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Unit 1Unit 1

Introduction and System Fundamentals

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Unit 1 OverviewUnit 1 Overview

GSM System Standards

GSM Services

Basic System

Elements and Principles

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Unit 1 ObjectivesUnit 1 Objectives

Identify the objectives for the Global System for Mobile Communication (GSM) standard

Define the basic terms relating to wireless and cellular communication

Compare and contrast GSM with other wireless services

Relate basic technical concepts to their use in cellular systems

Identify the components of a cellular system and their functions

Understand the phased release of GSM specifications

Recognise the types of services supported by GSM

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Unit 1 Section 1Unit 1 Section 1

Basic System Elements andPrinciples

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Global System for Mobile

Communications (GSM)

Global System for Mobile

Communications (GSM)

Definition

The Global System for Mobile Communications (GSM) provides a common standard that enablesusers to roam from one country to another and obtain seamless telecommunications coverage and

services

Objectives Integrated European system with international roaming

Increase available cellular system capacity

Take advantage of digital price/performance and economies of scale Accommodate new technology and services

- ISDN services

- short messaging services

- user data and fax

- information privacy and secure access

- smart-card technology

- enhanced coding techniques

Apply to Cellular and Personal Communications Network services (GSM 900, GSM 1800, PCS 1900)

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Major Worldwide Mobile

System Standards

Major Worldwide Mobile

System StandardsFirst Generation - Analogue

Amps (US), TACS (UK), JTACS (Japan), NMT (Nordic)- existing Analogue FM Standards

Second - Generation Digital

GSM (European Digital Standard)- new 900 MHz Spectrum, TDMA, 271 kb/s

- new 1800 MHz Spectrum, TDMA, 271 kb/s

- PCS 1900, Air Interface Specification for 1.8 to 2.0 GHz Frequency Hopping Time Division

Multiple Access (TDMA) for Personal Communications Services, ANSI, J-STD-007

IS-136 (North American TDMA Digital Standard)- existing 850 MHz Bands, TDMA, 48 kb/s

- IS-136 Based, Air Interface Compatibility PCS 1900 MHz Standard, ANSI, J-STD-011

IS-95 (North American CDMA Digital Standard)- existing 850 MHz Bands, CDMA, 1.23 Mb/s

- Personal Station-Base Station Compatibility requirement for 1.8 to 2.0 GHz Code Division

Multiple Access (CDMA) Personal Communications, ANSI, J-STD-008

PDC (Japanese Digital Cellular Standard)- similar to IS-136 on the radio side and GSM on the network side

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Layout of a Basic Cellular NetworkLayout of a Basic Cellular Network

ToTelephone NetworkRadio Link

Land Links

Mobile-services Switching Centre

Mobile Station (Mobile Unit)

Base Station System(Cell site)

MSC

MS

BSS

MSCBSS

BSS

BSS

BSS

BSSBSS

BSS

MS

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Typical MSC FunctionsTypical MSC Functions

Provide switched connections between mobile and fixed (PSTN) phones

Provide switched connections between mobile subscribers

Provide coordination over signalling with mobiles

Coordinate the location and handover process

Provide custom services to mobile users

Collect billing data

Collect traffic data

Provisioning/service orders

Maintenance functions

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Typical Base Station System

Functions

Typical Base Station System

Functions

Provide RF transmission and reception

Provide data communications with the MSC and mobile stations

Locate mobiles

Perform routine maintenance testing

Perform equipment control and reconfiguration functions

Perform voice-processing functions

Perform set-up, supervision and termination functions

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Typical Mobile Station FunctionsTypical Mobile Station Functions

Provide a telecommunications interface to subscribers

Provide RF transmission and reception

Transmit and receive user information and control data

Perform voice-processing functions

Perform initialisation and self-test functions

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Cellular Concepts

The key ways inwhich a cellular

system can meet

its objectives are

through: The architecture

of the cellular

system

Frequency re-use

Providing call

handover

capabilities

Roaming

capabilities

Base StationRural

City

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Frequency Assignment

Available spectrum islimited

Need to support large

number of users

The challenge is toassign the available

frequencies across the

network while

minimising the co-channel reuse distances

The example shows a

repeat pattern of 7 cells

196 channels spread across cells

gives 28 channels per cell

1-28

1-28

29-56

29-5657-84

57-84

Brown gets channels 1-28 and

these can be re-used 2 cells away,

and so on

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Frequency Re-useFrequency Re-use

Depends on:

Number and size of cells

more smaller cells carry more

total traffic

enables frequencies to be

reused more

increased system cost

Frequency re-use achieved

tighter reuse gives increased

capacity (more bandwidth per

cell)

downside is increased

interference

Total available spectrum

4 cell repeat

3 cell repeat

D

R

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Radio Frequency (RF)

Channel Reuse

Radio Frequency (RF)

Channel Reuse

1,5,9,

2,6,10,...

