## 4 Authentication Techniques in 6G Communication
### Physical Layer Authentication  
Most of the concepts related to authentication mechanisms are available in higher layers above the physical communication layer. However, it is observed that the implementation of authentication at the physical layer has evolved as an unconventional and stealthy mechanism, where the authentication process is achieved through the superimposition of a critically designed secret modulation on waveforms. Physical layer authentication has gained prominence due to robustness to interference and almost negligible bandwidth dependence [120]. The authors in [121] proposed a spread spectrum-based secret modulation for interference-resistant authentication. The scheme involves spread spectrum-enabled hardware; therefore the scheme has the limitation of compatibility with traditional modulation schemes. Performance of authentication schemes is critically dependent on channel estimation and dynamic compatibility of the protocol with variations in the communication link [122]. In [123], the authors presented an adaptive ML-based intelligent physical layer authentication technique for an improved authentication mechanism in time-varying scenarios. Here, machine learning is embedded in the physical layer through a physical attributes-based fusion model on a kernel machine. Physical layer security techniques emerge as a suitable framework for reduced complexity, low delay, and light footprint in the context-aware security paradigm. MIMO technology in 6G has potential to control physical wireless links at the individual device level as shown in Figure 6.  
\begin{table}
\begin{tabular}{p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt} p{56.9 pt}} \hline \hline
**Ref. ** & **Network Model** & **Technique** & **Features** & & **Limitations** \\ \hline
[115] & D2D communication & Randomly HMAC and ECC &  
The physical layer authentication mechanism provides exposure-free operation of higher layers in 6G-enabled networking architecture. PLS techniques result from channel characteristics' exploitation and randomness in wireless link parameters. Lightweight algorithms are considered ideal for implementation at the physical layer in resource-constrained IoTs. In [124], the authors suggested a Channel State Information (CSI)-based lightweight symmetric cipher-based authentication. It covers various attacks such as small integer stacks, spoofing and replay attacks. The scheme utilizes clustering properties instead of trusted party dependence. 6G-based physical layer security schemes are emerging as an unconventional futuristic paradigm of Human Bond Communication (HBC) and Molecular Communication, where related attributes of five human senses and biological characteristics are integrated into networks for remote analysis and medical procedures [125]. Physical layer security measures have evolved to cover all the foundational security requirements of the CIA triad in 6G networks [126]. Specifications of 6G are still in the evolution stage. 6G will be compatible with heterogeneous communication technologies, including LTE, 5G, B5G and other emerging communication technologies [127]. 6G has various challenges related to enormously high data rates reaching Tbps/THz. Physical Layer Modeling (PLM) and control of the authentication mechanism at the physical layer bring various further unknown research frameworks [128]. The authors in [129] provide a PLS based on mail-box theory with distributed learning. The scheme employs self-organization through a self-learning mechanism to reduce transmission quantity and transmission error. Orbital Angular Momentum (OAM) modes in joint implementation with MIMO have the potential to reach up to a 100 Gbps throughput in point-to-point transmission [130]. Emerging technologies using the THz spectrum are researched for physical security through hybrid free space optical (FSO)-THz communication. The authors in [131] employed OAM modes in mutually linked FSO and THz schemes.