# An Overview of Key Technologies in Physical Layer Security
## 3 Secure Multi-Antenna Techniques
### Cooperative Jamming  
Cooperative jamming is one of the techniques proposed to implement security in the physical layer to curb eavesdropping in wireless networks. A cooperative jamming network comprises the source that transmits its message to the intended receiver and a relay node that transmits a jamming signal to degrade the eavesdropper's channel and improve the secrecy rate. Cooperative jamming was derived from the conventional technique for user cooperation known as cooperative relaying. However, cooperative relaying is distinctive from cooperative jamming because it improves the security by enhancing channel quality between the transmitter and legitimate receiver. Some of the examples of cooperative relaying are decode-and-forward (DF) and amplify-and-forward (AF) schemes. A system model of a typical cooperative jamming scheme is shown in Figure 14, which consists of the transmitter [PERSON], a single trusted relay, an intended receiver [PERSON], and and eavesdropper [PERSON]. [PERSON] transmits a message signal \(s\) using a transmit power \(PA\), and the relay transmits a jamming signal \(z\) with a weighting vector \(\mathbf{w}\), simultaneously.  
The signal received by [PERSON] and [PERSON] is given by Equations (29) and (30), respectively  
\[y_{B}=\sqrt{P_{A}}\mathbf{h}_{AB}s+\mathbf{w}^{H}\mathbf{h}_{RB}z+n_{B} \tag{29}\]  
\[y_{E}=\sqrt{PA}\mathbf{h}_{AE}s+\mathbf{w}^{H}\mathbf{h}_{RE}z+n_{E} \tag{30}\]  
where \(\mathbf{h}_{AB}\) is the Alice-Bob channel, \(\mathbf{h}_{RB}\) is the relay-Bob channel, \(\mathbf{h}_{AB}\) is the Alice-Bob channel, \(\mathbf{h}_{AB}\) is the Alice-Bob channel, \(\mathbf{h}_{RE}\) is the relay-Eve channel, \(\mathbf{h}_{AE}\) is the Alice-Eve channel, and \(\mathbf{n}_{B}\) and are the AWG noise with variance \(\sigma^{2}\) at both Bob and Eve, respectively. The corresponding secrecy rate is given by  
\[R=\log_{2}\left(1+\frac{P_{A}|\mathbf{h}_{AB}|^{2}}{|\mathbf{w}^{H}\mathbf{h}_{RB }|^{2}+\sigma^{2}}\right)-\log_{2}\left(1+\frac{P_{A}|\mathbf{h}_{AE}|^{2}}{| \mathbf{w}^{H}\mathbf{h}_{RE}|^{2}+\sigma^{2}}\right) \tag{31}\]  
The problem of secrecy maximization has been extensively studied in the literature [76; 77; 78; 79; 80; 81; 82; 83]. Authors in [76; 77] investigated the problem of secrecy rate maximization of a secure wireless communication system in the presence of multiple eavesdroppers. [PERSON] et al. in [79] studied cooperative jamming for PLS enhancement in IoT, specifically considering a downlink transmission problem to tackle multiple passive and non-colluding eavesdroppers. The current research in cooperative jamming is focused towards integrating cooperative jamming with the current technologies in 5G and 6G, which can be found in [80; 81; 82; 83; 84]. In [80], they used joint cooperative jamming and secure channel training solutions to safeguard a two-user power domain non-orthogonal multiple access (NOMA) system against eavesdropping attacks coming simultaneously from inside and outside of the network. Another interesting study in [81] uses cooperative jamming to implement security for industrial wireless networks with mobile users and eavesdroppers. In the paper they employed an edge computing device to intelligently select an optimal cooperative node.