Outage Probability Analysis of MISO-NOMA Downlink Communications in UAV-Assisted Agri-IoT with SWIPT and TAS Enhancement

In the agricultural Internet of Things (Agri-IoT), the uncrewed aerial vehicle (UAV) can serve as a mobile relay to facilitate line-of-sight transmissions for agricultural sensors, especially in farmlands with complex terrain or numerous obstructions. Additionally, the integration of techniques such as simultaneous wireless information and power transfer (SWIPT) and multiple-input single-output (MISO), combined with non-orthogonal multiple access (NOMA) communications, not only supports a higher number of device connections but also provides an essential power supply to cell-edge sensors. Motivated by the above, we propose a collaborative MISO-NOMA communication mechanism in UAV-assisted Agri-IoT. Specifically, the UAV functions as a static relay, and the NOMA-enhanced decode-and-forward relay protocol and SWIPT technique are used in the cell-center relaying UAV. The selection combining technique is employed for cell-edge sensors to obtain the optimal quality signal from multiple antennas of the base station (BS). To further improve the channel capacity, we propose a transmit antenna selection (TAS) strategy for the base station equipped with multiple antennas. Different from existing strategies (such as maximizing harvested energy or direct-link performance), the proposed TAS strategy aims to achieve optimal outage performance at cell-edge sensors, rather than suboptimal performance. Then, we derive closed-form and approximate solutions for the outage probability of cell-edge sensors. These solutions can provide significant insights into the impact of MISO-NOMA communications in UAV-assisted Agri-IoT. Finally, the simulation results indicate that the proposed TAS strategy outperforms current state-of-the-art schemes in reducing the outage probability. Moreover, our simulation experiments verify that the derived approximate solution closely aligns with the closed-form solution.