For autonomous aerial vehicle (AAV) secure communications, traditional designs based on fixed position antenna (FPA) lack sufficient spatial degrees of freedom (DoF), which leaves the line-of-sight-dominated AAV links vulnerable to eavesdropping. To overcome this problem, this paper proposes a framework that effectively incorporates the fluid antenna (FA) and the artificial noise (AN) techniques. Specifically, the minimum secrecy rate (MSR) among multiple eavesdroppers is maximized by jointly optimizing AAV deployment, signal precoder, AN precoder, and FA positions. In particular, the worst-case MSR is considered by taking the channel uncertainties due to the uncertainty about eavesdropping locations into account. To tackle the highly coupled optimization variables and the channel uncertainties in the formulated problem, an efficient and robust algorithm is proposed. Particularly, the uncertain regions of eavesdroppers, whose shapes can be arbitrary, are disposed by constructing convex hull. In addition, two movement modes of FAs are considered, namely, free movement mode and zonal movement mode, for which different optimization techniques are applied, respectively. Also, both single-user and multi-user scenarios are considered. Numerical results show that, the proposed FA schemes boost security by exploiting additional spatial DoF rather than transmit power, while AN provides remarkable gains under high transmit power. The synergy between FA and AN results in a secure advantage that exceeds the sum of their individual contributions, achieving a balance between security and reliability under limited resources.

Aerial reconfigurable intelligent surfaces (ARIS), deployed on unmanned aerial vehicles (UAVs), could enhance anti-jamming communication performance by dynamically configuring channel conditions and establishing reliable air-ground links. However, large-scale ARIS faces critical deployment challenges due to the prohibitive computational complexity of conventional discrete optimization methods and sophisticated jamming threats. In this paper, we introduce a mean-field modeling approach to design the spatial configuration of ARIS by a continuous density function, thus bypassing high-dimensional combinatorial optimization. We consider an adaptive jammer which adjusts its position and beamforming to minimize the sum-rate. A key finding reveals that the jammer’s optimal strategy is governed by a proximity-directivity trade-off between reducing path loss and enhancing spatial focusing. To combat the jamming, we propose a robust anti-jamming transmission framework that jointly optimizes the BS beamforming, the ARIS reflection, and the ARIS spatial distribution to maximize the worst-case sum-rate. By leveraging variational optimization and Riemannian manifold methods, we efficiently solve the functional optimization problems. Our analysis further unveils that the optimal ARIS deployment follows a spatial water-filling principle, concentrating resources in high-gain regions while avoiding interference-prone areas. Simulation results demonstrate that the proposed framework remarkably improves the sum-rate. Furthermore, the computational complexity of the proposed algorithm is independent of the number of UAVs, validating its effectiveness for scalable ARIS-assisted anti-jamming communications.

This paper studies jammer-assisted covert transmission with both transmitter and the jammer equipping with fluid antenna arrays. Performance analyses of reliability and covertness are conducted, based on which both transmitter’s and jammer’s transmit powers, beamforming vectors, and antennas’ positions are jointly designed to optimize covertness under the reliability constraint for two scenarios. In the first scenario, warden’s position is known. The global optimal solutions of the aforementioned variables are obtained, based on which the advantages of fluid antennas over fixed-position antennas are analytically revealed, and a series of Pareto optimal solutions for balancing reliability and covertness are demonstrated. In the second scenario, warden’s position is uncertain and is modeled by a general distribution function. The resultant intractable optimization problem is addressed by invoking the constrained stochastic successive convex approximation method. Numerical results show significant performance gains from fluid antennas.

The active reconfigurable intelligent surface (RIS) offers a promising solution to overcome the double-fading attenuation inherent in passive RIS-aided systems. However, this capability can be exploited by adversaries to launch potent pilot spoofing attack (PSA). In this paper, we propose a novel active RIS-aided covert PSA scheme for time-division duplex systems, where a passive eavesdropper manipulates the channel state information (CSI) estimation at the legitimate transceiver during the uplink stage and steers downlink data towards itself during the downlink stage. Crucially, without requiring perfect instantaneous CSI, a practical challenge for eavesdroppers, we maximize the average eavesdropping signal-to-noise ratio (SNR) by jointly designing the RIS reflection coefficients for both stages. To ensure covertness, we integrate an anti-energy ratio detection (ERD) mechanism that constrains the detection probability below a predefined threshold. The resulting non-convex optimization problem is solved via an efficient alternating optimization algorithm combined with penalty methods, handling the rank-1 constraints and statistical CSI uncertainties. Simulations demonstrate that the proposed scheme achieves up to 26 dB SNR gain over passive RIS-aided PSA and reduces ERD detection probability compared to traditional PSA. This work reveals the dual-edged nature of the active RIS: while enhancing security, it introduces new attack schemes demanding advanced countermeasures.

