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8/12/2019 WCNPPTUnit-1

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Multiple Access Techniques forWireless Communication

FDMATDMA

SDMA

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Introduction

• many users at same time• share a finite amount of radio spectrum

• hi h erformance• duplexing generally required • frequency domain• time domain

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Frequency division duplexing (FDD)

• two bands of frequencies for every user • forward band • reverse band • duplexer needed • frequency seperation between forward band

and reverse band is constant

frequency seperation

reverse channel forward channel

f

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Time division duplexing (TDD)

• uses time for forward and reverse link • multiple users share a single radio channel

• reverse time slot• no duplexer is required

time seperationt

forward channelreverse channel

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Multiple Access Techniques

• Frequency division multiple access (FDMA)• Time division multiple access (TDMA)

• Space division multiple access (SDMA)• grouped as:

• narrowband systems• wideband systems

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Narrowband systems

• large number of narrowband channels• usually FDD

• Narrowband FDMA• Narrowband TDMA• FDMA/FDD• FDMA/TDD• TDMA/FDD• TDMA/TDD

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Logical separation FDMA/FDD

user 1forward channel

reverse channel

f

t

user n

forward channel

reverse channel

...

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Logical separation FDMA/TDD

user 1

forward channel reverse channel

f

t

user n


...

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Logical separation TDMA/FDD

forward forward

f

t

user 1 user nchannel

reverse

channel

channel

reverse

channel

...

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Logical separation TDMA/TDD

user 1 user n

f

t

forward

channel

reverse

channel

forward

channel

reverse

channel

...

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Wideband systems

Transmission b/w of a single channel is much largerthan the coherence b/w of the channel.large number of transmitters on one channel

TDMA -t me s ots to many tx’s on one c anne anonly one channel is allowed to acces channel.CDMA –allows all tx’s to access channel at same time

FDD or TDD multiplexing techniquesTDMA/FDD and TDMA/TDDCDMA/FDD and CDMA/TDD

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Logical separation CDMA/FDDIS-95

US narrow band spread spectrum

user 1


code

f

user n


...

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Logical separation CDMA/TDD(W-CDMA)

user 1


code

t

user n


...

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Multiple Access Techniques in use

Multiple Access

Technique

Cellular System

Advanced Mobile Phone System (AMPS) FDMA/FDD

Global System for Mobile (GSM) TDMA/FDD

US Digital Cellular (USDC) TDMA/FDD

Digital European Cordless Telephone (DECT) FDMA/TDDUS Narrowband Spread Spectrum (IS-95) CDMA/FDD

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Frequency division multiple access FDMA

• one phone circuit per channel• idle time causes wasting of resources

transmitting

• usually implemented in narrowband systems

• for example: in AMPS is a FDMA bandwidth of 30 kHz implemented

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FDMA compared to TDMA

• fewer bits for synchronization• fewer bits for framing

• higher costs for duplexer used in base

station and subscriber units

• FDMA requires RF filtering to minimizeadjacent channel interference

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Nonlinear Effects in FDMA

• many channels - same antenna• for maximum power efficiency operate near

• near saturation power amplifiers arenonlinear

• nonlinearities causes signal spreading• intermodulation frequencies





Nonlinear Effects in FDMA

• IM are undesired harmonics• interference with other channels in the

• decreases user C/I - decreases performance• interference outside the mobile radio band:

adjacent-channel interference• RF filters needed - higher costs





Number of channels in a FDMA system

N= Bt - Bguard

Bc

• N … number of channels• B t … total spectrum allocation

• Bguard … guard band • Bc … channel bandwidth

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Example: Advanced Mobile Phone System

• AMPS• FDMA/FDD

• 12.5 MHz per simplex band - B t

• Bguard = 10 kHz ; B c = 30 kHz

N= 12.5 E6 - 2*(10 E3)30E3 = 416 channels

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Time Division Multiple Access

• time slots• one user per slot

• noncontinuous transmission• digital data

• digital modulation

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Repeating Frame Structure

Preamble Information Message Trail Bits

One TDMA Frame

Slot 1 Slot 2 Slot 3 … Slot N

Trail Bits Sync. Bits Information Data Guard Bits

The frame is cyclically repeated over time.

