Page 404 - Satellite Communications, Fourth Edition

384 Chapter Twelve

The carrier-to-intermodulation-noise ratio is usually found experimen-
tally, or in some cases it may be determined by computer methods. Once
this ratio is known, it can be combined with the carrier-to-thermal-noise
ratio by the addition of the reciprocals in the manner described in Sec.
12.10. Denoting the intermodulation term by (C/N ) and bearing in mind
0 IM
that the reciprocals of the C/N power ratios (and not the corresponding
0
dB values) must be added, Eq. (12.61) is extended to
N 0 a N 0 b a N 0 b a N 0 b
C C U C D C IM (12.62)

A similar expression applies for noice-to-carrier (N/C) ratios.

Example 12.20 For a satellite circuit the carrier-to-noise ratios are uplink 23 dB,
downlink 20 dB, intermodulation 24 dB. Calculate the overall carrier- to-noise ratio
in decibels.
Solution From Eq. (12.62),

N 2.4 2.3 2
10 10 10 0.0019
C
c C d 10 log(0.0019)
N
17.2 dBHz

In order to reduce intermodulation noise, the TWT must be operated
in a BO condition as described previously. Figure 12.10 shows how the
[C/N ] ratio improves as the input BO is increased for a typical TWT.
0 IM
At the same time, increasing the BO decreases both [C/N ] and
0 U
[C/N ] , as shown by Eqs. (12.50) and (12.55). The result is that there
0 D
is an optimal point where the overall carrier-to-noise ratio is a maxi-
mum. The component [C/N ] ratios as functions of the TWT input are
0
sketched in Fig. 12.11. The TWT input in dB is [ ] [BO] , and there-

i
S
fore, Eq. (12.50) plots as a straight line. Equation (12.55) reflects the
,
curvature in the TWT characteristic through the output BO, [BO] 0
which is not linearly related to the input BO, as shown in Fig. 12.7.
The intermodulation curve is not easily predictable, and only the
general trend is shown. The overall [C/N ], which is calculated from
0
Eq. (12.62), is also sketched. The optimal operating point is defined
by the peak of this curve.
12.12 Inter-Satellite Links
Inter-satellite links (ISLs) are radio frequency or optical links that pro-
vide a connection between satellites without the need for intermediate