chapter 04 - multiple access techniques

8/17/2019 Chapter 04 - Multiple Access Techniques

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EKT 450Mobile Communication System

Chapter 4: Multiple Access Techniques

Dr. Azremi Abdullah Al-Hadi

School of Computer and Communication Engineering

[email protected]

1

mailto:[email protected]:[email protected]


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Introduction

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• Multiple access schemes are used to allowmany users to share simultaneously a finiteamount of radio spectrum.

• Sharing of spectrum is required to increase

capacity• For high quality communication this sharing ofspectrum should not degrade performance ofthe system

•high performance

• duplexing generally required

• frequency domain

• time domain


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Duplexing

3

• What is Duplexing ? - to talk and listensimultaneously.

• Classification of communication systems accordingto their connectivity:

o Simplex

o

Half-duplex

o Duplex

A B

A B

A B


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Frequency Division Duplexing (FDD)

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• Two bands of frequencies for every usero

Forward band (for traffic from Base stationto mobile unit)

oReverse band (for traffic from mobile unit

to Base station)• Duplexer needed.• Frequency separation between forward band

and reverse band is constant throughout thesystem.

frequency separation/split

reverse channel forward channel

f


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Time Division Duplexing (TDD)

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• Uses different time slots for forward andreverse link :o

Forward time slot

oReverse time slot

• No duplexer is required (a simple switch canbe used)

•

Communication is not full-duplex

time separation/split

reverse channel forward channel

f


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FDD versus TDD

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•FDD

o Provides individual radio frequencies to eachuser hence, transceiver should work on twofrequency bands

o Frequency allocation must be carefully

coordinated with Out-of-band userso Duplexer needed

• TDD

o Single frequency hence simple transceivero Duplexer not needed, a switch can do the jobo There is time latency, communication is not full-

duplex


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Multiple Access Techniques

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• Frequency division multiple access (FDMA)• Time division multiple access (TDMA)• Code division multiple access (CDMA)• Space division multiple access (SDMA)

• Grouped as:o Narrowband systems

oWideband systems


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Narrowband System

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• Bandwidth of the signal is narrow comparedwith the coherence bandwidth of the channel.

• In narrowband systems available radio

spectrum is divided into large number ofnarrowband channels usually FDD (largefrequency split):o

Narrowband FDMA

o

Narrowband TDMA


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Narrowband System

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• Narrowband FDMA:o a user is assigned a particular channel which is

not shared by other users.o if FDD is used then each channel has a forward

and reverse link (called FDMA/FDD)• Narrowband TDMA:

o Allows users to share the same channel butallocates a unique time slot to each user

FDMA/FDD

FDMA/TDD

TDMA/FDD

TDMA/TDD


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Logical Separation FDMA / FDD

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f

t

user 1

user n

forward channel

reverse channel

forward channel

reverse channel

...


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Logical Separation FDMA / TDD

11

f

t

user 1

user n

forward channel reverse channel


...


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Logical Separation TDMA / FDD

12

t

user 1 user n

forward

channel

reverse

channel

forward

channel

reverse

channel

...f


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Logical Separation TDMA / TDD

13

f

t

user 1 user n

forward

channel

reverse

channel

forward

channel

reverse

channel

...


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Wideband System

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• The transmission BW of a single channel ismuch larger than the coherence bandwidth ofthe channel.

• Users are allowed to transmit in a large part

of the spectrum.

• Large number of transmitters on onechannel.

• TDMA techniques allocates time slots todifferent transmitters.

• CDMA techniques allows the transmitters toaccess the channel at the same time.


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Wideband System

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TDMA/FDD

TDMA/TDD

CDMA/FDD

CDMA/TDD


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Logical Separation CDMA / FDD

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code

f

user 1

user n



...


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Logical Separation CDMA / TDD

17

code

t

user 1

user n



...


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Multiple Access Techniques

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TDMA

p o w e r

CDMA

p o w e r

FDMA

p o w e r


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Multiple Access Techniques in use

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Advanced Mobile Phone System (AMPS) FDMA/FDDGlobal System for Mobile (GSM) TDMA/FDD

US Digital Cellular (USDC) TDMA/FDD

Digital European Cordless Telephone (DECT) FDMA/TDD

US Narrowband Spread Spectrum (IS-95) CDMA/FDD

Cellular SystemMultiple Access

Techniques


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Frequency Division Multiple Access(FDMA)

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• One phone circuit per channel• Idle time causes wasting of resources• Simultaneously and continuously

transmitting• Usually implemented in narrowband systems• Complexity of FDMA mobile systems is

lower compared to TDMA

• FDMA uses duplexers• For example: AMPS is a FDMA system

with bandwidth of 30 kHz


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Nonlinear Effects in FDMA

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• Many channels - same antenna• For maximum power efficiency operate near

saturation• Near saturation power amplifiers are nonlinear

• Nonlinearities causes signal spreading• Intermodulation frequencies• IM are undesired harmonics• Interference with other channels in the FDMA

system• Interference outside the mobile radio band:

adjacent-channel interference• RF filters needed - higher costs


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Time Division Multiple Access(TDMA)

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• Time slots• One user per slot• Buffer and burst method

• Non-continuous transmission• Advantage:

o Total bandwidth is utilized• Disadvantage:

o Strict Burst Timing is required at theearth station


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Repeating Frame Structure

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Slot 1 Slot 2 Slot 3 … Slot N

Preamble Information Message Trail Bits

One TDMA Frame

Trail Bits Sync. Bits Information Data Guard Bits

The frame is cyclically repeated over time.


