A satcom or satellite modem is a modem that is useful for establishing data transfers using a communication satellite as a relay. The full form of modem is “modulator-demodulator” whose major function is to change an input bit stream to a radio signal and vice versa. Some devices include only a demodulator (having no modulator, hence facilitating satellite to download the data) are popular as “satellite modems”. Such devices offer one stop solution in satellite internet access (in this, the conventional ADSL modem or PSTN modem transfer uploaded data).
The major functions of a satcom modem are modulation and demodulation. Satellite communication standards likewise define framing formats and error correction codes. It includes the following:
• Binary phase shift keying
• Quadrature phase shift keying
• Orthogonal quadrature phase shift keying
• Quadrature amplitude modulation (QAM), mainly 16 QAM
An overview of DVB-S2 Modulator in Satellite Communication
Digital video broadcasting satellite is a digital television broadcast standard that has designed for the popular DVB-S system. The DVB-S2 contemplates for broadcasting services that include HDTV and standard, interactive services including data content distribution and internet access. The growth of DVB-S2 coincided with the introduction of HDTV and H.264. The vital features that have added compared to the DVB-S2 are as follows:
1. A robust coding scheme on the basis of modem LDPC code. For low encoding complexity, the DPC codes have unique structure, which is popular as Irregular repeat accumulate codes.
2. VCM (Variable Coding and Modulation) and ACM (Adaptive coding and modulation) modes that enable optimizing the bandwidth usage by dynamically changing transmission parameters.
Other specifications include powerful modulation schemes up to 32 APSK and the preface of a generic transport mechanism for IP packet data that includes MPEG-4 audio video streams with present MPEG-2 TS based transmission.
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 amodulation 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
• There are modulation types (like frequency shift keying) that can be demodulated without carrier recovery (non coherent 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.
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, with rates interleaving and outer Reed-Solomon code (block length: 204 bytes, information block: 188 bytes, can correct up to 8 bytes in the block).