Z. Ghassemlooy

Mobile Communication Systems

Professor Z Ghassemlooy

Faculty of Engineering and

Environment

University of Northumbria

U.K.

http://soe.ac.uk/ocr

Professor Z Ghassemlooy

Faculty of Engineering and

Environment

University of Northumbria

U.K.

http://soe.ac.uk/ocr

Part 7- Multiplexing

Z. Ghassemlooy

Contents

Multiple Access

Multiplexing

– SDM

– FDM

– TDM

– CDM

Wideband Schemes

Duplex Method

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Multiple Access

In today’s data communications systems there

is a need for several users to share a common

channel resource at the same time.

– The resource could be:

• high speed optical fibre links between continents

• frequency spectrum in a cellular telephone system

• twisted pair ‘ethernet’ cable in the office

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Multiple Access

For multiple users to be able to share a common

resource in a managed and effective way, it

requires:

– Some form of access protocol

• Defines how or when the sharing is to take place and the

means for identifying individual messages. Process is known

as multiplexing in wired networks and multiple access in

wireless digital communications.

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Multiplexing/Multiple Access

There four possible ways to divide the frequency

spectrum among many channels:

Space-division multiplexing (SDM)

Frequency-division multiplexing (FDM) / Frequency Division

Multiple Access (FDMA)

Time-division multiplexing (TDM) / Time Division Multiple Access

(TDMA)

Code-division multiplexing (CDM) / Code Division Multiple

Access (CDMA)

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Space-division Multiplexing (SDM)

s2

s3

s1f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki The spatial dimension is used for

multiplexing

Data stream are transmitted over,

non-overlapping transmission

channels

Uses spot beam antennas

Base station tracks user when moving

Cover areas with same frequency as TDMA,

CDMA, and FDMA

Can be achieved using:

Beam forming

Sectorization

• But, needs perfect adaptive antenna system:

infinitely large antenna needed

• Compromise needed

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Dividing the entire frequency spectrum into smaller bands

A frequency band (carrier) is allocated per channel for the entire transmission time

FDM, used in 1st generation systems (wastes spectrum), GSM and UMTS FDD

Mode

Two bands of frequencies for every user

Advantages:

– lower channel bit rate (than TDM)

means less susceptible to multi

path ISI

– requires coordination

– works also for analog signals

– in contrast to TDM, each

stream can continuously

transmit within its sub-

band

k2 k3 k4 k5 k6k1

f

t

code

Frequency Multiplexing I

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Frequency Multiplexing II

OFDM

– Is an efficient FDMs, which offers minimum spacing of the sub-

bands without ISI

Frequency Division Multiple Access

– In multiple access schemes, where different data streams belong to

different users

Disadvantages

– In-efficient use of bandwidth if the traffic is distributed unevenly

– Requires guard band between channels

– Cannot readily support variable user data rates, fixed channel width

means fixed bit rate

Number of channels in a FDMA systemN … number of channels

Bt … total spectrum allocation

Bguard … guard band

Bc … channel bandwidth

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Entire spectrum is allocated for a channel some of the time. multiple users share

a single radio channel

For 2nd generation

Uses time for forward and reverse link

Advantages:

– Only one carrier in the medium at any given time

– High throughput even for many users

– Common TX component design,

only one power amplifier

Disadvantages:– precise synchronization

necessary

– requires terminal to support a

much higher data rate than the user information rate

Time multiplexing I

f

t

code

k2 k3 k4 k5 k6k1

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Example TDMA System

GSM is a good example of a TDMA system– GSM (850-900 MHz) handsets transmit data at a rate of 270 kbit/s in a 200 kHz

channel using GMSK modulation.

– Each frequency channel is assigned 8 users, each having a basic data rate of

around 13 kbit/s

– Signal is divided by time using the fundamental unit of a burst period. This

burst period = 15/26 ms and is grouped together by 8 bursts into a frame.

– A single traffic channel is defined by grouping 26 frames together; giving a total

timeframe of 120 ms. These traffic channels are used to transfer speech and

data.

Number of channels

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TDMA Frame

TDMA used for the 3G air interface

A frame length: 4.615 ms and it consist of

• 64 1/64 time slots of length 72 usec

• 16 1/16 time slots of length 288 usec

Downlink Uplink

72ms 288ms Switching point between uplink and downlink

Efficiency: It is a measure of the percentage of transmitted data that contains information as opposed to

providing overhead for the access scheme

f = (1-bOH/bT)*100%f: Frame efficiency

bOH: Number of overhead bits per frame

bT: Total number of bits per frame

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Time and Frequency Multiplexing I

Combination of both methods– A certain frequency band for a given amount of time is allocated

per channel

– Example: GSM

Advantages:– Improved protection against tapping

and frequency selective interference

– Higher data rates compared to code

multiplex

Disadvantages:

– Requires precise

coordination

f

t

code

k2 k3 k4 k5 k6k1

Code Division Multiplexing

Each channel has a unique spreading code.

All channels use the same

spectrum at the same time.

Spreading codes should as far as possible

be orthogonal to each other to reduce interference

The codes, one/zero sequences, used to

differentiate signals, are designed and

generated at a much higher rate than the

baseband information.