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Features of TDMA

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• A single carrier frequency for several users• Transmission in bursts• Handoff process much simpler (can listen

when idle)•

Low battery consumption• Bandwidth can be supplied on demand• Compared to FDD : Switch instead of

duplexer• High synchronization overhead• Example: GSM


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FDMA versus TDMA

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• Fewer bits for synchronization

• Fewer bits for framing

• Higher costs for duplexer used in base

station and subscriber units

• FDMA requires RF filtering to minimize

adjacent channel interference


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Spread Spectrum Multiple Access(SSMA)

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• Spread Spectrum Multiple Access (SSMA) usessignals have transmission BW that is several ordersof magnitude greater than the minimum requiredBW.

• A Pseudo-noise sequence converts a narrow bandsignal to a wideband noise-like signal beforetransmission.

• SSMA not BW efficient when used by a single user.• Many users can share the same BW without

interfering with one another• Type of SSMA techniques:

• Frequency hoped multiple access FHMA).

• Direct sequence multiple access DS) or Code

division multiple access CDMA).


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Pseudo-Noise (PN) Sequence

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PN code sets can be generated from linearfeedback shift registers.

Modulo 2 adder

Stage 1 Stage 2 Stage 3Register

Output

Shift Register

Clock


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Frequency Hopped Multiple Access(FHMA)

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• It is digital multiple access system.• Carrier frequencies of individual users varied in

pseudorandom fashion with in a wideband channel.• Digital data broken into uniform sized bursts which are

transmitted on different carriers.• Instantaneous BW of any one transmission burst is

much smaller than the total spread BW.• Locally generated PN code is used to synchronize the

receiver frequency with that of transmitter.

• Erasures can occur when two or more users transmit onthe same channel at the same time

• FH(Frequency hopped) signal is immune to fading soerror control coding can be combined to guard against

erasures.


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Code Division Multiple Access(CDMA)

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• Narrowband signals is multiplied by a very large bandwidthsignal called the spreading signal.

• The spreading signal is pseudo noise code sequence that has achip rate which is orders of magnitudes greater than data rateof the message.

• All users use the same carrier frequency and transmitsimultaneously.

• Each user has its own pseudo random code word which isapproximately orthogonal to other codewords.

• Receiver performs time co-relation.• All other codewords appear as noise.• Receiver needs to know the code word used by transmitter.• Many users same frequency, TDD or FDD.• Soft capacity(capacity increases linearly)

• Self-jamming


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• Signal Spreading: Transmission bandwidth significantlyexceeds information bandwidthEach User is assigned a unique spreading Code.

• Processing Gain: Number of chips per data symbol.Processing gain reflects the ratio between the transmission

and information bandwidths.

DataSignal Spreading

Data

SpreadingCode

ReceivedSignal

SpreadingCode

TransmittedSignal

S(f)

f

S(f)

f

TSymbol

TChip

CDMA – Basic Concept

30


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• Signal De Spreading: Multiplying the received signal by thespreading code

De-spreading of the received signal with the same spreadingcode that was used for spreading restores the original data

De-SpreadSignal

Signal De-Spreading

Spreading

Code

De-SpreadSignal

SpreadingCode at Rx

ReceivedSignal

S(f)

f

S(f)

f

TSymbol

TChip

ReceivedSignal TChip

SpreadingCode at Tx



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• Signal De Spreading: Multiplying the received signal by thespreading codeDe-spreading of the received signal with a different spreadingcode than that was used for spreading does not restore theoriginal data and maintains bandwidth characteristics of

spread signal

De-SpreadSignal

Signal De-Spreading

Spreading

Code

De-SpreadSignal

SpreadingCode at Rx

ReceivedSignal

S(f)

S(f)

f

TSymbol

TChip

ReceivedSignal

SpreadingCode at Tx

f



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De-SpreadSignal

Symbol Detection: De-spreading using the same spreading

code that was used for spreading

SymbolT

0

TSymbol

4 -4 -4 4

De-SpreadSignal

SymbolT

0

TSymbol

0 0 0 0

Symbol Detection: De-spreading using a different spreading

code than that used for spreading



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m1(t)

c1(t)

m1(t)c1(t)

m2(t)

c2(t)

m2(t)c2(t)

Transmitter for User 1

Transmitter for User 2

SymbolT

0

Receiver for User 1

Wireless

Channel

m1(t)c1(t)+m2(t)c2(t)

SymbolT

0

Receiver for User 2

c1(t)

c2(t)

m1(t)+

m2(t)c1(t)c2(t)

m2(t)+m1(t)c1(t)c2(t)

m1(t)+e1(t)

m2(t)+e2(t)

m’1(t)

m’2(t)

mi(t): Information Message of User ici(t): Spreading code of user iei(t): Interference sensed at receiver of user I

m’i(t): Message detected at receiver

Important Note:

The value of ei(t) depends on the crosscorrelation properties between c1 & c2

ei(t)=0 if c1 & c2 are orthogonal

CDMA Operation


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The CDMA concept has been introduced asearly as 1970s in military applications toelude jamming signals

frequency

Spectraldensity

frequency

SpectraldensityJamming

signal

signal

signal

De-spreading

CDMA in Military Application


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Data

Symbol

Spreading Code

CommunicationChannelSignal Spreading

Interference Spreading Code

Symbol

Detection

Signal De-spreading

BW= BS BW= GBS BW= GBS BW= BS

CDMA in Wireless Communication


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Spreading Code Requirements

• Good CDMA spreading codes should be characterized byrelatively low cross-correlation properties to minimizemultiple access interference (MAI).

• Good CDMA spreading codes should be characterized bylow autocorrelation properties to minimize inter-symbolinterference due to multi-path channels

• Ideally it is desirable to have both correlation functionsto approach zero


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Example: Correlation Matrix of 31-bitPN Sequence

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5 10 15 20 25 30

5

10

15

20

25

30 -10

-5

0

5

10

15

20

25

30

chapter 04 - multiple access techniques

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