Low rate repeat-accumulate and unit memory codes.

摘要

Low rate codes can be used to achieve coding gains in non-bandlimited applications such as direct-sequence coded division multiple access (DS-CDMA). In this work, low rate codes based on repeat-accumulate (RA) codes and unit memory codes (UMC) are presented. We show that the performance of iteratively decoded low rate RA codes improves up to a rate of 1/8 and degrades with any further decrease in the code rate. This trend is explained by analyzing the probability distribution of messages being passed in the iterative process and by analyzing their iterative decoding thresholds. We look a low rate partial unit memory (PUM) turbo codes and present rate 1/5 and rate 1/9 PUM turbo codes that have thresholds of -0.55 dB and -0.40 dB, i.e, 0.41 dB and 0.85 dB away from the Shannon limit. We also show that further improvement in performance is obtained by using multiple concatenated PUM turbo codes.