Page 448 - Satellite Communications, Fourth Edition

428 Chapter Fourteen

Figure 14.5 Channeling arrangement for Intelsat SCPC system.

signal may be digital data or PCM voice signals (see Chap. 10). A pilot
frequency is transmitted for the purpose of frequency control, and the
adjacent channel slots on either side of the pilot are left vacant to
avoid interference. The scheme therefore provides a total of 798 one-
way channels or up to 399 full-duplex voice circuits. In duplex opera-
tion, the frequency pairs are separated by 18.045 MHz, as shown in
Fig. 14.5.
The frequency tolerance relative to the assigned values is within
±1 kHz for the received SCPC carrier and must be within ±250 Hz
for the transmitted SCPC carrier (Miya, 1981). The pilot frequency
is transmitted by one of the earth stations designated as a primary
station. This provides a reference for automatic frequency control
(AFC) (usually through the use of phase-locked loops) of the trans-
mitter frequency synthesizers and receiver local oscillators. In the
event of failure of the primary station, the pilot frequency is trans-
mitted from a designated backup station.
An important feature of the INTELSAT SCPC system is that each
channel is voice-activated. This means that on a two-way telephone
conversation, only one carrier is operative at any one time. Also, in long
pauses between speech, the carriers are switched off. It has been esti-
mated that for telephone calls, the one-way utilization time is 40 per-
cent of the call duration. Using voice activation, the average number of
carriers being amplified at any one time by the transponder traveling-
wave tube (TWT) is reduced. For a given level of intermodulation dis-
tortion (see Secs. 7.7.3 and 12.10), the TWT power output per FDMA
carrier therefore can be increased.
SCPC systems are widely used on lightly loaded routes, this type of
service being referred to as a thin route service. It enables remote earth
stations in sparsely populated areas to connect into the national tele-
phone network in a reasonably economical way. A main earth station
is used to make the connection to the telephone network, as illustrated