Transmitting Digital Television Signals by Satellite - DVB-S/S2 - Digital Video and Audio Broadcasting Technology - page 240

use of the same error protection mechanisms but in DVB-T, a much more
elaborate modulation method is used.
The DVB-S transmission method is defined in the ETSI Standard ETS
300421 “Digital Broadcasting Systems for Television, Sound and Data
Services; Framing Structure, Channel Coding and Modulation for 11/12
GHz Satellite Services” and was adopted in 1994.
14.1 The DVBS System Parameters
The modulation method selected for DVB-S was quadrature phase shift
keying (QPSK). For some time, the use of 8PSK modulation instead of
QPSK has also been considered in order to increase the data rate. In prin-
ciple, satellite transmission requires a modulation method which is rela-
tively resistent to noise and, at the same time, is capable of handling severe
nonlinearities. Due to the long distance of 36000 km between the satellite
and the receiving antenna, satellite transmission is subject to severe noise
interference caused by the free-space attenuation of about 205 dB. The ac-
tive element in a satellite transponder is a traveling wave tube amplifier
(TWA) which exhibits severe nonlinearities in its modulation characteris-
tic. These nonlinearities cannot be compensated for since this would be as-
sociated with a decrease in energy efficiency. During daylight, the solar
cells provide power both to the electronics of the satellite and to the batter-
ies. During the night, the energy for the electronics comes exclusively
from the backup batteries. If there are large amounts of nonlinearity, there-
fore, there must not be any information content in the amplitude of a
modulation signal.
Both in QPSK and in 8PSK, the information content is in the phase
alone. In the satellite transmission of analog TV, too, frequency modula-
tion was used instead of amplitude modulation for this reason.
A satellite channel of a direct broadcasting satellite usually has a width
of 26 to 36 MHz (e.g. 33 MHz in ASTRA 1F, 36 MHz in EUTELSAT Hot
Bird 2), the uplink is in the 14 ... 19 GHz band and the downlink is 11 ...
13 GHz. It is then necessary to select a symbol rate which produces a spec-
trum which is narrower than the transponder bandwidth. The symbol rate
selected is, therefore, often 27.5 MS/s. As QPSK allows the transmission
of 2 bits per symbol, a gross data rate of 55 Mbit/s is obtained.
gross_data_rate = 2 bits/symbol • 27.5 Megasymbols/s = 55 Mbit/s;