Page 390 - Satellite Communications, Fourth Edition

370 Chapter Twelve

12.7.2 Input backoff
As described in Sec. 12.7.3, where a number of carriers are present simul-
taneously in a TWTA, the operating point must be backed off to a linear
portion of the transfer characteristic to reduce the effects of intermodu-
lation distortion. Such multiple carrier operation occurs with frequency-
division multiple access (FDMA), which is described in Chap. 14. The
point to be made here is that backoff (BO) must be allowed for in the link-
budget calculations.
Suppose that the saturation flux density for single-carrier operation is
known. Input BO will be specified for multiple-carrier operation, referred
to the single-carrier saturation level. The earth-station EIRP will have
to be reduced by the specified BO, resulting in an uplink value of

[EIRP] U [EIRP ] [BO] i (12.49)
S U
Although some control of the input to the transponder power ampli-
fier is possible through the ground TT&C station, as described in Sec.
12.7.3, input BO is normally achieved through reduction of the [EIRP]
of the earth stations actually accessing the transponder.
Equations (12.48) and (12.49) may now be substituted in Eq. (12.39)
to give

c C d [ ] [A ] [BO] c G d [k] [RFL] (12.50)
S
0
i
N 0 U T U

Example 12.11 An uplink at 14 GHz requires a saturation flux density of
1
2
91.4 dBW/m and an input BO of 11 dB. The satellite [G/T] is 6.7 dBK ,
and receiver feeder losses amount to 0.6 dB. Calculate the carrier-to-noise
density ratio.
Solution As in Example 12.9, the calculations are best carried out in tabular
form.
2
[A 0 ] 44.37 dBm for a frequency of 14 GHz is calculated by using Eq. (12.44)
as in Example 12.10.

Quantity Decilogs
Saturation flux density 91.4
[A 0 ] at 14 GHz 44.4
Input BO 11.0
Satellite saturation [G/T ] 6.7
[k] 228.6
Receiver feeder loss 0.6
Total 74.5