Z. Ghassemlooy

Mobile Communication Systems

Professor Z Ghassemlooy

Scholl of Computing, Engineering and Information Sciences

University of NorthumbriaU.K.

http://soe.ac.uk/ocr

Professor Z Ghassemlooy

Scholl of Computing, Engineering and Information Sciences

University of NorthumbriaU.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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SDM

s2

s3

s1f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki

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Dividing the entire frequency spectrum into smaller bands A frequency band is allocated per channel for the entire

transmission time FDM, used in 1st generation systems, wastes spectrum Advantages:

– lower channel bit rate (than

TDM) means less susceptible

to multi path ISI

– Requires coordination

– works also for analog signals

k2 k3 k4 k5 k6k1

f

t

code

Frequency Multiplex I

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Frequency Multiplex II

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

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Entire spectrum is allocated for a channel some of the time For 2nd generation 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 multiplex I

f

t

code

k2 k3 k4 k5 k6k1

Time sl

ots

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Example TDMA System

GSM is a good example of a TDMA system• GSM 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

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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

• 16 1/16 time slots of length 288

Downlink Uplink

72s 288s Switching point between uplink and downlink

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Time and Frequency Multiplex 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

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Code Multiplex I

Each channel has a unique code. All channels use the same

spectrum at the same time. 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

k2 k3 k4 k5 k6k1

f

t

coding

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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 hoppingTime 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

Spreadingmodulation

Datamodulator

Data

DS-SS Transmitter

Codegenerator

Carriergenerator

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DS-SS Transmitter & Receiver

XWidebandmodulator

BinaryData

Codegenerator

Carriergenerator

Despreading Datademodulator

BinaryData

Codegenerator

Carriergenerator

CodeSynchronisation/

tracking

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CDMA Evolution

Early Stages

Narrowband

Wideband

1978 Cooper and Nettleton : cellular application of spread spectrum1980s Investigation of narrowband CDMA techniques for cellular applications1986 Formulation of optimum multiuser detection by Verdu1993 IS-95 standard

1995 - Europe : FRAMES FMA2Japan : Core-AUSA : cdma2000Korea : TTA I, TTA II

2000s Commercialization of wideband CDMA systems

WCDMA

1949 John Pierce : time hopping spread spectrum1949 Claude Shannon and Robert Pierce : basic ideas of CDMA1950 De Rosa-Rogoff : direct sequence spread spectrum1956 Price and Green : antimultipath “RAKE” patent1961 Magnuski : near-far problem1970s 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 s

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 2.4GHzEthernet Wireless LANConventional 2.4GHz

Ethernet Wireless LAN

5GHzEthernet Wireless LAN

(IEEE802.11)

5GHzEthernet Wireless LAN

(IEEE802.11)

Future 5GHzATM Wireless LAN

Future 5GHzATM Wireless LAN

25M bit/s25M bit/s

IMT 2000384kbit/s 〜 2Mbit/s

IMT 2000384kbit/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 Display Personal 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

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Questions and Answers

Tell me what you think about this lecture– fary@ieee.org