Nhiều giao thức truy cập đối với truyền thông di động P2

Multiple Access Protocols for Mobile Communications: GPRS, UMTS and Beyond
Alex Brand, Hamid Aghvami
Copyright

2002 John Wiley & Sons Ltd
ISBNs: 0-471-49877-7 (Hardback); 0-470-84622-4 (Electronic)
2
CELLULAR MOBILE
COMMUNICATION SYSTEMS:
FROM 1G TO 4G
The basic principles of cellular communications were explained in the introductory
chapter, and terms such as cluster size and reuse efficiency introduced. In the following,
some more considerations on the advantages and limitations of the cellular concept
will be made before reviewing first generation (1G) and second generation (2G) cellular
communication systems. Moving on to 3G, we will first discuss the initial requirements
according to which 3G systems were designed, and then to what extent they are likely
to be satisfied by first releases of ‘true’ 3G systems on the one hand, and evolved 2G
systems on the other. While special attention is paid to the GSM evolution route and to
UMTS, the initially European, but now global proponent for 3G cellular communications,
evolutionary routes from cdmaOne to cdma2000 are also reviewed.
The emergence of the Internet witnessed in the 1990s is expected to have a fundamental
impact on the telecommunications industry, including cellular communications. We will
examine how this affects 3G, what new requirements arise and how these can be met
through subsequent releases of UMTS. Finally, looking further into the future, possible
manifestations of 4G systems will also be discussed, including the role wireless LAN
technologies may play in this context.
2.1 Advantages and Limitations of the Cellular Concept
It was outlined in the previous chapter how the cellular concept allows use of the same
set of frequencies in multiple cells, and how it is in theory possible to arbitrarily increase
capacity to match growing demand for wireless communication services through cell
splitting. In practice, however, there are certain limitations. With smaller and smaller

cells as a result of cell splitting, it becomes increasingly difficult to place base stations at
the locations that offer the necessary radio coverage [1]. Furthermore, as the cell radius
decreases, the handover rate increases. This will place a costly burden onto the network
in terms of increased signalling load and, given the non-zero probability of handover
failures, the call-dropping probability may increase, particularly for fast moving mobiles.
Finally, it would be wasteful to deploy a large number of base stations covering small
cells in areas with low traffic density.
24
2 CELLULAR MOBILE COMMUNICATION SYSTEMS: FROM 1G TO 4G
All these considerations call for a network topology where different cell types are
deployed concurrently, to provide capacity, where required, and at the same time ensure
universal coverage. According to the topography and other conditions, base stations may
cover a macrocell (also called umbrella cell ), a microcell (typically to be found in city
centres, on highways or even indoors), or a picocell (usually deployed indoors), with
diameters of several kilometres, up to one kilometre and possibly as little as a few tens of
metres respectively. These cells of different types, each cell-type constituting a separate
layer of a multi-layered system, will serve overlapping coverage areas. In this way, a
fast moving mobile terminal, for instance, can be served by a macro- or umbrella cell,
in order to limit the number of handovers per call, while slower moving mobiles in the
same coverage area are normally supported by microcells.
For traffic management purposes, these multiple layers can be organised in a hierar-
chical cellular structure (HCS, see e.g. Reference [64]), in which overflow traffic from
microcells is handed ‘up’ to the hierarchically higher macrocellular layer. A space segment
may be added to the terrestrial segment, where satellite spot beams overlay clusters
of terrestrial macrocells [65], as illustrated in Figure 2.1. GSM, the most important 2G
cellular system in operation today, is one example of existing cellular systems that can
support multiple cell layers.
From these considerations, a first set of requirements for the efficient operation of
cellular communication systems can be inferred:
• the system should use available resources (i.e. frequency spectrum) as efficiently as

