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arXiv 1802.08663cs.ITcs.DSmath.IT
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同步字符串:插入和删除的列表解码

Synchronization Strings: List Decoding for Insertions and Deletions

Bernhard Haeupler, Amirbehshad Shahrasbi, Madhu Sudan

摘要:

我们研究在插入和删除下列表可解码的代码。具体来说,我们考虑的设置中,在一些有限的字母表的大小$q$的码字可能会遭受$δ$部分的对抗性删除和$γ$部分的对抗性插入。如果存在(有效)算法,该算法在给定接收到的字的情况下报告包括原始码字的$L$码字的列表,则称代码是$L$列表可解码的。使用前两位作者[STOC 2017]引入的同步字符串的概念,我们展示了一些令人惊讶的结果。我们发现,对于每个$0\leqδ<1$,每个$0\leqγ<\infty$和每个$ε>0$存在有效的代码率$1-δ-ε$和常数字母表(所以$q=O_{δ,γ,ε}(1)$)和子对数列表大小。我们强调,插入的分数可以是任意大的和率是独立的这个参数。我们的研究结果揭示了显着的不对称性之间的影响插入和删除从纠错的角度来看:而删除成本的代码的速率,插入成本是由对手承担,而不是代码!我们还证明了几个紧界的参数列表可解码的insdel码。特别是,我们表明,insdel码的字母表大小需要在$ε^{-1}$,其中$ε$是上述容量的差距是指数大。我们的结果甚至适用于唯一解码能力等于列表解码能力的设置,并且当它这样做时,它表明字母表大小需要在容量差距中呈指数级大。这与汉明误差模型形成鲜明对比,在汉明误差模型中,$ε^{-1}$中的字母表大小多项式足以进行唯一解码,并且还表明,在构造insdel码的先前工作中,对字母表大小的指数依赖实际上是必要的!

英文摘要:

We study codes that are list-decodable under insertions and deletions. Specifically, we consider the setting where a codeword over some finite alphabet of size $q$ may suffer from $δ$ fraction of adversarial deletions and $γ$ fraction of adversarial insertions. A code is said to be $L$-list-decodable if there is an (efficient) algorithm that, given a received word, reports a list of $L$ codewords that include the original codeword. Using the concept of synchronization strings, introduced by the first two authors [STOC 2017], we show some surprising results. We show that for every $0\leqδ<1$, every $0\leqγ<\infty$ and every $ε>0$ there exist efficient codes of rate $1-δ-ε$ and constant alphabet (so $q=O_{δ,γ,ε}(1)$) and sub-logarithmic list sizes. We stress that the fraction of insertions can be arbitrarily large and the rate is independent of this parameter. Our result sheds light on the remarkable asymmetry between the impact of insertions and deletions from the point of view of error-correction: Whereas deletions cost in the rate of the code, insertion costs are borne by the adversary and not the code! We also prove several tight bounds on the parameters of list-decodable insdel codes. In particular, we show that the alphabet size of insdel codes needs to be exponentially large in $ε^{-1}$, where $ε$ is the gap to capacity above. Our result even applies to settings where the unique-decoding capacity equals the list-decoding capacity and when it does so, it shows that the alphabet size needs to be exponentially large in the gap to capacity. This is sharp contrast to the Hamming error model where alphabet size polynomial in $ε^{-1}$ suffices for unique decoding and also shows that the exponential dependence on the alphabet size in previous works that constructed insdel codes is actually necessary!

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