Secure cache-aided multiuser UAV-relay networks with outdated channel state information.

In this paper, we investigate a secure communication of UAV-relaying systems, where multiple users attempt to deliver confidential messages to the destination with the aid of a relay, yet the confidential messages can be overheard by the eavesdropper. To enhance the physical-layer security of the considered system, we propose to equip the relay with cache, and accordingly, two user selection criteria are proposed with the purposes of maximizing the channel gain of direct main link and minimizing the channel gain of direct eavesdropper link, respectively. Moreover, we take into account the effects of outdated channel state information (CSI) on the user selection criteria, and derive the secrecy outage probabilities (SOPs) for the conventional and cache-aided systems regarding both user selection criteria. In further, the asymptotic SOPs in the high signal-to-noise ratio (SNR) and main-to-eavesdropper ratio (MER) region are provided as well to illustrate the difference between the secure performances of systems with and without cache. From the given SOPs, we find that cache-aided system can achieve better secure performance in comparison with the system without cache. In addition, the multiuser diversity gain is weakened by the outdated CSI, and therefore, the diversity order of system cannot exceed two regardless of the employment of cache.