# An Overview of Key Technologies in Physical Layer Security
## 2 Background  
### Concept and Evolution of PLS  
PLS dates back to the 1970s with a mathematical description of a wiretap channel [26]. Following the advancements in massive MIMO and integration of technologies such as IEEE 802.11n and long-term evolution (LTE), there has been major interest in PLS research over the last decade. The current research is focused towards exploiting MIMO spatial degrees of freedom in order to leverage security benefits [33]. A typical network in which PLS is employed comprises three nodes: a transmitter, a legitimate receiver, and an eavesdropper. With this setup, the transmitter under normal circumstances sends a confidential message to the receiver. The sent signal is protected from any form of interception by the eavesdropper [29]. The adopted general convention is that the transmitter is referred to as \"[PERSON]\", whilst the eavesdropper and the receiver are referred to as \"[PERSON]\" and \"[PERSON]\", respectively. This model is illustrated in Figure 4.  
[PERSON] Motivation of PLS  
It was shown in Section 1 that the inception of technologies such as quantum computing poses a major threat in existing security techniques in wireless networks, which means that wireless communication is not completely secure. PLS offers an additional solution to exploit the secrecy possibilities that the wireless channel offers. The main motivations behind PLS are (1) to find an alternative means to supplement the existing security measures in wireless networks which are based on [PERSON] algorithms, (2) employment of the physical layer of the network to improve security through enhancements of security methods implemented at the upper layer of the protocol stack, and (3) to find and develop new security technologies compatible with recent developments of the 5G and 6G wireless networks as well as MIMO communication. [PERSON] Information-Theoretic Security  
This is the kind of security that purely defines the fundamental limits of PLS measures from an information theory point of view. It was proposed by [PERSON] in 1949 [34]. [PERSON] defined the channel capacity as the maximum rate at which information can be transmitted over a communication channel with an arbitrarily low probability of error. For an additive white Gaussian noise (AWGN) channel, the channel capacity is given by  
\[C=Blog_{2}(1+\texttt{SNR}) \tag{1}\]  
Figure 4: Physical layer security (PLS) system model[PERSON]  
where \(B\) represents the channel's bandwidth (fixed quantity) in Hertz (Hz), and SNR is the signal-to-noise ratio. The SNR is mathematically denoted by Equation (2). From Equation (1), it can be seen that the channel capacity is directly proportional to the power of the signal. \[\texttt{SNR}=\frac{P}{\sigma^{2}}, \tag{2}\]  
where \(P\) denotes the power of the signal and \(\sigma^{2}\) is the noise power. The Shannon information content of an outcome, \(x_{i}\), is defined as  
\[h(x_{i})=\log_{2}\left(\frac{1}{p(x_{i})}\right)=-\log_{2}p(x_{i}) \tag{3}\]  
where the probability of the random variable \(X\) is denoted by \(p(x_{i})\). Figure 5 shows a graph of Shannon information content versus the different probabilities, and it illustrates that less probable or rare outcomes contain more information than common or highly probable outcomes. The entropy function \(\mathbf{H}(X)\) is the average of the Shannon information content and is given by  
\[H(X)=\sum_{i=1}^{M}p(x_{i})\log_{2}\left(\frac{1}{p(x_{i})}\right)=-\sum_{i=1} ^{M}p(x_{i})\log_{2}p(x_{i}) \tag{4}\]  
where \(M\) is the total number of possible outcomes. The mutual information, \(I(X;Y)\), defines the amount of information \(X\) conveys about \(Y\):  
\[I(X;Y)=H(X)-H(X\mid Y) \tag{5}\]  
Therefore, this implies the amount of information sent to [PERSON] by [PERSON] is given by Equation (6), whereas the same quantity between [PERSON] and [PERSON] is given by Equation (7). \[I(A;B)=H(A)-H(A\mid B), \tag{6}\]  
\[I(A;E)=H(A)-H(A\mid E), \tag{7}\]  
where random variable \(A\) represents the bit sent by [PERSON].