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Features of TDMA

• a single carrier frequency for several users• transmission in bursts• low battery consumption• handoff process much simpler for subscriber unit• FDD : switch instead of duplexer as tx and rx are

done on different time slots

• very high transmission rate• high synchronization overhead • guard slots necessary




Number of channels in a TDMA system

N= m*(B tot - 2*B guard)

Bc

• N … number of channels• m … number of TDMA users per radio channel• B tot … total spectrum allocation• Bguard … Guard Band • Bc … channel bandwidth

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Example: Global System for Mobile (GSM)

• TDMA/FDD• forward link at B tot = 25 MHz

c • if m = 8 speech channels supported, and • if no guard band is assumed :

N= 8*25E6200E3 = 1000 simultaneous users

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Efficiency of TDMA

• percentage of transmitted data that containinformation

•• usually end user efficiency < f ,

• Data tx has source and channel coding bits• How get f ?

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Efficiency of TDMA

• bOH … number of overhead bits

bOH = N r *b r + N t*b p + N t*bg + N r *bg

• r … num er o re erence urs s per rame• b r … reference bits per reference burst• N t … number of traffic bursts per frame

• b p … overhead bits per preamble in each slot• bg … equivalent bits in each guard time

intervall

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Efficiency of TDMA

bT = T f * R

• bT … total number of bits per frame• T f … frame duration

• R … channel bit rate

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Efficiency of TDMA

f = (1-b OH /bT)*100%

• f … frame efficiency• bOH … number of overhead bits per frame

• bT … total number of bits per frame

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Space Division Multiple Access

• Controls radiated energy for each user in space• using spot beam antennas

• cover areas with same frequency by usingTDMA or CDMA systems

• cover areas with same frequency by using NB-FDMA systems

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Reverse link problems

• general problem• different propagation path from user to base

each user to the base station required

• limits by battery consumption of subscriberunits

• possible solution is a filter for each user

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Multiple Access protocols

• Single shared broadcast channel• Two or more simultaneous transmissions by nodes: interference

– Collision if node receives two or more signals at the same time

• Distributed algorithm that determines how nodes share channel,i.e., determine when node can transmit

• Communication about channel sharing must use channel itself!

– No out-of-band channel for coordination

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Channel Partitioning

• Frequency Division Multiplexing – Each node has a frequency band

• – Each node has a series of fixed time slots

• What networks are these good for?

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Ideal Multiple Access Protocol

Broadcast channel of rate R bps1. When one node wants to transmit, it can send at

rate R.

. en no es want to transm t, eac can sen ataverage rate R/M

3. Fully decentralized: – no special node to coordinate transmissions

– no synchronization of clocks, slots4. Simple

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Pure (unslotted) ALOHA

• unslotted Aloha: simpler, no synchronization• when frame first arrives

– transmit immediately

• collision robabilit increases: – frame sent at t 0 collides with other frames sent in[t0-1,t 0+1]

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Pure Aloha efficiency

P(success by given node) = P(node transmits) .

P(no other node transmits in [t 0-1,t 0] .

P(no other node transmits in [t 0,t0+1]= p . (1-p) N-1 . (1-p) N-1

= p . (1-p) 2(N-1)

… choosing optimum p and then letting n -> ¥ ...

Efficiency = 1/(2e) = .18worse !

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Slotted ALOHAAssumptions• all frames same size• time is divided into equal

size slots, time to transmit 1frame

Operation• when node obtains fresh

frame, it transmits in next slot• no collision, node can send

• nodes start to transmitframes only at beginning ofslots

• nodes are synchronized • if 2 or more nodes transmit

in slot, all nodes detectcollision

• if collision, node retransmits

frame in each subsequent slotwith prob. p until success

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Slotted ALOHA

Pros• single active node can

continuously transmit at fullrate of channel

• highly decentralized: only slotsin nodes need to be in sync

• simple

Cons• collisions, wasting slots• idle slots• nodes may be able to

detect collision in lessthan time to transmit

packet• clock synchronization

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Slotted Aloha efficiency

• Efficiency is the long-run fraction of successful slotswhen there are many nodes, each with many framesto send

