# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 4 Review
### Resource Allocation  
Resource allocation can play a crucial role in both enhancing security and reducing power consumption in ISTNs [59]:  
* Dynamic Resource Allocation: ISTNs are inherently heterogeneous, with terrestrial links offering high bandwidth and satellite links providing wider coverage. By dynamically allocating resources based on the type of data and security requirements, network operators can prioritize security for sensitive information [60]. For instance, critical data can be routed through terrestrial links with stronger encryption, while less sensitive data can utilize satellite links with lighter security measures. * Cognitive Radio Techniques: Cognitive radios can intelligently sense the network environment and adapt resource allocation to optimize security. They can identify and exploit weaknesses in potential eavesdropper channels, thereby enhancing the overall security posture of the ISTN [41; 61]. * Energy-Efficient Resource Allocation: Resource allocation strategies can be designed to prioritize energy-efficient components of the network. For example, by offloading traffic to terrestrial links when possible and utilizing satellites only when necessary, network operators can reduce the reliance on power-hungry satellite communication [62]. * Resource Sharing and Power Control: The efficient sharing of resources between terrestrial and satellite networks can lead to significant power savings. Additionally, power control techniques can be employed to adjust transmission power based on traffic demands and channel conditions, further reducing energy consumption [63]. While each method offers potential benefits, it also comes with specific drawbacks. The choice of approach to adopt will depend on the specific context and the importance of different factors:  
* Dynamic Resource Allocation: Implementing dynamic resource allocation requires sophisticated algorithms and real-time monitoring of network conditions. Frequent resource reallocation might introduce additional latency, especially for time-sensitive applications. * Cognitive Radio Techniques: Developing and deploying cognitive radio systems can be technically challenging and expensive. Ensuring that cognitive radios do not interfere with other licensed users is a critical consideration. * Energy-Efficient Resource Allocation: Prioritizing energy efficiency might sometimes lead to lower network performance, especially in areas with limited terrestrial infrastructure. Determining the optimal balance between energy efficiency and performance requires careful consideration of economic factors. * Resource Sharing and Power Control: Coordinating resource sharing and power control between terrestrial and satellite networks can be complex. Ensuring that shared resources do not interfere with each other is a critical challenge. All of the proposed resource allocation techniques have the potential to significantly improve the security, performance, and energy efficiency of ISTNs. However, the optimal choice of techniques will depend on specific network requirements, technological capabilities, and economic constraints. A combination of these techniques, tailored to the unique characteristics of the ISTN, is likely to yield the best results. In Figure 2, we show a power-adaptive scheme for a LEO satellite. The parameters are considered using Telesat data [64]. As the elevation angle of LEO changes due to its movement, the power adapts based on the channel condition and elevation angle and the required SNR to be maintained. By adapting power to different elevation angles, the LEO satellite can maintain a very low SNR at the ground station, making it difficult for an eavesdropper. As the SNR remains low, the receiver will receive the message with a high bit-error rate. The simulation parameters are shown in Table 1. The gaseous attenuation has been calculated as per ITU recommendations [65]. The atmospheric channel is taken as the standard atmosphere model [66].