Optimal Cache Allocation for Femto Helpers with Joint Transmission Capabilities

As cellular network operators are struggling to keep up with the rapidly increasing traffic demand, two key directions are deemed necessary for beyond 4G networks: (i) extensive cell densification to improve spatial reuse, and (ii) storage of content as close to the user as possible to cope with the backhaul constraints and increased interference. However, caching has mostly been studied with an exclusive focus either on the backhaul network (e.g. the "femto-caching" line of work) or on the radio access (e.g. through coded caching or cacheaided CoMP). As a result, an understanding of the impact of edge caching on network-wide and end-to-end performance is lacking. In this paper we investigate the problem of optimal caching in a context where nearby small cells ("femto-helpers") can coordinate not just in terms of what to cache but also to perform Joint Transmission (a type of CoMP). We show that interesting tradeoffs arise between caching policies that improve radio access and ones that improve backhaul, and propose an algorithm that provably achieves an 1/2-approximation ratio to the optimal one (which is NP-hard), and performs well in simulated scenarios.