This paper studies the statistical characterization of ground-to-UAV (G2A) and reconfigurable intelligent surface (RIS)-assisted UAV-to-ground (A2G) communications in terrestrial and non-terrestrial networks under the impact of channel aging. We first model the G2A and A2G signal-to-noise ratios as non-central complex Gaussian quadratic random variables (RVs) and derive their exact probability density functions, offering a unique characterization for the A2G SNR as the product of two scaled non-central chi-square RVs. Moreover, we also find that, for a large number of RIS elements, the RIS-assisted A2G channel can be characterized as a single Rician fading channel. Our results reveal the presence of channel hardening in A2G communication under low UAV speeds, where we derive the maximum target spectral efficiency (SE) for a system to maintain a consistent required outage level. Meanwhile, high UAV speeds, exceeding 50 m/s, lead to a significant performance degradation, which cannot be mitigated by increasing the number of RIS elements.

翻译：本文研究在信道老化影响下，地面到无人机（G2A）与可重构智能表面（RIS）辅助的无人机到地面（A2G）通信在陆地和非陆地网络中的统计特性。我们首先将G2A与A2G信噪比建模为非中心复高斯二次型随机变量，推导出其精确概率密度函数，并针对A2G信噪比提出一种独特表征：将其描述为两个缩放非中心卡方随机变量的乘积。此外，我们发现当RIS单元数量较大时，RIS辅助的A2G信道可表征为单一路斯衰落信道。结果揭示了低速无人机场景下A2G通信中的信道硬化现象，并推导出系统在维持一致中断水平时所需的最大目标频谱效率。同时，当无人机速度超过50米/秒时，性能显著下降，且增加RIS单元数量无法缓解此退化。