Satellite systems provide the operators with a lot of benefits in offering services to end users. A natural ability of satellites to distribute signals to large areas of the Earth surface has been utilized for broadcasting of analog television and radio for decades. This ability mainly relates to geostationary satellites that are placed in a geostationary orbit i.e. in latitude about 36000 km (over the equator).
Each geostationary satellite appears for Earth user fixed in the sky so there is no need for an antenna tracking system.
On the other hand satellite transmissions suffer from error prone satellite links therefore every signal before transmitting has to be adapted for such difficult propagation conditions [20].
A communication payload of satellites consists of transponders. Their function is to receive, restore, amplify, process, re-modulate and sent signal back to Earth.
Currently, the conventional geostationary satellite contains about 20 to 30 transponders and a single transponder can most often have a bandwidth ranging from 26 to 72 MHz (e.g. 36 MHz on the ASTRA 3A satellite).
In case of the satellite analog television a single transponder took care of one TV channel. Applying the DVB technology to the satellite systems a single 33 MHz satellite transponder can carry 4 to 8 TV channels or 150 radio channels. A noisy satellite channel required from DVB project to define for the DVB-S systems efficient modulation techniques and error correction codes [22].
Figure above depicts a block diagram of a general DVB-S system. The MPEG-2 multiplexer multiplexes video and audio PES streams from the MPEG-2 coder with data in PES data streams coming from an IP gateway in the same way as was written in previous section. The multiplexed transport stream is randomized to spread its spectrum within its bandwidth. The randomized transport stream is equipped by an outer code (Reed Solomon code with a coding rate 188/204), interleaved (to make it more resistant to block errors) and encoded by an inner FEC code (convolutional code with a coding rate from 1/2 to 7/8). In the next phase the encoded transport stream is modulated on a carrier. DVB-S uses QPSK (Quaternary Phase Shift Keying) modulation, DVB-S2 uses 8-PSK, 16-APSK or 32-APSK (Amplitude and Phase Shift Keying) modulation. Afterwards, the signal is up-converted to a carrier from Ku band, amplified by a HPA (High Power Amplifier) and radiated by an antenna system to the satellite. The receiver performs opposite actions to demultiplex particular video, audio and data streams and to provide them to users using some end device (e.g. TV set, PC).