# Towards the Unmanned Aerial Vehicles (UAVs): A Comprehensive Review
## 3 Unmanned Aerial Vehicles (UAVs)  
In recent years, UAVs have gained significant attention. Generally, UAVs refer to controlled aerial vehicles without carrying a human pilot on them. It can be autonomously controlled and operated through sensors, microprocessors and other electronic gadgets [35]. Figure 1 depicts a typical UAV system architecture, showing how UAVs interact with satellites, ground control systems (GCS), smart phones, and computers via communication links. A human operator is used to control and operate a UAV remotely. UAVs can perform autonomous tasks in situations where human intervention is difficult or dangerous [36]. At present, UAVs have become a very convenient approach for logistics. In particular, there is a notable upsurge in the civilian market for UAVs. The key applications of UAVs include remote operations such as search and rescue, disaster monitoring, environmental monitoring, and delivery of airmail, medical items, and packages. Figure 2 presents the growing revenue of USA for commercial UAV market in different sectors. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Year** & **Research Focus** \\ \hline
[26] & 2019 & This study surveys UAV communication towards 5G/B5G wireless networks. It discusses space-air-ground integrated networks and associated challenges. It also identifies several open research problems and future research directions. \\ \hline
[27] & 2019 & This study presents a comprehensive survey on the integration UAV-enabled wireless networks and of 5G mmWave communication. It highlights existing research problems and cutting-edge solutions. This study also points out open issues and sheds new light on future research directions. \\ \hline
[28] & 2020 & This article presents a comprehensive study on UAV regulations, potential applications and challenges. Particularly, it addresses challenges including security, energy harvesting techniques, optimal trajectory, collision avoidance, interference mitigation, channel modeling and UAV standardization. Finally, it presents several problems and future research directions. \\ \hline
[29] & 2020 & This study focuses on Network Function Virtualization (NFV) and Software-Defined Network (SDN) technologies. In addition, it presents an in-depth analysis of use cases, classifications and challenges of UAVs. It also discusses NFV/SDN-assisted UAV systems along with different case studies and issues. Finally, it outlines open research problems, high level insights, and future research directions. \\ \hline
[30] & 2020 & This study is dedicated to the safety of UAVs from three different aspects such as communications, sensors and multiple UAVs. \\ \hline
[31] & 2020 & This survey presents a generic review on several applications of multi-UAV systems. Moreover, it provides architectural and nomenclature taxonomy. Finally, it discusses UAV challenges and current trends. \\ \hline
[32] & 2021 & This article carries out a comprehensive survey on green UAV communications for future 6G networks. In particular, it introduces the typical UAVs and their energy consumption models. It also discusses the typical trends and typical applications of UAVs. Finally, it investigates open research issues and several promising techniques. \\ \hline
[33] & 2021 & This study presents general architecture of UAV prototype along with experimental demonstration for UAV energy consumption model and air-to-ground channel models. In the end, it highlights some promising future directions for UAV prototype and experimental verifications. \\ \hline
[34] & 2021 & This study provides deep learning techniques for vehicle detection from UAV aerial images. It focuses on accuracy enhancement, computation overhead reduction and optimization objective. This study will help researchers in the area of traffic surveillance and artificial intelligence. \\ \hline \hline \end{tabular}
\end{table} 
Table 1: _Cont._Despite increasing attention, mostly UAVs are being controlled by human-aided remote controls.