130x1g2 - CCSDS

TM SYNCHRONIZATION AND CHANNEL CODING—SUMMARY OF CONCEPT AND RATIONALEThe x-axis shows the threshold value of E b /N 0 required to reach a FER of 10 -4 . The y-axisshows the average decoding speed, or reciprocally the average number of iterations. In thisfigure a decoder using a fixed 10 iterations achieves a speed of 300 Kb/s, and the decoder’saverage speed increases inversely as the average number of iterations is reduced byapplication of the stopping rule. The results in this figure are for a selection of recommendedTurbo codes with block lengths 1784 and 8920. The figure shows that effective stoppingrules can increase the decoder speed on the order of 50% to 100% with virtually nocompromise in the required value of E b /N 0 ; further increases in speed can also be obtained bytrading off additional SNR for increased speed.7.4.2 COMPARISON TO TRADITIONAL CONCATENATED CODESTurbo codes gain a significant performance improvement over the traditional Reed-Solomonand convolutional concatenated codes currently recommended by CCSDS. For example, toachieve an overall BER of 10 -6 with a block length of 8920 bits (depth-5 interleaving), therequired bit-SNRs are approximately 0.8 dB, 1.0 dB, and 2.6 dB for the DSN’s standardcodes consisting of the (255,223) Reed-Solomon code concatenated with the (15,1/6)convolutional code, the (15,1/4) convolutional code, and the (7,1/2) convolutional code,respectively. The performance gains achieved by the corresponding-rate Turbo codes infigures 7-6, 7-7, 7-8, 7-9, and 7-10 range from 0.9 dB to 1.6 dB.Figure 7-12 compares the performance of the recommended Turbo codes of block length1784 bits and rates 1/3 and 1/6 with the performance of the CCSDS concatenated code usedby Voyager and that of the non-CCSDS concatenated code used by Cassini and MarsPathfinder. The Voyager code consists of the recommended concatenation of the (255, 223)Reed-Solomon code with the (7,1/2) convolutional code. The Cassini/Pathfinder codeconsists of the same Reed-Solomon code concatenated with a (15, 1/6) convolutional codefor which the Viterbi decoder requires 2 8 = 256 times as many states as for the (7, 1/2) code.Performance for both concatenated codes is obtained using an interleaving depth of I = 1, notthe actual interleaving depths used in the Voyager/Cassini/Pathfinder missions, in order toprovide a fair comparison with the performance of the two Turbo codes with block length1784. In other words, a frame length of 1784 bits is assumed for all four curves in this figure.CCSDS 130.1-G-2 Page 7-11 November 2012