TABLE OF CONTENTS This chapter covers the critical topics of modulation and channel multiplexing (multiple access). These functions (in addition to forward error correction [FEC], discussed in Chapter 5) drive, in large measure, the performance of the satellite link, the effective channel throughput, and the service availability that one is able to obtain over the satellite link. This chapter only presents a basic introduction to these topics and focuses pragmatically only on commercial satellite applications. Each of these topics would require a textbook to begin to cover the discipline in a more complete manner.
Radio signals are electromagnetic waves that propagate in space with an underlying sinusoidal pattern defining the wave's instantaneous amplitude/frequency/phase value, as covered in Chapter 2. To transmit a content-bearing signal over a radio frequency (RF) link, a system needs to support three main stages [AG1200101]:
A carrier is generated at the transmitter. A carrier signal is a single-frequency signal that is used to carry the intelligence (data). There are three characteristics of a sinusoidal carrier signal that can be changed over time: amplitude, phase, and frequency. (Note, however, that phase and frequency are just different ways to view or measure the same signal change.)
The carrier is modulated with the information to be transmitted. Modulation is the process of overlaying a signal that has (some encoded) intelligence over an appropriate underlying carrier, thereby enabling the long-distance transport and reception of the intelligent signal. Any reliably detectable change in signal characteristics can carry information. When the underlying signal...
