Satellite Modem - Features

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.