Coded Caching in Hierarchical Cache-Aided Networks With Nonuniform User Distribution

Coded caching has emerged as a promising technique to alleviate traffic congestion by strategically creating coded multicasting opportunities, even for caches with different demands. For a two-layer cache-aided hierarchical network consisting of a central server, multiple helpers, and multiple users, prior works have characterized the fundamental performance limits of coded caching for this system with the constraint of a uniform user distribution (i.e., each helper serves an equal number of users). However, when the heterogeneity of user distribution is taken into account, there remain open questions. In this article, we consider a two-layer cache-aided hierarchical network with arbitrary user distributions, where a central server is connected via an error-free link to multiple helpers and each user can randomly access one helper. We introduce a new decentralized coded caching scheme and employ the cut-set technique to characterize lower bounds. Our results show that the gap between the upper and lower bound of the achievable rate from server to helpers is within a constant multiplicative (i.e., [1/32]) and additive (i.e., 2) factor, outperforming prior works under uniform user distribution. Moreover, we also show that the transmission rate from each helper to its attached users is at most a constant factor away from the corresponding lower bound. To our knowledge, this is the first work in hierarchical networks to eliminate the additive gap of the second layer. Finally, simulation results demonstrate the superiority of the proposed caching scheme compared with the state of the art.