Joint Cache Partitioning, Content Placement, and User Association for D2D-Enabled Heterogeneous Cellular Networks

The rapid growth of traffic demands has posed the challenges to both radio access networks (RANs) and backhaul links. While the device-to-device (D2D)-enabled heterogeneous cellular networks (HCNs) are expected to offer diverse radio access capabilities and to improve the transmission performance of user equipments (UEs) significantly, the backhaul links may still experience challenges in offering quality-of-service guaranteed services to UEs. To tackle these problems, caching technology, i.e., caching user contents at the infrastructures of RANs, is proposed as an effective approach. In this paper, we consider the joint cache partitioning, content placement, and user association problem in the D2D-enabled HCNs and propose a two-step algorithm. Aiming to improve the utilization of cache space at small base stations, we propose a bankruptcy game-based cache partitioning algorithm to obtain the optimal cache space allocation strategy, based on which we then propose a joint content placement and user association algorithm to achieve the minimum service delay of all the content request users. As the formulated optimization problem is a mixed integer nonlinear optimization problem which cannot be solved conveniently, we apply the McCormick envelopes and the Lagrangian partial relaxation method to decompose the optimization problem into three subproblems which can be iteratively solved by means of the modified Kuhn–Munkres algorithm and the unidimensional knapsack algorithm. Simulation results validate the effectiveness of our proposed scheme.