Low Latency Coding: Convolutional Codes vs. LDPC Codes

This paper compares the performance of convolutional codes to that of LDPC block codes with identical decoding latencies. The decoding algorithms considered are the Viterbi algorithm and stack sequential decoding for convolutional codes and iterative message passing for LDPC codes. It is shown that, at very low latencies, convolutional codes with Viterbi decoding offer the best performance, whereas for high latencies LDPC codes dominate - and sequential decoding of convolutional codes offers the best performance over a range of intermediate latency values. The "crossover latencies" - i.e., the latency values at which the best code/decoding selection changes - are identified for a variety of code rates (1/2, 2/3, 3/4, and 5/6) and target bit/frame error rates. For sequential decoding, both blockwise and continuous resynchronization procedures are used to allow the decoder to recover the correct path. The results indicate that sequential decoding substantially extends (beyond what is possible with Viterbi decoding) the range of latency values over which convolutional codes prove advantageous compared to LDPC block codes.