Transmitting Digital Television Signals by Satellite - DVB-S/S2 - Digital Video and Audio Broadcasting Technology - page 257

Additive
with Gaussian
noise (AWGN)

Nonlinearity
noise
IQ errors

DVBS
mod.

DVBS
rec.

Fig. 14.22. Influences affecting the satellite transmission link

14.6 Influences Affecting the Satellite Transmission Link

This section deals with the influences to be expected on the satellite trans-
mission link (Fig. 14.22.) and it will be seen that these influences are
mainly restricted to noise. However, let us first begin with the modulator.
This can be assumed to be ideal up to the IQ modulator. The IQ modulator
can exhibit different gains in the I and Q branches, a phase error in the 90 o
phase shifter and a lack of carrier suppression. There can also be noise ef-
fects and phase jitter coming from this circuit section. These problems can
be ignored, however, because of the rugged nature of the QPSK modula-
tion and will normally never reach an order of magnitude which will no-
ticeably affect the signal quality. In the satellite, the travelling wave tube
generates severe nonlinearities but these do not play a part, in practice. In
the region of the uplink and the downlink, however, where the DVB-S sig-
nal is attenuated severely by more than 200 dB due to the distance of
36000 km each way travelled by the signal, strong noise effects are experi-
enced. It is these noise effects, the additive white gaussian noise (AWGN)
becoming superimposed on the signal, which form the only influence to be
discussed.
In the part following, the satellite downlink will be analysed by way of
an example with respect to the signal attenuation and the resultant noise ef-
fects.
The minimum carrier/noise ratios (C/N) necessary and the channel bit
error rate needed are known and predetermined from forward error correc-
tion (FEC, Reed-Solomon and convolutional coding) (Fig. 14.23.).