Satellite modem
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A satellite modem or satmodem is a
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 link
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
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
Features
The main functions of a satellite modem are modulation and demodulation. Satellite communication standards also define
Popular modulation types being used for satellite communications:
- Binary phase-shift keying (BPSK);
- Quadrature phase-shift keying (QPSK);
- Offset 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 Sequential decoder);
- with
- interleaving;
- New modems support superior error correction codes (LDPCcodes).
Frame formats that are supported by various satellite modems include:
- Multiple data interfaces (like 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 how a modem works is to look at its internal structure. A block diagram of a generic satellite modem is shown on the image.
Analog tract
After a
The purpose of the analog tract in the receiver is to convert signal's frequency, to adjust its power via an
The input signal for the analog tract is at the
At last the signal passes through an anti-aliasing filter and is sampled or (digitized).
Modulator and demodulator
A digital modulator transforms a digital stream into a radio signal at the intermediate frequency (IF). A modulator is generally simpler 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 that 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 has 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, however, this method, known as noncoherent demodulation, 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
An FEC decoder decodes the
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
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.
Multiplexing
A multiplexer transforms several digital streams into one stream. This is often referred to as 'muxing.'
Generally, a
A demultiplexer achieves this goal by maintaining frame synchronization.
Applications
Satellite modems are often used for home
There are two different types, both employing the
- One-way satmodems (DVB-IP modems) use a return channel not based on communication with the satellite, such as 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
- Communications satellite
- Data collection satellite
- Yahsat
- Intelsat
- Satellite Internet access
- VSAT
External links
- ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.113, definition: satellite link International Telecommunication Union (ITU)
- Talal, M. (1997). Modem design for digital satellite communications (PDF) (PhD). Loughborough University. uk.bl.ethos.362760.