Page 453 - Satellite Communications, Fourth Edition

Satellite Access 433

Power limitation occurs where the EIRP is insufficient to meet the
[C/N] requirements, as shown by Eq. (12.34). The signal bandwidth will
be approximately equal to the noise bandwidth, and if the EIRP is below
a certain level, the bandwidth will have to be correspondingly reduced
to maintain the [C/N] at the required value. These limitations are dis-
cussed in more detail in the following two sections.

14.6.1 FDMA downlink analysis
To see the effects of output backoff which results with FDMA operation,
consider the overall carrier-to-noise ratio as given by Eq. (12.62). In
terms of noise power rather than noise power density, Eq. (12.62) states

a N b a N b a N b a N b (14.1)
C C U C D C IM
A certain value of carrier-to-noise ratio will be needed, as specified in
the system design, and this will be denoted by the subscript REQ. The
overall C/N must be at least as great as the required value, a condition
which can therefore be stated as

a N b a N b (14.2)
C REQ C
Note that because the noise-to-carrier ratio rather than the carrier-
to-noise ratio is involved, the actual value is equal to or less than the
required value. Using Eq. (14.1), the condition can be rewritten as

a N b a N b a N b a N b 14.3)
C REQ C U C D C IM
The right-hand side of Eq. (14.3) is usually dominated by the down-
link ratio. With FDMA, backoff is utilized to reduce the intermodulation
noise to an acceptable level, and as shown in Sec. 12.10, the uplink
noise contribution is usually negligible. Thus the expression can be
approximated by

a N b a N b
C REQ C D
or

a C b a C b (14.4)
N REQ N D