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Asatellite modem orsatmodem is amodem used to establishdata transfers using acommunications satellite as arelay. 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 insatellite Internet access (in this case uploaded data is transferred through a conventionalPSTN modem or anADSL 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 includesatellite antennas and frequency converters.

Data to be transmitted are transferred to a modem fromdata terminal equipment (e.g. acomputer). The modem usually hasintermediate frequency (IF) output (that is, 50-200 MHz), however, sometimes the signal is modulated directly toL band. In most cases, frequency has to be converted using anupconverter before amplification and transmission.

A modulated signal is a sequence ofsymbols, pieces of data represented by a corresponding signal state, e.g. a bit or a few bits, depending upon themodulation 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 firstlydownconverted (this is done by aLow-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 VDC.

The main functions of a satellite modem are modulation and demodulation. Satellite communication standards also defineerror correction codes andframing formats.

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:
  • withconstraint length less than 10, usually decoded using aViterbi algorithm (seeViterbi decoder);
  • withconstraint length more than 10, usually decoded using aFano algorithm (seeSequential decoder);
• Reed–Solomon codes usually concatenated with convolutional codes with aninterleaving;
• New modems support superior error correction codes (turbo codes andLDPC 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 inDVB)
• E1 andT1 framing

High-end modems also incorporate some additional features:

• Multiple data interfaces (likeRS-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 constantsignal to noise ratio at the remote end;
• Drop and insert feature for amultiplexed stream, allowing to replace some channels in it.

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 adigital-to-analog conversion in the transmitter, the signal passes through areconstruction 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 anautomatic gain control circuit and to get itscomplex envelope components.

The input signal for the analog tract is at theintermediate frequency, sometimes, in theL band, in which case it must be converted to an IF. Then the signal is eithersampled or processed by the four-quadrant multiplier which produces the complex envelope components (I, Q) through multiplying it by the heterodyne frequency (seesuperheterodyne receiver).

At last the signal passes through ananti-aliasing filter and issampled or (digitized).

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 amodulation 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 asymbol 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 (likefrequency-shift keying) that can be demodulated without carrier recovery, however, this method, known asnoncoherent demodulation, is generally worse.

There are also additional components in the demodulator such as theintersymbol interferenceequalizer.

If the analog signal was digitized without a four-quadrant multiplier, the complex envelope has to be calculated by a digitalcomplex mixer.

Sometimes a digitalautomatic gain control circuit is implemented in the demodulator.

Error correction techniques are essential for satellite communications, because, due to satellite's limited power asignal-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 anFEC encoder. The encoder applies an error correction code to the digital stream, thereby adding redundancy.

AnFEC decoder decodes theForward error correction code used within the signal. For example, theDigital Video Broadcasting standard defines a concatenated code consisting of inner convolutional (standard NASA code, punctured, with rates${\displaystyle 1/2}$,${\displaystyle 2/3}$,${\displaystyle 3/4}$,${\displaystyle 5/6}$,${\displaystyle 7/8}$), interleaving and outerReed–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 asPSK andQAM) 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 currentsymbol, 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 onlinear-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.

Amultiplexer transforms several digital streams into one stream. This is often referred to as 'muxing.'

Generally, ademultiplexer is a device that transforms onemultiplexed data stream into several. Satellite modems don't have many outputs, so ademultiplexer here performs adrop operation, allowing to the modem to choose channels that will be transferred to the output.

A demultiplexer achieves this goal by maintainingframe synchronization.

Satellite modems are often used for homeinternet access.

There are two different types, both employing theDigital 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 astelephone orcable.
• Two-way satmodems (DVB-RCS modems, also calledastromodems) employ a satellite-based return channel as well; they do not need another connection. DVB-RCS isETSIstandardEN 301 790.

There are also industrial satellite modems intended to provide a permanent link. They are used, for example, in the telephone network.

• Communications satellite
• Data collection satellite
• Yahsat
• Intelsat
• Satellite Internet access
• VSAT

• ITU Radio Regulations, Section IV. Radio Stations and Systems – Article 1.113, definition:satellite linkInternational Telecommunication Union (ITU)
• Talal, M. (1997).Modem design for digital satellite communications(PDF) (PhD). Loughborough University. uk.bl.ethos.362760.

• Satellite broadcasting
• Modems
• Telecommunications equipment
• Telecommunications infrastructure

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