possible, to limit infrastructure deployment and prevent some of the problems listed
in relation with small cell radii;
• handover procedures should be fast, require a minimum amount of signalling (prefer-
ably to be exchanged on dedicated signalling resources to avoid an impairment of
ongoing communications), and be robust to avoid dropped calls; and
• the system should be able to support multiple cell layers, for instance in the shape of
hierarchical cellular structures, to provide high-speed mobility and hot-spot capacity.
This list will be expanded in the following, first with respect to the initial 3G require-
ments identified in the early 1990s, according to which first releases of 3G systems were
designed, then with respect to additional requirements identified later on, mainly as a
result of the Internet revolution. Before we do that, the older 1G and 2G systems will be
reviewed briefly.
Macrocell
Microcell
Satellite spot
beam
Figure 2.1 Microcells, macrocells, and a satellite spot-beam
2.2 1G AND 2G CELLULAR COMMUNICATION SYSTEMS
25
2.2 1G and 2G Cellular Communication Systems
2.2.1 Analogue First Generation Cellular Systems
In the late 1970s and early 1980s, various first generation cellular mobile communication
systems were introduced, characterised by analogue (frequency modulation) voice trans-
mission and limited flexibility. The first such system, the Advanced Mobile Phone System
(AMPS), was introduced in the US in the late 1970s
1
. Other 1G systems include the
Nordic Mobile Telephone System (NMT), and the Total Access Communications System
(TACS). The former was introduced in 1981 in Sweden, then soon afterwards in other
Scandinavian countries, followed by the Netherlands, Switzerland, and a large number of

Central and Eastern European countries, the latter was deployed from 1985 in Ireland,
Italy, Spain and the UK.
While these systems offer reasonably good voice quality, they provide limited spectral
efficiency. They also suffer from the fact that network control messages — for handover
or power control, for example — are carried over the voice channel in such a way that
they interrupt speech transmission and produce audible clicks, which limits the network
control capacity [7]. This is one reason why the cell size cannot be reduced indefinitely
to increase capacity.
Such constraints did not prevent these systems from enjoying considerable success with
the public, so that subscriber numbers were still growing in the mid 1990s. In Italy, for
instance, they peaked at four million users in March 1998, corresponding to a penetration
of roughly 7%, and (while less impressive in absolute figures) penetration exceeded 10%
in most Scandinavian countries. However, they have been increasingly thrust aside by
2G systems in most parts of the world. In the meantime, after closing down 1G systems,
spectrum refarming from 1G to 2G has taken place in several countries.
2.2.2 Digital Second Generation Systems
Capacity increase was one of the main motivations for introducing 2G systems in the
early 1990s. Compared to the first generation, 2G offers [69]:
• increased capacity due to application of low-bit-rate speech codecs and lower frequen-
cy reuse factors (the cluster size can be as low as three compared to seven in analogue
systems for example, see also Subsection 2.3.2 on evolved 2G systems);
• security (encryption to provide privacy, and authentication to prevent unauthorised
access and use of the system);
• integration of voice and data owing to the digital technology; and
• dedicated channels for the exchange of network control information between mobile
terminals and the network infrastructure during a call, in order to overcome the limita-
tions in network control of 1G systems (note though, that handover-related signalling
still steals into the traffic channel in GSM).
1
According to Lee, the FCC released frequencies for cellular communication systems in the 800 MHz band

in 1974, and AMPS served 40 000 mobile customers in 1976 [66]. Other sources indicate later launch dates
though, for instance 1979 for a pre-operation AMPS system in Chicago [67]. See also Reference [68], which
provides a detailed history of cellular communications.
26
2 CELLULAR MOBILE COMMUNICATION SYSTEMS: FROM 1G TO 4G
At the time of writing, the major force driving the growth in cellular communications
are still such 2G systems, which were first introduced in Europe, Japan, and the US, and
are now in operation worldwide.
Initially, these systems operated only at 900 MHz (800 MHz in the US and Japan),
with up-banded versions at 1800 MHz (1900 MHz in the US, 1500 MHz in Japan)
coming soon after. These up-banded systems are aimed primarily at people moving
around in cities at pedestrian speeds with hand-held telephones. They are referred to
as Personal Communications Networks (PCN) in Britain and Personal Communications
Systems (PCS) in the US, to distinguish them from the ‘classical’ cellular systems oper-
ating below 1 GHz.
One of these 2G systems needs to be singled out, GSM, a TDMA-based system
with optional slow frequency hopping. The acronym stood initially for ‘Groupe Sp
´
ecial
Mobile’, but fortunately lends itself conveniently to ‘Global System for Mobile Commu-
nications’. With a subscriber number close to 500 millions in early 2001, a footprint
covering virtually every angle of the world, and a share of the digital cellular market
close to 70% in February 2001 (according to figures published by the GSM Associ-
ation [70]), it truly deserves this name. Introduced in early 1992, already by the end
of 1993, GSM networks had been launched in more than 10 European countries, and
outside Europe for instance in Hong Kong and Australia. From 1994, GSM gradually
conquered the markets in the remaining European countries and large parts of the rest of
the world.
The pan-European standardisation effort leading to GSM was initiated by the Conf´erence
Europ´eenne des Administrations des Postes et des T´el´ecommunications (CEPT) in 1982

