Page 452 - Satellite Communications, Fourth Edition

432 Chapter Fourteen

C, D, E,and F. Each earth station has the facility for generating any
one of the 794 carrier frequencies using frequency synthesizers.
Furthermore, each earth station has a memory containing a list of the
frequencies currently available, and this list is continuously updated
through the CSC. To illustrate the procedure, suppose that a call to sta-
tion F is initiated from station C in Fig. 14.9. Station C will first select
a frequency pair at random from those currently available on the list and
signal this information to station F through the CSC. Station F must
acknowledge, through the CSC, that it can complete the circuit. Once
the circuit is established, the other earth stations are instructed, through
the CSC, to remove this frequency pair from the list.
The round-trip time between station C initiating the call and station
F acknowledging it is about 600 ms. During this time, the two fre-
quencies chosen at station C may be assigned to another circuit. In this
event, station C will receive the information on the CSC update and will
immediately choose another pair at random, even before hearing from
station F.
Once a call has been completed and the circuit disconnected, the two
frequencies are returned to the pool, the information again being trans-
mitted through the CSC to all the earth stations.
As well as establishing the connection through the satellite, the CSC
passes signaling information from the calling station to the destination
station, in the example above from station C to station F. Signaling
information in the Spade system is routed through the CSC rather than
being sent over a voice channel. Each earth station has an equipment
called the demand assignment signaling and switching (DASS) unit
which performs the functions required by the CSC.
Some type of multiple access to the CSC must be provided for all the
earth stations using the Spade system. This is quite separate from the
SCPC multiple access of the network’s voice circuits. TDMA, described
in Sec. 14.7.8, is used for this purpose, allowing up to 49 earth stations
to access the common signaling channel.

14.6 Bandwidth-Limited and Power-Limited
TWT Amplifier Operation
A transponder will have a total bandwidth B , and it is apparent that
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this can impose a limitation on the number of carriers that can access the
transponder in an FDMA mode. For example, if there are K carriers each
of bandwidth B, then the best that can be achieved is K B /B. Any
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increase in the transponder EIRP will not improve on this, and the system
is said to be bandwidth-limited. Likewise, for digital systems, the bit rate
is determined by the bandwidth, which again will be limited to some
maximum value by B .
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