Coded Caching in Hybrid Networks

Caching networks have been extensively studied from an information-theoretic perspective for various network topologies. In particular device-to-device (D2D) topologies have been considered and it has been shown that rates similar to a centralized server setup can be achieved. Hierarchical networks with an intermediate layer of cache called mirrors between the server and the users have been looked at previously and approximately optimal schemes have been developed. In this paper we consider a more general hybrid network, where the mirrors in hierarchical network are connected to form a D2D network in between the server and the end users. We study the trade-off between the sum-rate in the D2D network and the server transmission rate, to understand the extent to which the D2D network can act as a substitute for the server. To this end, we propose an achievable scheme to operate the network for any values of mirror and user cache memory, and prove that is optimal up to a constant additive and multiplicative gap.