# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 3 Existing Surveys  
Recently, many excellent survey, review, and tutorial papers have been published regarding PLS. In [40], the authors showed how PLS can be utilized to achieve security goals such as confidentiality, integrity, authentication, and availability. It provides a background on cryptography and network security. Further, it explores hardware-based and statistical methods for user authentication, with a particular focus on techniques that leverage localization-based authentication and physical unclonable functions (PUFs). In [41], various challenges in the PLS of ISTNs is discussed in detail. The study shows a solution to the correlated-channel problem by using a dual-beam, dual-frequency transmission scheme and a near-ground, co-frequency, co-time, cooperative interference relay network. For the co-channel interference problem, the article provides solutions such as adaptive and cooperative beamforming, artificial noise, and multi-satellite scheduling. For the multi-user multi-eavesdropper scenario, they suggest relay and user selection. In [43], a survey of space-air-ground-sea integrated networks integrating satellite, aerial, terrestrial, and maritime networks was conducted. The article discusses the characteristics and potential challenges. A detailed discussion on security threats, attacks, and countermeasures is provided. Possible solutions, such as anti-jamming, secure routing, secured key management, handover schemes, and intrusion detection, are described. Future challenges, e.g., cross-layer attacks and countermeasures in such networks, are presented. In [44], the article examines current security challenges in satellite-terrestrial communication systems, proposes potential solutions, and suggests research directions to address existing security gaps. It provides an overview of the security landscape in ISTNs, highlighting key security challenges from three perspectives: satellite-to-satellite communications, satellite-to-ground station communications, and satellite-to-ground user equipment (UE) communications. It discusses potential solutions to these identified security challenges and identifies existing research gaps. Additionally, it offers insights into new technologies that could enhance security in these integrated networks. In [45], the study presents a comprehensive survey of satellite communications with a focus on PLS, providing background information and discussing challenges in the emerging integrated network architecture. It reviews popular satellite channel models and secrecy performance metrics, and categorizes state-of-the-art PLS research across different architectures: land mobile satellite communication networks, hybrid satellite-terrestrial relay networks, and satellite-terrestrial integrated networks. The article also identifies open research problems for future exploration. In [46], the authors provide a comprehensive survey of the convergence of satellite and terrestrial networks, identifying motivations and requirements, summarizing related architectures, classifying research taxonomy, and presenting performance evaluations. It also reviews the state of the art in standardization, projects, and key application areas of satellite-terrestrial networks, concluding with open issues and future directions. In [47], the authors discuss the potential of ISTNs to extend network coverage and reduce reliance on terrestrial infrastructure, which is key for 6G communication. However, challenges remain in areas such as architecture, air interface protocols, mobility management, and experimental validation. The article outlines four stages of ISTN architecture development and explores key technical enhancements. It also details an experiment using LEO satellites to link two 5G networks, demonstrating low latency and high data rates, showing the ISTN's potential for industrial applications. Future trends include programmable satellites and AI-driven networking.