Covert device-to-device (D2D) communication aims to hide the existence of D2D transmission behavior from the warden, which plays a pivotal role in bolstering D2D communication security. The existing research on covert D2D communication either considered the interference’s assistance with perfect channel state information (CSI) available at the warden, or considered channel distribution information at the warden without the assistance of interference. These setups facilitate the performance analyses and the parameters design, which, however, did not fully exploit the potential of covert D2D communication, since both channel uncertainty and interference uncertainty exist in practice. In this article, two covert schemes are proposed for covert D2D communication with dual uncertainties at the warden, where the warden has imperfect CSI. The base station transmits artificial noise to assist covert communication in the first scheme. In the second scheme, the cellular user adopts variable power to confuse the warden. For both schemes, the closed-form expressions of the warden’s approximate minimum detection error probability and the achievable D2D communication rate are derived. The achievable D2D communication rate is maximized by jointly optimizing the transmit power of different links with given covertness requirement. The results suggest that the proposed schemes achieve higher communication performance by considering dual uncertainties. Moreover, the scheme with variable cellular user’s transmit power performs better when the covert receiver can not well handle the interference from artificial noise.

Physical-layer key generation (PKG) technology offers a lightweight encryption solution for device-to-device (D2D) communications. However, it faces significant challenges due to the high correlation between the eavesdropping and legitimate channels, as well as the imperfect estimated channel, which cause substantial degradation of secret key capacity. To address the challenges, we propose a novel PKG framework that leverages the reflective beamforming of reconfigurable intelligent surface (RIS) for D2D communications. Specifically, we first derive closed-form expression for the secret key capacity under correlated and imperfect estimated channels. Then, we propose to maximize the minimum secret key capacity by optimizing the reflection coefficient matrix (O-RCM) of RIS, which is a non-convex optimization problem. Next, a semi-definite relaxation and successive convex approximation-based method for O-RCM (SSO-RCM) is proposed to tackle the non-convex max-min-min problem. To further reduce the computational complexity, we propose a low-complexity method based on the path-following algorithm for O-RCM (PFO-RCM). Simulation results show that under correlated and imperfect channels, both proposed methods significantly improve the minimum secret key capacity compared to the existing RIS-assisted methods. Moreover, the PFO-RCM method, while exhibiting lower computational complexity than the SSO-RCM method, incurs a small performance loss.

Jamming is regarded as an effective and popular way to assist covert wireless communications. However, if the warden uses a directional beam for detection, the effects of jamming may be alleviated or even be eliminated. This paper studies the jammer-assisted covert communication schemes against directional beam. Two specific collaborative unmanned aerial vehicle (UAV) communication scenarios are considered. In scenario 1, the warden’s beam direction and beamwidth are unknown. In scenario 2, the warden’s beamwidth is known, but his location is unknown. For both scenarios, the jammer deployment strategies for confronting directional beam are proposed. Closed-form expressions are developed to evaluate the warden’s detection performance, based on which conclusions regarding the optimal deployment of the UAVs are proved. The UAVs’ locations and transmit power are jointly optimized to enhance reception while satisfying covert constraints. Furthermore, scenario 1 is extended to the case where the warden’s location is imperfectly known. Both theoretical and numerical results reveal that collaborative beamforming can provide covert performance gains via dispersing the power over the space domain, which is similar to the role of direct sequence spread spectrum in frequency domain.

Physical layer security (PLS) enables secure communication which is independent of cryptography. It generally requires the legitimate channel being advantageous, otherwise, relaying techniques can be adopted to enhance the legitimate transmission and/or suppress the wiretap one. However, untrusted relays become new threatens since unknown passive eavesdroppers may approach either the transmitter or the receiver to wiretap the secure messages. Regarding the threatens in this setup, we propose a nonlinear transmit and relaying approach to protect the secure messages during the overall transmission. The essential idea is that Alice transmits a nonlinear transform of the original secure signal and the relays collaboratively invert the nonlinear transform at Alice when forwarding it to Bob. By carefully designing the layout of the relays, only a confined region where Bob locates is accessible to the non-distortion version of the original secure signal, and there is no secure signal leakage nearby Alice or the relays. Specifically, the proposed scheme consists of two phases. In the first one, a well-chosen nonlinear transform is applied to the secure signal of Alice, and she then broadcasts the transformed signal to both the relays and the eavesdropper. Each relay determines its forwarding approach by using either the Taylor series or the Lagrange inversion theorem on the nonlinear function of Alice to facilitate the perfect recovery of the original secure signal for Bob. In the second phase, the relays process the received signals using the well-designed forwarding approach and then cooperatively send them to Bob. The forwarding signals are aggregated at Bob and the original secure signal can be recovered directly without any further complicated equalization. We therefore define the region with this property by the secure-information-accessible region, and then provide the secrecy suggestions of laying the relays to shrink it. In contrast, the eavesdropper cannot get any useful secure information from either the first phase or the second one if it locates out of this region. Finally, we analyze both the bit error rate and the secrecy rate, and compare them with the existing secrecy methods. Numerical results validate the proposed nonlinear secrecy approach.

