Impact of Axial and Translatory UAV Instabilities in 5G Coverage Extension

With the growing interest in extending 5G coverage through aerial base stations using unmanned aerial vehicles (UAVs), enhancing the mid-haul link capacity between UAVs and gNodeBs (gNBs) emerges as a crucial challenge. Leveraging millimeter-wave (mmWave) communication technologies, such as 5G-FR2 and mmWave WLANs, present promising solutions. However, the usage of multi-antenna directional beamforming in mmWave communications, aimed at improving directional coverage, poses challenges such as misalignment due to UAV platform instabilities during hovering, leading to potential communication outages. This paper investigates the effects of platform instabilities, including axial rotation and translatory movements of a hovering UAV, on the mmWave gNB-to-UAV link coverage. In this paper, we establish a mathematical model to quantify misalignment caused by platform instabilities (limited to axial rotational and translatory disturbances along the azimuth plane) exhibited by UAVs when hovering at a fixed altitude, considering both indoor and outdoor scenarios. The misalignment model is then used to analyze communication outages when uniform linear array (ULA) antennas have varying antenna array orders. An analytical expression for the average coverage probability is presented, accounting for different transmitter misalignment scenarios in the presence of a dominant line of sight. Importantly, we investigate the influence of beam width on the link performance, considering diverse wind conditions leading to varying degrees of platform instabilities and misalignment. The performance analysis demonstrates that the analytical model closely aligns with simulation outcomes, with less than $2 \%$ error.