Joint User Association and Caching Placement for Cache-Enabling UAV Networks

Cache-enabling unmanned aerial vehicles (UAVs) are considered for storing popular contents and providing downlink data offloading in cellular net-works. In this context, we formulate a joint opti-mization problem of user association, caching place-ment, and backhaul bandwidth allocation for mini-mizing content acquisition delay with consideration of UAVs' energy constraint. We decompose the for-mulated problem into two subproblems: i) user as-sociation and caching placement and ii) backhaul bandwidth allocation. We first obtain the optimal bandwidth allocation with given user association and caching placement by the Lagrangian multiplier ap-proach. After that, embedding the backhaul band-width allocation algorithm, we solve the user asso-ciation and caching placement problem by a three-dimensional (3D) matching method. Then we decom-pose it into two two-dimensional (2D) matching prob-lems and develop low-complexity algorithms. The proposed scheme converges and exhibits a low com-putational complexity. Simulation results demonstrate that the proposed cache-enabling UAV framework out-performs the conventional UAV-assisted cellular net-works in terms of content acquisition delay and the proposed scheme achieves significantly lower content acquisition delay compared with other two benchmark schemes.