In this paper, we investigate a mobile relaying system assisted by an unmanned aerial vehicle (UAV) with a finite size of the buffer. Under the buffer size limit and delay constraints at the UAV relay, we consider a dual-hop mixed free-space optical/radio frequency (FSO/RF) relaying system (i.e., the source-to-relay and relay-to-destination links employ FSO and RF links, respectively). Taking an imbalance in the transmission rate between RF and FSO links into consideration, we address the trajectory design of the UAV relay node to obtain the maximum data throughput at the ground user terminal. Specifically, we classify two relaying transmission schemes according to the delay requirements, i.e., i) delay-limited transmission and ii) delay-tolerant transmission. Accordingly, we propose an iterative algorithm to effectively obtain the locally optimal solution to our throughput optimization problems and further present the complexity analysis of this algorithm. Through this algorithm, we present the resulting trajectories over the atmospheric condition, the buffer size, and the delay requirement. In addition, we show the optimum buffer size and the throughput-delay tradeoff for a given system. The numerical results validate that the proposed buffer-aided and delay-considered mobile relaying scheme obtains 223.33% throughput gain compared to the conventional static relaying scheme.

中文翻译：

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具有缓冲器的混合 FSO/RF 无人机辅助移动中继的吞吐量最大化

在本文中，我们研究了由具有有限缓冲区大小的无人驾驶飞行器 (UAV) 辅助的移动中继系统。在 UAV 中继的缓冲区大小限制和延迟约束下，我们考虑双跳混合自由空间光/射频 (FSO/RF) 中继系统（即源到中继和中继到目的地链路）分别采用 FSO 和 RF 链路）。考虑到 RF 和 FSO 链路之间传输速率的不平衡，我们解决了无人机中继节点的轨迹设计，以获得地面用户终端的最大数据吞吐量。具体来说，我们根据延迟要求将两种中继传输方案分类，即i）延迟限制传输和ii）延迟容忍传输。因此，我们提出了一种迭代算法，以有效地获得我们的吞吐量优化问题的局部最优解，并进一步介绍了该算法的复杂性分析。通过该算法，我们可以在大气条件、缓冲区大小和延迟要求上呈现结果轨迹。此外，我们展示了给定系统的最佳缓冲区大小和吞吐量-延迟权衡。数值结果证实，与传统的静态中继方案相比，所提出的缓冲辅助和延迟考虑的移动中继方案获得了 223.33% 的吞吐量增益。和延迟要求。此外，我们还展示了给定系统的最佳缓冲区大小和吞吐量-延迟权衡。数值结果证实，与传统的静态中继方案相比，所提出的缓冲辅助和延迟考虑的移动中继方案获得了 223.33% 的吞吐量增益。和延迟要求。此外，我们展示了给定系统的最佳缓冲区大小和吞吐量-延迟权衡。数值结果证实，与传统的静态中继方案相比，所提出的缓冲辅助和延迟考虑的移动中继方案获得了 223.33% 的吞吐量增益。