Impact of Cell Association on Energy-Efficiency and Hit Rate of Femto-Caching

Abstract

Content caching at femto base stations (F-BSs) enhances user experience. In practice, the storage size of F-BSs is often minuscule compared to the size of the content library, which calls for effective content placement strategy aligned with user association (UA), networking issues, and possible content retransmissions. The common assumption is to associate user equipments with the closes F-BS that holds the requested content—content-centric user association (CC-UA). We argue that CC-UA could deteriorate hit rate and energy efficiency (EE), and then propose three cache-agnostic UA schemes to tackle this issue: baseline (CA-B), Macro-to-femto (CA-M2F), and femto-to-femto (CA-F2F). Under CA-B if the requested content is not found at the cache, it is retrieved via backhaul. The CA-M2F scheme uses the communication between Macro-BSs and F-BSs to improve the performance. Finally, the CA-F2F scheme uses the direct F2F communication paradigm allowing F-BSs to share contents between themselves on-demand. By the aid of stochastic geometry we model and analyze all these UA schemes and derive associated hit rate and EE performances based on main system parameters. We then utilize the analysis to study the performance of the developed UA-C schemes under several (heuristic) probabilistic content placement strategies. For a chosen choice of the content placement strategy, the numerical results confirm that CA-F2F and CA-M2F outperform CC-UA and CA-B by large margins. For example, we observe that via CA-F2F one can achieve 1200 percent growth of EE compared to CC-UA.