Page 424 - Satellite Communications, Fourth Edition

404 Chapter Thirteen

or

c C d [E] [G ] [G (
)] [Y] D (13.3)
B
B
I D
The subscript D is used to denote downlink, and [E] is the difference
in dB between the [EIRP]s of the two satellites.

Example 13.1 The desired carrier [EIRP] from a satellite is 34 dBW, and the
ground station receiving antenna gain is 44 dB in the desired direction and 24.47
dB toward the interfering satellite. The interfering satellite also radiates an
[EIRP] of 34 dBW. The polarization discrimination is 4 dB. Determine the carrier-
to-interference ratio at the ground receiving antenna.
Solution From Eq. (13.3)

c C d (34 34) 44 24.47 4
I D
23.3 dB

13.2.2 Uplink
A result similar to Eq. (13.3) can be derived for the uplink. In this situ-
ation, however, it is desirable to work with the radiated powers and the
antenna transmit gains rather than the EIRPs of the two earth stations.
Equation (12.3) may be used to substitute power and gain for EIRP. Also,
for the uplink, G and G (
) are replaced by the satellite receive antenna
B
B
gains, both of which are assumed to be given by the 3-dB contour.
Denoting by [P] the difference in dB between wanted and interfering
transmit powers, [G ] the boresight transmit antenna gain at A, and
A
[G (
)] the off-axis transmit gain at C, it is left as an exercise for the
C
reader to show that Eq. (13.3) is modified to
c C d [P] [G ] [G (
)] [Y] U (13.4)
A
C
I U
Example 13.2 Station A transmits at 24 dBW with an antenna gain of 54 dB, and
station C transmits at 30 dBW. The off-axis gain in the S 1 direction is 24.47 dB,
and the polarization discrimination is 4 dB. Calculate the [C/I] ratio on the
uplink.
Solution Equation (13.4) gives

c C d (24 30) 54 24.47 4
I D
27.53 dB