Comtech EF Data SDM-300A - Manual (Page 239)

SDM-300A Satellite Modem Revision 6
Forward Error Correction (Options) MN/SDM300A.IOM
9–3
9.3 Sequential
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 kbits 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 lower data rates there are sufficient number of computational cycles per input symbol
to permit the decoding process to perform optimally. However, as the data rate increases,
there are fewer cycles available, leading to a reduction in coding gain. This is clearly
illustrated in the performance curves that follow. For data rates above ~1 Mbps, Viterbi
should be considered the better alternative. The practical upper limit at this time is 2.048
Mbps
.