Page 28 - Fundamentals of Communications Systems

xxvi Preface

this attitude. These concepts stayed in the back of my mind and fermented.
My research focus around 1994 went to multiple antenna modems and in
this field one of the most powerful and interesting ideas proposed was the
Alamouti signaling scheme [Ala98]. Alamouti signaling was a method to
multiplex data across antennas in an orthogonal fashion. Unfortunately, to
further upset my thinking, I started doing research on wireless orthogonal
frequency division multiplexing (OFDM) systems (2000) and at first the uni-
fication of all of these concepts was not apparent to my simple mind. Then
I struck upon the idea that Nyquist’s criteria, Alamouti signaling, OFDM,
and orthogonal spreading waveforms in CDMA systems were all just vari-
ations on a theme of orthogonality. This orthogonality allows bits to be sent
and decoded independently in a very simple way. Once the orthogonality
thread was put in place in my teaching then the relationship among all
these apparent disparate systems, with wideband signaling being the most
general modulation fell into place nicely. Finally, I hated the idea of calling
“normal” modulation “orthogonal time division multiplexing” and wanted
a shorter nomenclature. Since the idea was that bits were sent one after
another in time and that students were comfortable with the idea of stream-
ing video that comes with internet usage, I adopted the notation of stream
modulation. Many of my colleagues and book reviewers really were frus-
4
trated by my unified view and by my making up this new terminology . Also
the methodology of teaching a general concept (orthogonality) applicable to
multiple situations did not resonate with many professors’ teaching styles.
10. Pulse Shapes. My discussion of spectrally efficient digital modulations
again does not follow standard practice. The key ideas in pulse shaping re-
sult from orthogonality. I develope these ideas and introduce a cosine pulse
shape and a squared cosine pulse shape that can be used either in the time
or frequency domain. This approach gives a better understanding of why
the pulse shapes evolved but does not use the standard notation in the lit-
erature (e.g., spectral square root raised cosine). Hence I have introduced
new notation but this notation is only used to make the material more clear
conceptually.
11. OMWM. Communication engineers have approached Shannon’s limits by
adding structured redundancy into transmitted waveforms. I try to capture
this idea in this undergraduate book and keep a consistent theme by in-
troducing the concept of orthogonal modulations with memory (OMWM).
OMWM as a paradigm does not limit coding to time or frequency domain
signaling but enables a general approach. This general approach is different
from what has been done in the past and hence is not universally accepted
as the correct way to teach this subject. A new notation was introduced as
a way to highlight the important ideas of modern communications without
writing a toothless chapter on capacity approaching signaling.

4 The first time I taught stream modulation a student Googled the term with zero hits!