- Satellite modem
A satellite modem or sat modem is a
modemused to establish data transfers using a communications satelliteas a relay.
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 PSTNmodem or an ADSLmodem).
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 upconverterbefore 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.
The main functions of a satellite modem are modulation and demodulation. Satellite communication standards also define
error correctioncodes and framingformats.
Popular modulation types being used for satellite communications:
* Binary phase shift keying (
* Quadrature phase shift keying (
* Orthogonal quadrature phase shift keying (
Quadrature amplitude modulation(QAM), especially 16QAM.
The popular satellite error correction codes include:
constraint lengthless than 10, usually decoded using a Viterbi algorithm(see Viterbi decoder);
constraint lengthmore than 10, usually decoded using a Fano algorithm(see Sequential decoder);
Reed-Solomoncodes usually concatenated with convolutional codes with an interleaving;
* New modems support superior error correction codes (
turbo codes and LDPCcodes).
Frame formats that are supported by various satellite modems include:
Intelsat business service(IBS) framing
Intermediate data rate(IDR) framing
MPEG-2transport framing (used in DVB)
* E1 and T1 framing
High-endmodems also incorporate some additional features:
* Multiple data interfaces (like
RS-232, RS-422, V.35, G.703, LVDS, Ethernet);
* "Embedded Distant-end Monitor and Control" (
EDMAC), allowing to control the distant-end modem;
* "Automatic Uplink Power Control" (
AUPC), that is, adjusting the output power to maintain a constant signal to noiseratio at the remote end;
Drop and insertfeature for a multiplexed stream, allowing to replace some channels in it.
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.
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 controlcircuit and to get its complex envelopecomponents.
The input signal for the analog tract is at the
intermediate frequencyor 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 multiplierwhich produces the complex envelope components ("I","Q") through multiplying it by the heterodyne frequency.
At last the signal passes through an
anti-aliasing filterand sampled (digitized)..
Modulator and demodulator
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
modulationtype. 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:
timing recovery circuit, determining the borders of symbols;
carrier recovery circuit, which determines the actual meaning of each symbol. There are modulation types (like frequency shift keying) that can be demodulated without carrier recovery ( noncoherent demodulation) but this method is generally worse.
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 controlcircuit is implemented in the demodulator.
Error correctiontechniques are essential for satellite communications, because, due to satellite's limited power a signal to noiseratio 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 correctioncode that is used in the specific signal.For example, the Digital Video Broadcastingstandard defines a concatenated code consisting of inner convolutional (standard NASA code, perforated, with rates , , , , ), interleaving and outer Reed-Solomoncode (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 codingis 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.Descrambler restores the original stream from the scrambled one.
Scrambling shouldn't be confused with encryption, since it doesn't protect information from intruders.
multiplexertransforms several digital streams into one stream. This is often referred to as 'Muxing.'
demultiplexeris 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".
Satellite modems are often used for home internet access.
There are two different types, both employing the
Digital Video Broadcasting(DVB) standard as their basis:
*One-way satmodems (DVB-IP modems) use a return channel not based on communication with the satellite, such as
*Two-way satmodems (DVB-RCS modems, also called astromodems) employ a satellite-based return channel as well; they do not need another connection. DVB-RCS is
ETSIstandard EN 301 790.
There are also industrial satellite modems intended to provide a permanent link. They are used, for example, in the
Satellite Internet access
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