This paper concerns the problem of authenticating different transmitters at the physical layer in a multi-user scenario. In this paper, we propose two tag-based Physical-Layer Authentication (PLA) schemes in a multi-user scenario. We name the first scheme as the Multi-User Physical Layer Authentication and Classification (MU-PLAC) scheme, which not only detects an impersonation attack but also achieves multi-user classification. For further improving the performance of the MU-PLAC scheme for a two-user scenario, we propose an enhanced version of the MU-PLAC scheme by elaborately designing the tags of two legitimate users. We name the second scheme as the Enhanced Multi-User Physical Layer Authentication and Classification (EMU-PLAC) scheme. We provide the theoretical analysis of the proposed schemes over wireless fading channels and derive their closed-form expressions. We implement the proposed schemes and conduct extensive performance comparisons. Our simulation results show that, for a two-user scenario, the EMU-PLAC scheme provides better classification accuracy and authentication accuracy as compared with the MU-PLAC scheme.

This paper concerns the problem of improving compatibility in the tag-based Physical-Layer Authentication (PLA) without sacrificing robustness of the PLA schemes. The prior PLA schemes for improving compatibility often sacrifice robustness or introduce extra communication overhead and security vulnerabilities. The main objective of our approach is to reduce the modification ratio of the source message given the tag length via an encoding approach. Based on a tag-encoding function, we propose two encoded tag-based schemes for the scenario of a single block and the scenario of multiple blocks, respectively, which are named as the Encoded Tag-based PLA scheme for Single Block (ET-SB) and the Encoded Tag-based PLA scheme for Multiple Blocks (ET-MB), respectively. We theoretically analyze the performance of the proposed schemes over fading channels and derive the closed-form expressions of the performance analyses. We implement the proposed schemes and conduct extensive performance comparisons through simulations. Our simulation results show that the closed-form expressions of the theoretical results of the proposed schemes perfectly match the corresponding simulation results. When the SNR is 10 dB, the compatibility of the ET-SB and ET-MB schemes improves to 15.63% and 23.45%, respectively, compared to the prior scheme.