Page 30 - Fundamentals of Communications Systems

xxviii Preface

encountered by a communication engineer that cannot be solved by ap-
plication of signal and system theory (as angle modulation is a nonlinear
transformation). This is a great case study of how engineers gain insight
into complex problems by examining simple special cases, here a simple
periodic message signal. This analysis is important for both the insight
it provides and the demonstration of a critical engineering approach that
works in a wide variety of problems. Additionally, there is no better example
of how the Fourier series can help solve practical communication problems.
5. Pre-emphasis/De-emphasis in FM. My favorite example of how engi-
neering understanding can lead to significant performance gains is the idea
of pre-emphasis and de-emphasis in angle modulations. Pre-emphasis and
de-emphasis is used in broadcast radio because engineers realized that
in demodulation of frequency modulation (FM) that the noise spectrum
at the demodulator output was shaped in the frequency domain (a col-
ored noise). This colored noise and the constant envelope characteristic
of FM led to an improved signal processing technique that increased the
output demodulation fidelity. It is a great example of how true understand-
ing leads to a 10-dB gain in demodulation fidelity with little increase in
complexity.
6. Single Bit Modulation and Demodulation. I find the single bit trans-
mission and demodulation process very illustrative of the tasks communi-
cation engineering professionals must complete. The process of identifying
a need (sending a bit of information), building a mathematical model of
the processing (detection theory), and then completing a series of design
problems (best threshold, best filter, best signals) to optimize performance
of the system is very typical in a communication engineering career. The
issue related to single bit detection are all developed in detail to build a
comprehensive understanding of the communication engineering process.
7. Digital Communications and Shannon’s Bound. The generalization
of the modulation and demodulation of multiple bits is a straightforward
extension of single bit ideas. The interesting part occurs when examples
are examined and the realization is made that choices in modulation di-
rectly translate to different points in the fidelity versus spectral efficiency
performance space parameterized by Shannon’s bounds. Showing this re-
lationship between simple modulation ideas and the bound proposed by
Shannon demonstrates the power of Shannon’s theory.
8. Two Dimensional Digital Signaling. The idea of linear modulation is
a simple and insightful one. The mapping of bits into symbols in the com-
plex plane is simple to understand. The demodulation by computing the
distances between the received signal and all the constellation points is in-
tuitive. What really makes this a teachable moment is that probably more
than 90 percent of the digitial communication systems use linear modula-
tions in some form.