Page 393 - Satellite Communications, Fourth Edition

The Space Link 373

Solution Equation (10.16) gives
2B
R b
1
2 36 10 6

1.2
60 Mbps
Hence,

[R b ] 10 loga 60 10 6 b
1s 1

77.78 dBbps
5
For BER 10 , Fig. 10.17 gives an [E b /N 0 ] 9.6 dB.

From Eq. (10.24) the required C/N 0 ratio is

s C t s E b t [R b ]
N 0 N 0
77.78 9.6
87.38 dBHz
From Eq. (12.53),

G
[EIRP] D s C t s t [LOSSES] D [k]
N 0 D T D
87.38 32 200 228.6
> 26.8 dBW

12.8.1 Output back-off
Where input BO is employed as described in Sec. 12.7.2, a corresponding
output BO must be allowed for in the satellite EIRP. As the curve of
Fig. 7.21 shows, output BO is not linearly related to input BO. A rule of
thumb, frequently used, is to take the output BO as the point on the curve
which is 5 dB below the extrapolated linear portion, as shown in Fig. 12.7.
Since the linear portion gives a 1:1 change in decibels, the relationship
between input and output BO is [BO] [BO] 5 dB. For example, with
0
i
an input BO of [BO] 11 dB, the corresponding output BO is [BO]
i
0
11 5 6 dB.
If the satellite EIRP for saturation conditions is specified as [EIRP ] ,
S D
then [EIRP] [EIRP ] [BO] and Eq. (12.53) becomes
S D
0
D
c C d [EIRP ] [BO] c G d [LOSSES] [k] (12.55)
0
D
S D
N 0 D T D