Joint Optimization of File Placement and Delivery in Cache-Assisted Wireless Networks

In this paper, the downlink file transmission in one cell with the assistance of cache nodes is studied. Specifically, the base station (BS) reactively delivers files to cache nodes and a requesting user in a multicast manner. Therefore, one file transmission may lead to the cache status update, which further affects the future file transmissions. We consider the joint optimization of file placement and delivery. In particular, we first formulate the optimization of transmission power and time in one finite file lifetime as a Markov Decision Process (MDP) with a random number of stages, where the objective is to minimize the transmission resource at the BS. It is shown that the optimal solution can be obtained via a revised Bellman's equation. Due to the curse of dimensionality, a novel approximation approach is proposed, where the value functions of the Bellman's equation can be calculated from analytical expressions. Hence, iterative algorithms, which appear in the general approximate MDP solutions, can be avoided. Moreover, an bound on the approximation error is also provided.