# UAV Communication Recovery under Meteorological Conditions
## 1 Introduction  
In recent years, with the reduction in manufacturing costs, unmanned aerial vehicles (UAVs) have appeared in many applications. In the civil field, UAVs are widely used in aerial photography [1, 2], smart agriculture [3, 4], express transportation [5, 6], disaster relief [7], observing wild animals [8], etc. In addition, due to the advantages of flexible and rapid deployment, UAVs are also suitable for various applications in fifth-generation (5G) and beyond wireless communications. In cellular networks, UAVs can play two distinct roles: cellular-connected UAVs and UAV-assisted wireless communications [9]. The former treats the UAV as an aerial user accessing the cellular network for sky-based communication, while the latter utilizes the UAV as an aerial communication platform, acting as a base station (BS) or relay to provide data access from above. For example, the UAV in Ref. [10] is used as an air base station (ABS), and in Refs. [11, 12], the UAVs act as relays. This paper primarily focuses on UAVs serving as ABS. UAVs, when utilized as aerial base stations, can provide temporary communication services following natural disasters such as earthquakes, mudslides, floods, and forest fires. Furthermore, during civil emergencies such as power outages, UAVs can enable rapid communications and service restoration due to their high mobility and flexibility. However, common weather conditions, including rain, fog, and snow, cause the specific attenuationof electromagnetic waves [13; 14; 15], leading to reduced communications coverage between the UAV and ground users [16]. We believe that studying how to restore UAV-ground user communication coverage under various weather conditions to levels comparable to those in clear weather is a valuable endeavor. Several studies have explored the use of UAVs in communication with ground users. For instance, [PERSON] et al. [17] proposed an analytical method to optimize the heights of low-altitude platforms (LAP) for maximum terrestrial radio coverage. Similarly, [PERSON] et al. investigated the optimal height of a single drone small cell (DSC) for maximum coverage and minimum transmission power in [18]. In the case of two DSCs, the authors analyzed the effect of the distance between them on the coverage area, accounting for both no-interference and full-interference scenarios, and derived the optimal distance for maximum coverage. Furthermore, in [19], [PERSON] et al. explored the use of drone-based small cellular networks (DSCNs) to restore or fill coverage gaps in terrestrial cellular networks due to natural or man-made disasters. In another study, [PERSON] et al. investigated a downlink communication scenario in which a UAV base station (UBS) assists a ground base station (GBS) in providing services. Using stochastic geometry, they derived an explicit expression for the overall coverage probability [20]. In a cellular network with UAVs serving as base stations, [PERSON] et al. addressed the coverage recovery problem when one or more UAVs go offline due to energy replenishment or extreme environments [21]. The authors analyzed the relationship between the coverage radius, UAV height, and transmission power to mitigate the impact of offline UAVs. It is worth noting that the line-of-sight (LoS) probability expressions involved in the research of communication recovery in Refs. [19; 20; 21] are all based on Ref. [17]. The LoS probability expression in Ref. [17] is based on the assumption of evenly spaced buildings. This may not be appropriate in real urban environments, where buildings tend to be more randomly distributed. The communication recovery method proposed in this paper is based on our own proposed LoS probability expression under real urban environments. This is the first difference between this paper and the aforementioned three references. Secondly, this paper places an emphasis on the communication coverage of UAVs, which is intricately linked to the distance between the UAV and ground users.