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 networks. In this context, we formulate a joint optimization problem of user association, caching placement, and backhaul bandwidth allocation for minimizing content acquisition delay with consideration of UAVs' energy constraint. We decompose the formulated problem into two subproblems: i) user association 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 approach. After that, embedding the backhaul bandwidth allocation algorithm, we solve the user association and caching placement problem by a three-dimensional (3D) matching method. Then we decompose it into two two-dimensional (2D) matching problems and develop low-complexity algorithms. The proposed scheme converges and exhibits a low computational complexity. Simulation results demonstrate that the proposed cache-enabling UAV framework outperforms the conventional UAV-assisted cellular networks in terms of content acquisition delay and the proposed scheme achieves significantly lower content acquisition delay compared with other two benchmark schemes.