Multi Access Presn

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Multiple AccessMultiple AccessTechniques in wirelessTechniques in wireless

communicationscommunicationsPrepared byPrepared by

Ms. Sukruti KaulgudMs. Sukruti KaulgudMs. Geeta HayagreevMs. Geeta Hayagreev


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IntroductionIntroduction

In conventional telephone systems, it is possible to talk and listenIn conventional telephone systems, it is possible to talk and listen

simultaneously, called duplexing.simultaneously, called duplexing.

DuplexingDuplexing

Allow the possibility of talking and listening simultaneously.Allow the possibility of talking and listening simultaneously.

Frequency Division Duplex (FDD)Frequency Division Duplex (FDD)

Provides twoProvides two distinct bandsdistinct bands of frequencies for every userof frequencies for every user

Time Division Duplex (TDD)Time Division Duplex (TDD)

Multiple users share a signal channel by taking turns in timeMultiple users share a signal channel by taking turns in time

domaindomain

Each duplexing channel has both a forward time slot and aEach duplexing channel has both a forward time slot and a

reverse time slot to facilitate bidirectional communication.reverse time slot to facilitate bidirectional communication.


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IntroductionIntroduction


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Multiple Access TechniquesMultiple Access Techniques

For Wireless CommunicationFor Wireless Communication

Multiple access schemes are used toMultiple access schemes are used to

allow many mobile users to shareallow many mobile users to sharesimultaneously a finite amount ofsimultaneously a finite amount of

radio spectrum.radio spectrum.

For high quality communications, thisFor high quality communications, this

must be done without severemust be done without severe

degradation in the performance of thedegradation in the performance of the

system.system.


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FDMAFDMA TDMATDMA CDMACDMA SDMASDMA


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Narrowband systemsNarrowband systems

Wideband SystemsWideband Systems


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Multiple Access (MA)TechnologiesMultiple Access (MA)Technologies

used in Different Wireless Systemsused in Different Wireless Systems

Cellular SystemsCellular Systems MA TechniqueMA Technique

AMPS ( Advanced MobileAMPS ( Advanced Mobile

Phone system )Phone system ) FDMA / FDDFDMA / FDD

GSM ( Global System forGSM ( Global System for

Mobile )Mobile )TDMA / FDDTDMA / FDD

US DC ( U. S DigitalUS DC ( U. S Digital

Cellular )Cellular )TDMA / FDDTDMA / FDD

JDC ( Japanese DigitalJDC ( Japanese Digital

Cellular )Cellular )TDMA / FDDTDMA / FDD


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Multiple Access (MA) TechnologiesMultiple Access (MA) Technologies

used in Different Wireless Systemsused in Different Wireless Systems

Cellular SystemsCellular Systems MA TechniqueMA Technique

DECT ( Digital EuropeanDECT ( Digital EuropeanCordless Telephone )Cordless Telephone )

FDMA / FDDFDMA / FDD

ISIS 95 ( U.S Narrowband95 ( U.S Narrowband

Spread Spectrum )Spread Spectrum )CDMA / FDDCDMA / FDD


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FDMAFDMA


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Principles Of OperationPrinciples Of Operation

Each user is allocated a uniqueEach user is allocated a unique

frequency band or channel. Thesefrequency band or channel. These

channels are assigned on demand tochannels are assigned on demand tousers who request service.users who request service.

In FDD, the channel has twoIn FDD, the channel has two

frequenciesfrequencies forward channel &forward channel &reverse channel.reverse channel.


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During the period of the call, no otherDuring the period of the call, no other

user can share the same frequencyuser can share the same frequency

band.band.If the FDMA channel is not in use, thenIf the FDMA channel is not in use, then

it sits idle and cannot be used by otherit sits idle and cannot be used by other

users to increase or share capacity.users to increase or share capacity.This is a wasted resource.This is a wasted resource.


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Properties of FDMAProperties of FDMA

The bandwidth of FDMA channels isThe bandwidth of FDMA channels is

narrow (30 KHz) since it supports onlynarrow (30 KHz) since it supports only

one call/ carrier.one call/ carrier.

ISI is low since the symbol time is largeISI is low since the symbol time is large

compared to average delay spreadcompared to average delay spread

No equalization is required.No equalization is required.

FDMA systems are simple than TDMAFDMA systems are simple than TDMA

systems, but modern DSP is changingsystems, but modern DSP is changing

this factor.this factor.


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Properties of FDMA Properties of FDMA

Fewer bits are needed for overheadFewer bits are needed for overhead

purposes.purposes.

