Transmitting Digital Television Signals by Satellite - DVB-S/S2 - Digital Video and Audio Broadcasting Technology - page 253

DVB-S, the roll-off filtering is carried out with a roll-off factor of r = 0.35.
The signal rolls off with a root cosine squared shape within the frequency
band. The cosine squared shape of the spectrum actually required is only
produced by combining the transmitter output filter with the receiver filter
because both filters exhibit root cosine squared roll-off filtering. The roll-
off factor describes the slope of the roll-off filtering and is defined as r =
∆f/f N . After the roll-off filtering, the signal is QPSK modulated in the IQ
modulator, upconverted to the actual satellite RF and then, after power
amplification, fed to the satellite antenna. It is then uplinked to the satellite
in the 14...17 GHz band.
14.4 Signal Processing in the Satellite
The geostationary direct broadcasting satellites located permanently above
the equator in an orbit of about 36000 km above the Earth's surface re-
ceive the DVB-S signal coming from the uplink station and limit it first
with a band-pass filter. Since the uplink distance of more than 36000 km
results in a free-space loss of over 200 dB and, as a result, the useful signal
is correspondingly attenuated, the uplink antenna and the receiving an-
tenna on the satellite must exhibit corresponding gains. In the satellite, the
DVB-S signal is converted to the downlink frequency in the 11...14 GHz
band and then amplified by means of a TWA (Travelling Wave tube Am-
plifier). These amplifiers are highly nonlinear and, in practice, can also not
be corrected due to the power budget in the satellite. During the day, the
satellite is supplied with energy by solar cells and this energy is stored in
batteries. During the night, the satellite is then supplied only from its bat-
teries.
Before the signal is sent back to Earth, it is first filtered again in order to
suppress out-of-band components. The transmitting antenna of the satellite
has a certain pattern so that optimum coverage is obtained in the receiving
area to be covered on the ground. This results in a so-called footprint
within which the programs can be received. Because of the high free space
loss of about 200 dB due to the downlink distance of more than 36000 km,
the satellite transmitting antenna must exhibit a correspondingly high gain.
The transmitting power is about 60 … 80 W. The signal processing unit
for a satellite channel is called a transponder. Uplink and downlink are po-
larized, i.e. there are horizontally and vertically polarized channels. Polari-
zation is used in order to be able to increase the number of channels.