# Unmanned Autonomous Intelligent System in 6G Non-Terrestrial Network
## 4 Application of UAV-Aided Non-Terrestrial Networks in UAIS
### Unmanned Maritime Vehicles in UAV-Aided NTN Communication
#### 4.3.1 Unmanned Surface Vessels in NTN Communication  
Currently, maritime communication heavily relies on satellite and radio communications which are unable to meet the growing demand for broadband communication in maritime unmanned autonomous intelligent systems. However, satellite communication cannot support real-time maritime applications because of high propagation delays. On the other hand, radio communications, such as very high-frequency (VHF) communication, typically only support voice services, with limited bandwidth for high data rate communication. (1) **UAV-Aided Maritime Communication for USVs**  
Thanks to the successful application of UAV air-to-ground non terrestrial network, the maritime communication have seen possibilities of adopting UAVs for oceanic communication. The ultra-low latency feature of UAV-assisted networks will shed new light on the maritime UAIS applications such as control and coordination of multiple USVs. However, there is a long way to go for applying UAVs to maritime communication since the limitation of endurance, power consumption and all weather capabilities of UAVs. The UAVs usually are not designed for the salty and misty environments and long time of operation over the sea may cause mechanical and electrical failures. Secondly, the energy of UAVs is quite limited which prevent it from operating for long time network coverage for typical long endurance maritime applications. In addition, in extreme weather conditions, such as tornadoes and storms, very few kinds of UAVs are capable of normal operation. Thus, various obstacles appear in the path of UAV maritime communication applications. However, maritime radio channels are complicated and are seriously affected by adverse weather conditions, tide movements, ocean current activity and other factors, causing problems in air-to-sea channel modeling in oceanic communication [162]. **Topology optimization:** Due to the continuous maneuvering of oceanic nodes such as USVs, the design and maintenance of their topology is also challenging. Topology design is a fundamental issue that needs to be addressed, given the constantly changing nature of the maritime environment. In [163], an integrated communication framework for terrestrial, sea and HAP is proposed for multi-USV control. The authors of the paper considered the inter-USV distance and channel interference to maintain the stable handover of the network while USVs are maneuvering at high speed. The authors of [163] proposed an integrated network architecture for terrestrial, maritime, and high altitude platform for UAV and USV cooperative communication. The proposed architecture considered the switching cost caused by node mavuering and link interference using a reinforcement-learning-based whale optimization method. Furthermore [164], proposed a multi-USV group communication scheme in which the total communication cost was minimized using a nested-ring algorithm for topology optimization that considered the factor of occlusion, weather, and inter USV distance. **Formation control:** Formation control is a critical issue because multi-USV systems can accomplish complex tasks that a single USV cannot achieve. The main challenge of formation control is the need for dependable and low-latency communication for collaboration between USVs. In the USV group network, the interval distance between USVs and interference from nearby nodes tend to increase time delays and decrease network throughput. Therefore, unmanned aerial vehicles (UAVs) are introduced as effective means to support cooperative communication with USVs, building a combined network of air and sea using UAVs [165]. To create a secure and efficient network for UAV-USV formation, cooperative communication was employed, taking into account the time-varying topology and node locations [166]. The efficiency of formation tasks was severely affected by the communication between UAV and USV.