## 5 Open Problems and Future Opportunities for UAV Wireless Networks
### UAV Deployment and Trajectory  
We have surveyed recent studies on UAV deployment and trajectory in this paper to construct UAV wireless networks. As discussed in Section 4, the key consideration for UAV deployment on UAV wireless networks is coverage and throughput enhancement while there is a network lifetime for UAV trajectory. There have been lots of studies on UAV deployment and trajectory, but it has still open problems because it depends on environments, users' location, air-to-ground channel characteristics, and so on. Moreover, an air-to-ground channel characteristic is a function of UAV altitude, hence, the modeling of air-to-ground link and UAV deployment influence each other. Furthermore, the users' mobility and resource management have not been jointly considered on UAV deployment, which restricts actual use in a real environment. Considering all key factors on UAV deployment to optimize the performance of UAV wireless networks is a challenging task. When deploying multiple UAVs, it becomes more challenging. Other technologies on communication systems can be coupled with UAV deployment and trajectory to further improve the performance of UAV wireless networks. In [80], the cognitive radio (CR) technology is integrated into micro UAV-based communications to improve the spectral efficiency, and the UAV position and tranmit power are jointly optimized to minimize the UAV flying time, thereby achieving the energy efficiency at the same time. Ref. [81] also considers UAV-enabled CR networks and optimizes the UAV trajectory and transmit power while maximizing the average secrecy rate of the secondary network to improve the security on communications via UAV. Ref. [82] deals with the resouce allocation for the transmit power and channel time allocations for secondary ground user to maximize the total network throughput. The stationary UAV positions to hover and transmit are determined where the required throughput and no interference from the primary network on individual secondary ground user are guaranteed. Ref. [83] proposes the interference management framework for UAV-assisted networks based on the power control and UAV clustering, which could be combined with UAV deployment and trajectory to further improve the QoS in UAV wireless networks. In addition, non-orthogonal multiple access (NOMA) is applied in [84] to maximize the sum-rate of network by jointly optimizing the UAV deployment and power allocation. It has been well known that superposition-coded signal transmission and successive interference cancellation (SIC) in non-orthogonal transmission achieve superior performance over orthogonal transmission. A millimeter wave (mmWave) communication is considered in UAV wireless networks, and [85] provides reviews on research achievements for mmWave communications in UAV wireless networks as well as the technical advantages and challenges. Due to the characteristics of mmWave communications such as high FSPL, atmospheric attenuation, and blockage effect, mmWave communications are suitable for air-to-air links between UAVs. In addition, NOMA and mmWave communications are the key protocols in 5G standards, therefore, they enable UAV wireless networks to be integrated with 5G and beyond wireless networks to support the required performance demands. In [86], a simultaneous wireless information and power transfer (SWIFT) is employed in UAV-enabled two-way relay systems to harvest energy through the power splitting (PS) scheme. This study proposes PS factor optimization along with beamforming to maximize the sum-retained energy at two user equipments, and is extended in [87], considering multiple user equipments. ML techniques are also integrated into the construction of UAV wireless networks, specially UAV deployment and trajectory to satisfy the required performance. In [88], the joint optimization of multiple UAVs deployment and power control is proposed to maximize the sum transmit rate and to satisfy the users' rate requirement.