# Hybrid Satellite-Terrestrial Networks toward 6G: Key Technologies and Open Issues
## 2 Cooperative HSTN
### HSTN Relay Network
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **System Model** & **Performance Metrics** & **Other Factors** \\ \hline \multirow{3}{*}{[46]} & Multi-antenna satellite communicating & \multirow{3}{*}{\begin{tabular}{c} OP comparison of static and \\ mobile UAV relays under opportunistic \\ relay selection \\ \end{tabular} } & \multirow{3}{*}{Fixed altitude of UAVs} \\  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \hline \multirow{3}{*}{[47]} & HSTN with a DF-based aerial relay and a group of terrestrial receivers & \multirow{3}{*}{\begin{tabular}{c} Approximate OP expressions, \\ interference, and non-interference \\ coverage analysis \\ \end{tabular} } & Transmission power and Time \\  & & & \\ \cline{1-1}  & & & \\ \hline \multirow{3}{*}{[48]} & Three-layer space–air–ground & \multirow{3}{*}{\begin{tabular}{c} Link delay, bandwidth, and \\ connection time \\ \end{tabular} } & \multirow{3}{*}{Network traffic} \\  & integrated network & & \\ \hline \multirow{3}{*}{[49]} & DF-based terrestrial relay HSTN & \multirow{3}{*}{\begin{tabular}{c} MGF-based ASER analytical \\ expressions at high SNR \\ \end{tabular} } & \multirow{3}{*}{CCI} \\  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \cline{1-1}  & & & \\ \hline \multirow{3}{*}{[50]} & Three-node AF terrestrial & \multirow{3}{*}{ASER performance} & Path loss, multipath fading, and \\  & relay-based HSTN & & single and multiple CCI at relay \\ \hline \multirow{3}{*}{[51]} & Three-node distributed space–time coded & \multirow{3}{*}{\begin{tabular}{c} ASER performance \\ \end{tabular} } & \multirow{3}{*}{Fading} \\  & AF–terrestrial relay-based HSTN & & \\ \cline{1-1}  & & & \\ \hline \multirow{3}{*}{[52]} & Three-node AF hybrid satellite–terrestrial & \multirow{3}{*}{\begin{tabular}{c} ASER and diversity-order \\ calculations \\ \end{tabular} } & \multirow{3}{*}{Unavailable CSI} \\  & cooperative system with estimated & & \\ \cline{1-1}  & channel gains & & \\ \hline \multirow{3}{*}{[53]} & DF–terrestrial relay-based HSTN & \multirow{3}{*}{OP analysis} & Rain attenuation \\ \cline{1-1}  & Channel characterization for space/air & \multirow{3}{*}{\begin{tabular}{c} Average capacity and OP \\ system parameters \\ \end{tabular} } & \multirow{3}{*}{\begin{tabular}{c} Service continuity, ubiquity, and \\ scalability \\ \end{tabular} } \\  & & \\ \cline{1-1}  & & \\ \cline{1-1}  & & \\ \cline{1-1}  & & \\ \cline{1-1}  & & \\ \hline \multirow{3}{*}{[45]} & Multicast transmission scenario in & \multirow{3}{*}{\begin{tabular}{c} Maximum sum rate of all users \\ \end{tabular} } & \multirow{3}{*}{QoS requirements} \\  & space–air–ground integrated network & & \\ \cline{1-1}  & & \\ \hline \multirow{3}{*}{[42]} & UAV integration with existing MCNs & \multirow{3}{*}{\
\begin{tabular}{c} Minimum ergodic achievable rate \\ bits/\(\mathrm{s}/\mathrm{Hz}\) \\ \end{tabular} } & Coordination issues and limited \\ \cline{1-1}  & & & \\ \hline \multirow{3}{*}{[43]} & Hierarchical satellite–UAV–terrestrial MCN & Energy consumption & Slowly varying large-scale CSI \\ \cline{1-1}  & & & \\ \cline{1-1}  & & \\ \cline{1-1}  & & & \\ \hline \multirow{3}{*}{[44]} & Hybrid maritime network consisting of & \multirow{3}{*}{
\begin{tabular}{c} Minimum ergodic achievable rate \\ bits/\(\mathrm{s}/\mathrm{Hz}\) \\ \end{tabular} } & \multirow{3}{*}{Available CSI} \\  & mobile users (ships), UAVs, terrestrial BSs, & & \\ \cline{1-1}  & and satellites & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: List of major contributions to coordinated HSTN networks.