# Physical Layer Authentication in Wireless Networks-Based Machine Learning Approaches
## 3 Background
### Physical Layer Authentication  
The inherent broadcast nature of wireless communications raises security and privacy issues where adversaries can launch different types of attacks. Accordingly, authentication is an important issue in wireless communications [29]. Device identity authentication requires safeguarding wireless networks to validate whether the users are legitimate and allowing them to access the network while preventing malicious users [39]. Most existing wireless communication systems perform authentication through upper-layer authentication techniques that are typically implemented using cryptography-based authentication algorithms [39]. However, traditional authentication approaches depend on software addresses such as IP and MAC addresses, which can be tampered with or forged [44]. Once adversaries obtain the security credentials, they can pretend as legitimate users to reach private data and launch severe attacks on the wireless devices [44; 45]. However, upper-layer authentication mechanisms based on traditional cryptography-based algorithms are unsuitable for advanced wireless communication systems [29]. For example, cognitive radio networks, IoT, internet of vehicles (IoV), smart grids networks, and unmanned aerial vehicles (UAV) because of the following issues [6; 29; 39; 46]: With the advancement in computational power and cryptanalysis algorithms, the time it takes to crack a cryptography key has been drastically reduced. However, because the upper layer signaling is not altered, the replayed signal can successfully spoof the legitimate receiver. Therefore, the complicated cryptography techniques in upper-layer operations, e.g., encryption, decryption, and frequent authentication handovers, are unsuitable with limited capability for wireless devices. Furthermore, the process of key sharing and management introduces overhead concerns in massive ubiquitous computing scenarios, such as the amount of storing excessive keys or defending against the eavesdropping attacks of frequent exchanging keys. Wireless physical layer authentication is a method of validating a wireless transmitter by checking the physical layer characteristics of the communication [46]. A good authentication scheme should generally have three characteristics: covertness, robustness, and security [28], as demonstrated in Figure 2.  
* The covertness means that any authentication schemes should not significantly affect the performance of the standard data transmission, do not occupy too much communication overheads or extra computational resources, and do not harm the existing conventional higher-layer cryptographic-based techniques. * Robustness requires that the PLA framework is robust enough to mitigate channel fading and noise interference. * Security is the kernel of PLA systems, representing the ability to prevent the authentication procedure from being interrupted or invaded by eavesdroppers. Recently, PLA has attracted much research interest compared to traditional secret key-based authentication techniques because of the following advantages [29; 46]:  
* The PLA allows a legitimate receiver to easily distinguish between a legitimate and adversary transmitter without upper-layer processing, decreasing computational complexity and processing delay. * There is no key distribution and management need with PLA compared to conventional secret key-based authentication schemes. Instead, some existing physical layer authentication approaches rely on analog channel information and device-specific characteristics caused by manufacturing variability. * In a heterogeneous coexistence system, incompatible devices may not be able to decode each other's upper-layer signaling, but they should be able to decode the physical layer bit-streams. * The PLA presents information-theoretic security, where the physical layer puts adversaries in a state of uncertainty. Physical Layer Authentication Techniques  
The mostly studied authentication techniques can be classified into: radio frequency fingerprint-based and channel-based schemes.