# Hybrid Satellite-Terrestrial Networks toward 6G: Key Technologies and Open Issues
## 3 Cognitive HSTN
### HI and Interference
Cognitive HSTNs have been proposed due to their low cost, advanced network topology, and low complexity. However, cognitive HSTNs need continuous improvement in security performance, as there are more chances for attackers in cognitive radio technology. The data may be eavesdropped or altered without notice, and channels may be jammed or overused by adversaries. Major contributions to cognitive HSTNs are summarized in Table 5. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **System Model** & **Performance Metrics** & **Other Factors** \\ \hline \multirow{3}{*}{[104]} & Coexisting satellite and terrestrial & & Limited information exchange \\  & communication system with distributed & Average achievable rate & between satellite and \\  & base stations & & terrestrial network \\ \hline \multirow{3}{*}{[105]} & Overlay-spectrum-sharing cognitive HSTN & & \\  & with primary satellite and secondary & OP analysis & Multiple secondary networks \\  & terrestrial network & & within same spectrum and SNR \\ \hline \multirow{3}{*}{[107]} & EH-based overlay-cognitive HSTN & & \\  & spectrum-sharing system with primary & & \\  & satellite and secondary terrestrial network & & \\ \hline \multirow{3}{*}{[107]} & Underlay-cognitive HSTN with primary & & \\  & terrestrial and secondary satellite network & & \\  & & & \\ \hline \multirow{3}{*}{[115]} & Underlay-cognitive HSTN with primary & & \\  & terrestrial and secondary satellite network & & \\ \hline \multirow{3}{*}{[112]} & Uplink-underlay-cognitive HSTN & & \\  & consisting of one primary terrestrial & & \\  & network and one secondary & & \\  & satellite network & & \\ \hline \multirow{3}{*}{[118]} & Cognitive HSTN with FSS primary and & & \\  & terrestrial secondary network & & \\ \hline \multirow{3}{*}{[119]} & Underlay-cognitive LEO & & \\  & constellation HSTN & & \\  & & & \\ \hline \multirow{3}{*}{[143]} & Multiple-antenna cognitive HSTN with & & \\  & terrestrial secondary and satellite & & \\  & primary network & & \\ \hline \multirow{3}{*}{[130]} & Cognitive broadband HSTN with primary & & \\  & FSS and secondary terrestrial network & & \\ \hline \multirow{3}{*}{[145]} & UAV-aided cognitive HSTN with primary & & \\  & satellite and secondary terrestrial network & & \\ \hline \multirow{3}{*}{[102]} & IoT-enabled overlay-cognitive HSTN & & \\  & & & \\ \hline \multirow{3}{*}{[135]} & Cognitive HSTN relay network with & & \\  & multiple-user primary terrestrial network & & \\  & & & \\ \hline \multirow{3}{*}{[114]} & Underlay-cognitive & Energy efficiency and spectral & \\  & satellite–vehicular network & efficiency & \\ \hline \multirow{3}{*}{[147]} & & & UAV deployment, physical-layer \\  & Cognitive satellite–aerial–terrestrial & & security, interference \\ \cline{1-1}  & network & Energy efficiency & management, cooperative \\ \cline{1-1}  & & & beamforming, and \\ \cline{1-1}  & Cognitive broadband satellite system and & & non-cooperative beamforming \\ \hline \multirow{3}{*}{[113]} & Underlay-cognitive & & \\  & satellite system and & & \\ \cline{1-1}  & & & \\ \hline \multirow{3}{*}{[114]} & Underlay-cognitive & & \\  & satellite–vehicular network & & \\ \hline \multirow{3}{*}{[147]} & Cognitive satellite–aerial–terrestrial & & \\  & network & & \\ \cline{1-1}  & & & \\ \hline \multirow{3}{*}{[147]} & Cognitive broadband satellite system and & & \\ \cline{1-1}  & mmWave cellular network & & \\ \hline \multirow{3}{*}{[113]} & Integrated sensing-based cognitive satellite & & \\ \cline{1-1}  & terrestrial network & & \\ \hline \end{tabular}
\end{table} 
Table 5: List of major contributions to cognitive HSTN networks.