with the formation of the Groupe Sp
´
ecial Mobile (GSM) [3]. In 1988, the European
Telecommunications Standards Institute (ETSI) was founded, which was from then on
responsible for the evolution of the GSM standards [10]. ETSI is still formally respon-
sible for the GSM standards, but the technical work relating to system evolution is now
carried out within the framework of the Third Generation Partnership Project (3GPP), a
body created in 1998 to standardise UMTS. The GSM evolution work was added in the
year 2000. ETSI is a 3GPP member along with other standardisation bodies and interest
groups from around the world. In the context of cellular systems, ETSI’s role is now
to transform technical specifications created in 3GPP into regional standards for Europe,
whether this is for evolved GSM or for UMTS.
While GSM is currently the uncontested standard for 2G digital cellular communications
in Europe, there is no clear dominance of a single digital standard in most of the rest
of the world. In the US, for instance, there are essentially two types of 2G cellular
systems operating at 800 MHz, which are incompatible with each other. The first is a
TDMA system called North American Digital Cellular or Digital AMPS (D-AMPS), and
sometimes simply referred to as TDMA, according to the basic multiple access scheme
it is based on. This can create confusion, since there are other TDMA-based cellular
systems, notably GSM. The second system, which was launched later, is cdmaOne, the
first operational CDMA system [71]. The relevant air-interface specifications are the so-
called interim standards IS-136 (for D-AMPS) and IS-95 (for cdmaOne).
The fragmentation observed in the US was accentuated when the Federal Commu-
nications Commission (FCC) sold frequencies in the 1900 MHz band for PCS without
mandating the technology to be used. On top of up-banded D-AMPS and cdmaOne
systems, this allowed a 1900 MHz version of GSM to enter the US market. Similar
2.3 FIRST 3G SYSTEMS
27
developments could also be observed in the rest of the Americas, although Brazil, rather
than selling spectrum at 1900 MHz, decided in the year 2000 to auction 1800 MHz

spectrum for PCS. This decision favours GSM because, at least at the time of writing,
1800-MHz versions of D-AMPS and cdmaOne equipment were not available.
The first and most popular Japanese 2G digital standard is Personal Digital Cellular
(PDC). It was later complemented by the Personal Handyphone System (PHS), somehow
a hermaphrodite between mobile and cordless system, which caters only for low mobility,
but is popular for certain applications owing to relatively high data-rates (32 kbit/s,
later enhanced to 64 kbit/s). Both standards are TDMA-based and have not seen wide
deployment outside Japan. PDC has received some attention outside Japan, though, owing
to the tremendous success enjoyed by the ‘i-mode’ service since its launch in February
1999. This is a service similar to WAP (wireless application protocol) services, but (at
least at the time of writing) rather more popular. It enables access to some form of Internet
through mobile handsets, and runs on top of the packet-overlay added to PDC, referred
to as PDC-P. In the late 1990s, a cdmaOne system was launched in Japan. CdmaOne
systems enjoy considerable success also elsewhere in the Far East, most notably in South
Korea, where an IS-95 derivative dominates the 2G market.
2.3 First 3G Systems
2.3.1 Requirements for 3G
Already before the launch of 2G systems, the research community started to think about
requirements for a new, third generation of mobile communication systems and about
possible technological solutions to meet them (see for instance References [1,7,12,72,73]).
Before 3GPP was established, ETSI was one of the major players regarding the standardis-
ation of 3G systems. It called its 3G representative Universal Mobile Telecommunications
System (UMTS) and established a number of requirements, according to which such a
system should be designed. Those of interest here are the ones relating to the air interface
or radio access, the so-called UMTS Terrestrial Radio Access (UTRA), which are listed in
Reference [74]. This list appears to capture most of the 3G requirements stated in the liter-
ature of the early 1990s, when 3G emerged as a mainstream research topic. It is therefore
summarised in the following. The UTRA requirements pertain to four different cate-
gories, namely to bearer capabilities, operational requirements, efficient spectrum usage,
and finally complexity and cost.

