Optimal cache placement for VoD services with wireless multicast and cooperative caching

Wireless multicast and client caching are two promising approaches to provide scalable video-on-demand (VoD) services. In this paper, we consider VoD services with client pre-caching under asynchronous requests, where a client request watches the desired video from the beginning to the end. A requesting client can retrieve the data of the requested video from its local cache, other clients it meets via device-to-device (D2D) communication, and the BS multicast transmission. Under this model, we optimize the cache placement to minimize the average traffic rate of BS transmission by the optimal exploitation of BS multicast and cooperative caching. Specifically, we first formulate the cache placement problem into a separable concave minimization problem with convex constraints, which can be solved via convex underestimation and branch bound, where the global optimality is achieved at the cost of a very high computational complexity. To reduce the complexity, we further adopt the difference of convex algorithm. Moreover, two special cases are investigated, i.e., without D2D communication and without BS multicast. Simulation results verify the effectiveness of the proposed scheme which combines BS multicast and cooperative caching.