Secrecy Rate Optimization for Secure and Green Communication Systems Enabled by Cooperative Aerial Active RIS and STAR-RIS

Abstract

Aerial reconfigurable intelligent surfaces (ARISs), which integrate uncrewed aerial vehicles (UAVs) with RISs, have become a potential paradigm for secure wireless transmission. However, a single ARIS suffers from the inherent multiplicative fading effect and limited spatial beamforming flexibility, thereby significantly reducing its effectiveness in secure communications. In order to address these challenges, this paper introduces a novel cooperative ARISs-assisted secure and green communication architecture, where an aerial active RIS and an aerial active simultaneously transmitting and reflecting RIS (STAR-RIS) are jointly deployed to enable signal amplification and 360-degree full-space coverage. In particular, the cooperative gain achieved through the secondary reflection link between the two ARISs further improves communication security. Furthermore, the sum secrecy rate (SR) maximization problem is presented to jointly optimize the hovering locations of the two ARISs, the ARIS reflection coefficients, as well as the transmit beamforming at the base station for multiple eavesdroppers, while accounting for the onboard power constraints of the ARISs. To solve this highly coupled nonconvex problem, an alternating optimization (AO) algorithm is devised by integrating successive convex approximation (SCA), penalty-based semidefinite relaxation (PSDR), and Bayesian optimization (BO) techniques. Extensive simulations show that the proposed framework substantially enhances the sum SR under limited power budgets compared with existing benchmark schemes.