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Concept
Satellite modem
Summary
A satellite modem or satmodem is a modem used to establish data transfers using a communications satellite as a relay. A satellite modem's main function is to transform an input bitstream to a radio signal and vice versa.
There are some devices that include only a demodulator (and no modulator, thus only allowing data to be downloaded by satellite) that are also referred to as "satellite modems." These devices are used in satellite Internet access (in this case uploaded data is transferred through a conventional PSTN modem or an ADSL modem).
Satellite link
A satellite modem is not the only device needed to establish a communication channel. Other equipment that is essential for creating a satellite link include satellite antennas and frequency converters.
Data to be transmitted are transferred to a modem from data terminal equipment (e.g. a computer). The modem usually has intermediate frequency (IF) output (that is, 50-200 MHz), however, sometimes the signal is modulated directly to L band. In most cases, frequency has to be converted using an upconverter before amplification and transmission.
A modulated signal is a sequence of symbols, pieces of data represented by a corresponding signal state, e.g. a bit or a few bits, depending upon the modulation scheme being used. Recovering a symbol clock (making a local symbol clock generator synchronous with the remote one) is one of the most important tasks of a demodulator.
Similarly, a signal received from a satellite is firstly downconverted (this is done by a Low-noise block converter - LNB), then demodulated by a modem, and at last handled by data terminal equipment. The LNB is usually powered by the modem through the signal cable with 13 or 18 V DC.
The main functions of a satellite modem are modulation and demodulation. Satellite communication standards also define error correction codes and framing formats.
Popular modulation types being used for satellite communications:
Binary phase-shift keying (BPSK);
Quadrature phase-shift keying (QPSK);
Offset quadrature phase-shift keying (OQPSK);
8PSK;
Quadrature amplitude modulation (QAM), especially 16QAM.
Official source
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