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.
Read more about this topic: Satellite Modem
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