Xem mẫu
- 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)
15
Mobile Telephone
Networks
Being away from a telephone, a telex or a facsimile machine has become unacceptable for many
individuals, not only because they cannot be contacted, but because they may be deprived
also of
the opportunity to refer to others for advice or information. For these individuals, the advent of
mobile communications promises a era, one in which there will never be excuse for being
new an
‘out-of-touch’. This chapter discusses modern mobile radio communication technologies, covering
the principles of ‘radio telephone service’, ‘trunk mobile radio (TMR)’ cordless telephones, the
‘global system for mobilecommunication (GSM)’, as well as describing telephone communication
withships,aircraft, and trainsandtheemergingsatellitemobilenetworks(e.g.Iridiumand
Globalstar). Mobile datacommunication networks are covered in Chapter 24.
15.1 RADIO TELEPHONE SERVICE
The first radio telephone services were manually operated in the high frequency radio
band. These supported international telephone services, as well as communication with
ships and aircraft. Immediately after World War 2, a new breed of VHF (very high
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.
For calls made to or from the radiotelephone user, the public telephone network is
connected via mobile switching centres (MSCs)to a number of transmitter base stations
which emit and receive radio signals from the mobile telephones. Two radio channels
(using different frequencies)are needed to connect each telephone during conversation,
one radio channel for each direction of conversation. Figure 15.1 illustrates a typical
automatic radio telephone system of the late 1970s.
297
- 298 NETWORKS TELEPHONE MOBILE
Mobile
swltchlng
centre
(MSC) Public
switched
telephone
network
LOO
circutts
to PSTN
Figure 15.1 A simple radio telephonesystem
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 orreceived by the mobile telephone
station by monitoring a particular radio channel,called the signalling or call up channel.
When making a call, the mobile station signals a message to the base station in much
the same way as an ordinarytelephone sends an off-hook signal and a dialled digit train
(Chapter 7). The base station next allocates a free pair of radio channels to be used
between the base station and themobile for the subsequent period of conversation, and
the two switch over to the allocated channels simultaneously.
Attheendofthe call theradiochannelsare released for use onanother call.
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
network (PSTN). A particular area code within the normaltelephone numbering plan is
usually allocated to identify the mobile network, and all calls with this dialled
area code
arerouted the
to mobileswitchingcentre and via the appropriate base station
transmitter to reach the desired mobile receiver.
A hurdle to be overcome in the design of radio telephone systems is the need for
incoming calls to be routed only to the nearest base station. Failure to so causes the
do
failure of incomingcalls, because the mobile cannot guarantee tobe within the coverage
area of a particular transmitting base station. Early radio telephone systems overcame
the problem by requiring the caller to know the whereabouts of the mobile that he
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
wished the call to be routed to (i.e. according to which zone the caller thought the
mobile was in). Figure 15.2 illustrates this principle.
We see here that if the mobile customer’s number is ‘12345’, then when a caller believes
that the mobile in zone 1, the number033 1 12345 should be dialled. Similarly for zones
is
2 and 3, the numbers0332 12345 and 0333 12345 are appropriate. Some more advanced
- CELLULAR RADIO 299
Dial area code
Mobile as below plus
zone 2 0332
zone 3 0 333
Figure 15.2 Area code routing for incoming radio telephone calls
radio telephone systems were developed with the ability to ‘remember’ the last location
from which acall was made and route the callthere.Others used ‘trial-and-error’
methods, butthese 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
the caller to know where themobile was, but somedemandedthe‘press-to-speak’
action of the mobile user. The systems allowed the mobile user to continue to move
about during the call, but if he or she moved out of the coverage area of the base
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
of radiotelephonyinitsoriginal guise andtoits replacement with cellularradio,
discussed next. The technologyhas, however, survived for localized radionetwork
coverage, such as requiredby 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
life of the technology, as demonstrated in the recent development by ETSI of new
standards for T E T R A (trans-European trunk radio), which aims to provide relatively
low speed integrated voice and data networking capability to and from mobile stations
located within relatively large geographical radio coverage areas. T E T R A is discussed
more fully in Chapter 24.
