# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 4 Review
### Resource Allocation
The SNR is taken to be 5, 10, and 15 dB in this case; however, even lower SNRs can be considered, but then we have to assign more bits to error-correcting codes to receive messages with high fidelity. Figure 2: The power adaptive strategy for a LEO satellite communicating with a ground station at low SNRs of 5, 10, and 15 dB. This strategy is designed to mitigate eavesdropping attempts by an adversary (Eve) by dynamically adjusting the transmission power to optimize communication reliability while minimizing signal strength detectable by potential eavesdroppers. In [67], a support vector machine-based algorithm is presented for the detection of spectrum occupancy, and a convolutional neural network is proposed for spectrum prediction. The proposed system utilizes an intelligent resource management strategy that incorporates spectrum sensing, prediction, and allocation. This approach prioritizes user needs and optimizes spectrum allocation, resulting in lower error rates and improved spectrum efficiency. In [68], a dual-beam dual-frequency scheme is used to extend the difference between the main and wiretap channels. By limiting the satellite power and the user's QoS, the scheme is solved using convex semidefinite programming, which was shown to improve the secrecy capacity even when the eavesdropper is close to the legitimate user. In [69], PLS in satellite communication was studied under conditions of similar legitimate and eavesdropper channels. Using relay selection and power allocation, the secrecy outage probability is optimized. The relay selection is made based on the optimal choice of relays that the satellite has access to. Artificial noise is sent toward the eavesdropper to reduce the quality of the signal. In [70], a power-efficient beamforming scheme was studied for a cognitive satellite-terrestrial network. An optimal beamforming scheme was proposed to minimize the transmit power of the terrestrial base station while maintaining the secrecy rate of the satellite user and the communication rate of the terrestrial user. In [71], stochastic geometry was used to study the secrecy performance of a satellite-terrestrial network with random locations for one legitimate user and one eavesdropper. A closed-form expression of secrecy outage probability was derived, and maximum ratio combining achieved better results. In [72], the authors focused on designing PLS for next-generation LEO satellite links. The scenario involves an eavesdropper at a certain distance from the legitimate ground station (Bob) communicating with a LEO satellite (Alice). By exploiting the unique characteristics of the THz atmospheric channel, the study derived the optimal power spectral density (PSD) for the transmitted signal to maximize the normalized secrecy channel capacity. Using the higher losses from the atmospheric channel and the optimal PSD, the PLS of the satellite link was improved. In [73], a method to enhance cybersecurity for LEO satellite links operating at sub-THz frequency bands is presented. The short wavelength at these frequencies allows for highly directional antenna beams with small ground spots, making eavesdropping challenging. Further security is achieved by tuning the satellite transmitter power close to the noise floor and communicating within specific angles of arrival. This approach results in a lower SNR at the ground station receiver, increasing the likelihood of eavesdropper (Eve) outages. To improve Bob's SNR, an optimization algorithm is derived, which adapts the inter-element spacing of phased array elements to shape the gain pattern and minimize the required transmitter power for a given SNR. By judiciously distributing resources such as bandwidth, power, and computational capabilities, network operators can enhance security, reduce power consumption, and improve the overall network performance. Dynamic resource allocation strategies, which adapt to changing network conditions and user demands, are particularly effective in ensuring the efficient utilization of resources.