Page 391 - Satellite Communications, Fourth Edition

The Space Link 371

Note that [k] 228.6 dB, so [k] in Eq. (12.50) becomes 228.6 dB. Also, [RFL]
and [BO] i are entered as negative numbers to take account of the minus signs
attached to them in Eq. (12.50). The total gives the carrier-to-noise density ratio
at the satellite receiver as 74.5 dBHz.
Since fade margins have not been included at this stage, Eq. (12.50)
applies for clear-sky conditions. Usually, the most serious fading is
caused by rainfall, as described in Sec. 12.9.

12.7.3 The earth station HPA
The earth station HPAhas to supply the radiated power plus the transmit
feeder losses, denoted here by TFL, or [TFL] dB. These include waveguide,
filter, and coupler losses between the HPAoutput and the transmit antenna.
Referring back to Eq. (12.3), the power output of the HPA is given by

[P HPA ] [EIRP] [G ] [TFL] (12.51)
T
The [EIRP] is that given by Eq. (12.49) and thus includes any input
BO that is required at the satellite.
The earth station itself may have to transmit multiple carriers, and
its output also will require back off, denoted by [BO] HPA . The earth sta-
tion HPA must be rated for a saturation power output given by

[P HPA,sat ] [P HPA ] [BO] HPA (12.52)
Of course, the HPA will be operated at the backed-off power level so
that it provides the required power output [P HPA ]. To ensure operation
well into the linear region, an HPA with a comparatively high satura-
tion level can be used and a high degree of BO introduced. The large
physical size and high power consumption associated with larger tubes
do not carry the same penalties they would if used aboard the satellite.
Again, it is emphasized that BO at the earth station may be required
quite independently of any BO requirements at the satellite transpon-
der. The power rating of the earth-station HPA should also be sufficient
to provide a fade margin, as discussed in Sec. 12.9.1.

12.8 Downlink

The downlink of a satellite circuit is the one in which the satellite is trans-
mitting the signal and the earth station is receiving it. Equation (12.38) can
be applied to the downlink, but subscript D will be used to denote specifi-
cally that the downlink is being considered. Thus Eq. (12.38) becomes

c C d [EIRP] c G d [LOSSES] [k] (12.53)
D
D
N 0 D T D