2. Contents
Evolution of mobile communications....................................................................................................1
Contents................................................................................................................................................2
Mobile radio systems.............................................................................................................................3
Cellular...................................................................................................................................................3
First generation.....................................................................................................................................3
Second generation.................................................................................................................................3
Third generation....................................................................................................................................4
Beyond 3G or fourth generation mobile radio systems.........................................................................5
MIMO environment...............................................................................................................................6
Service area concept versus cellular concept........................................................................................6
References.............................................................................................................................................9
3. Mobile radio systems
The elementary target of a mobile radio system is provide seamless and qualitative
communication between mobile users or between mobile users and users of a fixed
communication network, by means of transmission of signals in the radio frequency (RF)
band. 100 years ago G. Marconi managed to set up a radio link across the Atlantic, an
accomplishment for which he was awarded the Nobel prize in 1909. A fact that G. Marconi
would probably not have guessed is that thanks to decisive advances in technology, mobile
communications is a radically changing field, dominantly present in every aspect of
worldwide research and economy. Representative about this phenomenon is that the
number of mobile cellular subscribers will surpass conventional fixed lines during the first
decade of this century as indicated by the forecasts.
In what follows a brief outline of the evolution of the mobile communications will be
performed.
Cellular
Cellular communication is based on distributing many base stations in the country to assure
proper coverage of the mobile communications service area and offer needed services to
available subscribers. This kind of mobile service has started late 70s and early 80s and from
that era until now, many evolutions have occurred that changed the face of this service
from usability, cost and quality and quantity of services it offers, below we will see a snap
shot of different generations of mobile telecommunications through the history of this
service.
First generation
In the 80’s several analogue cellular network came into operation around the world, based
on the cellular concept invented by Bell Labs in 1979. Frequency modulation (FM)
and frequency division multiple access (FDMA) were used. According to FDMA, active users
are separated in the frequency domain, by means of assignment of non overlapping
frequency bands to different users. The first generation of analog cellular systems included
the Advanced Mobile Telephone System (AMPS) in the USA, the Total Access
Communication System (TACS) in Europe, the C-450 system in Germany and Portugal, the
Nordic Mobile Telephones (NMT) in Scandinavian countries and the Nippon Telephone and
Telegraph (NTT) system in Japan.
Second generation
Parallel to the evolution of mobile communications, decisive progress in digital
communications took place. The increase of the device density in integrated circuits (ICs)
and the development of low rate speech coders spawned the second generation of mobile
radio systems. Due to this fact, the integration of the mobile radio systems in the digitalized
Public Switched Telephone Networks (PSTNs) could be performed more naturally. Another
improvement thanks to the digitalization was the provision of new services aside from
4. speech, such as data communication. In contrast to the first generation where FDMA was
used, in the second generation Time Division Multiple Access (TDMA) and Code Division
Multiple Access (CDMA) are used, thanks to the digital technology CDMA with analog
transmission applied in the signal processing techniques can be used.
In TDMA, the time axis is subdivided in separate non overlapping time slots. Each user is
assigned a separate slot to transmit and receive information, during which the user uses the
whole available bandwidth. Often TDMA can be combined with FDMA. CDMA uses a set of
orthogonal or quasi-orthogonal codes to spread the information to be transmitted in the
frequency domain. On the receiver, linear filtering with a synchronized replica of the
spreading code is applied to recover the information.
With the need of a transition from the multiple standards of many European national radio
systems characterizing the first generation to a Europe-wide standard for the second
generation of mobile radio systems the Groupe Speciale Mobile (GSM) was established by
the Conference Europeene des Postes et Telecommunications (CEPT) at 1982 which was
later renamed to Global System of Mobile communications. In 1988, the European
Telecommunication and Standardization Institute (ETSI) was founded and GSM became the
Technical Comittee Special Mobile Group (TC SMG).
In the United States an important factor considered by the standardization of second
generation mobile radio systems was the need of backwards the compatibility to AMPS due
to the large number of analog handsets already in operation. The Electronic Industry
Association (EIA) and the Telecommunications Industry Association (TIA) adopted the TDMA
based Interim Standard IS-54, also known as US-TDMA or digital AMPS. IS-136 is the version
of IS-54 with a digital control channel, and is the most commonly used term when referring
to US-TDMA. Backwards compatibility to the analog AMPS system was enabled by the use of
the same carrier spacing of 30 kHz.
