Satellite modem


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 that are also referred to as "satellite modems." These devices are used in satellite Internet access.

Satellite link

A satellite modem is not the only device needed to establish a communication channel. Other equipment that are 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. The modem usually has Intermediate frequency output, 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 is one of the most important tasks of a demodulator.
Similarly, a signal received from a satellite is firstly downconverted, 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

Features

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:
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 modem's work is to look at its internal structure. A block diagram of a generic satellite modem is shown on the image. Almost all parts are optional.

Analog tract

After a digital-to-analog conversion in the transmitter the signal passes through a reconstruction filter. Then its frequency is converted if needed.
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 or in the L-band, in the second case it must be firstly converted to IF. Then the signal is either sampled or processed by the four-quadrant multiplier which produces the complex envelope components through multiplying it by the heterodyne frequency.
See Superheterodyne receiver.
At last the signal passes through an anti-aliasing filter and sampled.

Modulator and demodulator

A digital modulator transforms a digital stream into a radio signal on Intermediate frequency. A modulator is generally a much simpler device than a demodulator, because it doesn't have to recover symbol and carrier frequencies.
A demodulator is one of the most important parts of the receiver. The exact structure of the demodulator is defined by a modulation type. However, the fundamental concepts are similar. Moreover, it is possible to develop a demodulator which can process signals with different modulation types.
Digital demodulation implies that a symbol clock at the receiving side have to be synchronous with those at the transmitting side. This is achieved by the following two circuits:
There are also additional components in the demodulator such as the Intersymbol interference equalizer.
If the analog signal was digitized without a four-quadrant multiplier, the complex envelope has to be calculated by a digital complex mixer.
Sometimes a digital automatic gain control circuit is implemented in the demodulator.

FEC coding

techniques are essential for satellite communications, because, due to satellite's limited power a signal to noise ratio at the receiver is usually rather poor. Error correction works by adding an artificial redundancy to a data stream at the transmitting side, and using this redundancy to correct errors caused by noise and interference.
A FEC encoder applies an error correction code to the digital stream, adding redundancy.
A FEC decoder decodes the Forward error correction code that is used in the specific signal.
For example, the Digital Video Broadcasting standard defines a concatenated code consisting of inner convolutional, interleaving and outer Reed-Solomon code.

Differential coding

There are several modulation types that have a phase ambiguity, that is, a carrier can be restored in different ways. Differential coding is used to resolve this ambiguity.
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.
Long '0'-only and '1'-only sequences create difficulties for timing recovery circuit.
Scramblers and descramblers are usually based on linear-feedback shift registers.
A scrambler randomizes the data stream to be transmitted.
A descrambler restores the original stream from the scrambled one.
Scrambling shouldn't be confused with encryption, since it doesn't protect information from intruders.

Multiplexing

A multiplexer transforms several digital streams into one stream. This is often referred to as 'Muxing.'
Generally, a demultiplexer is a device which transforms one multiplexed data stream to several streams which it consists of. Satellite modem doesn't have so many outputs, so a demultiplexer here performs a drop operation, allowing to choose channels that will be transferred to output.
A demultiplexer achieves this goal by maintaining frame synchronization.

Applications

Satellite modems are often used for home internet access.
There are two different types, both employing the Digital Video Broadcasting standard as their basis:
There are also industrial satellite modems intended to provide a permanent link. They are used, for example, in the Steel shankar network.