Wireless content caching for small cell and d2d networks. submitted

The fifth generation wireless systems must provide fast and reliable connectivity while coping with the ongoing traffic growth. It is of paramount importance that the required resources, such as energy and bandwidth, do not scale with traffic. An important observation to reduce the required resources is that different users tend to request the same content at different time instants. Therefore, caching the most popular content at the network edge is envisioned as a promising solution to reduce the traffic and the energy consumption of the backhaul links. In this paper, two scenarios are considered, where caching is performed either at a small base station, or directly at the user terminals, which communicate using device-to-device communications. In both scenarios, joint design of the transmission and caching policies is studied when the user demands are known in advance. This joint design offers two different caching gains, namely, the pre-downloading and local caching gains. It is shown that the finite cache capacity limits the obtainable gains and creates inherent tradeoff between the two. In this context, a continuous time optimization problem is formulated to determine the optimal transmission and caching policies that minimize a generic cost function at the macro base station, such as energy, bandwidth, or throughput. The jointly optimal solution is obtained by demonstrating that caching files at a constan