3,7,11,... 1,5,9,

3,7,11,...

4,8,12,... 2,6,10,...Subscriber

Set

MSC

OtherPLMN

Other MSCs

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Interference and Re-use

Distance

Interference and Re-use

Distance

The re-use distance D is directly related to theradius of the cell R

Clearly the re-use distance increases as the

cell repeat number goes up (D=R3N)

It is not too difficult to relate the carrier tointerference ratio to re-use distance it is given

approximately by C/I=1.5N2

The table summarises this for various re-use

numbersN D/R C/I

3.5 13.8dB

7 .6 18.7dB

12 6 23.3dB

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Cellular Architecture

Coverage area of celldepends on traffic demand

National coverage achieved

with mobile location

continually monitored Handover across cell

boundaries

Small cells and lower

transmit powers

High network capacity -

frequency re-use

Radio channels are trunked

To PSTN via

Mobile Switching

Centre

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Omni/Sectored Base Stations

Omni-directional Cells:

360 degree coverage

low network capacity

cost-effective

Sectored Cells:

120 degree coverage

increases network capacity

smaller coverage area

improved frequency

reuse

3 times as much equipment improved antenna gain

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Why Sectorise?

Effectively creates a

number of smaller cells,

increasing capacity without

needing extra sites

Less interference becausesector antenna are

directional

Can also increase range

Gain limited by antenna

leakage and handoverproblems

Typical GSM deployment

has some omni-cells and

some 3-sectored cells

C1

C2C3B1

B2B3A1

A2A3 C1 A3

A1

C3

A2A3A2

B1

B2B3

C1

The old omnis

C/I=4.5N2

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Network Capacity & Frequency

Reuse

C1

C2C3B1

B2B3A1

A2A3 C1 A3

A1

C3

A2A3A2

B1

B2B3

C1

D3

D2D1C1

C2A1

A2A3 B1 D1

D3

A3

B2B3C2

B1

B2B3

C1

C2 D1C3

C3

B2B3

D3

D2D1

A1

A2

C3

A2

A2

3/9 Re-use 4/12 Re-use

There are many

combinations of

sectorisation and re-

use patterns

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Frequency AllocationFrequency Allocation

Frequency Allocation for a Total of 27 Carriers

A1 B1 C1 A2 B2 C2 A3 B3 C3

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18

19 20 21 22 23 24 25 26 27

For sectored cells the frequencies must be allocated so that cells do not

use adjacent frequencies

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Mobile ControlMobile Control

Mobiles need a general channel to

Log on, initiate calls, accept calls,

etc

This is called the control channel

Each base station has at least one

When a call is established the mobile

is re-tuned to a traffic channel

During the call all signalling takes

place over the traffic channelIdle

Idle

Idle

Call

Control Channel

Traffic

Channel

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Call Handover

An essential part of any cellular

radio system

Enables conversations to

continue as mobiles move

between base station coverage

areas

Process controlled by thesystem

Decision mainly based on

measurements by the mobile of

the best available servers

A margin is allowed before thedecision is made - this prevents

ping-ponging

Base 1

Base 2

HandoverPoint

ReceivedPower

Base 1

Base 2

Distance

DecisionMargin

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GSM HandoverGSM Handover

From

Frequency 6

Time Slot 3

To

Frequency 9

Time Slot 7MSC

Subscriber

Set

Lanline switched at MSCFrequency and time slot changed at MS

MS

BSS

BSS

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Intra-network Roaming

This is simply the normal process whereby a

MS can move about within the coverage area

of its home network

The home network tracks and records which base

stations the mobile is served by at any given point

so that calls can be routed to and from it

Location Areas

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Inter-network RoamingInter-network Roaming

Here the mobile is moving

between two different networks -

usually in different countries

When the mobile arrives in the

foreign network the network

determines its identity and seeksinformation from its home network

to authenticate its request for

service

Any calls made to the mobile first

arrive at its home network before

being forwarded to the foreign

network in which it is roaming

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Network Coverage

Depends on:

system characteristics (e.g. antenna gains etc)

type of service required (i.e. on-street, in-building

etc)

terrain characteristics

surroundings (i.e. clutter - trees, buildings etc)

Typically use smaller cells in urban areas

high traffic

dense clutter Larger cells in rural areas

lower traffic

less clutter40 km radius

1 km radius

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Microcells

In city centres, all the spectrum

saving measures are not enough Capacity is measured in channels

per unit area

The smaller the cell, the higher the

capacity per given area

Main way to make cells small is tobring antennas below the rooftop

level

Result is signal constrained to up to

1km of street

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Microcells

More capacity but more cost

Make handover difficult

But allow massive capacity increase

Save mobile battery power

Product now available

Cell size is

determined by

the power of thebase station and

the mobile

There are many other waysof increasing capacity at

hot spots - cell splitting and

overlaid cells

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The Need for Medium Access

Control

The earlier example showed that

for a 10 MHz bandwidth and arepeat factor of 7 the maximum

number of calls was 28 per cell

But in a cell there might be

thousands of people with a phone

Therefore, each person only gets achannel when they need it

Access to the medium needs to be

controlled

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Multiple Access MethodsMultiple Access Methods

