Page 450 - Satellite Communications, Fourth Edition

430 Chapter Fourteen

14.4 Demand-Assigned FDMA
In the demand-assigned mode of operation, the transponder frequency
bandwidth is subdivided into a number of channels. Achannel is assigned
to each carrier in use, giving rise to the single-channel-per-carrier mode
of operation discussed in the preceding section. As in the preassigned
access mode, carriers may be frequency modulated with analog infor-
mation signals, these being designated FM/SCPC, or they may be phase
modulated with digital information signals, these being designated as
PSK/SCPC.
Demand assignment may be carried out in a number of ways. In the
polling method, a master earth station continuously polls all the earth
stations in sequence, and if a call request is encountered, frequency
slots are assigned from the pool of available frequencies. The polling
delay with such a system tends to become excessive as the number of
participating earth stations increases.
Instead of using a polling sequence, earth stations may request calls
through the master earth station as the need arises. This is referred to as
centrally controlled random access. The requests go over a digital orderwire,
which is a narrowband digital radio link or a circuit through a satellite
transponder reserved for this purpose. Frequencies are assigned, if avail-
able, by the master station, and when the call is completed, the frequen-
cies are returned to the pool. If no frequencies are available, the blocked
call requests may be placed in a queue, or a second call attempt may be ini-
tiated by the requesting station.
As an alternative to centrally controlled random access, control may
be exercised at each earth station, this being known as distributed con-
trol random access. A good illustration of such a system is provided by
the Spade system operated by INTELSAT on some of its satellites.
This is described in the following section.

14.5 Spade System
The word Spade is a loose acronym for SCPC pulse-code-modulated
multiple-access demand-assignment equipment. Spade was developed
by Comsat for use on the INTELSAT satellites (see, e.g., Martin, 1978)
and is compatible with the INTELSAT SCPC preassigned system
described in Sec. 14.3. However, the distributed-demand assignment
facility requires a common signaling channel (CSC). This is shown in
Fig. 14.8. The CSC bandwidth is 160 kHz, and its center frequency is
18.045 MHz below the pilot frequency, as shown in Fig. 14.8. To avoid
interference with the CSC, voice channels 1 and 2 are left vacant, and
to maintain duplex matching, the corresponding channels 1 and 2 are
also left vacant. Recalling from Fig. 14.5 that channel 400 also must be
left vacant, this requires that channel 800 be left vacant for duplex