2.3.1.1 Bearer Capabilities
(1) UMTS has to deliver services with bit rates up to 2 Mbit/s indoors, at least
384 kbit/s in suburban outdoor and at least 144 kbit/s in rural outdoor environ-
ments.
(2) UMTS should be flexible in terms of service provision, in particular:
— negotiation of bearer service attributes should be possible;
— parallel bearer services to enable service mix should be possible;
— circuit-switched and packet oriented bearers should be provided;
— variable-bit-rate real-time capabilities should be provided; and
28
2 CELLULAR MOBILE COMMUNICATION SYSTEMS: FROM 1G TO 4G
— scheduling (and pre-emption) of bearers according to priority should be
possible.
(3) UMTS has to provide services with a wide range of bit-rates in a variety of different
environments with bit error rates (BER) as low as 10
−7
for certain services.
To deliver optimum performance in all of these, a very flexible air interface is
required.
(4) UMTS should provide seamless handover between cells of one operator, possibly
even between cells of different operators. Seamless mean in this context, that the
handover must not be noticeable to the user.
2.3.1.2 Operational Requirements
(5) Compatibility with services from the following existing core networks must be
provided: ATM bearer services, GSM services, Internet Protocol (IP) based services,
and ISDN services.
(6) Automatic radio resource planning should be provided, if such planning is required.
2.3.1.3 Efficient Spectrum Usage
(7) The air interface should make efficient use of the radio spectrum for typical mixtures
of different bearer services.

(8) Given the asymmetric UMTS frequency allocation and the likely overall traffic
asymmetry (due to Web browsing, for instance), the air interface should support
operation in unpaired frequency bands or, according to Reference [74], ‘variable
division of radio resource between uplink and downlink resources from a common
pool [must be provided]’.
(9) UMTS should allow multiple operators to use the band allocated to UMTS without
coordination (this includes public, private, and residential operators).
(10) UMTS should allow flexible use of various cell types and relations between cells
(e.g. indoor cells, hierarchical cells) within a geographical area without undue waste
of radio resources.
2.3.1.4 Complexity and Cost
(11) Handportable and PCMCIA-card-sized UMTS terminals should be viable in terms
of size, weight, operating time, range, effective radiated power and cost.
(12) The development and equipment cost should be kept at a reasonable level, taking
into account the cost of cell sites, the associated network connections, signalling
load and traffic overhead.
Goodman stated in the early 1990s a vision for 3G as to ‘create a single network
infrastructure, that will make it possible for all people to transfer economically any kind
of information between any desired locations’, a statement which appears to cover the
essence of the above listed requirements. He added that ‘a unified wireless access [will
replace] the diverse and incompatible 2G networks with a single means of wireless access
to advanced information services’ [1]. The latter is something which may well fail to
materialise, as will be pointed out later on.
2.3 FIRST 3G SYSTEMS
29
Two questions, which will be discussed in the following, arise here:
(1) To what extent can evolved 2G systems meet these requirements?
(2) Do currently specified 3G systems meet these requirements?
2.3.2 Evolution of 2G Systems towards 3G
With the large subscriber base and the considerable investment in 2G infrastructure in

