## 2 Related Work  
In literature, there have been many studies on UAV in all aspects of communications and networks research [9; 10; 11]. Radio Propagation and Channel Models.The radio-wave, propagation, and antenna-related research results in UAV flying networks are well presented in [12]. The detailed summary of wireless channel and radio propagation models in UAV communication networks is given in [13] and, especially, air-to-ground UAV channel models are presented in [14]. For more details, non-stationaryair-to-air UAV wireless channel models are introduced [15]. In addition, three-dimensional (3-D) geometry-based stochastic models for UAV multi-input-multi-output wideband non-stationary are discussed in [16]. Moreover, the usage and corresponding channel models for millimeter-wave wireless technologies [17; 18; 19; 20] in UAV wireless networks under the consideration of hovering fluctuations is well-discussed in [21]. Lastly, the statistical channel modeling and radio-wave propagation of free-space optical communication (FSO) fronthaul under the consideration of UAV communications is studied in [22]. CommunicationsThe use of UAV systems for 5G millimeter-wave wireless networks and cellular networks is well-discussed in [23; 24; 25; 26], respectively. For more details, major communication research topics are in multi-antenna techniques [27; 28], the use of millimeter-wave networks for high-throughput UAV networks [21; 23; 29], interworking with satellite networks for coverage extension [30], the use of UAV for device-to-device networks [31; 32], and lastly, communication security for multi-UAV networking systems [33; 34]. NetworkingMost of UAV-based networking research topics and their related representative contributions are as follows. First of all, cell-edge user data offloading via UAV systems for non-uniform heterogeneous cellular networks is discussed in [35]. In addition, routing algorithms under the consideration of wireless charging nodes are designed in [36; 37]. Moreover, connectivity-aware robust routing and relay deployment algorithms in UAV networks are well-studied in [38]. Lastly, carrier-sensing-multiple-access with collision-avoidance (CSMA/CA)-based enhanced medium access control algorithms for UAV-relay networks are well-discussed in [39]. Mobility Management and Trajectory OptimizationOne of the major research topics in UAV networks is mobility management and trajectory optimization [40]. Under the consideration of high and unpredictable mobility in UAV, theoretical performance analyses and discussions are presented in [41; 42]. In addition, the joint UAV trajectory and flight-time optimization are presented in [43], and the paper also considered FSO communication, which is beneficial in terms of flexible network management. Moreover, energy-aware trajectory optimization with wireless power transfer methods is discussed in [44]. Furthermore, secure trajectory optimization for multi-UAV networks is discussed in [45; 46]. Energy-EfficiencyThe implementation and energy-aware system operations are discussed in [47]. Furthermore, energy efficiency is one of the key research topics in UAV networks due to the extreme UAV battery/energy limitations. To achieve this goal, an energy-efficient operation, especially for the Internet-of-Things (IoT) applications, is discussed in [48]. In terms of the minimization of wireless energy consumption, the proposed method in [49] pursues a sensor network lifetime extension. Under the consideration of a more realistic energy consumption model in UAVs, the proposed method in [50] minimizes the total flight time of the UAV while allowing sensors to successfully upload data. Moreover, considering the energy consumption of both the user and the UAV, the trade-off between the propulsion energy and wireless energy of the served IoT or user was discussed in [51]. Lastly, energy harvesting technologies are also used for the UAV energy-efficient networking, as presented in [52].