15.2 CELLULAR RADIO
A difficulty with early radio telephone systems was the small capacity in number of
users. Thisarose because theearly systems were designed to have very wide area
coverage, buthad a limited number of radiochannelsavailable within eachzone.
Furthermore, the re-use of radio channels in other zones was precluded by the risk of
- 300 NETWORKS MOBILE TELEPHONE
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
was (and still is) alimitedresource,therewaspressure for development of revised
methods, more efficiently using the radio bandwidth, therebygreatlyincreasingthe
available user capacity. The system that evolved has become known as cellular radio.
The basic components of cellular radio networks are mobile switching centres (MSCs),
base stations, and mobile units, as Figure 15.3 shows.
Cellular radio networks make efficient use of the radio spectrum, re-using the same
radio channel frequencies in a large number of base stations or cells. Oriented in a
'honeycomb'fashion,each cell is keptsmall, so thattheradiotransmitting power
required at the transmitting base station can be kept low. This limits the area over
whichthe radio signal is effective, and so reduces the area overwhichradiosignal
interference can occur. Outside the interfering zone of the transmitter, i.e. in a non-
adjacent cell at a sufficient distanceaway fromthe first,thesame radio channel
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 radiochannelsbetweenthe
various cells accordingto a pre-determined plans. A seven cell re-usepattern is shown in
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.
Figure 15.6 shows a 7-cell re-use pattern. Other patterns, some involving as few as
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
as trafficgrowswithdirectional antennasandmoreradiochannels. Usingdirec-
tional antennas helps to overcome radio wave shadows. For example, to locate three
Cell coverage
area
/
'Cells'
? / MSCs or
centres;
/\" I /
/ "
Mobile
handset Other base
statlons
Figure 1 . The basic components of a cellular radio network
53
- CELLULAR RADIO 301
Figure 15.4 Cellular radio carphone: mounted and in use in a car
Cell
re-using
channels
1-400
v
Figure 15.5 Cellular radio channel interference and re-use. BS = Base Station
- 302 NETWORKS TELEPHONE MOBILE
'Cluster'of Adjacent
seven cells 'cluster'
W
Figure 15.6 Cellular radio in re-use pattern
obstacle
Radio 'shadow'
Transmitter Mobilestation
Alternative radio path
Figure 15.7 Location of multiple base stations
directional antennas, one at each alternate corner of the cell, helps to overcome the
shadow effects that might otherwise occur near tall buildings by giving an alternative
transmission path, as Figure 15.7 shows.
A feature of cellular radio networks their ability to cope withan increasing level of
is
demand first by using more radio channels and more antennas in the cell, and then by
reducing the size of cells, splitting the old cells to form a multiplicityof new ones. Only a
limited number of radio channels can be made available in acell at the same time, and
thislimitsthenumber of simultaneoustelephoneconversations. By increasingthe
number of cells, the overall call capacity can be increased. The number of channels
needed in a given cell is determined by the normal Erlung formula (Chapter 30).
The total call demand during the busy hour of the day depends on the number of
callers within the cell at the time. Reducing the size of the cell has the effect of reducing
the number of mobile stations that is likely to be in it at any time, and so relieves
- RADIO MAKING CELLULAR CALLS 303
R---
/
1
/ Metropolitan Country
zone zone
l
‘--A
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
region surrounding a major metropolitan zone. When splitting cells in this manner, due
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 MAKINGCELLULAR RADIO CALLS
One or more control orpuging radio channels used to make or
are receive calls between the
mobile station. If a mobile user wants to makea call, the mobile handset scans the pre-
determined channelsto determine the strongest control channel, monitors to receive
and it
network status andavailability information. When the telephone number the destina-
of
tion has been dialled by the customer and the send key has been pressed, the mobile
handset finds a free control channel and broadcasts a request for a user (i.e. radio
telephone) channel. All the base stations use the same control channels, and monitor
them for call requests. On the receipt of such a request by any base station, amessage is
sent to the nearest mobile switching centre ( M S C ) , indicating both the desire of the
mobile station to place a call and the strength of the radio signal received from the
mobile. The MSC determines which basestationhas received the greatest signal
strength, and, based on this, decides which cell the mobile is in. It then requests the
mobile handset to identifyitself with an authorization number that canbe 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
appropriate cell forthecarriage of the call itself, andthe call is extended to its
destination on the public switched telephone network. Incidentally, the appropriate cell
need not necessarily imply the nearest base station; more sophisticated cellular net-
worksmight also choose to use adjacent base stations if this will help to alleviate
- 304 NETWORKS TELEPHONE MOBILE
Newstronger
signalradiopath
7 p a t hw
Ne
/ I switching
l
centre)
Old ell
c
P a t h re1 e a s e d
on h a n d off-
Figure 15.9 ‘Hand-off’ during a call
channel congestion. At the end of the call, the mobile station generates an on-hook or
end o call signal which causes release of the radio channel, and
f reverts the handset back
to monitoring the control and paging channel.