mind, it cannot come as a surprise that operators of such systems are keen to protect
this investment. Assuming that there is a market need for 3G systems meeting the above
requirements list (one would assume that operators having paid billions of dollars for 3G
licenses must have made this assumption), there appear to be two ways to achieve this:
• Standardise 3G in a manner so that at least part of the 2G network infrastructure
can be reused. In the case of GSM and UMTS, this has materialised to some extent.
Certain GSM core network nodes can potentially be reused for UMTS. Also, the
UMTS handover requirements state that handover to 2G systems, e.g. GSM, should be
possible. Furthermore, it was at least attempted to choose design parameters for UTRA
which ease implementation of dual-mode GSM/UMTS handsets; dual-mode operation
is expected to be a standard feature of most UMTS handsets. Correspondingly, it is
possible to deploy UMTS gradually in a GSM system, where in a first phase only
selected sites are equipped with UMTS base stations, while universal coverage is
provided by GSM.
• Evolve capabilities of 2G systems to meet 3G requirements, for instance through
enhancements to the air interface.
Given the importance of GSM and the large number of advanced features which have
been or are still being standardised for this system, it will be discussed briefly to what
extent such an evolved GSM system may fulfil 3G requirements from an air-interface
perspective. The main air-interface related enhancements to GSM, which are already
standardised (as discussed in detail in Chapter 4), are:
• higher data-rates for circuit-switched services through aggregation of several time-slots
per TDMA frame with High Speed Circuit-Switched Data (HSCSD) [75];
• efficient support of non-real-time packet-data traffic with the General Packet Radio
Service (GPRS), which entails enhancements to both the air interface [54] and the
network [76];
• higher data-rates on individual GSM physical channels through use of higher order
modulation schemes within the existing carrier bandwidth of 200 kHz, referred to
as Enhanced Data Rates for Global Evolution (EDGE). ‘Plain GSM’, HSCSD, and
GPRS can then exploit these higher data-rates (see Reference [77]).

Ignoring network constraints, with HSCSD, GSM could in theory offer user data-rates
of at most 8 × 14.4 kbit/s = 115.2 kbit/s for circuit-switched data traffic. With GPRS,
a data-rate of up to 8 × 21.4 kbit/s = 171.2 kbit/s can be provided for packet traffic
30
2 CELLULAR MOBILE COMMUNICATION SYSTEMS: FROM 1G TO 4G
for the case where no forward error correction coding is used [54]. Neither do these
data-rates meet the UMTS requirements, nor can they be achieved in an economical
manner, since aggregation of eight time-slots in a TDMA frame would result in rather
power-thirsty handsets and raise other issues such as how to dissipate the additional heat
being generated. A detailed discussion on realistic data-rates is provided in Chapter 4. By
applying EDGE to GPRS, without error coding, the data-rates per slot can be increased
to 59.2 kbit/s, hence with eight time-slots up to 473.6 kbit/s could be achieved, which
exceeds the UMTS requirements for all environments but indoors. Note though that apart
from requiring time-slot aggregation, due to lack of error protection, such throughput
levels can only be achieved at very high carrier-to-interference ratios, which has obviously
repercussions on cell planning and capacity. With mobile terminals capable of aggregating
eight time-slots, the suburban requirements of 384 kbit/s could be met while allowing for
moderate error protection, which would reduce the required CIR a bit.
GSM was initially designed with specific tele-services in mind, each of them being
individually standardised. GPRS and the general bearer service introduced mainly, but
not only for HSCSD [78] provide increased flexibility, as do other standardisation work
items related to services
2
. However, the constraints of the GSM air interface will not
allow the second and third requirement listed above to be satisfied fully. For instance,
GPRS was not designed for real-time packet-data services and in its early versions, it
is only suitable for real-time services with severe restrictions (if at all). Proper real-time
capabilities are being added to GPRS, but provision of high and variable bit-rate real-time
service will remain restricted.
Automatic resource planning is normally not provided, although dynamic channel

