# Towards the Unmanned Aerial Vehicles (UAVs): A Comprehensive Review
## 5 UAVs in 5G and IoT Networks  
With the rapidly growing advancements of UAV technology, the UAVs have proven their great stature in an extensive range of applications such as environmental monitoring, wireless communication and public safety. Apart from this, UAVs are extensively explored in conjunction with wireless networks and have shown promising solution to enhance the energy and spectrum efficiencies of the fifth generation (5G) wireless networks as they offer on-demand flexible platform to deploy aerial base stations. The unique attributes of UAVs, such as adaptive altitude, scalability, flexibility, mobility, high resilience, high reliability, and low cost, motivate their use for many applications. Orders of magnitude performance enhancement is envisaged to be attained in UAV-assisted networks as compared to conventional terrestrial system without UAVs. UAVs can be smartly controlled and remotely operated when needed. UAVs offer high probability of LoS communication which significantly enhances the transmission rate. Swift maneuverability of UAVs can ensure reliable and broadband wireless coverage during live stream events and natural disasters. Thus, UAVs can be used in 5G networks to offer offloading services with stringent requirements [96] in case of emergency operations. With aforementioned potencies, it is plausible to integrate UAVs into 5G networks. Extensive research endeavors are being performed on UAV-assisted 5G technology to reduce latency and extend network coverage, capacity and energy efficiency. However, there are certain challenges for this coexistence such as UAV's vulnerability to interference, dynamic network topology and mobility causing spatial and temporal dynamics. These challenges can lead to frequent intermittent connections or even transmission failures [97]. Moreover, UAVs are used to perform wireless energy transfer (WET) and wireless information transfer (WIT) to power ground terminals such as IoT terminal which are integral parts to develop flexible and sustainable 5G networks [98]. With the continuous development of UAVs in terms of maneuverability, agility, degree of autonomy, flying speed, flight time and mechanical performance, UAVs have emerged as attractive platforms to support various IoT applications [52]. For instance, UAVs can be incorporated with IoT terminals to get information about water, gas and power meters from sky, offering an unprecedented benefit to IoT regulators. UAVs equipped with specific sensors can gather information about air quality in a smart city and deliver this real-time data using machine type communication or LoRa. In ref. [99], the authors proposed an open-source smart IoT platform through UAV to support different smart city services. UAV has been considered as a promising enabler to support IoT vision due to its reliability, scalability and agile usability [100]. In contrast to other applications of 5G networks, IoT devices are randomly scattered and energy limited. These challenges will severely aggravate the connectivity issue for IoT networks. By leveraging the UAV's maneuverability and agile mobility, it is viable to get close to IoT terminals and gather the sensing information from them. Hence, UAV-assisted IoT networks can substantially reduce the overhead of IoT network and can solve the coverage issue. Moreover, cost-effectiveness of UAVs makes them promising candidates for emergency on-demand operations in IoT applications. Figure 9: Recharging of multiple UAVs through laser beams [95]. Despite these appealing advantages, using UAV in an IoT networks still suffers from several challenges of channel modeling, prior information of channel state and energy constraints of IoT terminals. Any small maneuverability error can significantly affect entire mission of UAV. UAV's maneuverability limitations have been experimentally analyzed in [52].