There is a wide range of satellite modems from cheap devices for home internet access to expensive multifunctional equipment for enterprise use.
A "modem" stands for "modulator-demodulator". 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 doesn't comprise any modulator) that are also referred to as "satellite modems". These devices are used in satellite Internet access (in this case data are transferred in other direction through a conventional PSTN modem or an ADSL modem).
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 (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.
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 a modulation. 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.
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.
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 (I,Q) through multiplying it by the heterodyne frequency.
A digital modulator transforms a digital stream into a radio signal on Intermediate frequency (IF). 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 (and, in most cases, an intermediate frequency generator) at the receiving side have to be synchronous with those at the transmitting side. This is achieved by the following two circuits:
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.
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 (standard NASA code, perforated, with rates , , , , ), interleaving and outer Reed-Solomon code (block length: 204 bytes, information block: 188 bytes, can correct up to 8 bytes in the block).
When differential coding is used, the data are deliberately made to depend not only on the current symbol, but also on the previous one.
A scrambler randomizes the data stream to be transmitted. Descrambler restores the original stream from the scrambled one.
Scrambling shouldn't be confused with encryption, since it doesn't protect information from intruders.
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.
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. They are used, for example, in the Steel shankar network.