Page 29 - Fundamentals of Communications Systems

Preface xxvii

Key Teachable Moments in the Book
I thought it useful to enumerate my view of the key teachable moments in an
undergraduate communications course. My view will likely give the users of
the book a better idea of how to get the most out of the book.

1. Communication Signals Are Two Dimensional. Bandpass communi-
cation signals are inherently two dimensional. Electronic communication
is about embedding information in these two dimensions and retrieving in-
formation from the two dimensions of a corrupted received bandpass signal
in a manner that best achieves a set of engineering trade-offs. Using the
complex envelope representation throughout the book emphasizes this idea
of communications signals being two dimensional in a continuous manner.
2. Fidelity, Complexity, and Spectral Efficiency. Communication engi-
neering is a vibrant field because there is no one best solution to all com-
munication problems but a wide variety of solutions that define a different
operating point within a three dimensional trade-off space. The dimensions
of the trade-off space for all communication systems are: (1) the fidelity of
message reconstruction, (2) the cost and complexity of the electronics to
implement the system, and (3) the amount of bandwidth used to accomo-
date the electronic transmission of information. An important concept that
must be understood is that this trade-off is a constantly changing trade-off.
The reason this trade-off evolves with time is that the cost of electronics is
decreasing for a fixed complexity with time or equivalently the complexity
is increasing for a fixed cost with time. It is important for the readers of this
book to understand engineering decisions made in the last century should
not be the same as the engineering decisions made in the coming century.
Consequently, communications engineering requires lifelong learning.
3. Filter Design to Improve Spectral Efficiency. When I was taught about
single-sideband and vestigial-sideband amplitude modulation (AM), I came
away with the impression that the modulation resulted from a particular
judicious choice of a bandpass filter. This gave me no insight into the prob-
lem and how I might apply the solution to different problems. I was directly
faced with the shortcoming of my education when I had a chance to work
on digital television broadcast where vestigial sideband transmission is one
option and I realized there was no straightforward way to generalize my
education to apply to digital transmissions. My approach in teaching these
concepts is to note that a degree of freedom is not used in traditional AM
(the quadrature channel) and to turn a desire to achieve better spectral effi-
ciency into a design problem with an intuitive answer. This approach gives
a much better flavor for what communication engineering tasks are like
as well as giving a nontrivial answer to why single-sideband and vestigial-
sideband amplitude modulations are used in communication systems.
4. Fourier Series to Analyze the Bandwidth of Angle Modulation.
Finding the spectrum of an angle modulated signal is the first problem