Covert communication conceals wireless transmission activity, but conventional multi-antenna designs rely mainly on spatial beamforming and can be constrained by limited spatial degrees of freedom. This paper investigates polarization-reconfigurable antenna (PRA)-aided multiple-input multiple-output (MIMO) covert communication, where Alice and Bob perform transmit and receive polarforming, respectively, and Willie uses a vertically polarized antenna for radiometric detection. We jointly optimize the transmit covariance matrix and the transmit and receive phase shift vectors to maximize Bob's achievable rate under the covertness and transmit power constraints. Under Gaussian signaling, perfect polarized channel state information, and known noise power at Willie, we derive his optimal radiometric threshold and exact minimum detection error probability (DEP) for a finite observation length, and convert the DEP requirement into a deterministic constraint on his received signal power. We further prove that, for a fixed transmission design, positive signal leakage becomes detectable as the observation length tends to infinity. To solve the design problem for a finite observation length, we develop an alternating optimization (AO) algorithm with a generalized water-filling covariance update and closed-form phase updates. Numerical results validate the exact DEP analysis and the rapid convergence of the proposed algorithm. Compared with the sufficient divergence-based condition, the exact covertness constraint permits a $0.99$--$1.12$~dB higher received signal-to-noise ratio (SNR) at Willie, while joint transmit and receive polarforming provides a rate gain of up to $53.8\%$ over the considered benchmark schemes.
Covert communication conceals wireless transmission activity, but conventional multi-antenna designs rely mainly on spatial beamforming and can be constrained by limited spatial degrees of freedom. This paper investigates polarization-reconfigurable antenna (PRA)-aided multiple-input multiple-output (MIMO) covert communication, where Alice and Bob perform transmit and receive polarforming, respectively, and Willie uses a vertically polarized antenna for radiometric detection. We jointly optimize the transmit covariance matrix and the transmit and receive phase shift vectors to maximize Bob's achievable rate under the covertness and transmit power constraints. Under Gaussian signaling, perfect polarized channel state information, and known noise power at Willie, we derive his optimal radiometric threshold and exact minimum detection error probability (DEP) for a finite observation length, and convert the DEP requirement into a deterministic constraint on his received signal power. We further prove that, for a fixed transmission design, positive signal leakage becomes detectable as the observation length tends to infinity. To solve the design problem for a finite observation length, we develop an alternating optimization (AO) algorithm with a generalized water-filling covariance update and closed-form phase updates. Numerical results validate the exact DEP analysis and the rapid convergence of the proposed algorithm. Compared with the sufficient divergence-based condition, the exact covertness constraint permits a 0.99--1.12~dB higher received signal-to-noise ratio (SNR) at Willie, while joint transmit and receive polarforming provides a rate gain of up to 53.8% over the considered benchmark schemes.
Covert communication conceals transmission of messages between Alice and Bob from an adversary, Willie, who tries to determine if a transmission took place or not. While covert communication in a basic, standard setting where all variables are known to Willie, results in the well-known square-root law, when a jammer is present and assists Alice by creating uncertainty in Willie's decoder, a strictly positive transmission rate is possible. In this work, we analyze the case where the jammer is equipped with multiple antennas. Specifically, we analyze the effect of multiple antennas at the jammer on Alice's transmission power and consequently on the transmission rate. We consider both the case where the channel knowledge of Willie is known as well as the case where it is unknown. We formulate several optimization problems for the transmission strategies of the jammer to maximize his assistance to Alice, in terms of maximizing Bob's received SNR and consequently the covert rate. When the channel information is known to the jammer, we show that under an achievable covertness scheme, the optimal strategy of the jammer is to perform beamforming towards a single direction with all his available power. This direction though, is not trivial, since it reflects a tradeoff point between minimizing the interference at Bob and maximizing the interference at Willie. When the channel knowledge is unknown, we show that the optimal strategy of the jammer is either to transmit isotropically to all directions or to the null-space of Bob, where this choice depends on certain channel conditions. This is in contrast to current schemes in the literature. Furthermore, we extend the optimization problems to the case where Bob is also equipped with multiple antennas, and provide insightful results, shown to be asymptotically optimal, accompanied by simulations.
