Introduction to Communication Systems

Modulation is the process of encoding information into a signal that can be transmitted (or recorded) over a channel of interest. In analog modulation, a baseband message signal, such as speech, audio, or video, is directly transformed into a signal that can be transmitted over a designated channel, typically a passband radio-frequency (RF) channel. Digital modulation differs from this only in the following additional step: bits are encoded into baseband message signals, which are then transformed into passband signals to be transmitted. Thus, despite the relentless transition from digital to analog modulation, many of the techniques developed for analog communication systems remain important for the digital communication systems designer, and our goal in this chapter is to study an important subset of these techniques, using legacy analog communication systems as examples to reinforce concepts.

From Chapter 2, we know that a passband signal carries information in its complex envelope, and that the complex envelope can be represented either in terms of I and Q components or in terms of envelope and phase. We study two broad classes of techniques: amplitude modulation, in which the analog message signal appears directly in the I and/or Q components; and angle modulation, in which the analog message signal appears directly in the phase or in the instantaneous frequency (i.e., in the derivative of the phase) of the transmitted signal. Examples of analog communication in space include AM radio, FM radio, and broadcast television, as well as a variety of specialized radios.