Joint Cache and Trajectory Optimization for Secure UAV-relaying with Underlaid D2D Communications

With the exponential growth of data traffic, the use of caching and device-to-device (D2D) communications has been regarded as an efficient approach for alleviating the backhaul congestion in unmanned aerial vehicle (UAV) assisted networks. In this paper, we investigate the security issue of a cache-enabled UAV-relaying network with D2D communications in the presence of eavesdropper. Specifically, a UAV and multiple D2D users are equipped with cache memory, which can pre-store some popular contents to cooperatively provide content transfer services for users. To achieve secure and fair transmission, an optimization problem is formulated with the aim of maximizing the minimum secrecy rate among receivers, by jointly optimizing the cache placement and UAV flight trajectory in a finite flight period. The joint design problem is a non-convex mixed-integer programming problem. To facilitate solving this problem, we propose an alternating iterative algorithm based on the block alternating descend and successive convex approximation methods. Numerical results show that the joint design scheme significantly outperforms other benchmark schemes in terms of the secrecy rate.