Fundamental limits of caching

Abstract

Caching is a technique to reduce peak traffic rates by prefetching popular content in memories at the end users. This paper proposes a novel caching approach that can achieve a significantly larger reduction in peak rate compared to previously known caching schemes. In particular, the improvement can be on the order of the number of end users in the network. Conventionally, cache memories are exploited by delivering requested contents in part locally rather than through the network. The gain offered by this approach, which we term local caching gain, depends on the local cache size (i.e., the cache available at each individual user). In this paper, we introduce and exploit a second, global, caching gain, which is not utilized by conventional caching schemes. This gain depends on the aggregate global cache size (i.e., the cumulative cache available at all users), even though there is no cooperation among the caches. To evaluate and isolate these two gains, we introduce a new, information-theoretic formulation of the caching problem focusing on its basic structure. For this setting, the proposed scheme exploits both local and global caching gains, leading to a multiplicative improvement in the peak rate compared to previously known schemes. Moreover, we argue that the performance of the proposed scheme is within a constant factor from the information-theoretic optimum for all values of the problem parameters.