FDMA systems have higher costFDMA systems have higher cost1.1. Cell site system due to single call/carrierCell site system due to single call/carrier

2.2. Costly band pass filters to eliminate spuriousCostly band pass filters to eliminate spurious

radiationradiation

3.3. Duplexers in both T/R increase subscriber costsDuplexers in both T/R increase subscriber costs

Nonlinear effects in FDMANonlinear effects in FDMA


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Number Of Channel SupportedNumber Of Channel Supported

By FDMA SystemBy FDMA System

gB gBtB

t g

c

g

c

B 2BN

B

B GuardBand

B ChannelBandwidth

!

p

p


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ExampleExample

In the US, each cellular carrier is allocatedIn the US, each cellular carrier is allocated

416 channels,416 channels,

!

!

! v v- ! !

v

t

g

c

6 3

3

B 12.5MHz

B 10K Hz

B 30K Hz

(12.5 10 ) 2(10 10 )N 416

30 10


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TDMATDMA


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TDMA systems divide the radioTDMA systems divide the radio

spectrum into time slots and each userspectrum into time slots and each user

is allowed to either transmit or receiveis allowed to either transmit or receivein each time slots.in each time slots.

Each user occupies a cyclicallyEach user occupies a cyclically

repeating time slots. TDMA can allowrepeating time slots. TDMA can allow

different number of time slots fordifferent number of time slots for

separate user.separate user.


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Properties Of TDMAProperties Of TDMA

TDMA shares the single carrierTDMA shares the single carrier

frequency with several users, wherefrequency with several users, where

each user makes use of noneach user makes use of non--overlapping timeslots.overlapping timeslots.

Data Transmission for user of TDMAData Transmission for user of TDMA

system is discrete burstssystem is discrete bursts

The result is low battery consumption.The result is low battery consumption.

Handoff process is simpler, since it isHandoff process is simpler, since it is

able to listen for other base stationsable to listen for other base stations

during idle time slots.during idle time slots.


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Properties Of TDMAProperties Of TDMA

Since different slots are used for T andSince different slots are used for T andR, duplexers are not required.R, duplexers are not required.

Equalization is required, sinceEqualization is required, sincetransmission rates are generally verytransmission rates are generally veryhigh as compared to FDMA channels.high as compared to FDMA channels.

Guard time should be minimizedGuard time should be minimized

High synchronization overhead isHigh synchronization overhead isrequiredrequired

Possible to allocate different numbersPossible to allocate different numbers

of time slotsof time slots


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TDMA Frame StructureTDMA Frame Structure

PreamblePreamble InformationInformation

messagemessage

Trail BitsTrail Bits

Slot 1Slot 1 Slot 2Slot 2 Slot NSlot N

Trail BitTrail Bit Sync BitSync Bit InformatioInformatio

n Bitn Bit

GuardGuard

BitsBits


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Components of 1 TDMA FrameComponents of 1 TDMA Frame

PreamblePreamble Address andAddress and

synchronization information for basesynchronization information for base

station and subscriber identificationstation and subscriber identificationGuard timesGuard times Synchronization ofSynchronization of

receivers between a different slots andreceivers between a different slots and

framesframes


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Efficiency of TDMAEfficiency of TDMA

Frame Efficiency :Frame Efficiency :

OH T

T OH

T

(1 b /b ) 100

(b b )100

b

! v

! v

fNo.ofbits/framecontaining transmitted data

TotalNumberofbits/frameL !


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Frame efficiency parametersFrame efficiency parameters

Tb Total Number ofbits perframe!

f=T R

OHb =Number ofoverhead bits /frame

fT=Frame duration

R=Channel bit rate

r r t p t g r g=N b N b N b N b


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Frame efficiency parametersFrame efficiency parameters

r

t

r

p

g

N Number ofreference bits perframe

N Number oftraffic bits perframe

b Number ofoverhead bits per reference burst

b Number ofoverhead bits per preamble in each slots

b Number ofequivalent bits

!

!

!!

! in each guard time interval


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Number of channels in TDMANumber of channels in TDMA

SystemSystem

tot guard

c

m (B -2B )

N= B

guard

m Maximum number ofT MA users supported on each radio channel

B Guard band to pr esent user at theedge of the band

from 'bleeding over' to an adjacent radio service

!

!


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ExampleExample

GSM System uses a TDMA / FDDGSM System uses a TDMA / FDD

system.system.

The GSM System uses a frameThe GSM System uses a frame

structure where each frame consist of 8structure where each frame consist of 8

time slots, and each time slot containstime slots, and each time slot contains156.25 bits, and data is transmitted at156.25 bits, and data is transmitted at

270.833 kbps in the channel. Find: 270.833 kbps in the channel. Find:


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ExampleExample

1.1. Time duration of a bitTime duration of a bit

2.2. Time duration of a slotTime duration of a slot

3.3. Time duration of a frame andTime duration of a frame and

4.4. How long must a user occupying aHow long must a user occupying a

single slot must wait between twosingle slot must wait between two

simultaneous transmissions?simultaneous transmissions?


