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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 (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.
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:
- Binary phase shift keying (BPSK);
- Quadrature phase shift keying (QPSK);
- Orthogonal quadrature phase shift keying (OQPSK);
- 8PSK;
- Quadrature amplitude modulation (QAM), especially 16QAM.
The popular satellite error correction codes include:
- Convolutional codes:
- with constraint length less than 10, usually decoded using a Viterbi algorithm (see Viterbi decoder);
- with constraint length more than 10, usually decoded using a Fano algorithm (see Sequential decoder);
- Reed-Solomon codes usually concatenated with convolutional codes with an interleaving;
- New modems support superior error correction codes (turbo codes and LDPC codes).
Frame formats that are supported by various satellite modems include:
- Intelsat business service (IBS) framing
- Intermediate data rate (IDR) framing
- MPEG-2 transport framing (used in DVB)
- E1 and T1 framing
High-end modems 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 noise ratio at the remote end;
- Drop and insert feature for a multiplexed stream, allowing to replace some channels in it.
Internal structure
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 (I,Q) through multiplying it by the heterodyne frequency.
At last the signal passes through an anti-aliasing filter and 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 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:
- 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 control circuit is implemented in the demodulator.
FEC coding
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.
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 1 / 2, 2 / 3, 3 / 4, 5 / 6, 7 / 8), interleaving and outer Reed-Solomon code (block length: 204 bytes, information block: 188 bytes, can correct up to 8 bytes in the block).
Differential coding
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
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.
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 (DVB) standard as their basis:
- One-way satmodems (DVB-IP modems) use a return channel not based on communication with the satellite, such as telephone or cable.
- 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 ETSI standard EN 301 790.
There are also industrial satellite modems intended to provide a permanent link. They are used, for example, in the Steel shankar network.
See also
External links
