Page 456 - Satellite Communications, Fourth Edition

436 Chapter Fourteen

Setting [ ] [BO] 0 gives [ ] 6 dB and hence from Eq. (14.11)

[K] [ ] [B TR ] [B]
6 75.56 64.77
4.79 dB

Hence
K 10 4.79/10 3 (rounded down).

If backoff was not required, the number of carriers which could be accommodated
would be B TR /B 12

14.7 TDMA
With TDMA, only one carrier uses the transponder at any one time, and
therefore, intermodulation products, which result from the nonlinear
amplification of multiple carriers, are absent. This leads to one of the
most significant advantages of TDMA, which is that the TWT can be
operated at maximum power output or saturation level.
Because the signal information is transmitted in bursts, TDMA is
only suited to digital signals. Digital data can be assembled into burst
format for transmission and reassembled from the received bursts
through the use of digital buffer memories.
Figure 14.10 illustrates the basic TDMA concept, in which the stations
transmit bursts in sequence. Burst synchronization is required, and in
the system illustrated in Fig. 14.10, one station is assigned solely for the
purpose of transmitting reference bursts to which the others can be syn-
chronized. The time interval from the start of one reference burst to the
next is termed a frame. A frame contains the reference burst R and the
bursts from the other earth stations, these being shown as A, B,and C
in Fig. 14.10.
Figure 14.11 illustrates the basic principles of burst transmission for
a single channel. Overall, the transmission appears continuous because
the input and output bit rates are continuous and equal. However,
within the transmission channel, input bits are temporarily stored and
transmitted in bursts. Since the time interval between bursts is the
, the required buffer capacity is
frame time T F
M R T F (14.15)
b
The buffer memory fills up at the input bit rate R during the frame
b
time interval. These M bits are transmitted as a burst in the next frame
without any break in continuity of the input. The M bits are transmitted