Multiple Access Techniques for Wireless Communications-940706

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Multiple Access Techniques for WirelessCommunications

2005/07/06

Weng Chien-Erh


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Wireless Access Tech. Lab.

CCUWireless Access Tech. Lab.

Table of Contents(1)

Introduction

FDMA

TDMA

CDMA SS (Spread Spectrum)

FHSS

DSSS

Hybrid


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Packet Radio Pure ALOHA

Slotted ALOHA

CSMA

Reservation Protocol Reservation-ALOHA

PRMA

NC-PRMA

Table of Contents(2)


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Introduction (1)

Multiple Access: Enable many mobile users to share simultaneously radio

spectrum.

Provide for the sharing of channel capacity between anumber of transmitters at different locations.

Aim to share a channel between two or more signals insuch way that each signal can be received withoutinterference from another.


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Introduction (2)


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Introduction (3)

In conventional telephone systems, it is possible totalk and listen simultaneously, called duplexing.

Duplexing Allow the possibility of talking and listening simultaneously.

Frequency Division Duplex (FDD) Provides two distinct bands of frequencies for every user

Time Division Duplex (TDD)

Multiple users share a signal channel by taking turns in time domain Each duplexing channel has both a forward time slot and a reverse time

slot to facilitate bidirectional communication.


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Introduction (4)


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Each transmitter is allocated a channel with aparticular bandwidth.

All transmitters are able to transmit simultaneously.

Frequency Division Multiple Access (FDMA)(1)


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Allocation of separate channels to FDMA signals


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Time-frequency characteristic of FDMA



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Features of FDMA (Cont.) For continuous transmission, fewer bits are needed for

overhead purposes (such as synchronization and framingbits) as compared to TDMA.

FDMA uses duplexers since both TX and RX operate atthe same time.



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Transmitter share a common channel.

Only one transmitter is allowed to transmit at a

time. Synchronous TDMA: access to the channel is restricted toregular.

Asynchronous TDMA: a station may transmit at any timethat the channel is free.

Time Division Multiple Access (TDMA) (1)


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Allocation of time slot in TDMA


W l h b


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Time-frequency characteristic of synchronousTDMA


W l T h b


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Features of TDMA (Cont.)

TDMA systems divide the radio spectrum into time slots.

Each user occupies a cyclically repeating time slot.

Transmit data in a buffer-and-burst method, thus thetransmission for any user is not continuous.

TDMA has TDD and FDD modes.

Wi l T h b


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TDMA Frame Structure


Wi l A T h b


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TDMA Frame Structure (Cont.) In TDMA, the preamble contains the address and

synchronization information that both the base stationand the mobiles use to identify each other.

Different TDMA standards have different TDMA framestructures.


Wi l A T h L b


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Features of TDMA (Cont.)

Share a single carrier frequency with several users.

Data transmission is not continuous, but occurs in bursts.

No duplexers is required since users employ differenttime slots for transmission and reception.

TDMA can allocate different numbers of time slots perframe to different users, allowing bandwidth be suppliedon demand to different users.


Wi l A T h L b


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Combined used of synchronous TDMA and FDMA


Wi l A T h L b


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Asynchronous TDMA: Carrier-Sense MultipleAccess (CSMA) Allows a transmitter to access the channel at any time

that is not being used by another transmitter.


Wi l A T h L b


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Transmitter may transmit at the same time, in thesame channel.

Each signal is modified by spreading it over a largebandwidth. This spreading occurs by combining the transmitter signal

with a spreading sequence.

Code Division Multiple Access (CDMA) (1)

Wi l A T h L b


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Wir l A T h Lab


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example


Wireless Access Tech Lab


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Features of CDMAMany users of a CDMA system share the same frequency.

The symbol (chip) duration is very short and usually much

less than the channel delay spread.

The near-far problem occurs at a CDMA RX if anundesired user has a high detected power as compared tothe desired user.




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Frequency Hopping (1)

Frequency hopping is a form of FDMA

Each transmitter is allocated a group of channels,

known as hop set.

The transmitter transmits data in short bursts,choosing one of these channels on which to

transmit each burst.



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Time-frequency characteristic of a singletransmitter.




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Signal received form a pair of frequency-hoppingtransmitters.




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Each bit is transmitted as a single pulse, with thevalue ofj-th bit determined by whether it arrivesbefore or after the reference time tj.

Time Hopping (1)



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Time Hopping (2)

TH-PPM

t

Ts

Tc

Tf

i

Ns

j

icjiNfstrtr dTcjTiTtwtS s

1

0

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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A transmission technique in which a PN code,independent of information data, is employed as amodulation waveform to spread the signal energyover a bandwidth much greater than the signal

information bandwidth.

At the receiver the signal is despread using asynchronized replica of the PN code.

Direct Sequence Spread Spectrum (DSSS)

Frequency Hopping Spread Spectrum (FHSS)

Spread Spectrum Multiple Access (1)



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Direct Sequence Spread Spectrum (DSSS) A carrier is modulated by a digital code in which the code

bit rate is much larger than the information signal bit rate.These systems are also called pseudo-noise systems.

Also called code division multiple access (CDMA)

A short code system uses a PN code length equal to adata symbol.

A long system uses a PN code length that is much longerthan a data symbol.




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Direct Sequence Spread Spectrum (DSSS)



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Basic principle of DSSS For BPSK modulation




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Frequency Hopping Spread Spectrum (FHSS) It divides available bandwidth into N channels and hops

between these channels according to the PN sequence.

Fast hopping

Slow hopping




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Frequency Hopping Spread Spectrum (FHSS)



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Modulation




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Wi eless Access Tech. Lab.