Each mobile switching centre controls a numberof radio base stations. If, during the
course of a call, the mobile station moves from one cell to another (as is highly likely,
because the cells are small), then the MSC is able to transfer the call to route via a
different base station, appropriate to the position. Theprocess of changeover to the
new
new cell occurs without disturbance to the call, and is known as hand-oflor handover.
This is one of the most importantcapabilities of a mobile telephone network. Hand-off
is initiated either by the active base station, or by the mobile station, depending upon
the network and system type. The relative strengths of signals received at all the nearest-
base-stationsarecompared withone anothercontinuously,and whenthecurrent
station signal strength falls below a pre-determined threshold, or is surpassed by the
signal strength available via an adjacent base station, then hand-ofSis initiated. The
mobile switching centre establishes a duplicate radio and telephone channel in the new
cell, and once established, the call is transferred to the new radio path by a control
message to the mobile handset. When confirmed on the new channel and base station,
the original connection is cleared, as Figure 15.9 illustrates.
15.4 TRACING CELLULAR RADIOHANDSETS
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
- AR EARLY 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
by its home MSC, the local MSC (i.e. the nearest to the mobile user) undertakes a
registrationprocedure, in which itinterrogatesthe home MSC (orthe intelligent
network database associated with it) for details of the mobile, including the authoriza-
tion number and other information. The information is held by the home MSC in a
database called the home location register, or HLR. It contains the mapping informa-
tion necessary for completing calls to the mobile user from the PSTN (its network
identity, authorization and billing information). The local MSC duplicates some of this
information in a temporary visitor location register or VLR, until the caller leaves the
MSC area. Once the visiting location register has been established in the local MSC,
outgoing calls may be made by the mobile user.
The registration procedure is a crucial part of the mechanism used for tracing the
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
locate the exact cell in which themobile is currently located. A suitablefreeradio
channel may then be selected for completion of the call.
Both the handsets and the network infrastructure needed to supportcellular radio are
complex and expensive, although the increase in user demand is reducing the cost. The
mobile handsets come in a number different forms, from the traditional car-mounted
of
telephone, to the pocket versions. The latter areexpensive not only because of the feats
ofelectronicsminiaturizationthathas been necessary butalso because thetrends
towards pocket telephones hasnecessitated advanced battery technology and theuse of
sophisticated battery conservation methods (which, in effect, turn much of the unit off
forasmuch of the time as possible, to save power). The mobileswitchingcentres
(MSCs) rely on advanced computers capable of storing and updating large volumes
of customer information, and alsoof rapidly interrogating other MSCs for the location
of out-of-area, or roaming mobiles. The interrogation relies on the use of the mobile
applicationpart ( M A P ) andthe transactioncapability ( 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
common o f the systems currently in use worldwide are as follows.
A M P S (Advanced Mobile Telephone System)
This was first introduced in Chicago in 1977 by Illinois Bell, at that time one o f the
operatingcompanies of AT&T in theUnitedStates.Developed by AT&T’s Bell
- 306 MOBILE TELEPHONE NETWORKS
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
900 MHz radio bands, at a channel spacing of 30 kHz.
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
and to some extent replaced after 1986 with the later NMT-900 version.
