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Sec. 8–5 Satellite Communication Systems 593
= Earth station
Figure 8–13
Interleaving of bursts in a TDMA satellite.
station, which means that the time delay is actually changing with time. Another disadvantage
of TDMA is that the Earth stations are probably transmitting data that have arrived from
synchronous terrestrial lines; consequently, the Earth station equipment must contain a large
memory in which to buffer the data, which are read out at high speed when the packet of
information is sent to the satellite.
A typical TDMA frame format for the data being relayed through a satellite is shown in
Fig. 8–14. In this example, station B is sending data to stations A, E, G, and H. One frame consists
of data arriving from each Earth station. At any time, only one Earth station provides the
time reference signal for the other Earth stations to use for computation of their transmitting
time for their data bursts (frame synchronization). The burst lengths from the various stations
could be different depending on the traffic volume. The second part of the figure shows an
exploded view of a typical burst format that is transmitted from station B. This consists of two
main parts, the preamble and data that are being sent to other Earth stations from station B. The
preamble includes a guard time before transmission is begun. Then, a string of synchronization
characters is transmitted that give the carrier sync recovery loops and bit timing recovery loops
(in the ground station receivers) time to lock onto this burst from station B. The end of the preamble
usually contains a unique word that identifies the burst as coming from station B and
might indicate the addresses (stations) for which the data are intended.