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W A


Performance in the presence of interference Narrowband interference

Wideband interference

Gaussian noise




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A





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A



Narrowband interference



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Wideband interference




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Gaussian noise




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Hybrid FDMA/CDMA (FCDMA): The available wideband spectrum is divided into a number of

subspectras with smaller bandwidths.

Each of these smaller suchannels becomes a narrowband

CDMA system having processing gain lower than the originalCDMA system.



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Time Division Frequency Hopping (TDFH) The subscriber can hop to a new frequency at the start of a

new TDMA frame.

Has been adopted in GSM.




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Packet Radio (1)

In packet radio (PR) access techniques, many subscribersattempt to access a single channel in an uncoordinated (orminimally coordinated manner.

Collision from the simultaneous transmissions of multipletransmitters are detected at the BS, in which case an ACK orNACK signal is broadcast by the BS to alert the desired user ofreceived transmission.

PR multiple access is very easy to implement but has lowspectral efficiency and may include delays.

The subscribers use a contention technique to transmit on acommon channel.



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Packet Radio (2)

ALOHA protocols, developed for early satellite systems, alloweach subscriber to transmit whenever they have data to sent.

The transmitting subscribers listen to the acknowledgementfeedback to determine if transmission has been successful or

not.

If a collision occurs, the subscriber waits a random amount oftime, and then transmits the packet.

The performance of contention techniques can be evaluated bythroughput (T), which is defined as the average number ofmessage successfully transmitted per unit time, and the averagedelay (D) experienced by a typical message burst.



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Packet Radio Protocols

, vulnerable period is defined asthe time interval during which thepackets are susceptible tocollisions with transmission form

other user.

Packet A suffer a collision ifother terminals transmit packetsduring the period to

pV

1t 1 2t

Packet Radio (3)



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Packet Radio (4)

Assume that packet transmissions occur with Poissondistribution having mean arrival rate of packets per secondand is the packet duration in seconds. The traffic occupancyor throughput Ris given by .

Ris the normalized channel traffic (measured in Erlangs) and ifR> 1, then the packets generated by the users exceed themaximum transmission rate of the channel. For reasonablethroughput, 0 < R< 1.

Under normal loading, the throughput Tis the same as the totaloffered load L.

The load Listhe sum of the newly generated packets and theretransmitted packets that suffered collisions.

R



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Packet Radio (5)

The normal throughput is given as the total offered load timesthe probability of successful transmission, i.e.

The probability that n packets are generated by the userpopulation during a given packet duration interval is assumed toPoisson distributed and is given as

The probability that zero packets are generated (i.e., nocollision) during this interval is given by

Pr[ ] Pr[ ]T R nocollision nocollision

Pr(0) Re

Pr( )!

Re

nn



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Packet Radio (6)

Type of Access Contention protocols are categorized as:

Random Access: there is no coordination among that users andthe messages are transmitted from the users as they arrive at

the transmitter.

Scheduled Access: based on a coordinated access of users onthe channel and the users transmit messages within allottedslots or time intervals.

Hybird Access: a combination of random access and scheduled.



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Packet Radio (7)

Pure ALOHA The pure ALOHA protocol is random access protocol used for

data transfer and a user accesses a channel as soon as amessage is ready to be transmitted.

After a transmission, the user waits for an acknowledgment on

either the same channel or a separate feedback channel. In case of collisions, the terminal waits for a random period of

time and retransmits the message.

For pure ALOHA, the vulnerable period is double the packetduration: ,



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Packet Radio (8)

Slotted ALOHA In slotted ALOHA, time is divided into equal time slots of

length greater than the packet duration .

The subscribers each have synchronized clocks and transmit

a message only at the beginning of a new time slot. The vulnerable period of slotted ALOHA is only one packet

duration, since partial collisions are prevented throughsynchronization.

The probability that no other packets will be generated

during the vulnerable period is . The throughput for the case of slotted ALOHA is thus given

by .

R

e

RT Re



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Packet Radio (9)

Carrier Sense Multiple Access (CSMA) CSMA protocols are based on the fact that each terminal on

the network is able to monitor the status of the channelbefore transmitting information.

In CSMA, detection delay and propagation delay are twoimportant parameters. Detection delay is a function of he receiver hardware and is the

time required for a terminal to sense whether or not thechannel is idle.

Propagation delay is a relative measure of how fast it takes fora packet to travel from a BS to a MS.



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Packet Radio (10)

Several variations of the CSMA strategy 1-persistent CSMA

Non-persistent CSMA

p-persistent CSMA

CSMA/CD

Data sense multiple access(DSMA)



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Packet Radio (11)

Reservation Protocols Reservation ALOHA (R-ALOHA) R-ALOHA is a packet scheme based on time division multiplexing.

Two phase: contention phase and transmission phase

Mobiles contend the channel in reservation phase (slotted-ALOHA)

Mobiles that succeed in making reservation can transmit without

interference

ACK

Frame N

. . .B-M B-M

M-B M-B

ACK

. . .

Reservation

phase

Transmission

phase



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Packet Radio (12)

PRMA (Packet Reservation Multiple Access) A combination of TDMA and reservationALOHA

Ask channel resource in the talkspurt

Release channel resource in the silent gap

Permission probability Effect of voice activity detector

Frame N

B->M ACK B->M B->M ACK ACK...........

M->B M->B ...

M->B



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Packet Radio (13)

NC-PRMA (Non-Collision PacketReservation Multiple Access) The existing users inform the

BS about their demands in anon-collision manner (time-

frequency signaling scheme)

Ii : Information slotCMi : Control minislot

C : Control slot

UID : Uplink user identifier

DID : Downlink user identifier

Multiple Access Techniques for Wireless Communications-940706

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