# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 4 Review
### Cooperative Communication  
Cooperative communication offers a promising approach to enhancing PLS in ISTNs for 6G networks [47]. While PLS leverages the inherent randomness of the wireless channel for security, it faces certain limitations in complex ISTN environments:  
* Uncertainty in Channel Characteristics: The diverse nature of terrestrial and satellite links within ISTNs can lead to significant variations in channel characteristics like fading and noise. This makes it challenging to exploit these variations for optimal security using traditional PLS techniques [90]. * Limited Range of Terrestrial Networks: Terrestrial base stations may not always have a direct line of sight to users, especially in remote areas. This can weaken the signal strength and limit the effectiveness of PLS [91]. By exploiting the combined channel variations across different paths in the network, cooperative communication creates a more complex and unpredictable environment for potential eavesdroppers [92]. Cooperative communication strategies can address these limitations by enabling collaboration among different network elements to establish a more robust and secure communication channel. The potential applications are as follows:  
* Relaying: Nodes (relays) strategically positioned within the network can receive signals from the source and retransmit them toward the destination. This can help \begin{table}
\begin{tabular}{l l l} \hline \hline
**Reference** & **Optimization Variable** & **Constraints** \\ \hline
[83] & Signal-to-noise ratio & Maximum transmit power and secrecy rate \\
[84] & Achievable sum rate & Intercept probability and transmit power \\
[85] & Total transmit power & QoS requirement and secrecy rate \\
[86] & Secrecy outage probability & \\
[87] & Power consumption & Secrecy outage probability \\
[88] & Optimal multibeam design & Energy-efficient allocation \\
[89] & Weighted sum of capacity and & \\  & minimum capacity-to-demand ratio & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: References using beamforming to improve the PLS of the communication network. overcome signal weaknesses from long distances or obstructions in terrestrial links, improving the overall channel quality and strengthening PLS mechanisms. Relay nodes can extend the reach of terrestrial networks, enabling secure communication even in remote areas where relying solely on terrestrial links might be challenging for PLS [93; 94; 95]. Such a scenario is depicted in Figure 5.
* Joint Signal Processing: Network elements can collaborate to jointly process and analyze the received signals. By combining information from multiple sources, they can gain a more accurate understanding of the channel characteristics and exploit them more effectively for security purposes. Relaying and joint processing techniques can enhance signal strength and mitigate channel fading, leading to a more reliable and secure communication channel [94; 96]. * Diversity Techniques: Cooperative communication allows for the creation of spatial diversity by utilizing multiple transmission paths. This makes it harder for attackers to exploit specific channel weaknesses and enhances the overall security of the communication link. In [97], the authors utilized relay nodes to help forward information from the satellite to the weak user, who then uses maximum ratio combining to boost signal strength. Two relay protocols, i.e., decode-and-forward (DF) and amplify-and-forward (AF), were investigated. They derived closed-form and asymptotic expressions for the outage probability and analyzed the diversity order. The results show that DF outperforms AF in outage probability, though with a higher computational load. In [98], the authors explore using LEO satellites as jammers to enhance PLS. An iterative scheme is proposed to maximize system secrecy energy efficiency (SEE) by optimizing transmit power allocation and user association.