Page 171 - Fundamentals of Communications Systems

Analog Communications Basics 5.11

frequencies spaced at 200 kHz spacings and the US Federal Communications Comission
(FCC) allows each station to use about 180 kHz of bandwidth (B T = 180 kHz). Since
a high fidelity audio has a bandwidth of around W = 15 kHz, FM broadcast in the
United States achieves a bandwidth efficiency of
15
E B = = 8.3% (5.13)
180

EXAMPLE 5.8
The most common example of video transmission in the United States is TV broadcast-
ing. TV broadcasting in the United States has many noncontiguous bands for transmis-
sion (e.g., 54–88 MHz for channels 2–6 and 174–220 MHz for channels 7–13). The US
Federal Communications Comission (FCC) allows each station to use about 6 MHz of
bandwidth (B T = 6 MHz). Since video has a bandwidth of around W = 4.5 MHz, TV
broadcast in the United States achieves a bandwidth efficiency of
4.5
E B = = 75% (5.14)
6

It is clear from the preceding examples that communication engineers have
made different choices for system designs for different applications. From the
development in Chapter 4 it is clear that the center frequency of the trans-
mission has little impact on system design so the marked differences seen in
system designs must be due to the differences in the signal characteristics, the
desired fidelity of the communication, and the time varying nature of engineer-
ing trade-offs. The time variations in communication system design trade-offs
are mostly due to the advances in technology to implement communication sys-
tems and the increasing scarcity of bandwidth that can be used to communicate.
As the different analog modulation techniques are discussed in this book, con-
stant comparisons will be made to the complexity, the fidelity of the message
reconstruction, and the spectral efficiency. This will help make the trade-offs
available in the communication system readily apparent.

5.4 Preview of Pedagogy

The next three chapters introduce specific techniques in analog modulation and
demodulation. This will be done assuming no noise is present in the waveform
observed at the receiver. While no communication system operates without
noise, experience has shown that students learn best by immediately starting
to discuss the modulation and demodulation process. Consequently, for the next
three chapters the observations, y c (t)or y z (t), will be represented with lower
case letters to represent the fact that they are not random but determinisitc.
These chapters will give an understanding of the trade-offs that various ana-
log modulations schemes offer for two of the important performance metrics,
complexity, and spectral efficiency.