# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 2 Background  
Congestion at lower frequencies will require satellites to be designed at mm-wave bands. Such frequencies offer vast bandwidth, which is crucial for supporting high-data-rate services in satellite communications [8; 9]. Understanding propagation characteristics in the mm-wave band is essential for efficient satellite link design and beamforming. MIMO technology can significantly improve spectral efficiency and capacity in satellite systems, enabling higher data rates and supporting more users [10]. MIMO techniques can be used for advanced beamforming, allowing for precise antenna pattern control and improved link quality. They also provide diversity gains, which can mitigate the effects of fading and interference, especially in satellite channels [11]. In Release 15, 3 GPP laid the foundation for 5G networks, with the ITU defining some of the KPIs: (a) a more than 10 Gbps peak data rate for eMBB, (b) a connection density of 1 million/km\({}^{2}\) for MMTC, and (c) URLLC with a latency of less than 1 ms [12]. Extended LTE support was provided to aerial vehicles to support cellular connectivity [13; 14]. The release also laid the foundation for URLLC to satisfy the technical requirements of new applications, such as virtual reality, industrial automation, and intelligent transportation [15]. Release 16 supported industrial IoT and mission-critical communication [16; 17]. The surge in the popularity of IoT led to a massive increase in the number of IoT devices, which could extend beyond 75 billion by 2025. Such expansion led to enhanced URLLC in Release 16 [18]. Some of the notable enhancements compared to Release 15 are MIMO, improved beamforming, dynamic spectrum sharing, dual connectivity, carrier aggregation, and user equipment power saving [19]. The standardization process also encompassed V2X communication through sidelink communication, enabling direct communication between two user devices without the need for the involvement of a base station [20]. In Release 17, 3 GPP provided robust support for NTNs, encompassing satellites, high-altitude platforms, and unmanned aerial vehicles. With NTN capabilities, 3 GPP protocols for broadband (NR protocol) and massive IoT (NB-IoT and eMTC) can seamlessly extend to satellite communications. The release laid a strong foundation for establishing communication between satellites, smart phones, and other user equipment. The integration of TNs with NTNs will provide faster and more reliable eMBB to smartphones and IoT [21]. The added NTN capabilities will address the surging need for connectivity in remote and underserved regions where conventional terrestrial networks may be absent, such as deserts, oceans, and mountainous regions [22]. The satellite system consists of GEO, MEO, and LEO based at different altitudes. Release 18, which supports multi-RAT systems, was introduced to provide satellite and terrestrial RANs with their own dedicated radio resource management [23]. A 5G network slice instance is composed of a set of virtual network function (VNF) instances to form the end-to-end virtual network for the slice to operate independently [24]. In a network slice instance, both terrestrial and non-terrestrial RANs can be created by the network operator [25]. Satellites can be configured to operate in either the transparent mode or the regenerative mode, and the choice depends on the specific requirements of the communication system. In the transparent mode, they act as a simple relay, amplifying and forwarding the received signal without modifying or processing the data. In the regenerative mode, the satellite can demodulate the received signal, extract the data, and then remodulate and transmit it [6]. It will also allow for functionalities like error correction, thus improving signal quality. Three use cases were defined in 18: (a) channel state feedback information, (b) beam management, and (c) positioning [26].