Caching is an efficient way to reduce network traffic congestion during peak hours, by storing some content at the user’s local cache memory, even without knowledge of user’s later demands. Maddah-Ali and Niesen proposed a two-phase (placement phase and delivery phase) coded caching strategy for broadcast channels with cache-aided users. This paper investigates the same model under the constraint that content is placed uncoded within the caches, that is, when bits of the files are simply copied within the caches. When the cache contents are uncoded and the users’ demands are revealed, the caching problem can be connected to an index coding problem. This paper focuses on deriving fundamental performance limits for the caching problem by using tools for the index coding problem that were either known or are newly developed in this work. First, a converse bound for the caching problem under the constraint of uncoded cache placement is proposed based on the “acyclic index coding converse bound.” This converse bound is proved to be achievable by the Maddah-Ali and Niesen’s scheme when the number of files is not less than the number of users, and by a newly derived index coding achievable scheme otherwise. The proposed index coding achievable scheme is based on distributed source coding and strictly improves on the widely used “composite (index) coding” achievable bound and its improvements, and is of independent interest. An important consequence of the findings of this paper is that advancements on the coded caching problem posed by Maddah-Ali and Niesen are thus only possible by considering strategies with coded placement phase. A recent work by Yu et al has however shown that coded cache placement can at most half the network load compared to the results presented in this paper.

中文翻译：

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使用未编码缓存放置进行缓存的索引编码方法

缓存是一种减少高峰时段网络流量拥塞的有效方法，通过将一些内容存储在用户的本地缓存中，即使不知道用户以后的需求。Maddah-Ali 和 Niesen 为具有缓存辅助用户的广播频道提出了一种两阶段（放置阶段和交付阶段）编码的缓存策略。本文在内容未编码放置在缓存中的约束下研究了相同的模型，也就是说，当文件的位被简单地复制到缓存中时。当缓存内容未编码并暴露用户需求时，缓存问题可以连接到索引编码问题。本文重点介绍通过使用本工作中已知或新开发的索引编码问题工具来推导缓存问题的基本性能限制。第一的，基于“非循环索引编码逆界”，提出了未编码缓存布局约束下的缓存问题逆界。证明了当文件数不小于用户数时，Maddah-Ali 和 Niesen 方案可以实现该逆界，否则通过新导出的索引编码可实现方案。所提出的索引编码可实现方案基于分布式源编码，严格改进了广泛使用的“复合（索引）编码”可实现边界及其改进，具有独立意义。本文研究结果的一个重要结果是，Maddah-Ali 和 Niesen 提出的编码缓存问题的进展因此只有通过考虑具有编码放置阶段的策略才有可能。