This rate is referred to

as a chip rate

rather than a bit rate.

k2 k3 k4 k5 k6k1

f

t

coding

Spreading factor = Chip rate/Data rate

Code Division Multiplexing

Advantages:

– bandwidth efficient and good power control

– no need for coordination and synchronization

– good protection against interference and

tapping

Disadvantages:

– lower user data rates

– more complex signal regeneration

Implemented using spread spectrum technology

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CDMA Classification

CDMA : direct sequence (DS)

CDMA : frequency hopping (FH)

– Carrier frequency changes periodically, after T secs

– Hopping pattern determined by spread code

CDMA : time hopping (TH)

– Data transmitted in rapid bursts

– Time intervals determined by code

Direct sequence

Frequency

hopping

Time hopping

Time

Frequency

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Direct Sequence CDMA

Directly modulated, discrete time, discrete valued

code signal

Analogue or Digital

Code bits are ‘chips’ (1)

Rate of Code >> Rate of Data

PSK, BPSK, D-BPSK,

QPSK or MPSK

Spreading

modulation

Data

modulatorData

DS-SS Transmitter

Code

generator

Carrier

generator

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DS-SS Transmitter & Receiver

XWideband

modulatorBinary

Data

Code

generator

Carrier

generator

DespreadingData

demodulatorBinary

Data

Code

generator

Carrier

generator

Code

Synchronisation/trac

king

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CDMA Evolution

Early Stages

Narrowband

Wideband

1978 Cooper and Nettleton : cellular application of spread spectrum

1980s Investigation of narrowband CDMA techniques for cellular applications

1986 Formulation of optimum multiuser detection by Verdu

1993 IS-95 standard

1995 - Europe : FRAMES FMA2

Japan : Core-A

USA : cdma2000

Korea : TTA I, TTA II

2000s Commercialization of wideband CDMA systems

WCDMA

1949 John Pierce : time hopping spread spectrum

1949 Claude Shannon and Robert Pierce : basic ideas of CDMA

1950 De Rosa-Rogoff : direct sequence spread spectrum

1956 Price and Green : antimultipath “RAKE” patent

1961 Magnuski : near-far problem

1970s Several developments for military field and navigation systems

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Wideband-CDMA

frame #i frame #i+1

timeslot #0 timeslot #1 timeslot #2 timeslot #13 timeslot #14

Radio Frame (10ms)

Time Slot (2560*Tc)

Tc = chip time = 1 / 3.84 ms

Framing structure

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High Speed Wireless Access

Mobile communication system

Up to 30 Mbps

Using the SHF and other band (3-60 GHz)

Used for mobile video telephone conversations

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Ultra High Speed Wireless LAN

Wireless LAN

Up to 156 Mbps

Using the millimeter wave radio band

(30-300 GHz)

Used for high quality TV conferences.

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5GHz Band Mobile Access

Two types

– ATM type Wireless Access

– Ethernet type Wireless LAN

Using 5GHz band

Each system can transmit at up to 20-25Mbps

Used for multimedia information

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High Data Rate Wireless LAN Evolution

ATMATM

Gigabit Ethernet

（1G bit/s)

Gigabit Ethernet

（1G bit/s)

Fast Ethernet

（100M bit/s)

Fast Ethernet

（100M bit/s)

Ethernet

（10M bit/s)

Ethernet

（10M bit/s)

Ethernet (10M bit/s)Ethernet (10M bit/s)

Conventional 2.4GHz

Ethernet Wireless LAN

Conventional 2.4GHz

Ethernet Wireless LAN

5GHz

Ethernet Wireless LAN

(IEEE802.11)

5GHz

Ethernet Wireless LAN

(IEEE802.11)

Future 5GHz

ATM Wireless LAN

Future 5GHz

ATM Wireless LAN

25M bit/s25M bit/s

IMT 2000

384kbit/s〜2Mbit/s

IMT 2000

384kbit/s〜2Mbit/s

36Mbit/s36Mbit/s

2M bit/s2M bit/s

ARIB, Japan, 1999

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Wireless Home-Link

Wireless Home-Link

Up to 100Mbps

Using the SHF and other band(3-60GHz)

Between PCs and Audio Visual equipments

Multimedia information.

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Home Link Concept

SatelliteTuner

CATV DVDVTR

Telephone line

5 GHz

5 GHz

Personal Computer DisplayPersonal Computer

Display

DisplayPersonal Computer

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Duplex Methods

Separating the send and receive signals (remember full duplex). Two approaches:

– Frequency Division Duplex (FDD)• Uses a pair of frequency bands – one for uplink and another for

downlink

– used in all second generation cellular systems

– requires good frequency separation filters - diplexer

– Time Division Duplex (TDD)• Uses a single frequency band for both uplink and downlink –

sharing the transmission time

– propagation delay limits cell size

– very efficient for asymmetric traffic, e.g. internet download

– used in cordless systems (DECT) and wireless LANs

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What is Universal Mobile Telecommunication System ?

European name for third generation (3G) radio system(1G = analog, 2G = digital voice and low speed data (GSM))

Key features with respect to 2G:

• Integration of fixed and mobile networks

• Expanded range of services (Packet, Internet, Multimedia)

Bit rates:

• Rural outdoor: 144 kb/s, 500 km/h

• Suburban outdoor: 384 kb/s, 120 km/h

• Indoor, low range outdoor: 2Mb/s, 10 km/h

• Flexibility:

• Variable bit rates

• Circuit switched and packet oriented bearers

• Negotiation of bearer service attributes

(bearer type, bit rate, delay BER, up/down symmetry, protection)

• Adaptability to quality, traffic, network load & radio conditions

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Summary

Multiple Access - sharing resources

– Frequency Division Multiple Access - FDMA

– Time Division Multiple Access - TDMA

• [Code Division Multiple Access – CDMA]

Duplex Methods

– Frequency Division Duplex - FDD

– Time Division Duplex - TDD

Z. Ghassemlooy

Questions and Answers

Tell me what you think about this lecture

– fary@ieee.org