Page 444 - Satellite Communications, Fourth Edition

424 Chapter Fourteen

channel through the multiple-access transponder). Circuits may be pre-
assigned, which means they are allocated on a fixed or partially fixed
basis to certain users. These circuits are therefore not available for gen-
eral use. Preassignment is simple to implement but is efficient only for
circuits with continuous heavy traffic.
An alternative to preassignment is demand-assigned multiple access
(DAMA). In this method, all circuits are available to all users and are
assigned according to the demand. DAMA results in more efficient over-
all use of the circuits but is more costly and complicated to implement.
Both FDMA and TDMA can be operated as preassigned or demand
assigned systems. CDMA is a random-access system, there being no
control over the timing of the access or of the frequency slots accessed.
These multiple-access methods refer to the way in which a single
transponder channel is utilized. A satellite carries a number of transpon-
ders, and normally each covers a different frequency channel, as shown
in Fig. 7.13. This provides a form of FDMA to the whole satellite. It is
also possible for transponders to operate at the same frequency but to
be connected to different spot-beam antennas. These allow the satellite
as a whole to be accessed by earth stations widely separated geograph-
ically but transmitting on the same frequency. This is termed frequency
reuse. This method of access is referred to as space-division multiple
access (SDMA). It should be kept in mind that each spot beam may
itself be carrying signals in one of the other multiple-access formats.

14.2 Single Access
With single access, a single modulated carrier occupies the whole of the
available bandwidth of a transponder. Single-access operation is used on
heavy-traffic routes and requires large earth station antennas such as the
class A antenna shown in Fig. 8.7. As an example, Telesat Canada pro-
vides heavy route message facilities, with each transponder channel being
capable of carrying 960 one-way voice circuits on an FDM/FM carrier, as
illustrated in Fig. 14.1. The earth station employs a 30-m-diameter

Figure 14.1 Heavy route mes-
sage (frequency modulation—
single access). (Courtesy of Telesat
Canada, 1983.)