# Review of Physical Layer Security in Integrated Satellite-Terrestrial Networks
## 4 Review
### Performance Parameters  
In this section, we discuss key performance indicators that are required to assess the security provided to a communication system using a PLS scheme. Such parameters include the following:* Secrecy Capacity (\(C_{s}\)): This metric measures the maximum rate at which confidential information can be transmitted reliably over a communication channel while keeping it secret from unauthorized users. It takes into account both the legitimate channel and the eavesdropper's channel. It is defined as [49] \[C_{s}=\ \log_{2}(1+\mathrm{SNR}_{\mathrm{Bob}})-\log_{2}(1+\mathrm{SNR}_{ \mathrm{Eve}})\] (1) where \(\mathrm{SNR}_{\mathrm{Bob}}\) and \(\mathrm{SNR}_{\mathrm{Eve}}\) denote the SNRs of Bob and Eve, respectively.
* Secrecy Outage Probability (\(P_{s}\)): The secrecy outage probability represents the probability that the secrecy capacity falls below a certain threshold. It indicates the likelihood of failing to maintain secure communication due to channel conditions. This metric is expressed as [50] \[P_{s}=P(C_{s}<R_{0})\] (2) where \(R_{0}\) is the threshold or target secrecy rate.
* Secrecy Diversity Gain (\(G_{s}\)): Secrecy diversity gain, also known as secrecy diversity order, measures the improvement in secrecy capacity achieved by employing multiple antennas or multiple transmit/receive paths. It quantifies the robustness of the system to eavesdropping attacks and is defined as [51] \[G_{s}=\lim_{\mathrm{SNR}_{\mathrm{Bob}}\rightarrow\infty}\  
where \(P\) is the total power consumption, and \(\Delta E\) is the amount of energy consumed over the duration \(\Delta T\).
* Probability of Nonzero Secrecy Capacity (\(P_{NZC}\)): This metric indicates the probability that the secrecy capacity is nonzero, meaning that secure communication is possible. It reflects the reliability of achieving secrecy in the system. The metric is defined as \[P(C_{s}>0)=\int_{0}^{\infty}\int_{0}^{\mathrm{SNR_{[PERSON]}}}p(\mathrm{SNR_{[PERSON]}}, \mathrm{SNR_{[PERSON]}})d\mathrm{SNR_{[PERSON]}}d\mathrm{SNR_{[PERSON]}}\] (8) where \(p(\mathrm{SNR_{[PERSON]}},\mathrm{SNR_{[PERSON]}})\) denotes the joint pdf of the SNRs of [PERSON] and [PERSON].
* Equivocation Rate: The equivocation rate quantifies the uncertainty an eavesdropper has about the actual message sent by [PERSON] to [PERSON] after it has been transmitted through a physical channel.
* Normalized Secrecy Capacity (\(\overline{C}\)): This parameter takes into account the relative strength of [PERSON]'s signal compared to [PERSON]'s signal and is defined as follows [57]: \[\overline{C}=\frac{\log_{2}(1+\mathrm{SNR_{[PERSON]}})-\log_{2}(1+\mathrm{SNR_{ [PERSON]}})}{\log_{2}(1+\mathrm{SNR_{[PERSON]}})}\] (9)
* Physical Layer Authentication Rate: While secrecy focuses on keeping information confidential, authentication ensures the legitimacy of the communicating parties. This metric assesses the effectiveness of PLS techniques in verifying the identity of the intended receiver, taking into account true positives (TP), true negatives (TN), false positives (FP), and false negatives (FN). It is defined as [58] \[\mathrm{AR}=\frac{\mathrm{TP}+\gamma\times\mathrm{TN}}{(\mathrm{TP}+\mathrm{ FN})+\gamma\times(\mathrm{TN}+\mathrm{FP})}\] (10) where \(\gamma\) is defined as \[\gamma=\frac{\mathrm{TP}+\mathrm{FN}}{\mathrm{TN}+\mathrm{FP}}\] (11)