Physical-Layer Authentication (PLA) attracts a lot of research interests because of its significant advantages over upper-layer authentication mechanisms: high security and low complexity. The PLA schemes can be categorized into passive and active schemes. In this paper, we extensively leverage the advantages of both the active and passive schemes as a reference scheme, named as the Direct Hybrid (DH) scheme. Although the DH scheme improves the authentication performance of the prior PLA schemes, it has limitations, e.g., high communication overhead. Then, we further propose two hybrid PLA schemes to overcome the limitations of the DH scheme. The first proposed scheme further uses the advantage of the Challenge-Response Authentication Mechanism (CRAM) scheme, named as the CR-based Hybrid (CRH) scheme. Although both DH and CRH schemes significantly improve the authentication performance of the prior PLA schemes, they do not address one significant limitation of the active scheme, i.e., to set the power allocation of a tag empirically. Thus, based on the CRH scheme, we further propose the Adaptive CR-Based Hybrid (ACRH) scheme to adaptively set the parameter instead of the empirical setting. Moreover, we provide the theoretical analysis of the proposed schemes over wireless fading channels and derive their closed-form expressions in terms of the Probability of Detection (PD), Probability of False Alarm (PFA), and optimal threshold, respectively. At last, we discuss the advantages and disadvantages of the proposed schemes and give some useful suggestions for seeking a better tradeoff. Our experimental results show that, in comparison with the active scheme, the DH scheme has better robustness, and the CRH scheme has better both robustness and compatibility but it sacrifices the security. The ARCH scheme achieves a better tradeoff than the remaining schemes.

Spatial Modulation (SM) is a promising low-complexity modulation scheme for Multiple-Input Multiple-Output (MIMO) systems. In this paper, we address the problem of authenticating the transmitter device in the SM. We propose an authentication approach for an SM system by using Physical-Layer Authentication (PLA) mechanisms because the PLA has the following advantages: high security and low complexity. Based on the features of an SM system, we propose two PLA schemes:PLA with Superimposed Authentication Tag(PLA-SAT) andPLA with Superimposed Imaginary authentication Tag(PLA-SIT). We provide performance analyses of our schemes over fading channels in terms of robustness, compatibility, and security. Moreover, we derive their closed-form expressions under both perfect and imperfect channel estimates, including the Probability of Detection (PD), Probability of False Alarm (PFA), and Average Error Probability (AEP). Although the two proposed schemes have the same robustness and security, the PLA-SIT scheme has better compatibility than the PLA-SAT scheme. Our schemes were implemented and extensive performance comparisons through simulations were conducted. We observe that the simulation results of the two proposed schemes perfectly match their corresponding theoretical analyses. The authentication accuracy of the two proposed schemes is close to one when the received SNR is greater than 20 dB and the security performances of the two proposed schemes improve as the variance of estimation errors increases.

This paper addresses the problems of both authentication and privacy in a Non-Orthogonal Multiple Access (NOMA) system by using a privacy-preserving Physical-Layer Authentication (PLA) scheme. The prior scheme has the following limitations: no privacy, low authentication performance, management problem of different secret keys, and constraint to the two-user scenario. In this paper, we propose two privacy-preserving PLA schemes: the Privacy-Preserving PLA (PP-PLA) scheme and the Enhanced Privacy-Preserving PLA (EPP-PLA) scheme, which improve both the privacy and authentication performance as compared to the prior scheme. The fundamental difference between the EPP-PLA and PP-PLA schemes is how to encrypt the source message of a certain user while the other operations of the two proposed schemes keep the same. We provide the theoretical analysis of the proposed schemes in terms of robustness, compatibility, privacy, and security, and derive their closed-form expressions. Moreover, we provide the theoretical comparisons between the proposed schemes and the prior scheme. We optimize the parameters of the proposed schemes to achieve both message-rate fairness and authentication-accuracy fairness. We implement the proposed schemes and conduct extensive performance comparisons through simulations. Experimental results show that the theoretical results perfectly match the corresponding simulation results. Specifically, we take the second user, acting as an administrator for a NOMA system with three users, as an example, where SNR = 16 dB. In comparison with the prior scheme, both proposed schemes have the same compatibility, whereas they improve the robustness by 4.72%. Moreover, the EPP-PLA and PP-PLA schemes improve the security by 32.8%, whereas they improve the privacy by 3000.75% and 1000.75%, respectively.