Frequency Division Multiple Access (FDMA)

Frequency 1 ch

Frequency 2 ch

Frequency N ch

Time Division Multiple Access (TDMA)

Time Time Time

Slot 1 Slot 2 Slot N

ch ch ch

Code Division Multiple Access (CDMA)

Code Sequence 1 ch

Code Sequence 2 ch

Code Sequence N ch

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MAC Alternatives - FDMA

er

Fre enc

an i th

Centre Fre

ChCh Ch Ch ChCh

ach Carrier Carries neTra ic Channel

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MAC Alternatives - TDMA

1 2 3 4 5 6 7 8 1 2 3 4 5 6

1 2 3 4 5 6 7 8 1 2 3 4 5 6

1 2 3 4 5 6 7 8 1 2 3 4 5 6

1 2 3 4 5 6 7 8 1 2 3 4 5 6

1 2 3 4 5 6 7 8 1 2 3 4 5 6

1 2 3 4 5 6 7 8 1 2 3 4 5 6

Fr

Time

Carrier

Carrier

Carrier

Carrier

Carrier

Carrier

ac Time- l t Carries O eTrafficC annel

TDM Fr ame Lengt

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MAC Alternatives - CDMA

Code 1 Code 2 Code 3 Code 4

All Channels ShareSame RF Band

Ch 1 Ch 2 Ch 3 Ch 4

Power

Freq

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MAC Summary

Take an example of 2 MHz of bandwidth

Frequency division multiple access could

divide this into 40 bands, each 50 kHz wide

Time DMA could divide this into 40 time slots,

each 25ms wide

Code DMA could divide this into 40 codes,

each causing the information to be spread by

40

GSM uses a mixture - FDMA/TDMA

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CDMA

Not easy to understand

Easier to hear your colleague at a cocktail partywhen everyone else is speaking a different language

Digital signal generated by the speech encoder

Signal multiplied by the code allocated and

transmitted

Received signal multiplied by the same code

Result passed to the speech decoder

Spreading the signal by 40 means it takes up 40

times the bandwidth but can tolerate 40 times the

interference

Spread Spectrum

One cell repeat pattern

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TDMA vs CDMA

An impassioned debate over the last few years

GSM capacity easy to calculate

CDMA much more difficult - softer

Practical deployments suggests that CDMA may be around 30%

better than GSM

But

GSM hardware cheaper

world-wide roaming

lower risk and can be deployed now

CDMA is the chosen basis for the next generation

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Channels forTwo-Way

Communications

Channels forTwo-Way

Communications

Frequency Division Duplex

Uplink

Downlink

1 2 3

Uplink RF carrier channels

1 2 3

Downlink RF carrier channelsfrequency

Frequency separation

between uplink and

downlink channel pairs

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Digital Radio SystemDigital Radio System

Information

Transmit

Processing

Modulation

Processing

Information

Input

Information

ReceiveProcessing

Demodulation

Processing

Information

Output

Receiver

Transmitter

Received

RF Signal

Transmitted

RF Signal

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Physical Channel Structure

Used in P-GSM900

Physical Channel Structure

Used in P-GSM900

Frequency1 ch ch

Slot0 Slot1

ch ch

Slot7

Frequency2 ch ch ch ch

Frequency124 ch ch ch ch

Downlink

Frequency1 ch ch

Slot0 Slot1

ch ch

Slot7

Frequency2 ch ch ch ch

Frequency124 ch ch ch ch

Uplink

992Duplex Physical Channels Available

Time Domain

ARFCN1

Frequency

Domain

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TDMA Operation in GSMTDMA Operation in GSM

Full Rate

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Frame (Count) Frame (Count + 1)

0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7

Frame (Count) Frame (Count + 1)

MS7MS1

MS5MS0

BS

UPLINK

DOWNLINK

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Simplified DigitalTDMA

Implementation

Simplified DigitalTDMA

Implementation

Downlink

Timeslot detectionand Baseband

processing

ModTimeMUX

BasebandProcessing

1

BasebandProcessingM

M time

slots d1f

ModTimeMUX

BasebandProcessing

1

BasebandProcessing

M M timeslots dNf

Demod

d1f

Combiner

Timeslot detectionand Baseband

pro

cessing

Demod

dNf

MS MxN

MS1

Mobile ReceiversBase Transmitters

N frequency carriersM time slots