C (Network C )
Network C was a developmentof Network B, a digitally controlled mobile radio system
introduced in the Federal Republic Germany in 1971. Network Bwas 450 MHz based.
of
Its replacement, Network C can be either 450 MHz or900 MHz based (hence C-450 and
C-900); it is still in use in Germany and Portugal, butbeing replaced by GSM systems.
TACS (Total Access Communication System)
This is a derivative of the American AMPS standard, modified to operate in the 900
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-
patible derivative of TACs which gives greater channel availability, particularlyin very
congested areas like the metropolitan London area. The system is also used in Austria,
Italy and Spain.
Other mobile telephone standards include N A M T S (Nippon Automatic Mobile
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
AMPS C 450 NMT 450 NMT 900 TACS
(USA) (Germany) (Scandinavia) (Scandinavia) (UK)
Uplinkband 824-849 MHz 450-455 MHz
453-458 890-915 890-915
MHz MHz MHz
Downlink
869-894 461-466 463-468 935-960 935-960
MHz MHz MHz MHz MHz
band
Channel 30 kHz kHz 20 25 kHz 25 kHz 25 kHz
spacing
Multiplexing FDMA FDMA
FDMA FDMA
Modulation
FSK PSK PSK
FSK FSK
Number of 833 222 180 1000 1000
channels
- SYSTEM GLOBAL COMMUNICATIONS
FOR MOBILE 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.
Subsequently there has been pressure to digitalize the radio speech channels, as well
as the control channel, to improve radio spectrum usage. Digital channels allow closer
channel spacing, dueto reduced interferencebetween channels. Theadoption of
digital
technology enabled, greater
first, efficiency in the use of
available radio
bandwidthandtherefore higher traffic volumes;second, dramatic pricereductions
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
simultaneouslyresulted in muchreducedpricesformobiletelephone services. The
result has been aworldwide boom in mobile telephony. The predominant technical
standardsarethose of GSM (globalsystemfor mobilecommunication) and PCN
(personal communications network, also known as DCS-1800: digital cellular system/
1800 MHz).
15.6 GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS (GSM)
In 1982, CEPT (the European Conference of Posts and Telecommunications) decided
to start work onspecifying new technical standards for the support of a pan-European
mobile telephone service, based entirely on digital radio transmission. The task group
set up to performthespecificationwork was theCEPTiGSMgroup, GSM being
initially an acronym of Groupe Speciale-Mobiles. It was an ambitious programme which
targetted a network spanning the whole of Europe by 1991, which would allow mobile
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
Union, who recognized its potential for spurring standardization and deregulation of
the telecommunications marketin Europe. In particular, GSM became the potential for
standardization of common networks across Europe, thus opening a common market
for end user telephone equipment. In parallel, it presented an opportunity for govern-
ments of the European Union member countries to commence deregulation of their
monopoly telephone markets without undue threat the vast established terrestrial (or
to
f i x e d ) telephone networks. Finally, the programme was further reinforced European
by
Commission pressure on member countries to participate in a memorandum o under- f
standing ( M O U ) initially signed in mid-1988, committingthenetworkoperators in
European countries to ensure operation of the networks by 1991, allowing in addition,
full roaming of mobile users between the networks and countries.
Although the 1991 target was not met by all countries, it led to a flurry of second
operator licensing across Europe, and to the rapid deployment of GSM networks.
In most European countries, at least two GSM networks have been established, one
owned by the monopoly operator, one operated by a new competing carrier. Most of
the networks have been fully operational since 1993, with extensive coverage equalling
or bettering the previous analogue mobile networks. With call prices under pressure
from competition, the GSM mobile telephone boom took hold in late 1993.
- 308 NETWORKS TELEPHONE MOBILE
A major attraction of the GSM system has been the ability of its users to make and
receive telephone calls anywhere in the GSM world (anywhere in Europe, Australia,
New Zealand, South Africa, Hong Kong, etc.) without having to advise callers of any
change in telephone number (this is called international roaming).
