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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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Multiple Access TechniquesMultiple Access Techniques
Multiple Access TechniquesMultiple Access Techniques
FDMAFDMA TDMATDMA CDMACDMA SDMASDMA
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Multiple Access TechniquesMultiple Access Techniques
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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Principles Of OperationPrinciples Of Operation
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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Principles Of OperationPrinciples Of Operation
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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Spread Spectrum MultipleSpread Spectrum Multiple
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
Spread Spectrum MultipleSpread Spectrum Multiple
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.
Spread Spectrum MultipleSpread Spectrum Multiple
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.
Spread Spectrum MultipleSpread Spectrum Multiple
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.
Spread Spectrum MultipleSpread Spectrum Multiple
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: