Page 26 - Fundamentals of Communications Systems

xxiv Preface

used other communication texts do not appreciate the usage of Erfc(•) but
frankly this book is written for students first learning communications.
This notation serves these students much better (even while it irritates
reviewers) as it gives a more consistent view betweeen the text and the
common communication tools.
6. Signal Space Representations. A major deficiency in my approach, ac-
cording to some reviewers, is that I do not include signal space represen-
tations of digital signals. While I understand the advantages and insights
offered by this approach I think signal space representations lead the stu-
dents off course. Specifically, I do not know of a single communication sys-
tem that uses the signal space concepts in designing a demodulator or a
modulator other than to exploit orthogonality to send bits independently
(see orthogonal modulations below). The best example of a high dimension
signaling scheme is a direct sequence code division multiple access (DS-
CDMA) system. To the best of my knowledge no DS-CDMA system does
“chip” level filtering and then combining as would be suggested by a signal
space approach but directly implements each spreading waveform or each
spreading waveform matched filter. All demodulators I am familiar with
are based on the concept of the matched filter. I feel taking the matched
filter approach leads to a more consistent discussion, while many of my
colleagues feel a signal space approach is necessary for their students to
comprehend digital communications.
7. Noncoherent and Differentially Coherent Detection in Digital Com-
munications. These subjects never enter this introductory treatment of
communication theory as they are really secondary topics in modern com-
munications theory. When I started my career there were three situations
where tradition ruled that noncoherent or differentially coherent tech-
niques were mandatory for high performance communications: (1) in the
presence of jamming, (2) with short packets, and (3) in land mobile wire-
less communications. In the past 15 years I have worked on these types of
systems in both an academic environment and as part of commercial en-
gineering teams and not once were noncoherent or differentially coherent
2
techiques used in modern communication systems. I decided that rather
than confuse the student with a brief section on these topics that I would
just not present them in this book and let students pick up this material,
if needed, in graduate school or with experience.
8. Cyclostationarity and Spectrum of Digital Modulations. This is a
sensitive subject for many of my professional colleagues. I am strongly of
the opinion that spectral efficiency is a key component of all digital com-
munication discussions. Consequently, all digital transmissions must have
an associated bandwidth. Interestingly nowhere in the previous teaching

2 Except to support legacy systems.