Chapter 7. Forward Error Correction Options

Viterbi

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Sequential

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TURBO Product Codec

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Uncoded Operation

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7.1INTRODUCTION

As standard, the CiM-550 Modem is equipped with two Forward Error Correction Decoders, Viterbi and Sequential. The constraint lengths and encoding polynomials are compatible with the vast majority of existing modems from other manufacturers, and Comtech EF Data has performed compatibility testing to ensure inter-operability.

As an option, the CiM-550 may be fitted with two additional Forward Error Correction (FEC) devices. The first is a concatenated Reed-Solomon Codec. This is always used in conjunction with, rather than instead of, the main FEC method (Viterbi or Sequential).

The second option is Comtech EF Data’s Turbo Product Codec, which is an entirely stand-alone method of FEC. These options are small daughter cards, and may be fitted at the factory at the time of order, or may be added by the user at a later date.

7.2VITERBI

The combination of convolutional coding and Viterbi decoding has become an almost universal standard for satellite communications. The CiM-550 complies with the Intelsat IESS 308/309 standards for Viterbi decoding with a constraint length of seven. This is a de facto standard, even in a closed network environment, which means almost guaranteed inter-operability with other manufacturer’s equipment. It provides very useful levels of coding gain, and its short decoding delay and error-burst characteristics make it particularly suitable for low data rate coded voice applications. It has a short constraint length, fixed at 7, for all code rates. (The constraint length is defined as the number of output symbols from the encoder which are affected by a single input bit.)

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