Transmitting Digital Television Signals by Satellite - DVB-S/S2 - Digital Video and Audio Broadcasting Technology - page 260

Thus, a C/N of about 10 dB can be expected in the example. Actual C/N
values can be expected between 9 … 12 dB.
The following equations form the basis for the C/N calculation:
Free space attenuation:
L[dB] = 92.4 + 20•log(f/GHz) + 20•log(d/km);
f = transmission frequency in GHz;
d = Transmitter-receiver distance in km;
Antenna gain of a parabolic antenna:
G[dB] = 20 + 20•log(D/m) + 20•log(f/GHz);
D = antenna diameter in m;
f = transmission frequency in GHz;
Noise power at the receiver input:
N[dBW] = -228.6 + 10•log(b/Hz) + 10•log((T/ 0 C +273)) + F;
B = bandwidth in Hz;
T = temperature in 0 C;
F = noise figure of the receiver in dB.
Fig. 14.23. shows the minimum C/N ratios as a function of the code rate
used. In addition, the pre-Viterbi, post-Viterbi (= pre-Reed-Solomon) and
post-Reed-Solomon bit error rates are plotted. A frequently used code rate
is 3/4. With a mimum C/N ratio of 6.8 dB, this results in a pre-Viterbi
channel bit error rate of 3 -2 . The post-Viterbi bit error rate is then 2 -4 which
correponds to the limit at which the subsequent Reed-Solomon decoder
still delivers an output bit error rate of 1 -11 or better. This approximately
corresponds to one error per day and is defined as quasi error-free (QEF).
At the same time, these conditions also almost correspond to the “fall off
the cliff” (or “brickwall effect”). Slightly more noise and the transmission
breaks down abruptly.
In the calculated example of the C/N to be expected on the satellite
transmission link, there is, therefore, still a margin of about 3 dB available
with a code rate of 3/4. The precise relationship between the channel bit er-
ror rate, i.e. the pre-Viterbi bit error rate, and the C/N ratio is shown in Fig.
14.24.