130x1g2 - CCSDS

TM SYNCHRONIZATION AND CHANNEL CODING—SUMMARY OF CONCEPT AND RATIONALEFigure 4-6: Bit Error Rate Performance of the CCSDS Rate-1/2 Convolutional Codewith Quantization Strategy 1 and Different Quantizers10 0 Hard Decision10 -1BIT ERROR RATE10 -210 -310 -410 -510 -63-Bit Quant. Soft Decision6-Bit Quant. Soft DecisionUnquantized Soft Decision10 -70 1 2 3 4 5 6E b/N 0(dB)Figure 4-7: Bit Error Rate Performance of the CCSDS Rate-1/2 Convolutional Codewith Quantization Strategy 2 and Different QuantizersIn principle, the Viterbi decoder should operate on the entire received sequence. This,however, would result in unacceptably long latency and excessive memory storage for thesurvivor sequences. In fact, since all survivor paths tend to merge into one single path whenexploring the trellis at sufficient depth, practical implementations use a truncated Viterbialgorithm that forces the decision on the oldest symbol of the minimum metric path after afixed and sufficiently long delay D. Actually, this quantity has the meaning of decodingdelay, since it represents the number of bits that must be waited before each decoded bitbecomes available at the output of the Viterbi decoder. It differs from the concept ofdecoding latency for block codes, which instead measures the time needed for completingreception of a codeword and performing decoding of the whole codeword. Just to avoidpossible misunderstanding, it is better to denote this quantity by the term ‘truncation length’,which is also adopted in the literature. Computer simulations show that using a truncationlength on the order of 5 times the constraint length (i.e., D=5K) is enough to obtainnegligible degradations.For the CCSDS rate-1/2 convolutional code, the dependence of the BER on the truncationlength is shown in figure 4-8. Using a truncation length of only D=30 bits, i.e., 5 times thememory m, the performance exhibits a very small degradation. Using D=60 bits nearlyoptimum performance is obtained. (All the curves have been obtained with unquantized softdecision.) For this reason, D=60 has been fixed in the following. The effect of furtherincreasing the truncation length is discussed in 4.7.CCSDS 130.1-G-2 Page 4-8 November 2012