Security-Reliability Tradeoff in UAV-Carried Active RIS-Assisted Cooperative Networks

In this letter, we propose an unmanned aerial vehicle (UAV)-carried active reconfigurable intelligent surface (RIS)-assisted cooperative network (UARCN) with a source (S), a trustworthy user (U), and an untrustworthy user (E), which is bridged by multiple half-duplex decode-and-forward relays and an active RIS mounted on a hovering UAV. In the UARCN, both U, E, and relays receive the signal from S through the active RIS merely in the first time slot; then one out of the successful decoding relays is selected to further transmit information via the direct link and RIS in the second time slot. The outage probability and intercept probability are jointly analyzed to investigate the security and reliability tradeoff (SRT) of the system. Simulation results validate our theoretical analysis, revealing that the active RIS can bring greater improvement to the system SRT than passive or hybrid RIS, and its location can be further optimized.

I. Introduction

To address the rising demands of data traffic, reliability, and security, the reconfigurable intelligent surface (RIS) has been proposed and thoroughly investigated over the past few years [1]. However, the widely known passive RIS-assisted systems have a double fading issue. Therefore, many researchers have combined relays with passive RIS to combat the double fading as well as harness the benefits of both. The authors of [2] utilized a decode-and-forward (DF) relay to improve the achievable rate of the system with only a weak link between the RIS and the destination. Besides, it has been demonstrated that the throughput of a RIS-relay-assisted cooperative system can be greatly improved by jointly optimizing the relay selection and the reflection coefficients of RIS using deep reinforcement learning [3]. The authors of [4] proposed both the joint and integrated RIS and relay transmission schemes, and verified that the bit error rate and achievable rate are improved significantly compared to the single RIS networks. Moreover, several buffer-aided relays were used to enhance the RIS-assisted system in [5], and the authors maximized the secrecy rate with a delay constraint by jointly optimizing the relay selection and RIS coefficients.