Editing Satellite modem (section)

== Internal structure ==
[[File:Satellite modem block diagram.png|right|thumb|500px|Satellite modem's 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 [[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 [[Sampling (signal processing)|sampled]] or processed by the four-quadrant multiplier which produces the complex envelope components (''[[I and Q|I, Q]]'') through multiplying it by the heterodyne frequency (see [[superheterodyne receiver]]).

At last the signal passes through an [[anti-aliasing filter]] and is '''[[sampling (signal processing)|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]] [[Equalization (communications)|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|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 <math>1/2</math>, <math>2/3</math>, <math>3/4</math>, <math>5/6</math>, <math>7/8</math>), interleaving and outer [[Reed–Solomon error correction|Reed–Solomon]] code (block length: 204 bytes, information block: 188 bytes, can correct up to 8 bytes in the block).

=== Differential coding ===
{{Main|Differential coding}}

There are several modulation types (such as [[phase-shift keying|PSK]] and [[quadrature amplitude modulation|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 ===
{{Main|Scrambler (randomizer)}}
[[Scrambler (randomizer)|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 register]]s.

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 [[demultiplexer]] is a device that transforms one [[multiplexing|multiplexed]] data stream into several. Satellite modems don't have many outputs, so a ''demultiplexer'' here performs a [[drop and insert|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''.