In 1982, the main governing body of the European telecommunication operators, knownas CEPT (Confrence Europene des Postes et Tl communications), created the Groupe Spcial Mobile (GSM) committee and tasked it with specifying a pan-European cellular radio system to operate in the 900 MHz band. The system was conceived to overcome the perceived capacity limitations of the successful analogue systems already deployed in several European countries (e.g. the Nordic Mobile Telephone system, NMT, in the Nordic countries). The pan-European cellular standard would support international roaming and provide a boost for the European telecommunications industry. The power centres behind the proposed system were the 12 countries of the European Economic Community (EEC), the 26 countries involved in CEPT and the French and German PTTs. There was also strong support from the Nordic countries and the UK Government and industry. The French and German alliance, formed in 1983, was joined by Italy in 1985, and in 1986 the UK joined to form the Quadripartite [1].
After initial discussions, three working parties (WPs) were created to deal with specific aspects of the system definition, and later on a fourth WP was added. In 1986, a permanent nucleus was set up in Paris to co-ordinate the efforts of the working parties and also manage the generation of the system recommendations. The WPs were required to define the system interfaces that would allow a mobile, in the form of either a hand-held or vehicular mounted unit, to roam throughout the countries where the new system had been deployed and have access to the full range of services. Compared with the existing analogue systems, the new system was required to have a higher capacity, comparable or lower operating costs and a comparable or better speech quality. The system was also required to co-exist with the analogue systems. A common pan-European bandwidth allocation for the new system of 890–915 MHz and 935–960 MHz was agreed; however, by the time the system was to be deployed, parts of this band would be occupied by analogue cellular systems in some countries (e.g. the Total Access Communications System, TACS, in the United Kingdom). In these countries only a portion of the band would be used initially for GSM.
Although studies in various European countries had concluded that digital systems were to
be preferred over analogue systems, the choice of the multiple access scheme was not as clear-cut. It was decided that a number of different system proposals, put forward bycompanies and consortia from a number of different European countries, should be evalu-ated in prototype form. There were eight different system proposals. The MATS-D systemproposed by TEKADE incorporated three different multiple access schemes, namely code division multiple access (CDMA), frequency division multiple access (FDMA) and time division multiple access (TDMA). The CD900 system proposed by SEL was a wideband TDMA system in conjunction with spectral spreading [2, 3]. The remaining six proposals were all based on narrow-band TDMA. The SFH900 system proposed by LCT used fre- quency hopping in combination with Gaussian minimum shift keying (GMSK) modulation, Viterbi equalisation and Reed–Solomon channel coding. Bosch proposed the S900-D sys-tem, which used four-level frequency shift keying (FSK) modulation, and Ericsson proposed the DMS90 system which used frequency hopping, GMSK modulation and an adaptive de-cision feedback equaliser (DFE). The Mobira system and the MAX II system proposed by Televerket were similar to the DMS90 system. Finally, the system proposed by ELAB of Norway employed adaptive digital phase modulation (ADPM) and a Viterbi equaliser to combat the effects of intersymbol interference (ISI). Some of the basic features of the eight
different systems are given in Table 2.1 [4].
The different systems were trialled in Paris at the end of 1986 and the most spectrally effi-
cient (and ‘unofficial winner’) was the system proposed by ELAB. During 1987 the results
of the trial were discussed and eventually agreement was reached on the main characteris-
tics of the new system. The wideband solutions advocated by the French and Germans were
not adopted for a number of reasons, including the probability that the 1 μm VLSI technol-
ogy, needed to support the complex baseband signal processing required by these systems,
might not be available within the proposed time-scales. By June 1987 there was complete
agreement that the system should employ narrow-band TDMA and that it would have many
of the features of the ELAB system. The system would initially support eight channels per
carrier with eventual evolution to 16 channels per carrier.
