chapter 5 multiple access systems

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Chapter Five

MultipleAccess

Techniques for Mobile

Communication Systems

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Objectives of the chapter

How multiple users share the same medium

Single shared broadcast channel

Going into higher data rates require proper multiplexing of lower

data rates

How two or more “simultaneous” transmissions possible

How to multiplex users

What are the degrees of freedom to choose from

Problems associated with multiplexing

Interference

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

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Lecture Outline

Introduction to multiple access techniques

FDMA techniques

TDMA techniques

CDMA techniques

Summary

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Used Acronyms

FDMA: Frequency division multiple access

TDMA: Time division multiple access

CDMA: Code division multiple access

DSSS: Direct sequence spread spectrum

FHSS: Frequency hoping spread spectrum

THSS: Time hoping spread spectrum

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Introduction to Multiple Access Techniques

Multiplexing: Allowing many (mobile) users to share a given resources

For high quality communication, this must be done without severe

degradation in the performance of the system

Multiple users want to communicate in a common geographic area

Cellular Example: Many people want to

talk on their cell phones. Each phone would

communicate(link) with a BS.

Problem: How should we share our

resources so that as many users as possible

can communicate simultaneously

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Hence multiplexing is the process of combining a number of

communication channels and transmitting them over one physical

medium

Where as de-multiplexing is the process used to separate and

recover the original channels at the receiver

Main types of multiplexing techniques

are: Frequency division multiplexing,

FDM Time division multiplexing, TDM

Code division multiplexing, CDM

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Example: MA techniques in wireless cellular communication

systems

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Frequency Division Multiple Access (FDMA)

FDMAis an analog MAtechnique where each transmitter is assigned a

portion of the frequency spectrum (or band) The transmitted signal spectra component must be confined to the

allocated frequency band

Different users are separated in the frequency domain.

To separate the channels, a guard band may be used

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Example: AMPS (1G, analog) used 30KHz for each user.

Pros

Very simple to design

Narrowband (no ISI)

Synchronization is easy

No interference among users

in a cell

Cons

Static spectrum allocation

High Q analog filters or larg

guard band is required

e

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Implementation example: NorthAmerican FDM telephone

Designed to transmit a large number of analog voice channels

Basic channel is called Voice Channel and has a BW of 0-4 kHz

The voice channel is modulated (frequency shifted) to occupy a

specific frequency band.

With three successive levels of multiplexing, a total of 600 voice

channels are multiplexed together.

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Drawbacks of FDMA

The bandwidths of FDMA systems are generally narrow

FDMAis usually implemented in a narrow band system

Prone to noise problems

Overtaken by TDMA, which is better suited for digital data

Demultiplexing requires a series of band pass filters

FDMA requires tight filtering to minimize adjacent channel

interference.

These filters are relatively complicated and expensive

As a result, receivers in FDMAsystem are generally expensive

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Time Division MultipleAccess Techniques (TDMA)

Each user is allowed to transmit only within specified time

intervals (Time Slots).

Different users transmit in different Time Slots.

When users transmit, they occupy the whole frequency

bandwidth (separation is performed in the time domain).

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TDMAsystem may be in two modes: FDD (Frequency division

duplex) and TDD (Time division duplex).

FDD and TDD are the techniques used to separate uplink and

downlink channel signals.

TDMAFDD

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TDMATDD


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TDMATDD: has a better advantage in case where the

asymmetry of the uplink and downlink data speed is variable.

As the amount of uplink data increases, more bandwidth can

be allocated to that and as it shrinks it can be taken away.

TDMAFDD: is much more efficient in the case of symmetric

traffic.

Uplink and downlink sub-bands are would be separated by the

"frequency offset".

Makes radio planning easier and more efficient since BS do not

hear each other (as they transmit and receive in different sub-

bands) and therefore will normally not interfere each other.


