28-05-2013, 03:33 PM
Cellular Communications from 1G to 3G
INTRODUCTION
Mobile systems have seen a change of generation, from fi rst to second to third,
every ten years or so (see Figure 1.3). At the introduction of 1G services, the
mobile device was large in size, and would only fi t in the trunk of a car. All
analog components such as the power amplifi er, synthesizer, and shared antenna
equipment were bulky. 1G systems were intended to provide voice service and
low rate (about 9.6 kbps) circuit-switched data services. Miniaturization of
mobile devices progressed before the introduction of 2G services (1990) to the
point where the size of mobile phones fell below 200 cubic centimeters (cc). The
fi rst-generation handsets provided poor voice quality, low talk-time, and low
standby time. The 1G systems used Frequency Division Multiple Access (FDMA)
technology (see Chapter 6) and analog frequency modulation [8,20].
The 2G systems based on TDMA and CDMA technologies [6] were primarily
designed to improve voice quality and provide a set of rich voice features. These
systems supported low rate data services (16–32 kbps).
For second-generation systems three major problems impacting system cost
and quality of service remained unsolved. These include what method to use for
band compression of voice, whether to use a linear or nonlinear modulation scheme,
and how to deal with the issue of multipath delay spread caused by multipath
propagation of radio waves in which there may not only be phase cancellation but
also a signifi cant time difference between the direct and refl ected waves.
The swift progress in Digital Signal Processors (DSPs) was probably fueled
by the rapid development of voice codecs for mobile environments that dealt with
errors. Large increases in the numbers of cellular subscribers and the worries of
exhausting spectrum resources led to the choice of linear modulation systems.
To deal with multipath delay spread, Europe, the United States, and Japan
took very different approaches. Europe adopted a high transmission rate of
280 kbps per 200 kHz RF channel in GSM [13,14] using a multiplexed TDMA
system with 8 to 16 voice channels, and a mandatory equalizer with a high
number of taps to overcome inter-symbol interference (ISI) (see Chapter 3). The
United States used the carrier transmission rate of 48 kbps in 30 kHz channel, and
selected digital advanced mobile phone (DAMP) systems (IS-54/IS-136) to reduce
the computational requirements for equalization, and the CDMA system (IS-95)
to avoid the need for equalization. In Japan the rate of 42 kbps in 25 kHz channel
was used, and equalizers were made optional.
Road Map for Higher Data Rate Capability in 3G
The fi rst- and second-generation cellular systems were primarily designed for
voice services and their data capabilities were limited. Wireless systems have since
been evolving to provide broadband data rate capability as well.
GSM is moving forward to develop cutting-edge, customer-focused solutions
to meet the challenges of the 21st century and 3G mobile services. When
GSM was fi rst designed, no one could have predicted the dramatic growth of the
Internet and the rising demand for multimedia services. These developments have
brought about new challenges to the world of GSM. For GSM operators, the
emphasis is now rapidly changing from that of instigating and driving the development
of technology to fundamentally enable mobile data transmission to that
of improving speed, quality, simplicity, coverage, and reliability in terms of tools
and services that will boost mass market take-up.
People are increasingly looking to gain access to information and services
whenever they want from wherever they are. GSM will provide that connectivity.
The combination of Internet access, web browsing, and the whole range of mobile
multimedia capability is the major driver for development of higher data speed
technologies.
Wireless 4G Systems
4G networks (see Chapter 23) can be defi ned as wireless ad hoc peer-to-peer
networking with high usability and global roaming, distributed computing, personalization,
and multimedia support. 4G networks will use distributed architecture
and end-to-end Internet Protocol (IP). Every device will be both a transceiver
and a router for other devices in the network eliminating the spoke-and-hub
architecture weakness of 3G cellular systems. Network coverage/capacity will
dynamically change to accommodate changing user patterns. Users will automatically
move away from congested routes to allow the network to dynamically and
automatically self-balance.
Recently, several wireless broadband technologies [20] have emerged to
achieve high data rates and quality of service. Navini Networks developed a wireless
broadband system based on TD-SCDMA. The system, named Ripwave, uses
beamforming to allow multiple subscribers in different parts of a sector to simultaneously
use the majority of the spectrum bandwidth. Beamforming allows the
spectrum to be effectively reused in dense environments without having to use
excessive sectors. The Ripwave system varies between QPSK, 16 and 64-QAM,
which allows the system to burst up to 9.6 Mbps using a single 1.6 MHz TDD
carrier. Due to TDD and 64-QAM modulation the Ripwave system is extremely
spectrally effi cient. Currently Ripwave is being tried by several telecom operators
in the United States.
Future Wireless Networks
As mobile networks evolve to offer both circuit and packet-switched services, users
will be connected permanently (always on) via their personal terminal of choice to
the network. With the development of intelligence in core network (CN), both voice
and broadband multimedia traffi c will be directed to their intended destination with
reduced latency and delays. Transmission speeds will be increased and there will
be far more effi cient use of network bandwidth and resources. As the number of
IP-based mobile applications grows, 3G systems will offer the most fl exible access
technology because it allows for mobile, offi ce, and residential use in a wide range
of public and nonpublic networks. The 3G systems will support both IP and non-IP
traffi c in a variety of modes, including packet, circuit-switched, and virtual circuit,
and will thus benefi t directly from the development and extension of IP standards
for mobile communications.