Page 454 - Satellite Communications, Fourth Edition

434 Chapter Fourteen

Consider the situation where each carrier of the FDMA system occu-
pies a bandwidth B and has a downlink power denoted by [EIRP] .
D
Equation (12.54) gives

c C d [EIRP] c G d [LOSSES] [k] [B] (14.5)
D
D
N D T D
≈ B. This can be written in terms of the
where it is assumed that B N
required carrier-to-noise ratio as

c C d [EIRP] c G d [LOSSES] [k] [B] (14.6)
D
D
N REQ T D
To set up a reference level, consider first single-carrier operation. The
satellite will have a saturation value of EIRP and a transponder band-
width B , both of which are assumed fixed. With single-carrier access,
TR
no backoff is needed, and Eq. (14.6) becomes

c C d [EIRP ] c G d [LOSSES] [k] [B ] (14.7)
TR
D
S
N REQ T D
or
c C d [EIRP ] c G d [LOSSES] [k] [B ] 0 (14.8)
S
D
TR
N REQ T D
If the system is designed for single-carrier operation, then the equality
sign applies and the reference condition is

c C d [EIRP ] c G d [LOSSES] [k] [B ] 0 (14.9)
TR
D
S
N REQ T D
Consider now the effect of power limitation imposed by the need for
backoff. Suppose the FDMA access provides for K carriers which share
the output power equally, and each requires a bandwidth B. The output
power for each of the FDMA carriers is

[EIRP ] [BO] [K] (14.10)
[EIRP] D S 0
The transponder bandwidth B TR will be shared between the carriers,
but not all of B TR can be utilized because of the power limitation. Let
represent the fraction of the total bandwidth actually occupied, such that
KB B , or in terms of decilogs
TR
[B] [ ] [B ] [K] (14.11)
TR