15.7 GSM TECHNOLOGY
Figure 15.10 presents the structure and main elements of a GSM network.
In general, the system works exactly as described earlier in the chapter. The mobile
switching centres (MSCs) are the main controllingelements of the networks (a national
network typically comprises 10-25 MSCs). Each controls a given geographic area over
which a number of base transmitter stations ( B T S ) are spread (typically 5-8 km radius
cells). The BSC (base station switching centre) is the control element for the base trans-
mitter stations, but need not be collocated with the BTS. Thus in a dense metropolitan
area, several antenna sites may be used, but they require only one small BSC switching
radio part
mobile B!
I m-
I-
interface U, A 0
AuC
authorization
centre HLR Home locationregister
BSCbasestationswitchingcentre MSCmobileswitchingcentre
BSS station
base sub-system NSS networkandswitchingsub-system
BTS transmitter
base station OMCoperationsandmaintenancecentre
EIR equipment identity
register OSS operationsandsupportsub-system
Figure 15.10 Main components of a GSM network
- GSM TECHNOLOGY 309
site. Home location registers ( H L R ) and visitor location registers ( V L R ) have the func-
tions described earlier in this chapter. Typically one HLR and one VLR is associated
with each MSC. As explained in Chapter 12, the mobile application part ( M A P ) is used
in communications between MSC, HLR and VLR.
The MSC provides for call control and switching, and for gateway functions to other
mobile or fixed networks.TheGSM system hasanumber of additional security
features compared with its predecessors. These aim to reduce the problem of stolen
handsets.A SZM card, containinga small user identificationchip and information
concerning the users configuration, must be inserted into the user’s handset before it
will operate. This chip enablesan authorization procedure to be carried out at each call
set up between mobile station, MSC and the equipment identfication register ( E I R ) .
Should a handset or a card lost or stolen, then the EIR may be updated to record this
be
information.TheEIRcontains a blacklist of barredequipmentand a grey list of
equipment not functioningcorrectly or for which no services are registered. A white list
contains all registered users and relevant subscription services.
The radio part of the GSM system uses a 25MHz radio band. The uplink channels
(mobile to base station) occupy the band 890-9 15 MHz. The downlink (base station to
mobile) channels are between 935-960 MHz, whereby uplink and downlink channels of
a particular channel pair are separated by 45 MHz.
The radio bandis subdivided into 124 carrier frequency pairs, each of 200 kHz uplink
and 200 kHz downlink bandwidth. Each carrier is coded digitally using TDMA (time
division multiple access as explained in Chapter 8). The normal TDMA frame used in
GSM is illustrated in Figure 15.1 1.
The usable individual circuit bitrate is around 24 kbit/s (1 14 bits per circuit, every
4.61 5 ms). Frequency hopping (Chapter 8) provides for protection against radio fading
caused by multipath effects, and also gives some protection against criminal overhearing
of the radio signal. A further GSM measure against overhearing is a key-coded data
encryption, whereby the key is held on the SIM card in the mobile station and the
authorization centre ( A u C ) provides the encryption algorithm.
-
F 4.615 ms 4
-
I O I 1 1 2_ -- 1 3 1- - _ 1 5 1 6 1 7 1
4
__---
__--- ---. _ _
- - - --.__
- - m
_ _ - -_ _ - -
__-- - -- - - - _ _ _ 1 - _
--- 0.577 ms ---
4
156.25 bits I
I
guard guard l
bit I
I bit
guard training guard
TB encrypted data encrypted data TB
bits sequence bits
8.25 3 57 1 26 1 57 3 8.25
Figure 15.11 TDMA frame used in GSM
- 310
c+
NETWORKS TELEPHONE
voice or data
ISDN S-interface .-1q+
i MOBILE
ISDN R-interface
(V- or X-series terminals)
Figure 15.12 Mobile station termination types for GSM
The radio modulation is GMSK (Gaussian minimum shft keying), a form of phase
shift keying(PSK, binaryphase modulation). The P C M (pulse codemodulation) coding of
the speech signal uses an A D P C M (adaptive dyerentialP C M ) algorithm (see Chapter
38) with a bitrate around10 kbit/s. The remaining bitrate needed for signalling, error
is
correction, encryption and other protocol functions (e.g. hand-of, etc.).
