# Machine Learning for Physical Layer in 5G and beyond Wireless Networks: A Survey
## 4 Security and Privacy in 5G
A simple framework model for generating the secret key in full-duplex mode is presented by considering the nonreciprocity embedded in the channel dimension. The simulation results illustrated that the key rate following the full-duplex approach was higher than the half-duplex approach while considering different correlation coefficients. Several efforts have been performed for reducing channel correlation, and preprocessing techniques are implemented to achieve the desired results. Wireless technologies such as full-duplex communication that provide enhanced spectral efficiency can be utilized for 5G infrastructure because they have the potential to increase efficiency up to n-squared times, especially when a structure has a point-to-point link [312]. If deployed correctly, FD can also be used to reduce crosstalk and interference issues. On the other hand, it can become a factor in reducing potential gains in case of poor deployment. Thus, to reap the benefits of FD, it is essential to limit the crosstalk issue that lies between the receiver and transmitter. Furthermore, improved designs of TX and RX are significant to fully utilize the FD stack [313]. In [314], the authors tried to achieve high agreement uncorrelated keys by following preprocessing component analysis. To obtain this, they studied signal processing algorithms with an independent eavesdropper. In the beginning, they performed principal component analysis (PCA) and then a discrete cosines transform (DCT), and at the end, wavelet transforms (WT) were used. Among theses, principal component analysis outperformed the other approaches regarding overall key considerations, mathematical expenses, security leakage, and key conformity. The numerical results proved that the presence of PCA with a common eigenvector algorithm among two legal communicators creates an optimized secret key and has a limited key error rate and high key generation rate while reducing computational cost and information leakage in contrast to PCA with a private eigenvector algorithm. Physical layer security offers confidentiality and authentication by benefiting from the channel randomness of the wireless medium. It is a secure way to develop protected transmission compared to the conventional cryptographic method because it does not require any extra security controls, complex algorithms, and schemes to perform security functions on the high layers. Table 10 shows the security and privacy threats related to 5G technologies. A survey given in [332] mentioned that physical layer security carries multiple ways to enrich the security of wireless transmission media via the properties of wireless channels such as fading, reflection, noise, and interference. Multiple antennas were used in this paper to amplify the quality of the received signal and its power with legitimate transmission and an eavesdropper concurrently. Multiple antenna techniques were analyzed for the multi-user, dual-hop, heterogeneous, and point-to-point schemes. A physical layer authentication method is presented in [333] which calculates the distance of surrounding noise sources with the communicating parties. The author considers a maximum likelihood approach to compute the associated positions of these sources by adopting received signal strength (RSS) and angle of arrival (AOA) methods. Simulation results are obtained by considering both active and passive attacks. Results showed that the proposed design is suitable for dynamic conditions. The above existing papers highlight the importance of wireless channels in the context of wireless channels.