# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 2 Background
In Release 19, 3 GPP continued with the advancement of 5G and introduced improvements in spectral efficiency and coverage across the sub-7 GHz and mm-wave spectrum [27]. It aimed to boost capacity in massive MIMO radio by enabling the sharing of the same time and frequency resources. A cost-effective transceiver would enable a distributed MIMO system. Additionally, 5G-Advanced would, for the first time, have one standard for TNs and satellites. With satellites, it focused on increasing downlink coverage. AI has now become imperative to improving the functionality and automation of communication networks. In 19, for both NR-NTNs and IoT-NTNs, it provided support for regenerative load and increased uplink capacity. Ambient IoT and extended boundless XR were considered [28]. With Apple and Samsung launching SOS services via iPhone and smartphones, respectively, the importance of satellites will keep growing in the near future [29; 30]. To identify its work on developing 6G specifications, 3 GPP unveiled a new logo [31]. The evolution to 6G presents many challenges in terms of reliability, availability, and responsiveness that are difficult to address alone by terrestrial cellular networks, especially in the case of natural disasters and terrorist attacks [32]. Satellites can provide (a) wider coverage, (b) backhaul links, and (c) edge computing. Moreover, 6G is expected to play a more holistic and human-centric role by providing services like human bond communication and tactile and holographic communication. Additionally, it will improve experiences over 5G, such as conventional mobile communication, indoor positioning, and mMTC for vertical industries [33]. As NTNs and TNs will be integrated to form a seamless, coherent network in 6G as shown in Figure 1, security challenges will be very complex in nature. The complexity of such a network opens many attack surfaces, for which a multi-layered approach would be preferable; i.e., the authentication scheme should be designed across the layer. Due to its unique characteristics, an NTN faces challenges due to its extended coverage, propagation environment, dynamic topology, and resource constraints [34; 35]. The vast coverage area of satellite networks increases the attack surface, making it difficult to monitor and protect all potential vulnerabilities. Satellite channels are susceptible to jamming, spoofing, and eavesdropping due to the open nature of the space environment. The constantly changing satellite constellation and user locations pose challenges for secure key management and authentication. Satellite platforms have limited power, computational resources, and storage capacity, which impact the feasibility of complex security algorithms. Strong authentication mechanisms are essential to verify the identity of users, devices, and network entities. This can be achieved through multi-factor authentication, digital certificates, and secure key management protocols. The unique identification and authentication of satellites are crucial for preventing spoofing and unauthorized access. Secure user authentication methods are required to protect user data and prevent unauthorized access to network resources. PLS can play a significant role in such a scenario [36]. PLS leverages the physical characteristics of a communication channel to ensure secure data transmission. Unlike traditional cryptographic methods that operate at higher layers of the network stack, PLS operates directly at the physical layer, exploiting the inherent properties of the communication medium. PLS exploits channel impairments such as noise, fading, and interference as a means to secure communication. Such properties make PLS an effective mechanism to address vulnerabilities introduced by the advent of quantum computing. While quantum computing poses a significant threat to traditional cryptographic algorithms based on computationally hard problems, it has a limited impact on PLS.