Figure 15.13 Modern GSM handheld telephone (handy). (Courtesy of Siemens A G )
- PERSONAL
COMMUNICATIONS
NETWORK
(PCN) AND DCS- 1800 31 1
The U,-interface is the mobile telephone network equivalent the ISDNU-interface
of
(Chapter 10). It is the standard radiointerface and protocol thatallows mobile handsets
from manydifferent manufacturers access to the mobile network. Figure 15.12 illustrates
the classification of the various ISDN-like end user termination tapes.
The boom in number of GSM subscribers and traffic volumes has prompted
fastfurther
development of the G S M system. The initial offering of carphones.
requiring 20 Watt battery power have been rapidly supplanted by 8 Watt and 5 Watt
portable equipments, and finally by 2 Watt handies(handheldterminals). These are
the palm-sized mobile telephones now available in most electrical stores. The lower
output power of the mobilestationsequatesto lower range and thereforesmaller
effective cell size. However, as the boom in traffic has demanded smaller cells any-
way (thenumber of calls per cell being limited), this is no longerapractical or
economic problem.
New developments of the GSM system will increase the intelligence and functionality
of the system. Already, sophisticated mailbox services, short message services (displayed
on the display of the mobile station as a short text), fax and data services are becoming
common network offerings.
15.8 PERSONALCOMMUNICATIONS NETWORK (PCN)
ANDDCS-1800
The PCN system, nowalsoknown as DCS-I800 (digitalcommunications system at
1800 M H z ) , was originally intended to herald a new era of ‘personal communications’.
In the late 1980s a mass-market was foreseen for handheld mobiletelephones (like
GSM-handies), but the 9 0 0 M H z G S M standards were not thought at the time to be
capable of satisfying the demand, because it was not thought that the mobile sets could
be created small enough and with sufficient power and battery conservation to work
effectively. An initiative for PCN commenced, and a new radio band at 1800 MHz was
allocated.
In the event, the PCN initiative was largely overtaken by GSM, and the DCS-1800
system is little mure than a modified version of G S M for the 1800 MHz radio frequency
band. The network licensing programme of European governments then sealed PCN
technology as a direct competitor to standard G S M by issuing further mobile network
licences to new network operators. The interest of these network operators has largely
been to ‘cash-in’ on the mobile telephone boom of the G S M operators, so that DCS-
1800 network operators such as E-plus in Germany and One-to-one and Orange in UK
cannot bedistinguished by mostcustomersfromthe classical cellular and G S M
operators D1 and D2 in Germany and Cellnet and Vodujone in UK. A further reduc-
tion in market prices has resulted.
Table 15.2 comparesthe DCS-1800 system with thatof G S M and the USand
Japanese digital cellular systems.
One of the major lessons learnedfrom both DCS-1800 and GSM has been
how quickly viable alternativenetworkstothetraditional,terrestrial-basedpublic
telephone can be established. GSM operators have become very adept at finding and
buying or leasing small footprint sites where they may erect radio towers which serve
- 312 NETWORKS MOBILE TELEPHONE
Table 15.2 Comparison of DCS-1800 (PCN) with GSM, USDC and PDC
~ ~ -~ ~
Parameter GSM DCS- 1800 USDC (ADC) PDC (JDC)
(US or American (Japanese
digital cellular personal digital
system) cellular system)
Uplink 890-915
band MHz 1710-1785MHz 824-849 MHz 940-960 MHz
Downlink
935-960 MHz 1805-1 855 MHz 869-894 MHz 8 10-830 MHz
band
Channel kHz200 200 kHz 30 kHz 25 kHz
spacing
Duplex spacing 45 MHz 95 MHz 45 MHz 130 MHz
Multiplexing
TDMAiFDD TDMA/FDD TDMAiFDD TDMAiFDD
Modulation GMSK GMSK DQPSK DQPSK
Speech data 13 kbit/s 13 kbitis < 13 kbitis < 1 1 kbitis
rate (6.5 kbit/s)
Frequencies 124 374 832 800
Time slots per 8 (16) 8 3 3
radio channel
Data service
9.6 kbit/s 9.6 kbit/s 4.8 kbit/s 4.8 kbit/s
Maximum
km/h
250 250 km/h 100 km/h 100km/h
speed of
mobile station
output of 2, 5 , 8 or 0.25 or 1 Watt
handheld unit 20 Watt
simultaneously as base stations for the radio links to mobile stations and masts for the
installation of point-to-point microwave radio links as backhaul links to mobile
switching centres.
