# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 5 Future Directions
### Emerging Techniques  
Another key area of focus is the integration of Quantum Key Distribution (QKD), which represents a groundbreaking advancement in secure communication [111; 112; 113]. QKD leverages the principles of quantum mechanics, where quantum bits (qubits) are used to transmit encryption keys [114]. The unique property of quantum states is that they cannot be observed or measured without altering their state, which means any eavesdropping attempt would be immediately detectable by the communicating parties. This makes QKD theoretically immune to many of the vulnerabilities that affect classical encryption methods [115]. However, implementing QKD in satellite communications presents significant challenges, particularly over long distances, where maintaining quantum coherence is difficult due to atmospheric interference and the vast distances involved in space. Research is therefore focused on developing robust satellite-based QKD systems that can maintain quantum states over long distances, exploring methods such as free-space optics and satellite relays to extend the range and reliability of QKD [116]. Dynamic spectrum access and cognitive radios are another area of active research, with the goal of enabling satellite communication systems to dynamically adjust their spectrum usage in response to threats or interference [117]. Cognitive radios are designed to sense their electromagnetic environment and intelligently select the best frequency bands to use, thereby avoiding congestion and potential attacks [118]. This adaptability is particularly important in contested environments where spectrum access may be limited or under threat from adversaries. Anti-jamming and anti-spoofing techniques are being further developed to protect satellite communications from deliberate disruptions. Spread spectrum techniques, such as frequency hopping and direct-sequence spread spectrum methods [119; 120], are being refined to make it more difficult for adversaries to jam or intercept signals. These techniques, combined with advanced encryption methods, provide a robust defense against a wide range of attacks. Additionally, machine learning algorithms are being employed to detect and respond to jamming and spoofing attempts in real time, allowing for rapid adaptation to emerging threats [121; 122]. Secure satellite network architectures are being explored to ensure that satellite communications are inherently secure at the physical layer. This includes the development of decentralized network designs that reduce the risk of single points of failure and the integration of blockchain technology to ensure the integrity and confidentiality of data transmitted across satellite networks. Blockchain can provide a transparent and tamper-resistant record of transactions and data exchanges, which is particularly valuable in ensuring the security of communication in a decentralized network [123; 124]. Resilient satellite communication protocols are also being developed to maintain operation even when the system is under attack. These protocols incorporate features such as error correction, redundancy, and secure routing [125] to ensure that communications can continue even in the presence of physical layer attacks [126]. By incorporating these features into the design of communication protocols, researchers aim to create systems that can degrade gracefully and continue to function in a compromised environment. The integration of AI and machine learning into satellite communication systems is increasingly seen as vital for enhancing security. AI and machine learning can be used to detect, classify, and mitigate physical layer attacks by analyzing patterns in communication signals and identifying anomalies that may indicate an attack. These technologies can also be used to optimize system performance and resource allocation in real time, further enhancing the resilience of satellite communications [127; 128].