---

Authentication plays a pivotal role in ensuring communication security. Cryptographic methods are frequently employed to fulfill this purpose. These methods, implemented at upper network layers, encounter challenges including complexity, power consumption, and overhead. Particularly for users with limited computational power, these methods encounter challenges. A novel solution to overcome these challenges is physical layer authentication (PLA), which involves utilizing physical layer features to embed a tag in the transmitted signal for authentication, leveraging various channel characteristics such as position, velocity, noise, etc. In this paper, a review of previous research is provided, highlighting the differences between physical layer and upper-layer authentication. Furthermore, existing categorizations for PLA and a novel classification based on covertness levels are provided. Moreover, possible attacks and corresponding countermeasures are investigated, followed by suggestions for future research in this area.