Third generation
The need for high data rates and spectrum efficiencies as well as for a global standard
initiated in 1992 research and standardization activities for mobile radio systems of the third
generation (3G). The term initially used to describe the 3G systems in International
Communication Union (ITU) was Future Public Land Mobile Telephone System (FPLMTS)
which was later renamed to International Mobile Telecommunication 2000 (IMT-2000). The
3G Partnership Project (3GPP) was initiated in 1998 to coordinate research activities and
standardization around the world. 3GPP does not contribute directly to ITU and is formed by
Organizational partners, such as ETSI (Europe), Association of Radio Industries and Business
(ARIB) and the Telecommunications Technology Association (TTA) (Korea) and T1 (USA).
Several companies take part in 3GPP as market representation partners and other
standardization bodies. In Europe, research concerning 3G mobile radio system is known
under the term Universal Mobile Telephone System (UMTS), began in 1990. In 1998,
WCDMA was selected for the FDD mode and time division CDMA (TD-CDMA) for the TDD
mode of UMTS. An important target of the standardization of UMTS is that the bit rates
offered should be determined in accordance with the Integrated Services Digital Network
(ISDN) rates. In particular, 144 Kpbs (rate of 2B+D ISDN channels) is offered with full
coverage and supporting full mobility, and for limited coverage and mobility, 1920 Kbps
(rate of H12 ISDN channel) should be available.
5. Beyond 3G or fourth generation mobile radio systems
The next evolution that is expected to be released soon is the 4th generation which is based
on LTE (Long Term Evolution) and Wi Max technologies that are promising an internet speed
that reaches 233 Mbit/s for mobile users.
A 4G system will be a complete replacement for current networks and be able to provide a
comprehensive and secure IP solution where voice, data, and streamed multimedia can be
given to users on an "Anytime, Anywhere" basis, and at much higher data rates than
previous generations.
As 3G systems are already operating in some parts of the world, research activities directed
towards the definition and design of beyond 3G systems are being discussed in many parts
of the world. With the expected development of new mobile multimedia services in the
coming years, new technical approaches will be necessary for the future mobile
communications systems. Looking the approximately 10 years of time span observed for 2G
or 3G from first research to the deployment of the system, a new air interface and complete
network concepts for beyond 3G systems are already being discussed in research since last
year 2000. Due to the new mobile multimedia services, data services will dominate over
pure voice services. Moreover, in the future the allotted frequency bands will be a scarce
resource, the support of high data rates requires system designs which make optimum use
of the assigned frequency spectrum and thus guarantee a high spectrum efficiency.�_�_�_�_�_�_�_�_�_�_�_�_�_�_�_�_�_�_�_�_�_�_�
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The 4G working group has defined the following as objectives of the 4G wireless
communication standard:
• A spectrally efficient system (in bits/s/Hz and bits/s/Hz/site)
• High network capacity: more simultaneous users per cell
• A nominal data rate of 100 Mbit/s while the client physically moves at high speeds
relative to the station, and 1 Gbit/s while client and station are in relatively fixed
positions as defined by the ITU-R
• A data rate of at least 100 Mbit/s between any two points in the world
• Smooth handoff across heterogeneous networks
• Seamless connectivity and global roaming across multiple networks
• High quality of service for next generation multimedia support (real time audio, high
speed data, HDTV video content, mobile TV, etc)
• Interoperability with existing wireless standards, and
• An all IP, packet switched network
Orthogonal Frequency Division Multiplexing (OFDM) transmission techniques at the physical
layer with interference suppression is considered by the majority of the scientific
community to be the leading the candidate for the beyond 3G mobile radio systems, due to
6. its inherent ability to mitigate the effects of multipath propagation, which pose a limit at the
achievable data rates.
MIMO environment
Another standardized capability is MIMO, a technique that employs multiple transmit
antennas and multiple receive antennas, often in combination with multiple radios and
multiple parallel data streams. The most common use of the term “MIMO” applies to spatial
multiplexing. The transmitter sends different data streams over each antenna.
Whereas multipath is an impediment for other radio systems, MIMO—as illustrated in
Figure 1—actually exploits multipath, relying on signals to travel across different
uncorrelated communications paths. This results in multiple data paths effectively operating
somewhat in parallel and, through appropriate decoding, in a multiplicative gain in
throughput.
Figure 1. MIMO using multiple paths
Service area concept versus cellular concept
Mobile radio systems have to serve a large number of mobile subscribers. To cope with the
problematic regarding the efficient coverage of the theoretically infinite geographical area,
the cellular system invented by Bell Labs in 1979 is applied in the mobile radio systems of
the first, second and third generation. According to the cellular system, mobile radio
operators distribute a number of base stations (BSs) over the geographical area of
responsibility in order to accomplish radio coverage. Mobile terminals (MTs) are served by
the nearest BS and the area responsability of each BS is termed cell.