The boom in demand for mobile telephones has caused operational problems for the
cellular radio network operators, who in some cases have not been able to expand their
networks in step with the growing demand. The result has congestion. At first, the
been
relief of congestion could be brought about by reducing cell sizes and so introducing
more cells. However, as time has progressed, the radio bandwidth available has come
to be aproblem.Initially, DCS was seen as asolution.Nowadays, DECT (digital
European cordless telephony described in Chapter 16) technology is increasingly being
seen as a complement to GSM. The idea is that a dual-mode telephone handset could log
into whichever network was available in a particular area. The claimed advantage of
DECT is that much higher subscriber density can be supported, so overcoming the
hotspot problem currently existing in some pure GSM networks.
- AERONAUTICAL AND MARITIME
MOBILE
COMMUNICATIONS
SERVICES 313
15.9 AERONAUTICAL AND MARITIME MOBILE
COMMUNICATIONS SERVICES
High frequency radio communications have long been used in aeronautical and mari-
time applications for navigational purposes and distress Furthermore, manypublic
calls.
telephone network operators have for many years offered ordinary telephone customers
the opportunity to place or receive H F radio telephone calls to and fromships in coastal
waters. Unfortunately high frequency radio systems, when used for communication over
long distances at sea, suffer from poor signal propagation, atmospheric disturbance,
interference, and signal fading. As a result, the ships’ telephone radio service is of poor
quality, and unreliable.
As a means of getting over this problem, a number of organizations during the late
1960s and early 1970s were studying the use of satellites for aeronautical and maritime
communications. These studiesled in 1976 to the launchof M A R I S A T , the world’s first
commercialmaritime satellite system. It was conceived by aconsortiumofUnited
States companies, and it comprised three M A R I S A T satellites in geostationary orbit,
providing communications services for military and civilian use. At the outsetheavy use
of the system was made by the US Navy.
Further worldwideinterest, followed up an initiative of theInter-Governmental
Maritime Consultative Organisation (IMCO) in 1973, led to the signing of an agree-
ment by 26 countries in 1976, and the establishment in 1979 of the International Mari-
time Satellite Organization ( I N M A R S A T ) .
I N M A R S A T ’ s satellites have revolutionized the world of communications with ships.
Geostationary satellites now placed over the Indian, Atlantic and Pacific Oceans make
possible communication to any suitably equipped ship, no matter where it is located
around the globe. Automatic telephone calls, dialled directly by customers of some
countries’public switched networks can be made to people aboard ships; theonly
requirement is that the caller knows which ocean the ship is in, so that the appropriate
ocean country code number (part the international telephone number) can dialled,
of be
directing the call via the appropriate ocean’s satellite. Similarly, persons aboard ships
may make direct-dialled calls to other customers connected to the international public
telephone network.
Figure 15.14 illustratesthemaincomponents of themaritime satellite network,
comprising earth station equipments both on shore and aboard ship, together with a
sophisticated Access Control and Signalling Equipment. The A C S E performs similar
functions to those of cellular radio network base stations and mobile switching centres
(MSCs). It allocates radio channels for individual calls and carries out billing, account-
ing and other administrative functions.
A cheap form of satellite communication, allowing ‘telex-like’ communication to and
from terminals on boardsmaller craft, vehicles and lorries is the INMARSAT Standard
C service. This allows two-way communication using an omnidirectional antenna about
the size of a saucepan.