assignment (DCA) and dynamic resource assignment (DRA) could be used to ease the
planning process. Alternatively, a combination of slow frequency hopping with fractional
loading may allow the deployment of a one site/three sector (1/3) reuse-pattern for carriers
not carrying broadcast channels, eliminating the need for frequency planning for these
carriers [79,80]. A system scenario for GSM with two hierarchical layers is described in
Reference [81], where 1/3 reuse is applied to hopping channels of macrocells, 1/3 or even
1/1 reuse to those of microcells, and carriers with broadcast channels for microcells are
planned adaptively. The GPRS COMPACT mode [82], a stand-alone data-only solution
(i.e. without support of GSM circuit-switched services) relies also on a 1/3 reuse. Such
matters are discussed in detail in Chapter 4.
An evolved GSM system will fail to meet 3G requirements on two more counts.
Firstly, asymmetric frequency allocation is not possible (it is possible to provide asym-
metric services, but the total resources managed by base stations are always symmetric).
Secondly, for public operation, GSM certainly does not allow multiple operators to use
the total allocated band for GSM without co-ordination (i.e. without reserving for each
operator a dedicated part of the available spectrum). However, if the respective require-
ment (number (9) in the list) is interpreted in this manner, it will also not be met by
UMTS. Residential (or private) use without co-ordination is a different matter; a GSM-
based cordless telephony system is a part of the GSM evolution story. As far as efficient
use of the radio spectrum is concerned, refer to the discussion of multiple access schemes
2
Tele-services are fully specified end-to-end services providing the complete capability, including terminal
equipment functions, for communication between users. Bearer services, by contrast, provide only the capability
for the transmission of signals between user–network interfaces; they provide bearers used by tele-services.
2.3 FIRST 3G SYSTEMS
31
in Chapter 3 for comments on spectral efficiency of TDMA used in GSM, and CDMA,
the main alternative.
In conclusion, 2G systems continue to evolve and the boundary between advanced 2G
and ‘true’ 3G systems is increasingly being blurred. GSM, for instance, with its wealth

of implemented and imminent features, is now rightly referred to as an advanced 2G
system (or alternatively, a generation 2.5 system [72]), and may, in certain manifestations,
become an integral part of 3G systems. However, as stand-alone systems, 2.5G systems
will struggle to meet all 3G requirements. In particular (at least in the case of an evolved
GSM system), they will only to a limited extent be able to provide efficient support of
high and variable bit-rate multimedia services.
2.3.3 Worldwide 3G Standardisation Efforts
These high and variable bit-rate multimedia services are, at least from today’s perspective,
exactly those services that may create market demand for new systems. This is one reason
why the mobile communication communities in Europe, the Far East, and the US specified
‘true’ 3G systems. There are other reasons, as well. For instance, handset and infrastructure
manufacturers must naturally be interested in deployment of new systems.
Japan was pushing particularly hard for 3G and leads on 3G deployment, mainly because
of overcrowded 2G systems, but likely also motivated by a wish to break out of a tech-
nological isolation in which its industry found itself with PDC and PHS.
The International Telecommunications Union (ITU), which refers to 3G systems as
either Future Public Land Mobile Telecommunications System (FPLMTS) or, more handy,
International Mobile Telecommunications 2000 (IMT-2000), initially had the intention of
controlling the 3G standardisation process in a manner such that a single system would
emerge. This would have allowed, for the first time, worldwide roaming with a single
handset, as envisaged by Goodman in Reference [1]. The idea was that the different
regions of the world would submit system proposals capable of meeting a given set of
requirements. The proposal best meeting these requirements would then be selected or,
if this was not possible, an attempt would be made to merge different proposals into a
single one in a consensus building phase.
While all bodies standardising such systems actually submitted their proposals to the
ITU in the middle of 1998 [83], it became clear that the ITU would not be in a posi-
tion to enforce a unified system. Instead, it would essentially have to approve all viable
proposals meeting the core ITU 3G requirements, which the regional bodies intend to
implement. To complicate matters, while ETSI in Europe and the Association of Radio

