# An Overview of Key Technologies in Physical Layer Security
## 3 Secure Multi-Antenna Techniques
### Artificial Noise (AN) Precoding  
The notion of using artificial noise to enhance security in the physical layer was first proposed in [70]. They identified AN-based transmission as an effective technique that can be deployed in PLS to ensure secure communication in wireless networks. The technique involves deliberately degrading the quality of the channel of the eavesdropper by generating an interference signal, which is used to interrupt their eavesdropping capabilities. In the AN precoding scheme, the transmitter Alice divides the transmission power between transmitting the information to the intended recipient, [PERSON], and transmitting the noise signal towards the eavesdropper, [PERSON], is shown in Figure 13.  
Generating AN depends on the transmitter's knowledge of the eavesdroppers' channel state information. In a case where the eavesdropper's CSI is unknown, the isotropic AN is generated. The generated AN is designed such that it lies in the nullspace of the intended receiver and directed in the range space of the unintended receivers. This is done to cancel out its effect at the intended receiver such that only the eavesdropper's channel is degraded [70]. Another form of AN generation is called spatially selective AN, which is applicable in the event that the transmitter knows the the eavesdropper's CSI [71]. The AN generation technique's major strength is that the provided secrecy scales well with the SNR since an increase in SNR at Eve will increase the received AN power along with the message power. The conventional AN scheme can be represented in general as follows:  
\[\mathbf{x}=\mathbf{w}s_{a}+\mathbf{v}s_{j}, \tag{22}\]  
where \(\mathbf{x}\) is the signal transmitted by [PERSON]. The source information is denoted by \(s_{a}\) and \(s_{j}\) denotes the AN jamming signal, which is chosen to be independent of the source information, i.e., \(s_{a}\
eq s_{j}\). Beamforming vectors for the information and jamming signals are represented by \(\mathbf{w}\) and \(\mathbf{v}\), respectively. Therefore, the signals received by [PERSON] and [PERSON] are given by Equations (23) and (24), respectively. \[\mathbf{y}_{B}=\mathbf{h}_{B}^{H}\mathbf{w}\mathbf{s}_{a}+\mathbf{g}_{B}^{H} \mathbf{v}s_{j}+n_{B} \tag{23}\]  
\[\mathbf{y}_{E}=\mathbf{h}_{E}^{H}\mathbf{w}\mathbf{s}_{a}+\mathbf{g}_{E}^{H} \mathbf{v}s_{j}+n_{E} \tag{24}\]  
Figure 13: Secrecy beamforming with artificial noise (AN). where \(\mathbf{h}_{B}^{H}\) and \(\mathbf{g}_{E}^{H}\) denote the channel responses of [PERSON] and [PERSON], respectively. The independent, identically distributed complex Gaussian noise for [PERSON] and [PERSON] with zero mean and variance \(\sigma^{2}\) are denoted by \(n_{B}\) and \(n_{E}\), respectively. The corresponding secrecy rate is given by  
\[R=\log_{2}\left(1+\frac{|\mathbf{h}_{B}^{H}\mathbf{w}|^{2}}{|\mathbf{g}_{B}^{H} \mathbf{v}|^{2}+\sigma^{2}}\right)-\log_{2}\left(1+\frac{|\mathbf{h}_{E}^{H} \mathbf{v}|^{2}}{|\mathbf{g}_{E}^{H}\mathbf{v}|^{2}+\sigma^{2}}\right) \tag{25}\]  
A novel approach of the AN scheme that offers better secrecy rate performance, where the jamming signal is generated to be dependent on the information signal, was presented in [72]. The scheme significantly improved the signal strength at Bob and, at the same time, canceled the received signal at the eavesdropper.