• ,transmits in slot with probability p

• prob that node 1 has success in a slot = p(1-p) N-1

• prob that any node has a success = Np(1-p) N-1

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Optimal choice of p

• For max efficiency with N nodes, find p* thatmaximizes

Np(1-p) N-1

• For many nodes, take limit of Np*(1-p*) N-1 as N goesto infinity, gives 1/e = .37

• Efficiency is 37%, even with optimal p

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Carrier Sense Multiple Access

CSMA : listen before transmit:If channel sensed idle: transmit entire frame• If channel sensed busy, defer transmission

• Human analogy: don’t interrupt others!

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Carrier Sense Protocols• Use the fact that in some networks you can sense the medium to check whether

it is currently free – 1-persistent CSMA – non-persistent CSMA – p-persistent protocol

– CSMA with collision detection (CSMA/CD): not applicable to wireless systems• 1-persistent CSMA

– when a station has a packet:• it waits until the medium is free to transmit the packet• if a collision occurs, the station waits a random amount of time

– first transmission results in a collision if several stations are waiting for the channel

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Carrier Sense Protocols ( Cont’d )• Non-persistent CSMA

– when a station has a packet:• if the medium is free, transmit the packet• otherwise wait for a random period of time and repeat the algorithm

– higher delays, but better performance than pure ALOHA• p-persistent protocol

– • transmit the packet with probability p• wait for next slot with probability 1- p

– better throughput than other schemes but higher delay• CSMA with collision Detection (CSMA/CD)

– stations abort their transmission when they detect a collision – e.g., Ethernet, IEEE802.3 but not applicable to wireless systems

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CSMA collisions

collisions can still occur: propagation delay meanstwo nodes may not hear each other’s transmission

collision:entire packet transmissiontime wasted

note:role of distance & propagation delay

in determining collision probability

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CSMA/CD (Collision Detection)

CSMA/CD: carrier sensing, deferral as in CSMA – collisions detected within short time – colliding transmissions aborted, reducing channel wastage

• collision detection: – easy in wired LANs: measure signal strengths, compare

transmitted, received signals – difficult in wireless LANs: receiver shut off while

transmitting

• human analogy: the polite communicator

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CSMA/CD collision detection

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Demand Assigned Multiple Access

• Channel efficiency only 18% for Aloha, 36% for Slotted Aloha(assuming Poisson distribution for packet arrival and packet length)

• Reservation can increase efficiency to 80% – a sender reserves a future time-slot – -

Wireless Networks Spring 2005

– reservation also causes higher delays – typical scheme for satellite links

• Examples for reservation algorithms: – Explicit Reservation (Reservation-ALOHA) – Implicit Reservation (PRMA) – Reservation-TDMA

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Explicit Reservation•Explicit Reservation (Reservation Aloha):

– two modes:• ALOHA mode for reservation:

competition for small reservation slots, collisions possible• reserved mode for data transmission within successful reserved slots (no collisions

possible) – it is im ortant for all stations to kee the reservation list consistent at an oint in


time and, therefore, all stations have to synchronize from time to time

Aloha reserved Aloha reserved Aloha reserved Aloha

collision

t

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PRMA•Implicit reservation (PRMA - Packet Reservation MA):

– a certain number of slots form a frame, frames are repeated – stations compete for empty slots according to the slotted aloha principle – once a station reserves a slot successfully, this slot is automatically assigned to

this station in all following frames as long as the station has data to send – competition for this slots starts again as soon as the slot was empty in the last


frame

frame 1

frame 2

frame 3

frame 4

frame 5

1 2 3 4 5 6 7 8 time-slot

collision atreservationattempts

A C D A B A F

A C A B A

A B A F

A B A F D

A C E E B A F D t

ACDABA- F

ACDABA- F

AC- ABAF-

A- - - BAFD

ACEEBAFD

reservation

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