Page 443 - Satellite Communications, Fourth Edition

Chapter
14

Satellite Access

14.1 Introduction
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 transpon-
der to be loaded by a number of carriers. These may originate from a
number of earth stations geographically separate, and each earth sta-
tion 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 sig-
nals will be received simultaneously at an earth station, but by using
the key to the code, the station can recover the desired signal by means
of correlation. The other signals occupying the transponder channel
appear very much like random noise to the correlation decoder.
Multiple access also may be classified by the way in which circuits are
assigned to users (circuits in this context implies one communication
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