Industries and Businesses (ARIB) in Japan put in place their own procedures to select
one such proposal, there were no concerted efforts in the US towards 3G standardisa-
tion. As with 2G systems, it was believed that the marketplace should choose a system,
resulting in several 3G proposals from the US. For these reasons, ITU then advocated
‘a “family of systems” concept, defined as a federation of systems providing IMT-2000
service capabilities to users of all family members in a global roaming offering’ [83]. The
latter entailed efforts during the consensus building phase at least to enable worldwide
roaming, for instance by facilitating the implementation of multi-mode terminals, given
that a complete harmonisation seemed unachievable.
Eventually, two main camps (with various sub-streams) formed. The first one is united
in the original Third Generation Partnership Project (3GPP) mentioned previously, dealing
32
2 CELLULAR MOBILE COMMUNICATION SYSTEMS: FROM 1G TO 4G
with the standardisation of UMTS and the evolution of GSM, the second one in a similar
structure named 3GPP2, dealing with cdma2000, an evolution of cdmaOne. The systems
being developed by these two organisations are based on different core network standards.
Moderately successful air-interface harmonisation efforts have taken place in the frame-
work of an ‘operator harmonisation group’, but although similar air-interface technologies
are considered for UMTS and cdma2000, the systems remain essentially incompatible.
Other harmonisation efforts in the same framework resulted in the introduction of ‘hooks’
and extensions in the relevant standards, allowing the 3GPP air interface to be deployed
on a 3GPP2 network infrastructure and vice versa, as agreed in the second quarter of
1999. What relevance this option will have in practise remains to be seen.
2.3.4 The Third Generation Partnership Project (3GPP)
In Europe, several radio interface proposals were seriously considered within ETSI for
UMTS. Five concept groups were set up in ETSI SMG2 during 1997, classifying the
different proposals according to the basic multiple access schemes employed. These were
wideband CDMA (WCDMA), wideband TDMA, hybrid TDMA/CDMA (referred to as
TD/CDMA), orthogonal frequency-division multiplexing (OFDM) with a TDMA element
for multiple access, and opportunity driven multiple access (ODMA). Strictly speaking,

ODMA is not a basic multiple access scheme, but rather a technique that transforms
mobile terminals into relay stations. Signals from a terminal far away from a base station
can be relayed by other terminals nearer to it (potentially over multiple hops) to improve
coverage and lower transmission power, thereby reducing interference and thus increasing
spectral efficiency.
The strongest contenders were WCDMA and TD/CDMA, while ODMA was eventually
suggested as an option on top of whatever basic multiple access scheme would be chosen.
After a heated debate, it became evident that a unanimous decision for only one proposal
was not possible. While a majority preferred WCDMA, it was appreciated that TD/CDMA
would lend itself better to time-division duplexing suitable for operation in unpaired
frequency bands. As a compromise choice, rather than picking both frequency-division
duplex (FDD) and time-division duplex (TDD) mode of one of the two proposals, it was
decided to choose the WCDMA FDD mode together with the TD/CDMA TDD mode.
This is reflected in the ETSI candidate submission [84] to ITU. In the following, these
two modes are referred to as UTRA FDD and UTRA TDD respectively.
Japan, in its own selection process, was also considering various proposals and finally
opted for a WCDMA based system [85] very similar to the ETSI WCDMA mode. Since
Japanese companies had also contributed to the WCDMA concept in ETSI and were eager
to avoid finding themselves in similar technological isolation as with PDC, it was only
natural that they decided to join forces with ETSI. This led eventually to the creation of
3GPP in 1998, which took over detailed standardisation of the UTRA FDD and TDD
modes from ETSI (with UTRA now standing for universal terrestrial radio access rather
than UMTS terrestrial radio access). Apart from ETSI and Japanese bodies, 3GPP was
also joined by some of the US and the South Korean WCDMA proponents. Further
information on this topic can be found in Reference [86].
The core network to be used for the 3GPP system is an evolved GSM core network. As
a result, work on specifications dealing with protocols and network components common
to GSM and UMTS was transferred from ETSI to 3GPP in 1998. 3GPP can therefore be