Satellite modem
A satellite modem's main function is to transform an input bitstream to a radio signal and vice versa.
A satellite modem is not the only device needed to establish a communication channel.
The modem usually has intermediate frequency (IF) output (that is, 50-200 MHz), however, sometimes the signal is modulated directly to L band.
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.
Satellite communication standards also define error correction codes and framing formats.
Popular modulation types being used for satellite communications: The popular satellite error correction codes include: Frame formats that are supported by various satellite modems include: High-end modems also incorporate some additional features: Probably the best way of understanding how a modem works is to look at its internal structure.
A block diagram of a generic satellite modem is shown on the image.
The purpose of the analog tract in the receiver is to convert signal's frequency, to adjust its power via an automatic gain control circuit and to get its complex envelope components.
The input signal for the analog tract is at the intermediate frequency, sometimes, in the L band, in which case it must be converted to an IF.
A modulator is generally simpler than a demodulator because it doesn't have to recover symbol and carrier frequencies.
This is achieved by the following two circuits: There are also additional components in the demodulator such as the intersymbol interference equalizer.
The encoder applies an error correction code to the digital stream, thereby adding redundancy.
There are several modulation types (such as PSK and QAM) that have a phase ambiguity, that is, a carrier can be restored in different ways.
When differential coding is used, the data are deliberately made to depend not only on the current symbol, but also on the previous one.
Scrambling is a technique used to randomize a data stream to eliminate long '0'-only and '1'-only sequences and to assure energy dispersal.
There are two different types, both employing the Digital Video Broadcasting (DVB) standard as their basis: There are also industrial satellite modems intended to provide a permanent link.
