframetransmissionusing
theRTS/CTSaccessmethodwillbelikethefollowing:
Performance analysis of IEEE 802.11 protocol (Cont.):
RTS: Request To Send.
SIFS: Short Inter-Frame Space.
CTS: Clear To Send.
DIFS: Distributed Coordination Function.

29. Source
Destination
t0
Other
SIFSCTSSIFS
DATA
SIFS
RTS: Request To Send.
SIFS: Short Inter-Frame Space.
CTS: Clear To Send.
DIFS: Distributed Coordination Function.
NAV (RTS)
NAV (CTS)
NAV (DATA)
DIFS
RTS
ACK
Defer Access Back-off

30. Performance analysis of IEEE 802.11 protocol (Cont.):
Abbreviations:

31. Performance analysis of IEEE 802.11 protocol (Cont.):
For simplicity, I assumed that the value of τ will be known and we will
start by calculating the value of p:
Because we have n stations and all of them will transmit with a
probability of τ , then the value of will be:tr
P

32. Performance analysis of IEEE 802.11 protocol (Cont.):
Theprobabilityofsuccessfultransmissionwillbeequaltothe
probabilitythatonestationistransmittingwhiletheremainingstations
staysilent:
If istheframeheader,theaveragetimedelays
forRTS/CTSwillbe:

33. Performance analysis of IEEE 802.11 protocol (Cont.):
Thethroughputwillbeequaltothetimeneededtotransmitthe
payloadinformationdividedbytheaveragelengthofslottime:

34. Major Development in CDMA:
Wideband Code Division Multiple Access (W-CDMA):
It has a radio channel of 5 MHz wide and a chip rate of 3.84 Mcps.
It supports frequency division (FDD) and inter-cell asynchronous
operation.
It employs coherent detection on both the uplink and downlink.
It has a Variable mission on a 10 ms frame basis.
It supports Multi-code transmission.
It uses a direct sequence CDMA transmission technique.

35. Major Development in CDMA (Cont.):
Time-division –CDMA (TD-CDMA):
Itusesacombinedtimedivisionandcodedivisionmultipleaccess
scheme.
Thesignalsfromdifferentusersareseparatedinbothtimeandcode
domain.
Ithasaframelengthof10msanditisdividedinto15timeslots.
Eachslothasdurationof666microsecondandlengthof2560chips.
ItusesQuadraturePhaseShiftKeying(QPSK)tomodulatethedata.

36. Major Development in CDMA (Cont.):
Time Division Synchronous Code Division Multiple Access (TD-
SCDMA):
ItisanalternativetoW-CDMAinthemobiletelecommunications
networks.
Itisbettersuitedfordenselypopulatedareas.
Itusestime-divisionduplexing(TDD).
Itcanaccommodateasymmetrictrafficwithdifferentdatarateson
downlinkanduplink.
ItusesbothTDMAandCDMA.