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TDMAFrame Structure: TDMArequires a centralized control

node, whose primary function is to transmit a periodic reference

burst that defines a frame and forces a measure of

synchronization of all the users.

The frame so-defined is divided into time slots, and each user is

assigned a Time Slot in which to transmit its information.

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TDMAframe structure

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TDMA guard times: Since there are significant delays between users,

each user receives the reference burst with a different phase, and its

traffic burst is transmitted with a corresponding different phase within

the time slot.

There is, therefore, a need for guard times to take account of this

uncertainty.

Each Time Slot is therefore longer than the period needed for the actual

traffic burst, thereby avoiding the overlap of traffic burst even in the

presence of these propagation delays.

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TDMAPreamble: Since each traffic burst is transmitted independently

with an uncertain phase relative to the reference burst, a preamble is

required at the beginning of each traffic burst.

The preamble allows the receiver to acquire the coarse

synchronization provided by the reference burst with a fine estimate

of timing and carrier phase.

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TDMAframe structure

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TDMA reference transmitter scheme

Buffer: the symbols of the original signal are organized in groups of

Nbps symbols. Each group is transmitted in a single Time Slot of

duration TS. Time Slots are organized in frames of duration TF.

Coder: the position in time , each group is modified according to the

TDMAcode, that is assigned to the user. The TDMA code indicates

which slot inside each frame must be occupied by the user.

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Types of TDM

Synchronous and asynchronous types

Synchronous TDM ( STDM)

Each source is repeatedly assigned a portion of the channel

TDM is understood to imply STDM

Used in circuit switched telephone networks

Asynchronous TDM( ATDM)

Each source is assigned a portion of the channel only when it is

needed

ATDM is used in statistical multiplexers, packet switches, and

asynchronous transfer mode (ATM)

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TDM Implementation

Bit interleaving: Each channel is assigned a time slot

corresponding to a single bit

Byte/word interleaving: Each channel is assigned a longer time

slot corresponding to some larger number of bits called word.


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Example1: T1 digital system

Analog voice signal is sampled at 8-kHz rate, i.e., the time

between samples is 125 μ second

Each sample is then companded, quantized, and represented by

8 bits (i.e., a time slot)

• Thus one voice call/channel is represented by 64-kbps stream

Digital Signalling 0 (DS0): is a basic digital signalling at the rate

of 64 kbps, corresponding to the capacity of one voice channel

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In the T1 system, 24 time slots are multiplexed together to form a

frame

Here byte interleaving is used

Each time slot is an 8-bit encoded word

One more bit is added for frame synchronization and alignment

If we combine these 64kbps channels together

• 24 voice time slots x 8 bits per slot = 192 bits

• 192 bits + 1 framing bit = 193 bits / frame

• 8000 frames/second x 193 bits/frame= 1.544 Mbps


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We can achieve wideband transmission rate of 1.544 Mbps

The frame structure is called T1 or DS1 (digital signalling 1)

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Hierarchical multiplexing from the basic T1 signal is used to get

higher data rates

This is based on bit interleaving

Example 2: T2 or DS2 is generated by multiplexing four T1 lines

in M12 multiplexer. Here, 17 bits added for frame synchronization

(193x4 + 17)x8000 = 6.312 Mbps

T3 or DS3: is formed from seven T2 lines in multiplexer

M23 69 bits for frame synchronization and plus stuffing

results in 5592 bits frame (789x7+69)

T3 bit rate is 5592x8000 = 44.736 Mbps

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T4 or DS4: from six T3 lines and adds 720 bits for

synchronization and pulse stuffing

• Thus, T4 rate is 274.176 Mbps

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Advantage of TDMATechniques

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Data transmission is bursty: low battery consumption,

• Tx can be turned off when it is idle

Hand off is simple: b/c BS can listen other BS during idle time

•

MS can only listen and broadcast for its own time slot.• On the rest of the time it can carry out measurement on the

network.

