# Unmanned Autonomous Intelligent System in 6G Non-Terrestrial Network
## 3 UAV-Assisted Non-Terrestrial Networks
### UAV Path Planning for Aerial Mobile Relay
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Literature** & **Topics** & **Contributions** & **Year** \\ \hline \multirow{3}{*}{[32]} & \multirow{3}{*}{UAV Base Staion} & Investigated a distributed framework based on DRL to control and optimize UAV base stations considering ground users’ fairness & 2021 \\ \cline{3-4}  & & & Proposed a flow-level model for UAV base station network and a DRL-optimizing method for network traffic balancing based on the realistic UAV radio model. & 2019 \\ \hline \multirow{3}{*}{[34]} & \multirow{3}{*}{UAV Base Staion} & Demonstrated a circle trajectory setup for multiple ground uses coverage. & 2020 \\ \cline{3-4}  & & & Designed a deep Q learning network and greedy algorithm-based UAV trajectory optimization paradigm for unevenly distributed ground users. & 2021 \\ \hline \multirow{3}{*}{[35]} & \multirow{3}{*}{UAV Base Staion} & Emphasized a multi-target coverage dynamic trajectory optimization scheme utilizing knowledge-incorporated approach & 2021 \\ \cline{3-4}  & & & \\ \hline \multirow{3}{*}{[36]} & \multirow{3}{*}{UAV Relay} & Proposed an iterative solution for joint optimization of UAV trajectory, power consumption and time-slot assignment. & 2019 \\ \cline{3-4}  & & & Considered a path planning and power consumption optimization paradigm for a space-air-ground integrated relay network in a non-orthogonal multiple access scheme & 2018 \\ \hline \multirow{3}{*}{[39]} & \multirow{3}{*}{UAV Relay} & Investigated a novel solution for trajectory design and transmitting power minimization for a UAV-assisted relay & 2021 \\ \cline{3-4}  & & & The paper proposed an efficient algorithm designed for UAV relay’s three-dimensional trajectory optimization in order to achieve higher throughput. & 2021 \\ \hline \multirow{3}{*}{[41]} & \multirow{3}{*}{UAV Relay} & The paper studied a multi-UAV trajectory optimization framework in which UAVs acted as both aerial base stations and wireless relays. & 2022 \\ \cline{3-4}  & & & The paper proposed an analytical model to demonstrate the influence of trajectory and altitude on the quality of service in UAV aerial cellular users & 2020 \\ \hline \multirow{3}{*}{[45]} & \multirow{3}{*}{UAV User Equipment} & The paper investigated an iterative method for trajectory planning to minimize the time to fulfill the task and overall power consumption for UAV aerial users & 2021 \\ \cline{3-4}  & & & The paper proposed an online trajectory optimizing scheme for minimal power consumption based on outage probability map reconstructed from sparse sampling. & 2023 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary of existing work on UAV trajectory/path planning in NTN communication. Recently, reconfigurable intelligent reflection surfaces (IRSs) and UAVs have been combined together to achieve communication coverage enhancement in challenging environments [57]. In [58], the trajectory planning of the UAV with IRS onboard is addressed using the DRL method. Ref. [59] proposed UAV optimal path planning for maximizing the reflecting rate considering the UAV motion limit by means of a heuristic method.