Premier Mounts CDM-600 specifications Sequential, Viterbi Decoding Summary, For Against

CDM-600 Satellite Modem	Revision 7
Forward Error Correction Options	MN/CDM600.IOM

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 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 expansion. 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). This is not true of the Sequential decoding method, as explained in the section below. Note that in BPSK mode, the CDM-600 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 7-1. 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 (~100 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.

7.3Sequential

Although the method of convolutional coding and Sequential decoding appears to be very similar to the Viterbi method, there are some fundamental differences. To begin with, the convolutional encoder is said to be systematic - it does not alter the input data, and the FEC overhead bits are simply appended to the data. Furthermore, the constraint length, k, is much longer (Rate 1/2, k=36. Rate 3/4, k= 63. Rate 7/8, k=87). This means that when the decoding process fails (that is, when its capacity to correct errors is exceeded) it produces a burst of errors which is in multiples of half the constraint length. An error distribution is produced which is markedly different to that of a Viterbi decoder. This gives rise to a pronounced threshold effect. A Sequential decoder does not fail gracefully - a reduction in Eb/No of just a few tenths of a dB can make the difference between acceptable BER and a complete loss of synchronization. The decoding algorithm itself (called the Fano algorithm) uses significantly more path memory (4 kbps in this case) than the equivalent Viterbi decoder, giving rise to increased latency. Furthermore, a fixed computational clock is used to process input symbols, and to search backwards and forwards in time to determine the correct decoding path. At

7–2