Page 425 - Satellite Communications, Fourth Edition

Interference 405

13.2.3 Combined [C/I] due to interference
on both uplink and downlink
Interference may be considered as a form of noise, and assuming that
the interference sources are statistically independent, the interference
powers may be added directly to give the total interference at receiver
B. The uplink and the downlink ratios are combined in exactly the same
manner described in Sec. 12.10 for noise, resulting in

I
I
I
a b a b a b (13.5)
C ant C U C D
Here, power ratios must be used, not decibels, and the subscript “ant”
denotes the combined ratio at the output of station B receiving
antenna.

Example 13.3 Using the uplink and downlink values of [C/I] determined in
Examples 13.1 and 13.2, find the overall ratio [C/I] ant .
Solution For the uplink, [C/I] 27.53 dB gives (I/C) U 0.001766, and for the
downlink, [C/I] 23.53 dB gives (I/C) D 0.004436. Combining these according
to Eq. (13.5) gives
I
a b 0.001766 0.004436
C ant
0.006202

Hence

c C d 10 log 0.006202
I ant
20.07 dB

13.2.4 Antenna gain function
The antenna radiation pattern can be divided into three regions: the
mainlobe region, the sidelobe region, and the transition region
between the two. For interference calculations, the fine detail of the
antenna pattern is not required, and an envelope curve is used
instead.
Figure 13.4 shows a sketch of the envelope pattern used by the FCC.
The width of the mainlobe and transition region depend on the ratio of
the antenna diameter to the operating wavelength, and Fig. 13.4 is
intended to show only the general shape. The sidelobe gain function in
decibels is defined for different ranges of
. Specifying
in degrees, the