By the end of 1987 the number of INMARSAT signatory countries hadincreased to
50. INMARSAT’s originalaim was to providemaritimecommunications,thereby
improving the safety and management of ships. In 1985 the charter was expanded to
include aeronautical satellite communications.
- 314 MOBILE TELEPHONE NETWORKS
-
Radlo transparent
cover l radome 1
\ Satellite
CF
antenna Ship’s antenna
(detail )
d
Antenna
Ship
ACSE
Access
Controland
Signalling
Equipment
‘Earth station’
Figure 15.14 Maritime satellite communications
A number of initiatives have provided aeronautical satellite services. These bring the
public telephone network to passengers and crew aboard commercial airliners.
15.10 IRIDIUM, GLOBALSTAR ANDTHE EVOLUTION TOWARDS
THE UNIVERSAL MOBILE TELEPHONE SERVICE (UMTS)
With the mobile telephone boom of theearly 1990s in full swing inthedeveloped
countries, attention swung towards extending the possibilities of mobile roaming to
worldwide coverage and to the increased penetration of mobile telephone services in
less developed parts of theworld. The result has been anumber of new satellite
technology initiatives. In these, the base transmitter station ( B T S ) and base switching
centre ( B S C ) functions of GSM are combined into one unit which is now carried by a
low orbiting satellite. The low orbit is necessary to reduce the transmit power needed in
the mobile stations and also to reduce the undesirable long propagation time of signals
transmitted to and from geostationary orbits at 37 000 km altitude (Chapter S).
The low orbit creates a significant technological challenge, because each individual
satellite now moves at considerable velocity relative to the earth’s surface, being at best,
‘visible’ from a particular point on the ground for a maximum of 15 minutes per orbit.
Like GSM, each of the proposed satellite systems, divides the coverage area (the
of
earth’s surface) into a number cells, within each of which a number of radio channels
permit individual users to make telephone calls. The difference is that the cells are now
typically the size of a European country, so that the capacity in terms of traffic density
is much less than for a terrestrial-based GSM system. The advantage, of course, of the
satellite systems, is the global coverage and roaming capability. Figure 15.16 illustrates
the concept in simple terms and Table 15.3 compares four of the systems proposed to go
into service during the 1998-2000 timeframe.
- IRIDIUM GLOBALSTAR
AND THE EVOLUTION
TOWARDS
UMTS
- 315
Figure 15.15 Inmarsatship’santenna.Theradomecoverprotectingaship’sINMARSAT
telecommunications satellite antenna. (Courtesy of British Telecom)
- 316 TELEPHONE MOBILE NETWORKS
K-band 10.9-36 GHz
L-band 1.6-2.1 GHz
I \ / I I / \ .
I
I
earth
station
mPaw car or
house office mobile
Figme 15.16 The Universal Mobile Telephone Service (UMTS)
Table 15.3 Comparison of satellite mobile telephone systems
Globalstar
Iridium Teledesic
Odyssey
Consortium Leader Motorola Loran, Qualcom TRW-Matra Bill Gates
Number of 66 48 10 840
satellites
Orbit type and LEO (low earth, LEO (low earth, ME0 (medium (low
LEO
altitude circular orbit), circular orbit), earth, circular earth orbit)
780 km, 1414km, orbit) 10355 km, 840 km
6 orbit paths 14 orbit paths 3 orbit paths
Satellite inclination 86.4" 47/65" 55"
Minimum elevation 8" 40" 10" 18"
2.6 ms 4.7 ms 34.5 ms
No of 48 beams
spot 6 163
(cells/satellite)
Multiplexing FDMAiTDMA FDMAjCDMA FDMAiTDMA TDMA/SDMA
QPSK QPSK Modulation
kbit/s
2.4/4.8
Bitrate 4.8/9.6 kbit/s 4.8 kbit/s
switching
transparent
on-board
Satellite
transparent
weight 262
Satellite kg 689 1130kg
kg
200 1999 1998 Date of service
1998 1
nguon tai.lieu . vn
