## 2 Related Work and Survey Scope
### Survey Scope  
Therefore, as evident from the preceding discussion, a significant research void exists in the relevant literature, defining the domain that this current work aims to address: the realm of connectivity within the IoT context, with a particular focus on the crucial aspects of energy efficiency and sustainability. Regarding the paper structure, Section 3 provides a detailed description of the characteristics of non-terrestrial networks, laying the foundation for understanding their significance in the context of IoT. Moving forward, Section 4 identifies and clarifies the key challenges involved in integrating NTNs within IoT systems. To address these challenges, Sections 5-7 present comprehensive surveys of IoT systems based on Space-borne NTN Networks, Aerial NTN Networks, and Hybrid NTN Networks, respectively. Each survey investigates the unique objectives, characteristics, and  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & **Reference** & **Year** & **Main Focus** & \multicolumn{1}{c}{**NTNs**} & \
\begin{tabular}{c} **Coverage of** \\ **IoRT** \\ **Connectivity** \\ **Solutions** \\ \end{tabular} &
\begin{tabular}{c} **Coverage of** \\ **Energy** \\ **Efficiency** \\ **Solutions** \\ \end{tabular} \\ \hline
1 & [39] & 2015 & \begin{tabular}{c} Overview of satellite technologies for IoT Applications \\ **100** \\ \end{tabular} & \
\begin{tabular}{c} ✓ \\ **\end{tabular} &
\begin{tabular}{c} ✕ \\ **\end{tabular} & Partially & ✕ \\
2 & [40] & 2017 & Utilization of LEO satellites in IoT & Partially & ✕ & Partially & ✕ \\
3 & [41] & 2019 &
\begin{tabular}{c} Utilization of satellite and UAV technologies in 5G- \\ IoT networks \\ \end{tabular} & Partially & ✓ & Partially & ✕ \\
4 & [42] & 2020 & AI-based techniques to improve the performance of NTN-based IoT services & Partially & ✓ & Partially & Partially \\
5 & [43] & 2021 & Hybrid Satellite–Terrestrial Networks (HSTNs) & ✓ & ✓ & Partially & Partially \\
6 & [44] & 2021 & Satellite-based IoT solutions for remote/rural areas & ✓ & ✕ & Partially & ✕ \\
7 & [45] & 2021 & \
\begin{tabular}{c} Integration status of IoT and space communication technologies \\ **100** \\ \end{tabular} &
\begin{tabular}{c} ✓ \\ **\end{tabular} & ✓ & ✓ & Partially & Partially \\
8 & [24] & 2021 & IoT Maritime Communication Networks (MCNs) & ✓ & ✓ & Partially & Partially \\
9 & [46] & 2022 & UAV-enabled Wireless Sensor Networks (U-WSNs) & ✕ & ✓ & Partially & Partially \\
10 & [47] & 2022 & Evolution of NTNs in 5G context & ✓ & ✓ & Partially & Partially \\
11 & [48] & 2022 & Communication solutions for IoT applications in cellular, wide-area, and non-terrestrial networks & Partially & ✓ & Partially & Partially \\
12 & [49] & 2023 &
\begin{tabular}{c} Space–air–ground–sea-integrated (SAGSI) networks deployment resource optimization analysis \\ \end{tabular} & ✓ & ✓ & Partially & Partially \\
13 & [50] & 2023 & Satellite communications overview for multi-orbit IoT & ✓ & ✕ & Partially & ✕ \\
14 & This Survey & 2024 & Non-terrestrial networks, including UAV and satellite technologies, for energy-efficient connectivity of remote IoT Devices & ✓ & ✓ & ✓ \\ \hline \hline \end{tabular}
\end{table}
Table 2: Related surveys and reviews.  
challenges associated with these NTN implementations in IoRT. Section 8 examines future research trends and challenges in IoRT, while Section 9 focuses on the 3 GPP standardization roadmap, emphasizing the importance of research and standardization efforts in both bridging remote connectivity gaps and fostering energy-efficient IoRT environments in the 6G landscape. Finally, Section 10 concludes the paper by summarizing our work and identifying future research directions.