# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 4 Review
### Cooperative Communication
Extensive experiments demonstrate that the optimization scheme significantly improves SEE and determines optimal power allocation and user association strategies. In [99], the authors study secure communication in a satellite-terrestrial network overlaid on a legacy cellular network using mm-wave frequencies. The aim was to maximize the secrecy rate of an eavesdropped satellite service by designing a cooperative secure transmission beamforming scheme involving adaptive beamforming, artificial noise, and cooperative beamforming by base stations (BSs). A non-cooperative beamforming scheme was also considered. The secrecy rate maximization problem was addressed with an approximation and iteration-based genetic algorithm. Simulations show that multiple antenna arrays and cooperative beamforming significantly improve secure communication and validate the algorithm's efficiency. In [100], the authors study the security and reliability of a satellite-terrestrial network, where a satellite sends confidential information to a ground user via multiple relays, with an eavesdropper trying to intercept. A friendly jammer enhances security. Using generalized Rician fading channels and imperfect CSI, the study derives closed-form expressions for outage and intercept probabilities with  
Figure 5: This diagram illustrates a cooperative communication scenario where a relay node (R) is used to enhance the signal strength between the sender ([PERSON]) and the receiver ([PERSON]). [PERSON] transmits her signal directly to [PERSON] but also indirectly to the relay node. The relay node then amplifies and retransmits the received signal to [PERSON]. This indirect path helps to overcome obstacles or poor channel conditions between [PERSON] and [PERSON], resulting in improved overall signal quality and reliability. amplify-and-forward relaying. Monte Carlo simulations validate the results, examining the effects of channel estimation errors, transmit power, number of relays, and fading severity. In [101], the authors propose an adaptive power-bandwidth cooperative scheme to combat eavesdropping on LEO satellite links communicating with ground stations when drones are present in the line of sight between the satellite and ground station. The algorithm dynamically adjusts resources, i.e., power and bandwidth, to maximize the normalized secrecy capacity while maintaining a reasonable SNR at the legitimate receiver, using a nearby terrestrial party for signal amplification. Simulations show a significant performance improvement of over 70% compared to non-adaptive methods across various elevation angles. We summarize the works that have used cooperative communication to enhance the PLS of the ISTNs in Table 4. Overall, cooperative communication offers a promising approach to enhancing PLS in ISTNs for 6G networks. While PLS leverages the inherent randomness of the wireless channel for security, it faces certain limitations in complex ISTN environments, such as uncertainty in channel characteristics and the limited range of terrestrial networks. Cooperative communication can address these limitations by enabling collaboration among different network elements, establishing a more robust and secure communication channel. Through techniques like relaying, joint signal processing, and diversity techniques, cooperative communication can improve signal strength, mitigate channel fading, and create a more unpredictable environment for potential eavesdroppers. This can enhance the overall security of the communication link and make it more difficult for attackers to intercept or disrupt data transmission. While cooperative communication offers significant potential, its implementation can add complexity to network management. Further research is needed to develop efficient resource allocation algorithms, ensure synchronization and coordination among network elements, and address challenges related to data exchange.