Page 43 - Fundamentals of Communications Systems

Introduction 1.9

Many of my professional colleagues have made the suggestion that analog
modulation concepts should be removed from the modern undergraduate cur-
riculum. Comments such as “We do not teach about vacuum tubes so why should
we teach about analog modulations?” are frequently heard. I heartily disagree
with this opinion but not because I have a fondness for analog modulation but
because analog modulation concepts are so important in modern communica-
tion systems. The theory and notation for analog signals learned in this text is a
solid foundation for further explorations into modern communication systems
because modern digital communications use analog waveforms. For example,
in the testing of modern communication systems and subsystems analog modu-
lation and demodulation concepts are used extensively. In fact, most of my good
problems for the analog communication chapters have come as a result of my
work in experimental wireless communications even though my research work
has always been focused on digital communication systems! Another example
of the utility of analog communications is that I am unaware of a synthesized
signal generator that does not have an option to produce amplitude modulated
(AM) and frequency modulated (FM) test signals. While modern communica-
tion engineers do not often design analog communication systems, the theory is
still a useful tool. Consequently, this part of the book focuses on analog commu-
nications but using a modern perspective that will provide students the tools
to flourish in their careers.

1.5.3 Noise in Communication Systems
Understanding the effects of noise and interference on communication systems
what makes a communication system engineer uniquely trained. I have always
been struck by the fact that engineering technicians have a training in time
domain analysis, Fourier analysis, modulation techniques, and demodulation
techniques. The main thing a technician does not understand is how to char-
acterize how noise impacts the trade-offs that must be made in system design.
On the other hand, the understanding of noise is often a frustrating subject for
students as the level of mathematics and abstraction can often seem not worth
the gains in useful skills. The approach taken in this text is to introduce the
minimal amount of abstraction necessary to get useful results for engineering
practice.
There are four topics/chapters that are presented in this section to introduce
the techniques to analyze the impact of noise and interference. The first chapter
focuses on the characterization of Gaussian stationary random processes and
how linear filters impact this characterization. This material builds heavily on
probability and random variable concepts. This text offers little new insights
than has been available since the 1950s [DR87, Pap84] other than a reordering
of topic presentation. The next chapter generalizes the concepts of random pro-
cesses to the case of noise in bandpass communication receivers. The impact of
filters in the receiver on the noise characteristics is explored. After these prelim-
inary tools are in place, a revisiting of all the forms of analog communications in