# Unmanned Autonomous Intelligent System in 6G Non-Terrestrial Network
## 4 Application of UAV-Aided Non-Terrestrial Networks in UAIS
### 1.2.1. **USV-assisted maritime communications**
#### 4.3.2 Unmanned Underwater Vehicles in Underwater Communications
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Literature** & **Topics** & **Contributions** & **NTN Communication Type** \\ \hline
[169] & UAV-USV Cooperation &
\begin{tabular}{c} Performance and reliability evaluation of \\ communications for USV–UAV \\ cooperation tasks \\ \end{tabular} & UAV-USV \\ \hline
[170] & UAV-USV Cooperation &
\begin{tabular}{c} UAV–USV cooperative tracking and landing \\ scheme using model-based control. \\ \end{tabular} & UAV-USV \\ \hline
[171] & UAV-USV Cooperation &
\begin{tabular}{c} Collaborative surface coverage of oceanic area \\ utilizing UAV and USV \\ \end{tabular} & UAV-USV \\ \hline
[172] & UAV-USV Cooperation &
\begin{tabular}{c} USV–UAV marine cooperative search and \\ control by means of visual information \\ \end{tabular} & UAV-USV \\ \hline \hline \end{tabular}
\end{table} 
Table 5: _Cont._emphasized on the angle of arrival model for cooperatively extracting positions of multiple targets. The proposed scheme evaluated three-dimensional position information through D-optimality criterion. The authors of [187] examined the impact of changes in propagation speed relative to water depth and clock synchronization on achieving improved accuracy of position estimation. In [188], the proposed approach combined maximum likelihood estimation and the Gaussian belief method to mitigate the effects of noise and non-linearity in the formula. **USV-AUV Cooperation:** Form recent literature, AUVs are typically utilized in coordination with USVs or other platforms to collectively fulfill tasks as USVs have stronger communication and perception capabilities compared to AUVs. Furthermore, USVs are also considered as communication hubs for accessing maritime radio networks and underwater communication simultaneously, performing the role of air-to-underwater gateway. Researchers [189] emphasized the influence of the random wave fluctuation in underwater acoustic communication for USVs in underwater node communication. Ref. [190] proposed an AUV-USV system for search tasks in which acoustic communication is used for the USVs to transmit perception information to AUVs, while wireless radio was utilized among a group of USVs. In this application, satellite communication was also introduced to assist USV localization, thereby forming a space-sea-underwater network architecture. In reference [191], a communication scheme for UAVs and underwater drones was introduced, and the pros and cons of multiple underwater communication methods were summarized. Open issues still exist in the domain of underwater unmanned autonomous system communication, such as cross-medium communication, visible light communication (VLC) [192], software-defined underwater networks [193], and information security. Cross-medium communication is derived from the challenge of directly merging underwater access networks with contemporary terrestrial networks with water-air signal conversion. Acoustic millimeter-wave communication is realized as the water-air interface [194]. Ref. [195] proposed a blockchain-enabled secure underwater communication system for unmanned underwater drones in oceanic special classification. Nevertheless, the aforementioned issues require further discussion.