## 1 Introduction
### Related Work  
The literature on UAV-assisted communication systems is thoroughly evaluated in this subsection. In recent years, numerous number of studies were done in the field of UAV-assisted wireless networking [16; 17; 18; 19; 20; 21; 22; 23; 24; 25], but only a few studies have looked at the flying regulations [26; 27]. In [8], the authors presented a survey on the most recent research possibilities and problems in the field of UAV aided wireless networks. The key difficulties in UAV-assisted networking are investigated, including 3D deployment, performance analysis, channel modeling, and energy efficiency. These topics are investigated due to huge demands in terms of network performances for 5G services, which results, among other factors, in a considerable increase in energy consumption. Consequently, energy saving is a fundamental design requirement for 5G networks. Although, in the case of ground base stations, solutions as low-cost, low-complexity cell sleep scheduling algorithms may help to minimize the energy consumption while guaranteeing the service requirements [28; 29], for UAV-assisted networks it is still an open-issue. For the coexistence of UAVs and under- laid device-to-device (D2D) communication networks, a tractable analytical framework is proposed in [22]. The authors showed that flying a UAV at the ideal altitude can result in the highest system sum-rate and coverage probability. Furthermore, an optimal trajectory design can reduce transmit power; however, networking under the UAV altitude regulations did not receive enough attention in the literature. The lowest and maximum authorized altitudes of flying UAVs vary by country; for example, European laws for flying UAVs establish the limits of minimum and maximum allowed altitudes, which may fluctuate in different regions of the world, and in [26], the authors looked at the status of UAV-related regulations. In the previous few years, numerous surveys and tutorials were released. The findings reveal that creating air route networks is a scientifically sound and efficient way to standardize and improve the efficiency of low-altitude UAV operations [27]. The most significant approach for UAV regulation in urban regions, in terms of safety and efficiency, is to enhance research that heavily relies on urban remote sensing and Geographic Information System (GIS) technology, as well as application demonstrations of low-altitude public air route networks [26]. In [27], the authors discussed the standardization initiatives for UAV-assisted UEs, UAV-assisted BSs, UAV communication prototypes, and UAV-assisted cellular communications cyber-physical security. The usage of UAV-assisted communication was suggested as a possible approach for Internet of Things (IoT) networks in the literature [8; 30; 31; 32]. In [30], it was demonstrated how to collect data in an energy-efficient manner for IoT networks, and the best way to deploy and move several UAVs was examined. The authors developed a framework for concurrently optimizing UAV 3D positioning and mobility, device-UAV association, and uplink power regulation in their paper. Firstly, the ideal UAV location is identified based on the locations of active IoT devices at each time instant. Next, the optimal UAV mobility patterns were studied to dynamically serve the IoT devices in a time-varying network. The goal is to utilize as little energy as possible for the UAVs' mobility while serving IoT devices. For the coverage and rate analyses, a tractable analytical framework is developed [21], wherein the UAV's coexistence with a D2D communication network is taken into account. The authors in [33], reviewed the literature of machine learning integration to UAV-assisted communications, and they discussed various aspects including physical layer, security, resource management, and positioning. The interfering UAVs are considered in [34], while in [35], the authors investigated the optimal 3D placement of multiple UAVs, that use directional antennas, to maximize total coverage area.