| Wireless communication is one of the most vibrant areas in the communication
field today. While it has been a topic of study since the 1960s,
the past decade has seen a surge of research activities in the area. This is
due to a confluence of several factors. First, there has been an explosive
increase in demand for tetherless connectivity, driven so far mainly by cellular
telephony but expected to be soon eclipsed by wireless data applications.
Second, the dramatic progress in VLSI technology has enabled small-area
and low-power implementation of sophisticated signal processing algorithms
and coding techniques. Third, the success of second-generation (2G) digital
wireless standards, in particular, the IS-95 Code Division Multiple Access
(CDMA) standard, provides a concrete demonstration that good ideas from
communication theory can have a significant impact in practice. The research
thrust in the past decade has led to a much richer set of perspectives and tools
on how to communicate over wireless channels, and the picture is still very
much evolving.
There are two fundamental aspects of wireless communication that make
the problem challenging and interesting. These aspects are by and large not
as significant in wireline communication. First is the phenomenon of fading:
the time variation of the channel strengths due to the small-scale effect of
multipath fading, as well as larger-scale effects such as path loss via distance
attenuation and shadowing by obstacles. Second, unlike in the wired
world where each transmitter–receiver pair can often be thought of as an
isolated point-to-point link, wireless users communicate over the air and there
is significant interference between them. The interference can be between
transmitters communicating with a common receiver (e.g., uplink of a cellular
system), between signals from a single transmitter to multiple receivers
(e.g., downlink of a cellular system), or between different transmitter–receiver
pairs (e.g., interference between users in different cells). How to deal with fading
and with interference is central to the design of wireless communication
systems and will be the central theme of this book. Although this book takes
a physical-layer perspective, it will be seen that in fact the management of
fading and interference has ramifications across multiple layers. |
