Reconfigurable intelligent surface (RIS) and simultaneously transmitting and reflecting RIS (STAR-RIS) have emerged as key enablers for enhancing wireless coverage and capacity in next-generation networks. When mounted on unmanned aerial vehicles (UAVs), they benefit from flexible deployment and improved line-of-sight conditions. Despite their promising potential, a comprehensive performance comparison between aerial RIS and STAR-RIS architectures has not been thoroughly investigated. This letter presents a detailed performance comparison between aerial RIS and STAR-RIS in three-dimensional wireless environments. Accurate channel models incorporating directional radiation patterns are established, and the influence of deployment altitude and orientation is thoroughly examined. To optimize the system sum-rate, we formulate joint optimization problems for both architectures and propose an efficient solution based on the weighted minimum mean square error and block coordinate descent algorithms. Simulation results reveal that STAR-RIS outperforms RIS in low-altitude scenarios due to its full-space coverage capability, whereas RIS delivers better performance near the base station at higher altitudes. The findings provide practical insights for the deployment of aerial intelligent surfaces in future 6G communication systems.

翻译：可重构智能表面（RIS）和同时透射反射智能表面（STAR-RIS）已成为提升下一代网络无线覆盖与容量的关键使能技术。当部署于无人机平台时，它们得益于灵活的布设方式和优化的视距传输条件。尽管具备广阔的应用前景，空中RIS与STAR-RIS架构之间的系统性性能对比尚未得到深入研究。本文针对三维无线环境，对空中RIS与STAR-RIS进行了详尽的性能比较。通过建立包含定向辐射模式的精确信道模型，深入分析了部署高度与方位角对系统性能的影响。为优化系统总速率，我们针对两种架构分别构建了联合优化问题，并提出基于加权最小均方误差与块坐标下降算法的高效求解方案。仿真结果表明：在低空场景中，STAR-RIS凭借其全空间覆盖能力表现优于RIS；而在高空基站邻近区域，RIS则展现出更佳性能。本研究为未来6G通信系统中空中智能表面的实际部署提供了重要参考依据。