The speech codec was chosen based on a subjective comparison of six different codecs at
16 kb/s. The two codecs which performed significantly better than the others were a residual
excited linear prediction (RELP) codec and a multipulse excitation linear prediction codec
(MPE-LPC). These two designs were merged to produce a regular pulse excitation LPC
(RPE-LPC) with a net bit rate of 13 kb/s.
Table 2.1: Some basic features of the GSM prototype systems.
The success of the ELAB system in the Paris trials focused attention on the Viterbi
equaliser, which out-performed the DFE used in other systems. Although Reed–Solomon
channel coding was heavily favoured amongst the prototype systems, the high levels of syn-
ergism between the Viterbi equaliser and the convolutional decoder, which also employs the
Viterbi algorithm, meant that convolutional channel coding was chosen for the new system.
The adaptive digital phase modulation (ADPM) scheme employed by the ELAB system
was initially chosen as the main candidate for the new system. However, GMSK was later
preferred because of its improved spectral efficiency.
The initial drafts of the GSM specifications became available around the middle of 1988
and by the end of that year the GSM working parties and the associated expert groups
had completed a substantial part of the specifications of the pan-European system. Around
this time it became clear that it would not be possible to fully specify every feature of the
proposed system in time for the launch in 1991. For this reason, the system specification was
divided into two phases. The most common services (e.g. call forwarding and call barring)
were included in the Phase 1 specifications which were frozen in 1990. The remaining
services (e.g. supplementary services and facsimile) were delayed until the Phase 2 release.
The second phase was also used to rectify faults in, and improve the performance of, the
Phase 1 system.
At the request of the United Kingdom, a version of GSM, operating in the 1800 MHz
band, was included in the specification process, tailored to the requirements of the emerg-
ing Personal Communications Networks (PCN). This system became known as the Digital
Cellular System at 1800 MHz (DCS1800). From this point onwards we shall use the term
GSM900 to describe the system operating in the 900 MHz band to distinguish it from the
1800 MHz system. The term GSM will be used to refer to both systems. It is important to
note that the term GSM1800 is also commonly used to refer to the DCS1800 system. The
DCS1800 system adaptation began in 1990 and, in 1991, Phase 1 of the DCS1800 system
specifications were frozen and were subsequently released as a set of amendments to the
GSM900 Phase 1 specifications. The amendments were termed delta recommendations or
∆-recs. In Phase 2 of the specifications, which were frozen in June 1993, the GSM900
system and the DCS1800 system were combined into the same set of documents.
During the development of the Phase 2 specifications it became clear that the task of
revising the specifications for a third time would be huge. For this reason it was decided
that beyond Phase 2 the GSM system should gradually evolve as new features arrive and this
continual evolution has become known as Phase 2+. Some of the more significant features
proposed for Phase 2+ included the half-rate speech coder, an increase in the maximum
mobile speed for reliable communications and a higher power 4 W mobile for the DCS1800
system.
In 1988, GSM became a Technical Committee of the newly created European Telecom-
munications Standards Institute (ETSI). Each of the four working parties became Sub-
Technical Committees (STCs). At the end of 1991 the scope of the GSM Technical Com-
mittee was widened to include the specification of a successor to GSM and, for this reason,
the technical committee was renamed the Special Mobile Group (SMG) with the STCs be-
coming SMG1 to 4. SMG5 was added with the task of specifying the Universal Mobile
Telecommunication System (UMTS), GSM’s successor [5]. Several other STCs have been
added and their responsibilities are summarised in Table 2.2. The term GSM is still used to
describe the system, but it has been renamed ‘The Global System for Mobile Communica-
tions’. SMG5 has since been discontinued and the task of developing the specifications for
UMTS has been distributed among the other committees.
In this chapter we give an overview of GSM. We have concentrated our description on the
GSM radio interface, since this has a direct impact on the capacity of a cellular system. The
reader wishing to learn more about GSM is either referred to books that are solely dedicated
to describing the system [6, 7], or the complete GSM specifications themselves which run
to some 5000 pages and describe all the complexities of GSM.
发表于 2009-4-8 19:44:33