Improving the covertness in the physical-layer authentication

A well-designed Physical-Layer Authentication (PLA) scheme should consider three properties: covertness, robustness, and security. However, the three properties always cause some dilemmas, e.g., higher covertness leading to lower robustness. This paper concerns the problem of improving the covert- ness without sacrificing the robustness. This problem is important because of the following reasons: reducing the errors in recovered source message, improving the security, and ease of constructing a multi-factor authentication system. In this paper, we propose three covert PLA schemes to address the problem. In the first scheme, we improve the covertness by reducing the modification ratio on the source message based on an encoding mechanism. In the second scheme, we improve the covertness by optimizing the superimposing angle, which maximizes the minimum distance between the tagged symbols and the boundary line of the demodulation decision for the source message. In the third scheme, referred to as the hybrid scheme, we further improve the covertness by jointly using the advantages of both the above two schemes. Our experimental results show that when the SNR at a legitimate receiver is 25 dB, as compared with the prior scheme, the first scheme improves the covertness by 17.74%, the second scheme improves the covertness by 28.79%, and the third scheme improves the covertness by 32.09%, while they have similar robustness as that of the prior scheme.