Physical layer security (PLS) is a promising paradigm for safeguarding 6G wireless networks by exploiting the inherent characteristics of wireless channels. However, the efficiency of conventional PLS is often limited by fixed orientation antennas. This paper investigates a rotatable antenna (RA)-aided secure multiple-input multiple-output (MIMO) communication system, where both the transmitter and the receiver are equipped with RAs in the presence of an eavesdropper. By dynamically optimizing the orientations of RAs, we can proactively reshape the effective MIMO channels to enhance legitimate transmission while simultaneously suppressing information leakage to the eavesdropper. We formulate a secrecy rate maximization problem by jointly optimizing the transmit beamforming, artificial noise (AN) covariance matrix, and the transmit/receive RA orientations, subject to the transmit power budget and antenna orientation constraints. To tackle the resulting highly coupled and non-convex problem, we first study a simplified single-input single-output (SISO) case to reveal the structure of the optimal RA orientation. For the general MIMO case, we develop an alternating optimization algorithm by reformulating the original problem through the minimum mean-square error framework. In particular, the transmit beamforming and AN covariance matrix are derived in semi-closed forms, while the RA orientations are updated via the Riemannian Frank-Wolfe method. The proposed design is further extended to the multi-receiver secure transmission scenario. Simulation results show that the proposed scheme converges rapidly and achieves significant secrecy rate gains over the conventional fixed-orientation scheme.
Covert communication enforces the transmitted signal to be indistinguishable from other signals under wireless environment, making it difficult for an eavesdropper to detect and intercept the message. The most common approach within covert systems is the augmentation of artificial noise (AN), ensuring reliable transmission. However, this strategy becomes problematic when the eavesdropper's position is uncertain, as continuous amplification of interference power can precipitate a substantial decline in covert communication capacity. In this paper, we investigate the covert communication between legitimate transmitter (i.e., Alice) and full duplex receiver (i.e., Bob) in the presence of the location uncertainties of the eavesdropper (i.e., Willie). We first derive the sufficient condition for achieving covert communication through robustness analysis. Then, we derive the effective throughput to measure the covert communication capacity. Afterward, by using joint optimization technology, the optimal AN power of Bob and the number of antennas for Alice are jointly designed to maximize the effective throughput subject to a covertness constraint. Theoretical analysis indicates that reducing the AN power while increasing the number of antennas helps to improve the system capacity, the effective throughput between Alice and Bob is inversely proportional to the distance between them. Finally, the validity of our analytical approach is corroborated by simulation results.
In this paper, we consider a covert communication system with a multi-antenna full-duplex (FD) receiver to enhance covert performance. More precisely, the receiver Bob (i.e., multiple antennas) selects the best antenna to receive the covert message, and then it sends artificial noise (AN) to the warden Willie causing uncertainty by utilizing an antenna among the remaining antennas. In order to take full advantage of multi-antenna technology, we consider two cases: (1) Willie does not know which antenna Bob chooses to send AN, and Willie knows only part of the channel information. (2) Willie knows which antenna Bob chooses to send AN, and Willie knows all the channel information. Based on the analysis of the optimal detection threshold, we derive the minimum detection error probability of the two cases. Furthermore, given the pre-determined convert constraints, the closed-form expression of the maximum effective convert rate is derived. When the power of AN sent by Bob is greater than that of covert message sent by the sender Alice, surprised research results are shown: (1) the detection error rate of case 1 is higher than that of case 2. (2) the effective covert rate of case 1 is always lower than that of case 2. The simulation results show that the proposed scheme is more conducive to sending covert messages for Alice; in addition, the numerical results imply that the maximum effective covert rate of the system using the multi-antenna FD receiver is significantly higher than that of the traditional dual-antenna FD receiver system.
This paper investigates the covert communication network assisted by the multi-antenna relay, by taking into account two typical scenarios of eavesdropping channel state information (ECSI). In this network, the relay operates in half-duplex amplify-and-forward (AF) relaying protocol, where the relay either forwards the Alice’s confidential signal to the Bob by designing the precoding matrix to enhance the communication performance or sends artificial noise (AN) to disrupt the warden’s detection. For this covert communication system with either instantaneous ECSI (I-ECSI) or statistical ECSI (S-ECSI), an optimization problem is formulated with the aim of maximizing the received signal-to-noise ratio (SNR) at the Bob while ensuring the constraints on the transmit power and covert performance. To solve the non-convex optimization problem, an alternating optimization method is proposed through jointly optimizing the transmit power at Alice and precoding matrix at the relay. Numerical results are provided to demonstrate the effectiveness of our proposed method. In particular, our method is superior to the conventional ones up to 49.6% with I-ECSI and 57.9% with S-ECSI.
This paper investigates the joint physical layer security and covert communication in multi-input single-output (MISO) systems, aiming to enhance both the security and covertness of the system. In particular the base station, Alice, covertly transmits confidential information to the legitimate receiver, Bob, via multiple antennas, while avoiding detection by Willie, the detector, and eavesdropping by Eve. For the considered system, we analyze the covertness constraint with a noise uncertainty model at Willie's location and derive the closed-form expression for the system's average detection error probability. Next, we propose and solve the optimization problem to maximize the average security rate subject to the covertness constraint. Simulation results show that the multi-antenna at Alice with maximum ratio beamforming scheme can significantly improves the system's both security and covertness, and that the noise uncertainty at Willie's location and the nominal noise level have a substantial impact on the system's average security rate.