Underwater sensor networks (UWSNs) are of interest to gather data from underwater sensor nodes (SNs) and deliver information to a terrestrial access point (AP) in the uplink transmission, and transfer data from the AP to the SNs in the downlink transmission. In this paper, we investigate a triple-hop UWSN in which autonomous underwater vehicle (AUV) and unmanned aerial vehicle (UAV) relays enable end-to-end communications between the SNs and the AP. It is assumed that the SN–AUV, AUV–UAV, and UAV–AP links are deployed by underwater optical communication (UWOC), free-space optic (FSO), and radio frequency (RF) technologies, respectively. Two scenarios are proposed for the FSO uplink and downlink transmissions between the AUV and the UAV, subject to water-to-air and air-to-water interface impacts; direct transmission scenario (DTS) and retro-reflection scenario (RRS). After providing the channel models and their statistics, the UWSN's outage probability and average bit error rate (BER) are computed. Besides, a tracking procedure is proposed to set up reliable and stable AUV– UAV FSO communications. Through numerical results, it is concluded that the RSS scheme outperforms the DTS one with about 200% (32%) and 80% (17%) better outage probability (average BER) in the uplink and downlink, respectively. It is also shown that the tracking procedure provides on average 480% and 170% improvements in the network's outage probability and average BER, respectively, compared to poorly aligned FSO conditions. The results are verified by applying Monte-Carlo simulations.

中文翻译：

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使用射频和光学无线链路的无人机辅助水下传感器网络


水下传感器网络（UWSN）的目的是从水下​​传感器节点（SN）收集数据并在上行链路传输中将信息传送到地面接入点（AP），并在下行链路传输中将数据从AP传输到SN。在本文中，我们研究了三跳 UWSN，其中自主水下航行器 (AUV) 和无人机 (UAV) 中继实现了 SN 和 AP 之间的端到端通信。假设SN-AUV、AUV-UAV和UAV-AP链路分别通过水下光通信（UWOC）、自由空间光学（FSO）和射频（RF）技术部署。针对AUV和UAV之间的FSO上行链路和下行链路传输，提出了两种场景，受水-空和空-水接口影响；直接传输场景（DTS）和后向反射场景（RRS）。提供信道模型及其统计数据后，将计算 UWSN 的中断概率和平均误码率 (BER)。此外，还提出了一种跟踪程序来建立可靠且稳定的AUV-UAV FSO通信。通过数值结果得出结论，RSS 方案优于 DTS 方案，上行链路和下行链路的中断概率（平均 BER）分别高出约 200%（32%）和 80%（17%）。研究还表明，与对齐不佳的 FSO 条件相比，跟踪程序使网络中断概率和平均 BER 平均分别提高了 480% 和 170%。通过应用蒙特卡罗模拟验证了结果。