Telecommunication

Wireless communication, despite the hype of the popular press, is a field
that has been around for over a hundred years, starting around 1897 with
Marconi’s successful demonstrations of wireless telegraphy. By 1901, radio
reception across the Atlantic Ocean had been established; thus, rapid progress
in technology has also been around for quite a while. In the intervening
hundred years, many types of wireless systems have flourished, and often
later disappeared. For example, television transmission, in its early days, was
broadcast by wireless radio transmitters, which are increasingly being replaced
by cable transmission. Similarly, the point-to-point microwave circuits that
formed the backbone of the telephone network are being replaced by optical
fiber. In the first example, wireless technology became outdated when a wired
distribution network was installed; in the second, a new wired technology
(optical fiber) replaced the older technology. The opposite type of example is
occurring today in telephony, where wireless (cellular) technology is partially
replacing the use of the wired telephone network (particularly in parts of
the world where the wired network is not well developed). The point of
these examples is that there are many situations in which there is a choice
between wireless and wire technologies, and the choice often changes when
new technologies become available.
In this book, we will concentrate on cellular networks, both because they are
of great current interest and also because the features of many other wireless
systems can be easily understood as special cases or simple generalizations
of the features of cellular networks. A cellular network consists of a large
number of wireless subscribers who have cellular telephones (users), that can
be used in cars, in buildings, on the street, or almost anywhere. There are
also a number of fixed base-stations, arranged to provide coverage of the
subscribers.
The area covered by a base-station, i.e., the area from which incoming
calls reach that base-station, is called a cell. One often pictures a cell as
a hexagonal region with the base-station in the middle. One then pictures
a city or region as being broken up into a hexagonal lattice of cells (see
Figure 1.1a). In reality, the base-stations are placed somewhat irregularly,
depending on the location of places such as building tops or hill tops that
have good communication coverage and that can be leased or bought (see
Figure 1.1b). Similarly, mobile users connected to a base-station are chosen
by good communication paths rather than geographic distance.
When a user makes a call, it is connected to the base-station to which it
appears to have the best path (often but not always the closest base-station).
The base-stations in a given area are then connected to a mobile telephone
switching office (MTSO, also called a mobile switching center MSC) by highspeed
wire connections or microwave links. The MTSO is connected to the
public wired telephone network. Thus an incoming call from a mobile user
is first connected to a base-station and from there to the MTSO and then to
the wired network. From there the call goes to its destination, which might
be an ordinary wire line telephone, or might be another mobile subscriber.
Thus, we see that a cellular network is not an independent network, but rather
an appendage to the wired network. The MTSO also plays a major role in
coordinating which base-station will handle a call to or from a user and when
to handoff a user from one base-station to another.
When another user (either wired or wireless) places a call to a given user, the
reverse process takes place. First the MTSO for the called subscriber is found,
then the closest base-station is found, and finally the call is set up through
the MTSO and the base-station. The wireless link from a base-station to the
mobile users is interchangeably called the downlink or the forward channel,
and the link from the users to a base-station is called the uplink or a reverse
channel. There are usually many users connected to a single base-station,
and thus, for the downlink channel, the base-station must multiplex together
the signals to the various connected users and then broadcast one waveform
from which each user can extract its own signal. For the uplink channel, each
user connected to a given base-station transmits its own waveform, and the
base-station receives the sum of the waveforms from the various users plus
noise. The base-station must then separate out the signals from each user and
forward these signals to the MTSO.
Older cellular systems, such as the AMPS (advanced mobile phone service)
system developed in the USA in the eighties, are analog. That is, a voice
waveform is modulated on a carrier and transmitted without being transformed
into a digital stream. Different users in the same cell are assigned
different modulation frequencies, and adjacent cells use different sets of frequencies.
Cells sufficiently far away from each other can reuse the same set
of frequencies with little danger of interference.
Second-generation cellular systems are digital. One is the GSM (global
system for mobile communication) system, which was standardized in Europe
but now used worldwide, another is the TDMA (time-division multiple access)
standard developed in the USA (IS-136), and a third is CDMA (code division
multiple access) (IS-95). Since these cellular systems, and their standards,
were originally developed for telephony, the current data rates and delays
in cellular systems are essentially determined by voice requirements. Thirdgeneration
cellular systems are designed to handle data and/or voice. While
some of the third-generation systems are essentially evolution of secondgeneration
voice systems, others are designed from scratch to cater for the
specific characteristics of data. In addition to a requirement for higher rates,
data applications have two features that distinguish them from voice:
• Many data applications are extremely bursty; users may remain inactive
for long periods of time but have very high demands for short periods of
time. Voice applications, in contrast, have a fixed-rate demand over long
periods of time.
• Voice has a relatively tight latency requirement of the order of 100 ms.
Data applications have a wide range of latency requirements; real-time
applications, such as gaming, may have even tighter delay requirements
than voice, while many others, such as http file transfers, have a much
laxer requirement.