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Spread Spectrum MultipleSpread Spectrum Multiple

Access Technologies (SSMA)Access Technologies (SSMA)

SSMA technologies uses techniquesSSMA technologies uses techniques

which has a transmission bandwidthwhich has a transmission bandwidththat is much greater than maximumthat is much greater than maximum

required RF bandwidth.required RF bandwidth.

This is achieved by pseudo noise (PN)This is achieved by pseudo noise (PN)

sequence that contents a narrowbandsequence that contents a narrowband

signal to a wideband noisesignal to a wideband noise--like signallike signal

before transmission.before transmission.


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Access Technologies (SSMA)Access Technologies (SSMA)

SSMA provides immunity to multipleSSMA provides immunity to multiple

interference and has robust multipleinterference and has robust multipleaccess capability.access capability.


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Frequency HoppingFrequency Hopping

Frequency hopping is a form of FDMAFrequency hopping is a form of FDMA

Each transmitter is allocated a group ofEach transmitter is allocated a group ofchannels, known aschannels, known as hop sethop set..

The transmitter transmits data in shortThe transmitter transmits data in shortbursts, choosing one of these channels onbursts, choosing one of these channels on

which to transmit each burst.which to transmit each burst.


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TimeTime--frequency characteristic of a singlefrequency characteristic of a single

transmitter.transmitter.

Frequency HoppingFrequency Hopping


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Time HoppingTime Hopping

THTH--PPMPPM

t

Ts

Tc

Tf

User1 : C(1)=[1 0 0 2] d1=0

User2 : C(2)=[0 1 2 0] d2=1

User3 : C(3)=[2 2 1 1] d3=0


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Frequency Hopping Spread SpectrumFrequency Hopping Spread Spectrum

(FHSS)(FHSS)

It divides available bandwidth into N channelsIt divides available bandwidth into N channels

and hops between these channels accordingand hops between these channels according

to the PN sequence.to the PN sequence.

Fast hoppingFast hopping

Slow hoppingSlow hopping


AccessAccess


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SSMASSMA

Frequency Hopped Multiple Access (Frequency Hopped Multiple Access (

FHMA )FHMA )

Direct Sequence Multiple Access ( CDMADirect Sequence Multiple Access ( CDMA

))


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DSSSDSSS


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DSSSDSSS


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DSSSDSSS


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Message SignalMessage Signal

m(t) is a time sequence of nonm(t) is a time sequence of non--

overlapping pulses of duration T, eachoverlapping pulses of duration T, each

of which has an amplitude (+/of which has an amplitude (+/--) 1.) 1.

The PN waveform consists of N pulsesThe PN waveform consists of N pulsesor chips for message symbol period T.or chips for message symbol period T.

NTNTCC = T= T

where Twhere TCC is the chip period.is the chip period.


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CDMACDMA


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CDMACDMA


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CDMACDMA


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Features of CDMAFeatures of CDMA


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Hybrid FDMA/CDMA (FCDMA):Hybrid FDMA/CDMA (FCDMA):

The available wideband spectrum is divided into aThe available wideband spectrum is divided into a

number of subspectras with smaller bandwidths.number of subspectras with smaller bandwidths.

Each of these smaller suchannels becomes aEach of these smaller suchannels becomes anarrowband CDMA system having processing gainnarrowband CDMA system having processing gain

lower than the original CDMA system.lower than the original CDMA system.


AccessAccess


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Hybrid Direct Sequence/FrequencyHybrid Direct Sequence/Frequency

Hopped Multiple Access (DS/FHMA)Hopped Multiple Access (DS/FHMA)

This technique consists of a direct sequenceThis technique consists of a direct sequencemodulated signal whose center frequency is made tomodulated signal whose center frequency is made to

hop periodically in a pseudorandom fashion.hop periodically in a pseudorandom fashion.

Having an advantage in that they avoid the nearHaving an advantage in that they avoid the near--farfar

effect.effect.


AccessAccess


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Time Division Frequency Hopping (TDFH)Time Division Frequency Hopping (TDFH)

The subscriber can hop to a new frequency atThe subscriber can hop to a new frequency at

the start of a new TDMA frame.the start of a new TDMA frame. Has been adopted in GSM.Has been adopted in GSM.


AccessAccess


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SDMASDMA

Space division

multiplexing.

Frequency reuse only

with a certain distance

between the base

stations

Users inside a cell use

FDM or CDM.

Standard model using 7

frequencies:

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