TABLE OF CONTENTS A transponder channel aboard a satellite may be fully loaded by a single transmission from an earth station. This is referred to as a single access mode of operation. It is also possible, and more common, for a transponder to be loaded by a number of carriers. These may originate from a number of earth stations geographically separate, and each earth station may transmit one or more of the carriers. This mode of operation is termed multiple access. The need for multiple access arises because more than two earth stations, in general, will be within the service area of a satellite. Even so-called spot beams from satellite antennas cover areas several hundred miles across.
The two most commonly used methods of multiple access are frequency-division multiple access (FDMA) and time-division multiple access (TDMA). These are analogous to frequency-division multiplexing (FDM) and time-division multiplexing (TDM) described in Chaps. 9 and 10. However, multiple access and multiplexing are different concepts, and as pointed out in CCIR Report 708 (1982), modulation (and hence multiplexing) is essentially a transmission feature, whereas multiple access is essentially a traffic feature.
A third category of multiple access is code-division multiple access (CDMA). In this method each signal is associated with a particular code that is used to spread the signal in frequency and/or time. All such signals will be received simultaneously at an earth station, but by using the key to the code, the station can recover the desired signal...
