Page 161 - Fundamentals of Communications Systems

Chapter
5

Analog Communications

Basics

Analog communication involves transferring an analog waveform containing
information (no digitization at any point) between two users. Typical examples
where analog information is transmitted in this fashion are

■ Music — broadcast radio
■ Voice — citizen band radio, amateur radio, walkie-talkies, cellular radio
■ Video — broadcast television
These message signals all have unique characteristics that engineers need to
understand to be able to build communication systems that can implement this
communication with high fidelity and low cost. The characterization of these
message signals build upon the foundation of an undergraduate signals and
systems education as reviewed in Chapter 2. Signals are typically characterized
in both the time and frequency domain and that is the approach that will be
taken in this chapter.

5.1 Message Signal Characterization
Characterizing the message to be transmitted will be important in understand-
ing analog communications. The information bearing analog waveform is de-
noted m(t) and it is assumed to be an energy signal with a Fourier transform
of M(f ) and energy spectral density of G m (f ), respectively. The message signal
is assumed to be a real valued signal. Figures 5.1 and 5.2 show an example
of a short time record of a message signal and an energy spectral density of a
message signal. This text will use these examples, time signal and spectrum,
to demonstrate modulation concepts. Often a DC value is not an important
component in an analog message signal so we will often use the fact that the

5.1
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