• Detect surrounding transmissions on different frequencies

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Code Division MultipleAccess (CDMA)

Alternative to FDMAand TDMA

What happen if we could allow users to share time and frequency

Eliminates need for tight synchronization among many

different users

Eliminates need for expensive analog filters

May have favourable impact on capacity

But:

How do we separate the users

Won’t they interfere with each other

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CDMA Principles: does not divide up the channel in time (as

TDMA), or frequency (as FDMA), but instead encodes data with a

certain code associated with a channel.

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Each user is assigned a unique code sequence (spreading code) to

encode its data signal.

The receiver, knowing the code sequence of the user, decodes the

received signal and recovers the original data.

The bandwidth of the coded data signal is chosen to be much

larger than the bandwidth of the original data signal.

Thus, the encoding process enlarges (spreads) the spectrum of

the data signal.

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CDMAis based on spread-spectrum modulation.

If multiple users transmit a spread-spectrum signal at the same

time, the receiver will still be able to distinguish between users,

provided that each user has a unique code that has a sufficient

low cross-correlation with the other codes.

Spread Spectrum Techniques:

Idea: “Spread” the signal to a wider bandwidth that is actually

necessary to avoid short, frequency selective distortions

o One form of frequency diversity exploitation

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Main implementation options of spread spectrum

1. Frequency and Time Hopping

2. Direct Sequence (is the basis for CDMA)

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Types of CDMA:

Direct Sequence CDMA (DS-CDMA): The original data signal

is multiplied directly by a high chip rate spreading code.

Frequency Hopping CDMA (FH-CDMA): The carrier

frequency at which the original data signal is transmitted is

rapidly changed according to the spreading code.

Time Hopping CDMA (TH-CDMA): The original data signal is

not transmitted continuously. Instead, the signal is transmitted in

short bursts where the times of the bursts are decided by the

spreading code.

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Direct Sequence CDMA (DS-CDMA): The original data signal

is multiplied directly by a high chip rate spreading code.35


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XOR of the signal with pseudo random number (chipping

sequence or keys)

Many chips per bit (e.g., 128) result in higher bandwidth of the

signal. The chipping sequence is termed as code or key

Advantage

• Reduces time-selective

fading by frequency diversity

Disadvantage

•

Precise power control is necessary

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DSSS: XOR of a “long” symbol with a chipping sequence/code

DSSS can also be determine by point wise product of vectors

when using +/-1 instead of 0/1

Assign: “1” = -1, “0” = +1

Formally: Transmitted signal,AS(t) for 0 ≤ t ≤ T,

• T symbol time

• Represent data by Ad(t) = constant over the duration of a

given symbol

•

Ak (t) is the chipping key for 0 ≤ t≤ T

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The resulting signal has more level changes per time

• Higher bandwidth required

Example 1: SenderAsendsAd = 1, chipping key Ak

=010011 Rewrite for only one value per chip as :

Ad = (-1, -1, -1, -1, -1, -1);A

k = (+1, -1, +1, +1, -1, -1)

Compute transmitted signal As = Ad * Ak (point wise product)

AS = Ad * Ak = (-1, -1, -1, -1, -1, -1) * (+1, -1, +1, +1, -1, -1)

= (-1, +1, -1, -1, +1, +1)

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Example 2: DSSS transmission on signal level

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DSSS at Receiver

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Example 1: CDMA in theory

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Example2: CDMA by equation

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Summary


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Summary

In communications, a channel access method is used to share a

communications channel or physical medium between multiple users.

The goal in the design of cellular systems is to be able to handle as many

calls as possible in a given bandwidth with some reliability.

In cellular system, each MS can distinguish a signal from the serving BS and

differentiate the signals from neighbouring BSs. To accommodate a number

of users, many traffic channels need to be made available.

In principle there are three different ways to allow access to the channel:

frequency, time, and code division multiplexing and is addressed by three

multiple access techniques; FDMA, TDMA, CDMA.

chapter 5 multiple access systems

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