CDM-Qx Satellite Modem

Revision 5

Forward Error Correction Options

MN/CDMQx.IOM

6.2Viterbi

The combination of convolutional coding and Viterbi decoding has become an almost universal standard for satellite communications. The Modem complies with the Intelsat 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 that are affected by a single input bit.) By choosing various coding rates (Rate 1/2, 3/4, or 7/8) the user can trade off coding gain for bandwidth compression. Rate 1/2 coding gives the best improvement in error rate, but doubles the transmitted data rate, and hence doubles the occupied bandwidth of the signal. Rate 7/8 coding, at the other extreme, provides the most modest improvement in performance, but only expands the transmitted bandwidth by 14 %. A major advantage of the Viterbi decoding method is that the performance is independent of data rate, and does not display a pronounced threshold effect (i.e., does not fail rapidly below a certain value of Eb/No). Note that in BPSK mode, the Modem only permits a coding rate of 1/2. Because the method of convolutional coding used with Viterbi, the encoder does not preserve the original data intact, and is called non-systematic.

Table 6-1. Viterbi Decoding Summary

FOR

AGAINST

Good BER performance - very useful coding gain.

Higher coding gain possible with

 

other methods

Almost universally used, with de facto standards for

 

constraint length and coding polynomials

 

Shortest decoding delay (~200 bits) of any FEC scheme -

 

good for coded voice, VOIP, etc

 

Short constraint length produce small error bursts - good

 

for coded voice.

 

No pronounced threshold effect - fails gracefully.

 

Coding gain independent of data rate.

 

6–2

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Mocomtech CDM-QX operation manual Viterbi Decoding Summary, For Against