Joint optimization of transmit beamforming and base station cache allocation in multi-cell C-RAN

Abstract

Cloud radio access network (C-RAN) with wireless backhaul has been considered as a possible solution for small cell deployment. Limited capacity of wireless backhaul encourages people to equip the base stations (BSs) with cache storages to mitigate peak-time data traffic. We study a downlink C-RAN consisting of a central processor (CP) and multiple cache-enabled BSs connected to the CP through wireless backhaul links. Through joint optimization of beamforming and cache size allocation, we aim to distribute cache sizes among BSs such that long-term delivery time of files is minimized. Assuming zero-forcing (ZF) beamforming is adopted at CP, cache size allocation problem becomes convex which can be solved by convex optimization techniques. Simulation results show that loss of optimality due to ZF assumption is negligible for large antenna sizes at CP. Moreover, the superiority of our proposed caching scheme over several heuristic ones, including proportional caching, is demonstrated by simulation.