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Wire and Wireless Communication Applications Chap. 8
stations by using a TDMA format, as shown in Fig. 8–12c. PA denotes the synchronizing preamble
that occurs at the beginning of each frame, and A, B, C, and so on denote the 1-msec time slots
that are reserved for transmission by Earth stations A, B, C, and so on. In this way 49 different
Earth stations may be accommodated in the 50-msec frame. For example, if Earth station B wishes
to initiate a call to Earth station D, station B selects a QPSK carrier frequency randomly from
among the idle channels that are available and transmits this frequency information along the
address of station D (the call destination) in the station B-TDMA time slot. If it is assumed that
the frequency has not been selected by another station for another call station D will acknowledge
the request in its TDMA time slot. This acknowledgement would be heard by station B about 600
msec after its TDMA signaling request since the round trip time delay to the satellite is 240 ms,
plus equipment delays and the delay to the exact time slot assigned to station D with respect to that
of station B. If another station, say station C, had selected the same frequency during the request
period, a busy signal would be received at station B, and station B would randomly select another
available frequency and try again. When the call is over, disconnect signals are transmitted in the
TDMA time slot and that carrier frequency is returned for reuse. Because the CSC signaling rate is
128 kbs and each time slot is 1 ms in duration, 128 bits are available for each accessing station to
use for transmitting address information, frequency request information, and disconnect signaling.
In practice, because only 49 time slots are available for the TDMA mode, a number of the
SCPC frequencies are assigned on a fixed basis.
In FDMA, such as the SPADE system, when the carriers are turned on and off on demand,
there is amplitude modulation on the composite 36-MHz-wide signal that is being amplified by
the transponder TWT. Consequently, the drive level of the TWT has to be “backed off” so that
the amplifier is not saturated and will be sufficiently linear. Then, the intermodulation products
will be sufficiently low. On the other hand, if a single constant-amplitude signal (such as a
single-wideband FM signal used in relaying television signals) had been used, IM products
would not be a consideration, and the amplifier could be driven harder to provide the fully saturated
power output level.
As indicated earlier, TDMA is similar to TDM, where the different Earth stations send
up bursts of RF energy that contain packets of information. During the time slot designated
for a particular Earth station, that station’s signal uses the bandwidth of the whole transponder.
(See Fig. 8–13.) Since the Earth stations use a constant envelope modulation technique,
such as QPSK, and since only one high-rate modulated signal is being passed through the
transponder during any time interval, no interfering IM products are generated (as compared
with the FDMA technique described earlier). Thus, the final TWT amplifier in the satellite
may be driven into saturation for more power output. This advantage of TDMA over FDMA
may be compared with the main disadvantage of the TDMA technique. The disadvantage of
TDMA is that strict burst timing is required at the Earth station in order to prevent collision of
the bursts at the satellite. In other words, the burst from a particular Earth station has to arrive
at the satellite in the exact time slot designated for that station, so that its signal will not interfere
with the bursts that are arriving from other Earth stations that are assigned adjacent time
slots. Because the ground stations are located at different distances from the satellite and may
use different equipment configurations, the time delay from each Earth station to the satellite
will be different, and this must be taken into account when the transmission time for each
Earth station is computed. In addition, the satellite may be moving with respect to the Earth