Satellite and unmanned aerial vehicle (UAV) integrated networks (SUINs) are considered as a promising method to offer various Internet of Remote Things (IoRT) applications. In this article, we investigate the downlink transmission of SUINs where the satellite-to-UAV link uses the free-space optical (FSO) technology with an equal gain combining (EGC) scheme while the links from UAV to IoRT devices exploit radio frequency (RF) with the space-division multiple access (SDMA) technique. Specifically, considering that only statistical channel state information (CSI) is available, we first formulate an optimization problem to maximize the ergodic sum rate (ESR) of the system, which is constrained by the total transmit power budget and IoRT devices’ rate requirements. Then, a beamforming (BF) scheme based on the alternating direction method of multipliers (ADMM) is proposed to solve the nonconvex problem. Furthermore, a zero-forcing (ZF)-based suboptimal approach is also presented to reduce the implementation complexity. Finally, by assuming that the FSO link and RF links are subject to Gamma–Gamma fading and Nakagami- mm fading, respectively, we derive closed-form ESR expressions for the considered network with the proposed BF schemes. Simulation results are provided to confirm the accuracy of the theoretical analysis. Moreover, it is revealed that our proposed EGC scheme for FSO communication and BF schemes for RF transmission can both achieve better performance than the existing works.

中文翻译：

---

物联网应用中卫星和无人机集成网络的波束成形设计和性能分析


卫星和无人机（UAV）集成网络（SUIN）被认为是提供各种远程物联网（IoRT）应用的有前途的方法。在本文中，我们研究了 SUIN 的下行链路传输，其中卫星到无人机的链路使用具有等增益组合 (EGC) 方案的自由空间光学 (FSO) 技术，而从无人机到 IoRT 设备的链路则利用射频（ RF）与空分多址（SDMA）技术。具体来说，考虑到只有统计信道状态信息（CSI）可用，我们首先制定一个优化问题来最大化系统的遍历和速率（ESR），该问题受到总发射功率预算和IoRT设备速率要求的约束。然后，提出了一种基于乘法器交替方向法（ADMM）的波束形成（BF）方案来解决非凸问题。此外，还提出了基于迫零（ZF）的次优方法来降低实现复杂性。最后，通过假设 FSO 链路和 RF 链路分别受到 Gamma-Gamma 衰落和 Nakagami-mm 衰落的影响，我们利用所提出的 BF 方案导出了所考虑网络的封闭形式 ESR 表达式。提供仿真结果以证实理论分析的准确性。此外，我们提出的用于 FSO 通信的 EGC 方案和用于 RF 传输的 BF 方案都可以实现比现有工作更好的性能。