# Hybrid Satellite-Terrestrial Networks toward 6G: Key Technologies and Open Issues
## 2 Cooperative HSTN  
### Coordinated HSTN  
While the terrestrial network has been effectively implemented in developed areas to offer broadband Internet access at low cost, the satellite network can offer ubiquity with wide coverage. Thus, these two systems can cooperate for mutual benefits. Figure 2 shows the structure of coordinated HSTN. Coordinated HSTN is a promising architecture because it makes use of the strengths of both networks instead of choosing only one of them. Using the same physical-layer protocols, unified terminals may effortlessly access either the satellite network or the terrestrial cellular network. Higher network efficiency and user experience can be achieved when integration advances from the top level to the bottom level, at the expense of higher implementation complexity and deployment costs. Broadband services can be supported via the terrestrial network at fairly low cost. However, due to financial and geographical limitations, the coverage of each BS is quite narrow, which restricts the expansion of terrestrial networks. On the other hand, because of its top-down design, the satellite network can offer extensive coverage. However, the satellite network generally experiences large signal attenuation and latency. To minimize the inherent communication delays, path loss, and deployment costs of the satellite network [38], aerial networks have been suggested as a mitigating part of HSTNs. In addition to their versatility and low cost, these elements have lower latency and can operate with spot beams delivering more capacity to ground users and a greater diversity of options. Due to their inexpensive cost and tremendous versatility, unmanned aerial vehicles (UAVs) have received much interest in recent years for use as aerial BSs. In contrast to cellular communications, UAVs can freely and adapt their movement in a 3D space, making it possible to establish line-of-sight (LOS) communication links and to prevent signal blockage and shadowing. UAVs are also seen as a possible method of achieving ubiquitous connectivity for IoT, which is fascinating from a technological perspective. However, aerial networks, including UAVs, incur significant energy consumption due to propulsion and hovering. Their intrinsic mobility could impair network performance without proper coordination and countermeasures [39]. Figure 3 shows the structure of an HSTN assisted by UAVs. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Contributions** & **Year** \\ \hline
[28] & Handover schemes in satellite networks & 2006 \\ \hline
[29] & Comparison of different mobile–satellite systems & 2010 \\ \hline
[30] & Design parameters, operational solutions, and inter-satellite communication of small satellite systems & 2016 \\ \hline
[31] & State-of-the-art architectures of converged satellite–terrestrial systems & 2016 \\ \hline
[32] & Optical communication techniques in integrated satellite–terrestrial network to mitigate atmospheric side effects & 2016 \\ \hline
[33] & State-of-the-art space–air–ground integrated networks & 2018 \\ \hline
[34] & Convergence of satellite–terrestrial networks, performance evaluation, and simulation platforms & 2019 \\ \hline
[35] & Non-terrestrial network (NTN) architectures and NTNs in cellular communications. & 2020 \\ \hline
[36] & Future research directions for a cell-free, hierarchical, decoupled HSTN and open issues to envision an agile, smart, and secure HSTN & 2021 \\ \hline
[37] & An environment-aware and service-driven hybrid satellite–terrestrial MCN for future maritime communications & 2021 \\ \hline Our work & Comprehensive review of cooperative and cognitive HSTNs centered on key technologies with their performance and open issues & 2022 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Comparison of surveys of HSTNs. To relay signals to the users, networks use different relaying schemes such as amplify-and-forward (AF), fixed decode-and-forward (FDF), and selective decode-and-forward (SDF) [40; 41].