Redundancy allocation in time-varying channels with long propagation delays

In this paper, we propose an optimal redundancy allocation over a binary symmetric channel by using recent results on a tight approximation of codeword error probability in the finite blocklength regime. We evaluate the performance of the proposed algorithm over experimental data where slowly time-varying channel conditions were measured. The rationale of this study is to enhance the performance of underwater acoustic communications, for which low bit rates are available. The performance of the proposed scheme is compared with those obtained for a constant rate allocation and a rateless scheme. Results confirm that in the presence of a time-varying channel with long propagation delays, the utilization of a robust codeword is inefficient in terms of bandwidth used and energy per bit. However, as it results in a small number of retransmissions, it also guarantees short delivery delays. Furthermore, in the case where a lower reliability can be tolerated, the proposed scheme is more efficient and also has shorter delivery delays.