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First used for applications such as the police, the fire service and in taxis, later development lead to their use as full radio telephones, connected to the public switched telephone n

15 Mobile Telephone

Networks

Being away from a telephone, a telex or a facsimile machine has become unacceptable for many

‘out-of-touch’ This chapter discusses modern mobile radio communication technologies, covering

15.1 RADIO TELEPHONE SERVICE

band These supported international telephone services, as well as communication with

frequency) radio transmitters and receivers (transceivers) were developed First used for

applications such as the police, the fire service and in taxis, later development lead to

their use as full radio telephones, connected to the public switched telephone network

(PSTN) for the receipt and generation of ordinary telephone calls

The technology used a radio mast located on a hill and equipped with a powerful

multi-channel radio transceiver The mobile stations were weighty but were nonetheless popular for commercial ‘car telephone’ service, which grew rapidly in popularity in the mid-1970s

(using different frequencies) are needed to connect each telephone during conversation,

automatic radio telephone system of the late 1970s

297

Networks and Telecommunications: Design and Operation, Second Edition.

Martin P Clark Copyright © 1991, 1997 John Wiley & Sons Ltd ISBNs: 0-471-97346-7 (Hardback); 0-470-84158-3 (Electronic)

swltchlng Mobile centre ( M S C ) Public

switched telephone network

L O O

circutts

to PSTN

Figure 15.1 A simple radio telephone system

Figure 15.1 illustrates a single mobile switching centre and a single transmitter base station, serving 3600 mobile customers within an area of 1200 km2, within a radius of about 20 km from the base station Calls are made or received by the mobile telephone

the same way as an ordinary telephone sends an off-hook signal and a dialled digit train

between the base station and the mobile for the subsequent period of conversation, and the two switch over to the allocated channels simultaneously

Incoming calls to the radio telephone connect in the same way An ordinary telephone number is dialled by an ordinary customer connected to the public switched telephone

transmitter to reach the desired mobile receiver

A hurdle to be overcome in the design of radio telephone systems is the need for

failure of incoming calls, because the mobile cannot guarantee to be within the coverage area of a particular transmitting base station Early radio telephone systems overcame

wished to call The same customer number would be used on all incoming calls, but a different area code would have to be used, depending on which base station the caller

2 and 3, the numbers 0332 12345 and 0333 12345 are appropriate Some more advanced

CELLULAR RADIO 299

Dial area code Mobile as below plus

zone 2 0 3 3 2 zone 3 0 3 3 3

Figure 15.2 Area code routing for incoming radio telephone calls

radio telephone systems were developed with the ability to ‘remember’ the last location

methods, but these systems were all superseded by the advent of cellular radio, which has

eliminated the market for old-style radio telephones used by roaming users

Another drawback of the early radio telephone systems was their low user capacity,

and their ‘unfriendly’ usage characteristics Not only did the automatic systems rely on

action of the mobile user The systems allowed the mobile user to continue to move

station, the call would be terminated There were no facilities for transferring to other

base stations during the course of a call Together, these drawbacks lead to the demise

coverage, such as required by taxi companies or regional haulage companies The terms

private mobile radio ( P M R ) or trunk mobile radio ( T M R ) (in Germany Bundelfunk) are

more commonly used nowadays Some interest has been shown to extend the usage and

standards for T E T R A (trans-European trunk radio), which aims to provide relatively

located within relatively large geographical radio coverage areas T E T R A is discussed

more fully in Chapter 24

15.2 CELLULAR RADIO

Furthermore, the re-use of radio channels in other zones was precluded by the risk of

interference except where base stations were separated by large distances Because the 1970s saw a boom in radio telephone demand, and because radio channel availability

base stations, and mobile units, as Figure 15.3 shows

Cellular radio networks make efficient use of the radio spectrum, re-using the same

frequencies may be re-used Figure 15.5 illustrates the interfering zone of a given cell

base station, and shows another cell using the same radio channel frequencies

A whole honeycomb of cells is established, re-using radio channels between the

Figure 15.6 Seven different radio channel frequency schemes are repeated over each cluster of seven hexagonal cells, each cell using a different set of frequencies By such planning the same radio frequency can be used for different conversations two or three cells away

three cells, and some more than thirty cells, can also be used Large repeat patterns are necessary to cater for heavy traffic demand in built-up areas where small non-adjacent cells may still interfere with one another

Each cell is served initially by a single base station at its centre and is complemented

tional antennas helps to overcome radio wave shadows For example, to locate three

Cell coverage area

statlons

Figure 15.3 The basic components of a cellular radio network

CELLULAR RADIO 301

Figure 15.4 Cellular radio carphone: mounted and in use in a car

C e l l channels re-using

1 - 4 0 0

v

Figure 15.5 Cellular radio channel interference and re-use BS = Base Station

T r a n s m i t t e r

'Cluster'of seven cells

Adjacent 'cluster'

Figure 15.7 Location of multiple base stations

shadow effects that might otherwise occur near tall buildings by giving an alternative

A feature of cellular radio networks is their ability to cope with an increasing level of demand first by using more radio channels and more antennas in the cell, and then by

needed in a given cell is determined by the normal Erlung formula (Chapter 30)

MAKING CELLULAR RADIO CALLS 303

Figure 15.8 Cell splitting to increase cell capacity

congestion Figure 15.8 shows a simple splitting of cells and a gradual reduction in cell size in the transitional region between a low traffic (country) area and the high traffic

care needs to be taken when allocating radio frequencies to the new cells, and a new frequency re-use plan may be necessary to prevent inter-cell interference

