# 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  
Unmanned underwater vehicles have much potential in fulfilling sophisticated missions that are dangerous or impossible for humans to accomplish. For applications of unmanned underwater vehicles (UUVs) or autonomous underwater vehicles (AUVs), communication is even more sophisticated, as underwater communication channels have been barely investigated by research scientists. The rapid development of wireless communications has extended from terrestrial to aerial spaces to enhance user connectivity in NTN. However, the underwater wireless communications are rarely discussed, despite being an essential component of 6G NTN networks. Underwater communication is achieved via multiple means including radio signals, acoustic communication, lasers, and cabled communication. However, due to physical constrains, underwater communication cannot be achieved using a straightforward means, since radio communication is nearly impossible in the underwater scenario. Underwater communication is typically achieved using USV and buoys, which serve as gateway for radio communication and various underwater communication methods. Ref. [180] proposed a hybrid underwater communication scheme integrating acoustic, optical, and fiber optic communications to support underwater applications. Cabled communication is a traditional method because conventional radio transmission is nearly impossible in underwater environments. An applicable network architecture was investigated by [181] utilizing a hybrid network between cable-assisted communications and wireless underwater communications in which cable-based sink nodes and tethered cable UUVs were utilized to provide short-distance communication for autonomous underwater vehicles. **Formation Control:** With the advancement of underwater communication in the 6G era, AUVs are widely utilized as nodes in underwater communication network, posing challenges related to formation control and collision avoidance of AUVs in complex underwater scenarios. The stable formation and safe operation of AUVs in underwater environments is beneficial for network topology management and network robustness. Ref. [182] investigated a formation control paradigm with a heterogeneous underwater communication method for AUVs under communication constraints. Ref. [183] proposed a framework utilizing a fusion control method with a distributed mechanism to accomplish real-time formations in underwater wireless communication environments. The authors of [184] considered the formation control and collision avoidance problem in a multi-agent system involving UAVs, USVs, and UUVs. In this work, to fulfill the formation control task, the communication between USVs and UUVs were achieved using moving buoys to maintain the cross-media communication topology. **Localization and Navigation:** Localization is a critical issue for AUVs in underwater NTN networks. However, the conventional satellite positioning method might be unsuitable for deep-water applications. The localization method can be divided into stand-alone and cooperative positioning methods. For the stand-alone localization method, AUVs estimate their position based on globally collected information. A simple cost-efficient method is proposed to globally estimate the AUV position in [185]. In [186], the authors