Physical layer covert communication is a crucial secure communication technology that enables a transmitter to convey information covertly to a recipient without being detected by adversaries. Unlike typical cryptography and physical layer security systems that concentrate on protecting the sent signal content, covert communications seek to conceal the existence of legitimate transmission. Thus, with beyond fifth-generation (B5G) wireless communications, covert communications can operate in tandem or as a supplement to conventional security techniques. We provide an extensive overview of the basic theories and several strategies in physical layer covert communications in this article. In particular, we go into great detail about the basic theories of physical layer covert communications, such as channel models, codes, secret keys, and covertness metrics, as well as various covert schemes in progressively more complicated scenarios, such as covert communications in single-antenna and multiantenna three-node systems and covert communications in jammer-and relay-aided systems. In addition, we identify the challenges and future directions for research on covert communications in B5G wireless networks.

A device-to-device (D2D) communication link is vulnerable because it is relatively easy to be compromised by adversaries due to the fact that D2D terminals are power limited nodes. This article proposes a covert communication scheme that allows D2D communication links to transmit covert signals (e.g., privacy sensitive data) to ensure a low probability of detection. We allocate transmit powers of cellular and D2D noncovert signals following a fading D2D channel to add the uncertainty in adversary’s background noises. With the help of base station, cooperative power domain nonorthogonal multiple access and successive interference cancellation are used to decode covert signal. Depending on whether a D2D transmitter has the knowledge of adversary’s detection threshold, we derive minimum error probability to measure the covertness performance. Furthermore, we evaluate covert throughput, which is defined as a maximum average transmission rate of covert signal, subject to a given covertness performance.

For the sake of enhancing the covertness and sensing performance in the integrated sensing and covert communications (ISCCs) system, we design a beamforming framework for reconfigurable intelligence surface (RIS) assisted ISCC. Specifically, the system intends to transmit information to a legitimate receiver (Bob) covertly and sense the target simultaneously while avoiding being detected by a warden (Willie). RIS can be applied to both traditional single-connected networks and a broad fully connected networks. By jointly optimizing the beamforming vector of the communication and the autocorrelation matrix of the sensing, and the phase shift matrix of the RIS, both the convert rate and the target’s probing power are maximized. And the covertness and the constant-mode constraints are considered. A multistrategy alternate optimization (MSAO) algorithm is proposed to solve the optimization problem based on quadratic constraint quadratic programming (QCQP) and semidefinite relaxation (SDR). Furthermore, we consider a more realistic application scenario where the legal party has imperfect channel state information of Willie. Simulation results show that deploying RIS in a generalized fully connected mode can achieve better transmission of beampatterns and increase upper limit of covert communications rate than conventional single-connected mode.

Magnetoresistance (MR) and paramagnetic (PM) resonance experiments for $({\mathrm{La}}_{1-x}{\mathrm{Y}}_x{)}_{2/3}{\mathrm{Ca}}_{1/3}{\mathrm{MnO}}_3$ $(x=0.15\mathrm{}$ and 0.2) show that associated with PM anomalies at $T<~T_{\mathrm{onset}},$ the MR becomes observable, especially, both MR and resonance linewidth exhibit similar T-dependent behavior in the temperature range of $T_C<~T<~T_{\mathrm{onset}},$ suggesting the evidence of the same underlying origin for both MR and PM anomalies. We argue both PM anomalies and sizable MR to be due to a substantial increase in the volume fraction of the more conductive ferromagnetic domains.

Wireless energy harvesting (WEH) enabled Device-to-Device (D2D) communication emerges as an effective technique to improve spectral and energy efficiencies. However, D2D users are usually power-constrained devices that may be monitored or attacked by adversaries easily. To confuse randomly distributed adversaries in WEH-enabled D2D underlaying cellular networks, power beacons (PBs) can be used to send jamming signals in idle time slots. A time slot is divided into two sub-slots, i.e., WEH sub-slot and covert transmission sub-slot. The energy collected by a D2D transmitter in the first sub-slot is used to support possible covert transmission in the second sub-slot. The performance of the proposed scheme is measured by covert throughput, which is defined as D2D communication rate with several constraints, e.g., wireless energy harvesting, covertness, and cellular link communication requirements. The closed-form expressions of energy outage probability, covert probability, and desired link connection outage probabilities are derived. The minimum covert probability is used to measure covertness. Moreover, an alternating optimization algorithm is adopted to maximize covert throughput. The analytical results are compared with Monte-Carlo simulation results to verify the analytical approach. In addition, the impacts of different parameters (e.g., density of PBs and covert signal transmit power) on covert performance are evaluated.