Malicious jamming attacks have been regarded as a serious threat to Internet of Things (IoT) networks, which can significantly degrade the Quality of Service (QoS) of users. This article utilizes an intelligent reflecting surface (IRS) to enhance anti-jamming performance due to its capability in reconfiguring the wireless propagation environment via dynamically adjusting each IRS reflecting elements. To enhance the communication performance against jamming attacks, a robust beamforming optimization problem is formulated in a multiuser IRS-assisted anti-jamming communications scenario with or without imperfect jammer’s channel state information (CSI). In addition, we further consider the fact that the jammer’s transmit beamforming can not be known at BS. Specifically, with no knowledge of jammers transmit beamforming, the total transmit power minimization problems are formulated subject to the outage probability requirements of legitimate users with the jammer’s statistical CSI, and signal-to-interference-plus-noise ratio requirements of legitimate users without the jammer’s CSI, respectively. By applying the decomposition-based large deviation inequality, Bernstein-type inequality, Cauchy–Schwarz inequality, and penalty nonsmooth optimization method, we efficiently solve the initial intractable and nonconvex problems. Numerical simulations demonstrate that the proposed anti-jamming approaches achieve superior anti-jamming performance and lower power-consumption compared to the non-IRS scheme and reveal the impact of key parameters on the achievable system performance.

The reconfigurable intelligent surface (RIS) is envisioned to create ultra-secure wireless networks. Previous works on the RIS-assisted security provisioning techniques assumed wiretap channel information at the transmitter, which is practically unavailable in a passive eavesdropping scenario. In this article, we consider a point-to-point anti-eavesdropping system in which a RIS is used to enable the secure transmission from a multi-antenna transmitter to a multi-antenna legitimate receiver. A passive eavesdropper, whose channel state information is completely unknown, attempts to decode the secret messages. We propose a security approach by using the reflection at the RIS as multiplicative randomness against the wiretapper. Specifically, the reflection coefficients in terms of amplitude and phase are updated in each transmission and kept private at the RIS. Through the reflection designs, the effective channel matrix is diagonalized at the legitimate receiver. In contrast, the eavesdropper receives coupled signals with the weights of the randomness at RIS. The main contributions of this article are three reflection designs and correspondingly three secure transmission schemes, which fulfills diverse requirements of the balance amongst performance metrics including the degrees of randomness, spectral efficiency, and reliability. The main benefits of the proposed secure transmission schemes are four-fold. First, the transmitter does not need to know the eavesdropper's channel states. Second, closed-form solutions for the reflection coefficients are provided. Third, the legitimate receiver has a linear decoding complexity. Fourth, the unauthorized wiretapper is unable to cancel out the multiplicative randomness and thus cannot extract much useful information. Numerical results show that exploiting the RIS as a source of multiplicative randomness provides a new perspective to improve the security of the wireless networks.

Joint transmitter-receiver spatial modulation (JSM) is an appealing transmission scheme in the spatial modulation family, where transmit diversity, receive diversity, and multiplexing gain are achieved simultaneously. However, the conventional JSM technique is neither spectrum efficient nor antenna efficient due to its limited candidates of bit-to-antenna mapping. In this paper, we propose a novel JSM approach that involves more mapping candidates to relax the implementation constraint and achieve better reliability. The proposed JSM scheme maximizes the minimum Euclidean distance (MED) between the received signals of different antenna mappings by optimizing the antenna selection and the power allocation over the transmit antennas, named MED-JSM. In particular, an optimal closed-form power allocation solution is derived when there are two receive antennas. For more receive antennas, a lower bound of the minimum Euclidean distance is derived, and a corresponding lower bound-approaching power allocation algorithm is proposed accordingly. Then, the proposed power allocation algorithm and antenna selection are used in a hybrid manner to further enhance the system reliability. Since the optimal MED-JSM requires an exhaustive search over all the received signals, we propose a suboptimal scheme with reduced computational complexity, which shrinks the search space via the Rayleigh-Ritz theorem. Numerical results show that the proposed MED-JSM outperforms the conventional JSM schemes in system's reliability.