Space-air-ground integrated networks (SAGIN) within UAV-assisted free-space optical (FSO) communication systems can efficiently accommodate massive connections and provide highly reliable and seamless communications. In this work, we investigate the performance of a UAV-assisted, asymmetric, dual-hop radio frequency (RF)/FSO system with the amplified-and-forward relay protocol for the SAGIN. Specifically, the shadowed Rician fading is utilized in this study to characterize the shadowing effect on the RF signal for the satellite-to-UAV link. Meanwhile, the atmospheric turbulence effect on the optical signal for the UAV-to-terrestrial user link is modeled by the Málaga distribution fading, in view of the pointing error impairments. For comparison, the heterodyne detection technique is employed, as well as the intensity modulation with direct detection technique, in improving performance of the relay system. Thus, we derive the closed-form expressions for the cumulative distribution function, probability density function, the moment generating function, and particularly, the moments of the end-to-end RF/FSO system, in terms of Meijer’s G-function. Utilizing these derived formulae, the precise closed-form expressions for the outage probability, the average bit error rate (BER) with various modulation schemes, and the ergodic capacity are given. In specific, the tight asymptotic results for the outage probability and the average BER at the high SNR regions are derived using the asymptotic expansion of Meijer’s G-function. Furthermore, closed-form expressions are presented for the case that the FSO link experiences Gamma-Gamma distribution by changing some specific parameters. Finally, extensive numerical results validate the theoretical results with the Monte-Carlo simulation.

中文翻译：

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无人机辅助天地一体化网络 RF/FSO 中继系统：性能分析


无人机辅助的自由空间光学（FSO）通信系统中的空空地综合网络（SAGIN）可以有效地容纳大量连接并提供高度可靠的无缝通信。在这项工作中，我们研究了无人机辅助、非对称、双跳射频 (RF)/FSO 系统的性能，该系统采用 SAGIN 的放大转发中继协议。具体来说，本研究利用阴影莱斯衰落来表征卫星到无人机链路射频信号的阴影效应。同时，考虑到指向误差损伤，大气湍流对无人机到地面用户链路光信号的影响通过马拉加分布衰落进行建模。为了进行比较，采用外差检测技术以及直接检测技术的强度调制来提高中继系统的性能。因此，我们根据 Meijer 的 G 函数推导了累积分布函数、概率密度函数、矩生成函数，特别是端到端 RF/FSO 系统矩的封闭式表达式。利用这些导出的公式，给出了中断概率、各种调制方案的平均误码率（BER）以及遍历容量的精确封闭式表达式。具体而言，使用 Meijer G 函数的渐近展开导出了高 SNR 区域的中断概率和平均 BER 的紧密渐近结果。此外，针对FSO链路通过改变某些特定参数而经历Gamma-Gamma分布的情况，给出了闭合形​​式的表达式。 最后，大量的数值结果验证了蒙特卡罗模拟的理论结果。