Fractional Dynamic Caching: A Collaborative Design of Storage and Backhaul

To reduce the average file delivery time under limited backhaul bandwidth and cache storage, we propose a fractional dynamic caching scheme, that coordinates the utilization of the backhaul and the cache storage. In the proposed scheme, only part of each file is pre-fetched and stored in the static storage segment of small base stations (SBSs), while remaining part of the file will be fetched into the dynamic storage segment when requested by users. We aim to minimize the average file delivery time by investigating the optimal prefetching policy, i.e., the size of the pre-fetched part of each file and the size of the dynamic storage. We formulate the file delivery time minimization problem as a convex optimization formulation. We compute the closed-form of the optimal pre-fetching policy when the wireless rate is constant. Then, a heuristic pre-fetching policy is derived, using the conditional average wireless rate. To overcome the issue that file popularity may be unknown or time-varying, we further provide an approximation of the heuristic pre-fetching policy which is irrelevant to the file popularity. We also discuss on the potential application of the proposed caching scheme in scenarios with heterogeneous file sizes. Numerical results show that the proposed scheme can substantially improve the backhaul utilization efficiency, and reduce the average file delivery time. The heuristic pre-fetching policy has similar performance with the optimal pre-fetching policy yet with much lower complexity.