Achievable Rates for Channels with Deletions and Insertions

TL;DR

This paper derives a computable lower bound on the capacity of a binary channel with deletions and insertions, providing the first achievable rate characterization for such channels and improving bounds for deletion probabilities up to 0.3.

Contribution

It introduces a mutual information decomposition approach to establish the first lower bounds for channels with insertions and deletions, enhancing understanding of their capacity.

Findings

Provides the first achievable rate bounds for channels with insertions and deletions.

Improves existing lower bounds for deletion channels with probabilities up to 0.3.

Offers a new method for analyzing synchronization issues in channels with insertions and deletions.

Abstract

This paper considers a binary channel with deletions and insertions, where each input bit is transformed in one of the following ways: it is deleted with probability d, or an extra bit is added after it with probability i, or it is transmitted unmodified with probability 1-d-i. A computable lower bound on the capacity of this channel is derived. The transformation of the input sequence by the channel may be viewed in terms of runs as follows: some runs of the input sequence get shorter/longer, some runs get deleted, and some new runs are added. It is difficult for the decoder to synchronize the channel output sequence to the transmitted codeword mainly due to deleted runs and new inserted runs. The main idea is a mutual information decomposition in terms of the rate achieved by a sub-optimal decoder that determines the positions of the deleted and inserted runs in addition to decoding the transmitted codeword. The mutual information between the channel input and output sequences is expressed as the sum of the rate achieved by this decoder and the rate loss due to its sub-optimality. Obtaining computable lower bounds on each of these quantities yields a lower bound on the capacity. The bounds proposed in this paper provide the first characterization of achievable rates for channels with general insertions, and for channels with both deletions and insertions. For the special case of the deletion channel, the proposed bound improves on the previous best lower bound for deletion probabilities up to 0.3.