As a novel technology, simultaneous transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) provides users with full spatial coverage communication by modulating reflected and refracted signals. This study proposes a novel UAV equipped with STAR-RIS-assisted non-orthogonal multiple access system that jointly optimized the cluster correlations with the power allocation coefficients and positions of STAR-RIS to maximize the user system sum rate. However, the application of this technique is limited by the complex nonconvex optimization problem, which was addressed by developing an alternating optimization iteration algorithm. The joint optimization problem related to cluster association and the power allocation coefficients and positions of STAR-RIS was solved by alternating iterations. The simulation results demonstrated that the proposed algorithm delivered a superior user system sum rate compared to single-power optimization, ultimately achieving near-optimal system performance. At the same time, we verified that the NOMA system equipped with STAR-RIS on UAV can achieve the best performance than the OMA system. Therefore, the proposed algorithm contributes to the optimization of STAR-RIS-assisted systems, which will benefit applications in wireless communications, IoT, and smart city infrastructure.

This paper addresses the secure communication problem in a distributed active-passive reconfigurable intelligent surface (RIS) assisted multiple-input single-output (MISO) scenario with a single-antenna malicious eavesdropper. The objective is to maximize the secrecy rate of the single-antenna legitimate user under constraints on the base station (BS) power and RIS amplification coefficients. The study investigates a joint optimization algorithm for BS beamforming and RIS phase-shift matrices. To address the complex non-convex fractional programming problem with coupled variables, an alternating optimization (AO) algorithm based on semi-definite relaxation (SDR) and the method of minorization maximization (MM) is proposed to obtain a suboptimal solution to the optimization problem. Simulation results demonstrate that the proposed AO algorithm achieves better secrecy performance with fewer iterations compared to traditional approaches. The design of the distributed active-passive RIS effectively mitigates the "double fading" effect, leading to a significant improvement in secrecy rate compared to existing active RIS and passive RIS designs.

This paper investigates secure and low-latency communications in UAV-mounted simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted urban vehicular networks, where severe blockage, high vehicle mobility, eavesdropping threats, and delay-sensitive traffic services coexist. In the considered system, the UAV is used not only as an aerial carrier for the STAR-RIS but also as a mobile intelligent control node that can dynamically adjust its horizontal aerial position according to vehicle distribution, blockage conditions, and eavesdropping threats. First, a UAV-STAR-RIS-assisted vehicular communication system model is developed by jointly considering urban blockage, vehicle mobility, passive eavesdropping attacks, queueing dynamics, and UAV flight constraints. Then, a high-dimensional, non-convex, and strongly coupled dynamic optimization problem is formulated to maximize the long-term average secure and low-latency utility through the joint optimization of the UAV trajectory, the STAR-RIS transmission–reflection partition ratio, the phase-shift matrices, and the transmit power allocation. Furthermore, the problem is modeled as a Markov decision process with continuous state and action spaces, and a hierarchical constrained soft actor–critic (HC-SAC)-based joint control algorithm is proposed to enable adaptive UAV movement, STAR-RIS configuration, and power control in complex dynamic environments. Simulation results demonstrate that the proposed method outperforms DDPG and several structural benchmark schemes. In the representative evaluation, the proposed HC-SAC achieves an average delay of 10.85 slots and a secrecy outage probability of 0.7160, compared with 11.72 slots and 0.8501 for PPO, and 11.94 slots and 0.8599 for DDPG. Although PPO provides the highest average secrecy rate and successful service ratio, the proposed method still maintains a competitive secure communication capability and service reliability. A normalized composite utility analysis further shows that HC-SAC attains the highest utility value of 0.9254, indicating a more favorable security–latency trade-off in complex urban vehicular scenarios.