15.3 MAKING CELLULAR RADIO CALLS

mobile station If a mobile user wants to make a call, the mobile handset scans the pre-

them for call requests On the receipt of such a request by any base station, a message is

sent to the nearest mobile switching centre ( M S C ) , indicating both the desire of the

strength, and, based on this, decides which cell the mobile is in It then requests the mobile handset to identify itself with an authorization number that can be used for call charging The authorization procedure eliminates any scope for fraud

Following authorization of an outgoing call, a free radio channel is allocated in the

destination on the public switched telephone network Incidentally, the appropriate cell

channel congestion At the end of the call, the mobile station generates an on-hook or end of call signal which causes release of the radio channel, and reverts the handset back

to monitoring the control and paging channel

Each mobile switching centre controls a number of radio base stations If, during the course of a call, the mobile station moves from one cell to another (as is highly likely,

new cell occurs without disturbance to the call, and is known as hand-oflor handover This is one of the most important capabilities of a mobile telephone network Hand-off

is initiated either by the active base station, or by the mobile station, depending upon

mobile switching centre establishes a duplicate radio and telephone channel in the new

message to the mobile handset When confirmed on the new channel and base station,

15.4 TRACING CELLULAR RADIO HANDSETS

Whenever the mobile handset is switched on, and at regular intervals thereafter, it uses the control channel to register its presence to the nearest mobile switching centre This

EARLY CELLULAR RADIO NETWORKS 305

enables the local mobile switching centre at least to have some idea of the location of the mobile user If outside the geographical area covered by the base stations controlled

registration procedure, in which it interrogates the home MSC (or the intelligent network database associated with it) for details of the mobile, including the authoriza-

database called the home location register, or HLR It contains the mapping informa-

identity, authorization and billing information) The local MSC duplicates some of this

MSC area Once the visiting location register has been established in the local MSC, outgoing calls may be made by the mobile user

whereabouts of mobile users, so that incoming calls can be delivered Incoming calls are first routed to the nearest mobile switching centre (MSC) to the point of origin (i.e the

caller) This MSC interrogates the home location register for the last known location of

the mobile user (this is known as a result of the most recent mobile registration) The call can then be forwarded to the mobile switching centre where the mobile was ‘last heard’, whereupon a paging mechanism, using the base station control channels, can

channel may then be selected for completion of the call

complex and expensive, although the increase in user demand is reducing the cost The

telephone, to the pocket versions The latter are expensive not only because of the feats

towards pocket telephones has necessitated advanced battery technology and the use of

( M S C s ) rely on advanced computers capable of storing and updating large volumes

of customer information, and also of rapidly interrogating other MSCs for the location

of out-of-area, or roaming mobiles The interrogation relies on the use of the mobile application part ( M A P ) and the transaction capability ( T C A P ) user parts o f SS7

signalling (which we discussed in Chapter 12)

15.5 EARLY CELLULAR RADIO NETWORKS

A number of different cellular radio standards have evolved, with the result that hand

portables purchased for use on one system are unsuitable for use on another The most

A M P S (Advanced Mobile Telephone System)

Laboratories, this became the most commonly used system in North America up to the early 1990s, when digital systems began to take over It operates in the 800 MHz and

N M T (Nordic Mobile Telephone Service)

This commenced service in the Nordic countries in 1981 and is used in other countries

in Europe (Austria, Belgium, Czechoslovakia, France, Hungary, Netherlands, Spain, Switzerland) The original system was 450 MHz based but has been gradually extended

C (Network C )

C-900); it is still in use in Germany and Portugal, but being replaced by GSM systems

TACS (Total Access Communication System)

MHz band, with a more efficient 25 kHz radio channel spacing TACs was the system introduced into the UK and Ireland during 1985 E T A C S or extended T A C S is a com-

congested areas like the metropolitan London area The system is also used in Austria, Italy and Spain

Telephone System used in Japan), Radiocom 2000 (used in France), R T M S (second

generation Mobile Telephone used in Italy), UNZTAX (used in China and Hong Kong) and Comvik (used in Sweden)

Table 15.1 Comparison of analogue cellular radio network types

GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS 307

The trend of the above systems to migrate to the 900 MHz follows the allocation of

frequencies in the band by the World Administrative Radio Council ( W A R C ) in 1979

as the control channel, to improve radio spectrum usage Digital channels allow closer

resulting from miniaturization and large scale production of components Further, the introduction of competing cellular radio telephone network operators as the first stage

of deregulation and competition for the traditional monopoly telephone companies

result has been a worldwide boom in mobile telephony The predominant technical

(personal communications network, also known as DCS-1800: digital cellular system/

1800 M H z )

15.6 GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS (GSM)

In 1982, CEPT (the European Conference of Posts and Telecommunications) decided

to start work on specifying new technical standards for the support of a pan-European mobile telephone service, based entirely on digital radio transmission The task group

initially an acronym of Groupe Speciale-Mobiles It was an ambitious programme which

handset users to roam anywhere on the European land mass and still be able to make and receive calls

The GSM programme was adopted by the European Commission of the European

the telecommunications market in Europe In particular, GSM became the potential for

for end user telephone equipment In parallel, it presented an opportunity for govern-

f i x e d ) telephone networks Finally, the programme was further reinforced by European

standing ( M O U ) initially signed in mid-1988, committing the network operators in European countries to ensure operation of the networks by 1991, allowing in addition, full roaming of mobile users between the networks and countries

operator licensing across Europe, and to the rapid deployment of GSM networks

the networks have been fully operational since 1993, with extensive coverage equalling