# A Survey on Machine-Learning Techniques for UAV-Based Communications
## 1 Introduction
### Previous Survey Works
\begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Reference** & **Short Description** & **Scope of AI/ML on UAV** \\ \hline [PERSON] et al. [4] & A survey on cyber-physical applications in multi-UAVs & ML for object detection and image recognition \\ \hline [PERSON] et al. [5] & UAV cellular communications & ML for trajectory and placement \\ \hline [PERSON] et al. [6] & Regulatory aspects of using UAVs for cellular communications & UAV trajectory and placement \\ \hline [PERSON] et al. [22] & A review of DL for UAVs & DL for feature extraction, planning, situational awareness, and motion control \\ \hline [PERSON] et al. [23] & DRL in wireless networks & UAV as an agent \\ \hline [PERSON] et al. [24] & A tutorial on UAVs for wireless networks & Trajectory and path planning \\ \hline [PERSON] et al. [25] & Wireless big data & No discussion on AI and ML \\ \hline [PERSON] et al. [26] & 6G vision & No concern about AI and UAV \\ \hline [PERSON] et al. [27] & A survey of 5G and beyond for communication of UAV & Nothing specific \\ \hline [PERSON] et al. [28] & A survey of V2X communications & Security in VANET with UAVs \\ \hline [PERSON] et al. [29] & A survey on UAV networks from the viewpoint of CP security & ML and RL at the physical layer and routing correspondingly \\ \hline [PERSON] et al. [30] & A comprehensive tutorial of ANN-based ML for wireless networks & RL for coverage, connectivity, trajectory, and path planning \\ \hline [PERSON] et al. [31] & Wireless and security challenges for different UAV use cases & ANN-based solutions for interference management \\ \hline [PERSON] et al. [32] & AI techniques for communication between robots & Intelligent swarm robot communication \\ \hline [PERSON] et al. [33] & AI on SAGIN & Based on routing on a case study of a complete network \\ \hline [PERSON] et al. [34] & An overview of 5G communications for V2X, drones, and healthcare & No discussion on ML and UAV \\ \hline This survey & A survey on AI/ML for UAV-based communications & AI/ML integration in UAV-based communications and open issues \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Relevant surveys and tutorials on artificial intelligence (AI)/machine learning (ML) and unmanned aerial vehicle (UAV) communications. Also, in Figure 1, various applications of the AI/ML solutions in UAV-enabled communications are depicted. Generally, the importance of distilling intelligence in wireless communication networks has been outlined in numerous works. In Reference [24], the authors have observed that the ever increasing heterogeneity and complexity of mobile networks has made monitoring and management of network elements intractable. Moreover, ML allows systematic mining of valuable information from mobile data and automatically identifies correlations that are too complex to be derived by human experts. Likewise, the work in Reference [33] has noted that, in wireless networks, ML enables the wireless devices to actively and intelligently monitor their environment, exploiting mobile data for training purposes in order to learn, predict, and adapt to the evolution of environmental features, including wireless channel dynamics, traffic and mobility patterns, as well as network composition, among others. In this way, they can proactively act towards maximizing the probability of satisfying different performance metrics. It can be seen that ML takes as input data from different sources and through the application of various learning techniques, i.e., supervised/unsupervised, deep or reinforcement learning, allows the network to adapt to the wireless environment in a dynamic and autonomous manner. Thus, especially in networks consisting of a large number of nodes, such as those consisting of swarms of UAVs [35], centralized coordination and excessive overheads that must be acquired and exchanged among the network nodes are avoided, paving the way for distributed network optimization where intelligence plays a key role.