Спутниковые модемы: принцип работы и устройство.

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).

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.

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.

After a digital to analog conversion in the transmitter, the signal passes through a reconstruction filter. Then, if needed, frequency conversion is performed. 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. 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 (see superheterodyne receiver). At last the signal passes through an anti aliasing filter and is sampled or (digitized).

Error correction 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. This is performed by an FEC encoder. The encoder applies an error correction code to the digital stream, thereby adding redundancy. An FEC decoder decodes the Forward error correction code used within the signal. For example, the Digital Video Broadcasting standard defines a concatenated code consisting of inner convolutional (standard NASA code, punctured, 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).

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. 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 transmitted data stream. 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.

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