Authentication is an important issue in wireless communications because the open nature of the wireless medium provides more security vulnerabilities. Recently, Physical-Layer Authentication (PLA) attracts many research interests because it provides information-theory security and low complexity. Although many researchers focus on the PLA and exploit its potential in enhancing wireless security, the literature is surprisingly sparse with no comprehensive overview of the state-of-the-art PLA and the key fundamentals involved. Thus, this paper provides a detailed survey of features and techniques that can be used in the PLA. We categorize the existing PLA schemes into two categories: passive and active schemes. In the passive schemes, a receiver authenticates the transmitter based on the physical-layer features of the received signals. We further divide the passive schemes into two sub-categories: device-based features and channel-based features. In the active schemes, a transmitter generates a tag based on a secret key and embeds it into a source message. Then, a receiver authenticates the transmitter based on the tag whether it exists in the received signal. We further divide active schemes into two sub-categories: non-covert schemes and covert schemes. Moreover, we also provide some future research directions.

This paper concerns the problem of authenticating the transmitter without a secret key. In comparison with traditional cryptographic-based authentication mechanisms, the Physical-Layer Authentication (PLA) has the following advantages: high security, low complexity, and high compatibility, since it exploits intrinsic and unique features of the physical layer to authenticate the transmitter rather than using a secret key. The prior channel-based PLA schemes use a quantization algorithm to deal with multiple channel-based features for simplicity. However, there are two main limitations in the prior schemes: performance loss due to quantization error and the difficulty of obtaining the optimal thresholds in closed-form. In this paper, we propose two multiple Channel Impulse Response (CIR) based PLA schemes to effectively overcome the aforementioned limitations of the prior schemes. The first scheme uses multiple CIRs to realize the PLA, which is named as the Multiple CIRs PLA (MCP) scheme. The MCP scheme has better authentication performance than the prior schemes, since it avoids to use a quantization algorithm. The second scheme further improves the authentication performance by exploiting the channel correlation coefficient, which is named as the Enhanced Multiple CIRs PLA (EMCP) scheme. We provide rigorous performance analysis of two proposed schemes. We implemented the proposed schemes and conducted extensive performance comparisons through simulations. Our experimental results show that the closed-form expressions of the theoretical results of the proposed schemes perfectly match the corresponding simulation results. The EMCP scheme has the best authentication performance and the MCP scheme is the second one, whereas the prior scheme is the worst one. As the SNR or the channel correlation coefficient declines, the performance gap among various schemes gradually increases.

Authentication is a key requirement for secure communications in modern wireless systems. Compared with the conventional authentication at the upper layer using a cryptographic tool, authentication at the physical layer has many advantages, including enhanced security through the introduction of uncertainty to adversaries and increased efficiency and compatibility through the avoidance of operations at the upper layer, particularly in heterogeneous coexistence environments, e.g., 5G wireless systems. In this paper, we investigate authentication at the physical layer under time-varying fading channels. Conventional authentication schemes at the physical layer operate poorly under fast fading or frequency selective fading channels. Furthermore, conventional schemes require additional complicated preprocessing such as channel estimation and message symbol recovery through demodulation and decoding. This paper proposes a new blind authentication scheme at the physical layer that combines the techniques of blind known interference cancellation (BKIC) and differential processing to implement authentication without requiring any of the above-described preprocessing. The proposed scheme utilizes both the smoothing (SM) technique and belief propagation (BP) technique to achieve BKIC through the distinct blind authentication schemes with a superimposed tag over pilots (BSUPs) referred to, respectively, as BSUP-SM and BSUP-BP. The proposed scheme not only effectively suppresses the deteriorate effect of fading channels without any additional preprocessing but is also covert to unaware users, robust to interference, and secure for identity verification. The tradeoffs of using the proposed system with respect to various goals are discussed and analyzed. The performance of the BSUP-SM scheme depends on the channel fading and the length of the pilot cluster, while the performance of the BSUP-BP scheme is not sensitive to the above factors but depends instead on the quantization step used. The BSUP-BP scheme works well in fast fading channels, even in frequency selective fading channels.

A high roll-off, third-order frequency selective surface (FSS) with quasi-elliptic bandpass response is presented. It is based on substrate integrated waveguide technology. Its unit cell includes three cavities that support at the TE ₁₀₁ mode. Interactions of resonators are dominated by magnetic couplings. The coupling matrix is analyzed and used to design FSS. Guidelines for a coupling-matrix-based FSS design are given. Transmission zeros realized close to the lower and upper narrow passband lead to high roll-off characteristics. In addition, a cross-polarization feature is easily achieved. Finally, a prototype is designed and fabricated to validate the proposed structure. A stable frequency response in passband has been obtained with an oblique incidence of 40° for TE polarization. Good agreement between the simulated and measured results verifies the design concept of bandpass FSS.

Authentication is a key requirement for secure communications. In this paper, we consider authentication in a wireless system at the physical layer. There are two main authentication methods at the physical layer in the existing literature, authentication with time division multiplexed tag (Auth-TDM) and authentication with superimposed tag (Auth-SUP). However, there are drawbacks in the existing authentication methods. The Auth-TDM method requires additional bandwidth. The Auth-SUP method corrupts the entire data message. Moreover, regarding the purpose of authentication, both methods require additional complicated preprocessing such as channel estimation and message symbol recovery through demodulation and decoding. This paper proposes a new authentication method at the physical layer, referred to as slope authentication at the physical layer. The proposed method not only overcomes the aforementioned drawbacks but also is designed to be covert to the unware user, robust to interference, and secure for identity verification. The tradeoffs by considering different goals are discussed and analyzed in block-fading channels.