To avoid interference situations between the individual radio links of the MTs of
neighboring cells utilizing the same frequencies, different frequency bands may be assigned
to each cell.
However, given the theoretically infinite size of the area to be covered, such a solution
would lead to a waste of resources. In the cellular concept, the frequency band assigned to
the mobile radio operator, is distributed among cells of a particular group, termed cluster
and the number of cells forming a cluster is called cluster size.
7. As attenuation of electromagnetic waves grows with the distance of propagation, a specific
partial frequency band of a cell is reused after a sufficiently large distance, because the
interference between MTs of the two cells using the same frequencies can be considered to
be negligible.
In this way, the whole geographical area is covered with clusters of cells. In GSM cluster size
of 4 is used but in 3G mobile radio system (UMTS), unity cluster size is used and the
resulting intercell interference is mitigated by the use of spread spectrum techniques in
each cell. Fig. 2 shows the architecture of a conventional cellular system. Each cell contains
a BS, and the MTs of each cell communicate solely with this BS. All BSs are connected to a
central entity termed core network in Fig. 2, which, in the case of GSM, consists of the base
station controllers and the mobile switching centers. The core network can be considered
the data source and data sink in the communication with the MTs.
An alternative air interface architecture to cellular systems are service area (SA) based
systems. In the SA based air interface architecture, instead of individual BSs access points
(AP) are introduced with groups of such APs being linked to a central unit (CU). The CUs in
their turn are connected to the core network. Each such group defines a SA, and the MTs of
each SA communicate with the SA specific CU via all APs of the SA.
Instead of a number of cells - each with a BS- of conventional cellular systems we now have
a SA with a number of APs, which are connected to a CU. Fig. 3 shows the architecture of a
SA-based system as opposed to the cellular system architecture, shown in Fig. 2 In the
uplink (UL), the transmit signals of the simultaneously MTs of a SA are received by APs of
the SA and fed to the CU, where they are jointly processed. The aim of this joint processing
consists in exploiting the signal energies received by the APs of the SA in an optimum way,
Figure 2. Conventional cellular system with 12 cells and cluster size 4
8. Figure 3. Architecture of a SA-based system, example with 3 SAs
and in simultaneously combating the impacts of intersymbol interference (ISI) and intra-SA
multiple access interference (MAI). The CU jointly detects the signals radiated by MTs of the
SA and provides the data transmitted by the MTs at its output. This means that in the UL the
CU performs joint detection (JD).
In the downlink (DL), each MT of a SA is supported by transmit signals radiated by APs of the
SA.
These signals are generated in the CU based on the data for each MT of the SA in such a way
that the transmit signals for each MT have minimum powers and cause minimum
interference at other MTs, and the complexity of the MTs can be kept low. This means that
in the DL the CU performs joint transmission (JT).
The rationale of SA based systems can be applied in both single, that is isolated SAs, and
conglomerates of SAs corresponding to conventional cellular networks. Each CU has to be
connected to a core network, into which- in the case of the UL - the data coming from the
MTs is fed, and which - in the case of the DL - provide the data to be fed to the MTs.
In the case of conventional cellular systems in each cell only the MAI originating in the cell,
that is intracell MAI, can be avoided or mitigated by schemes as JD and JT.
In the case of a SA-based system, intra-SA MAI, corresponding to the intercell MAI of cellular
systems, is combated by JD and JT, see above. Because in the case of a SA-based system the
SAs are larger than the cells of a conventional cellular system, a larger number of links is
included in the interference mitigation processes, producing an improvement of the
spectrum efficiency.
9. References
McDonald, V.: The cellular concept. The Bell System Technical Journal
Proakis, J. G.: Digital Communications. 3. Auflage. New York: McGraw-Hill, 1995
Padgett, J. E.; Günther, C. G.; Hattori, T.: Overview of wireless personal communications.
IEEE Communications Magazine
IMT-2000 official site. http://www.itu.int/imt
Partnership project description. http://www.3GPP.org
Wikipedia Mobile Communications, http://en.wikipedia.org/wiki/Mobile_telecommunications
10. References
McDonald, V.: The cellular concept. The Bell System Technical Journal
Proakis, J. G.: Digital Communications. 3. Auflage. New York: McGraw-Hill, 1995
Padgett, J. E.; Günther, C. G.; Hattori, T.: Overview of wireless personal communications.
IEEE Communications Magazine
IMT-2000 official site. http://www.itu.int/imt
Partnership project description. http://www.3GPP.org
Wikipedia Mobile Communications, http://en.wikipedia.org/wiki/Mobile_telecommunications
