A Lower Bound on the List-Decodability of Insdel Codes

TL;DR

This paper establishes a new lower bound on the list-decodability of insdel codes, revealing that previous bounds are not tight and advancing understanding of their decoding limits.

Contribution

It introduces a novel lower bound for insdel code list-decodability, improving upon previous bounds and addressing an open problem in the field.

Findings

The new bound is not tight, unlike the Johnson bound for other metrics.

If a code's list decoding radius is below this bound, it must be list-decodable with a given list size.

Application of the bound to well-known codes provides new insights into their insdel-list-decodability.

Abstract

For codes equipped with metrics such as Hamming metric, symbol pair metric or cover metric, the Johnson bound guarantees list-decodability of such codes. That is, the Johnson bound provides a lower bound on the list-decoding radius of a code in terms of its relative minimum distance $\delta$, list size $L$ and the alphabet size $q.$ For study of list-decodability of codes with insertion and deletion errors (we call such codes insdel codes), it is natural to ask the open problem whether there is also a Johnson-type bound. The problem was first investigated by Wachter-Zeh and the result was amended by Hayashi and Yasunaga where a lower bound on the list-decodability for insdel codes was derived. The main purpose of this paper is to move a step further towards solving the above open problem. In this work, we provide a new lower bound for the list-decodability of an insdel code. As a consequence, we show that unlike the Johnson bound for codes under other metrics that is tight, the bound on list-decodability of insdel codes given by Hayashi and Yasunaga is not tight. Our main idea is to show that if an insdel code with a given Levenshtein distance $d$ is not list-decodable with list size $L$, then the list decoding radius is lower bounded by a bound involving $L$ and $d$. In other words, if the list decoding radius is less than this lower bound, the code must be list-decodable with list size $L$. At the end of the paper we use such bound to provide an insdel-list-decodability bound for various well-known codes, which has not been extensively studied before.