# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 4 Review
### Physical Characteristics  
The physical characteristics of the communication channels themselves offer unique advantages for achieving PLS [102]. Such characteristics can be exploited to secure communication: \begin{table}
\begin{tabular}{l l l} \hline \hline
**Reference** & **Optimization Variable** & **Constraint** \\ \hline
[97] & Outage probability & \\
[98] & Secrecy energy efficiency & Transmit power and secrecy rate \\
[99] & Secrecy rate & Power and transmission quality \\
[100] & Outage probability & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: References using cooperative communication to improve the PLS of the communication network. * Fast Fading: Terrestrial links experience rapid changes in signal strength and phase due to multipath propagation. This inherent randomness can be utilized for PLS by embedding secret information into the fading patterns. An eavesdropper without knowledge of these patterns would struggle to decipher the transmitted data [103; 104]. * Slow Fading: Slower fading, often observed in satellite links due to longer propagation delays, can be used for key generation. By exploiting the time-varying nature of the fading channel, a shared secret key can be established between legitimate communicators without explicitly exchanging any information [105]. * Background Noise: The inherent noise present in all communication channels can be leveraged for security purposes. By adding carefully designed noise to the transmitted signal, it becomes difficult for eavesdroppers to distinguish the actual data from the background noise, enhancing the secrecy of the communication [106]. * Forward and Reverse Links: Terrestrial and satellite links often exhibit different channel characteristics in the forward (source to destination) and reverse (destination to source) directions. This asymmetry can be exploited to create a \"secrecy code\" embedded in the channel itself. Legitimate communicators with knowledge of this code can successfully decode the message, while eavesdroppers without this knowledge would be unable to do so. * Terrestrial vs. Satellite Links: The inherent differences between terrestrial and satellite links can be a valuable asset for PLS. By strategically exploiting the unique fading and noise characteristics of each type of link, communication networks can create a more complex and unpredictable environment for potential attackers. In [107], the authors propose physical layer authentication (PLA) for LEO satellites using Doppler frequency shift (DS) and received power (RP) characteristics. Hypothesis testing via thresholds or machine learning (ML) with a one-class classification support vector machine discriminates between legitimate and illegitimate satellites. Using real satellite data from the system tool kit, the results show that DS achieves higher authentication rates at low elevation angles, while RP is better at high angles. A machine learning algorithm provides a higher authentication rate than the threshold scheme with minimal outliers, but the threshold scheme excels with more outliers. In [103], the authors propose an advanced signal processing algorithm for authenticating satellite signals received at ground stations. This algorithm leverages the spatio-temporal characteristics of signals as they traverse atmospheric paths, enabling differentiation between satellites. Simulation studies at a ground station in Beer Sheva, Israel, using the GEO satellites AMOS-17 and AsiaSat-7, demonstrate an authentication rate exceeding 95%. By enhancing PLS, the algorithm safeguards satellite and terrestrial communication systems, ensuring robust and secure global services. In [108], the proposed method capitalizes on the unique physical characteristics of electromagnetic signal propagation through stochastic atmospheric channels. This approach aims to safeguard ground stations from machine-in-the-middle (MITM) attacks launched by aerial platforms (APs).