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In this letter, we explore the benefits of applying a random frequency diversity array (RFDA) in near-field communications. To this end, we first derive the small-scale channel power gains (SSCPGs) for near-field systems both with and without RFDA. Our analysis reveals that RFDA achieves a significantly reduction in SSCPG at non-targeted locations, demonstrating that a superior level of beamfocusing performance can be achieved by RFDA in near-filed communications. Furthermore, we also investigate the SSCPG of a far-field system incorporating RFDA, which highlights that RFDA offers a lower SSCPG at non-targeted locations in near-field systems than in far-field systems.

This letter examines the optimal pilot length for key generation in correlated wireless channels. We first deduce the secret key capacity as a function of the pilot length N and other system parameters. Then, derive two fixed-point equations to identify the optimal N with and without an eavesdropper. Our analysis and numerical results confirm the existence of the optimal N that maximizes the secret key capacity, and both results for optimal N are consistent. Furthermore, the optimal N generally decreases with average signal-to-noise ratio and maximum Doppler shift. Our examination can provide useful guidelines in designing practical key generation systems.

ABSTRACT To investigate the effect of freeze-thaw cycle on the mechanical and porosity properties of the recycled construction waste mixtures, a series of freeze-thaw cycles, cyclic loading and unloading and [...]

ABSTRACT Diversity in microscopic particle morphology and the particle contact between geomaterials can play an important role in terms of their respective mechanical properties. In this paper, a series of [...]

In this work, we consider an intelligent reflecting surface (IRS)-assisted covert wireless detection system, where a transmitter aims to improve the detection accuracy of its transmission at a legitimate receiver, but avoid this transmission being detected by a warden. Considering Rician fading channels and a bounded noise uncertainty model, the detection performance at the legitimate receiver and warden is thoroughly analyzed. Then, the optimal design of the transmit power and IRS reflection matrix is proposed for maximizing the detection performance at the legitimate transceiver subject to a covertness constraint (i.e., the detection accuracy at the warden is less than a specific tolerable level). Our design is solely based on the line-of-sight (LoS) channel components and thus partially eliminates the need for acquiring the IRS's instantaneous channels, which takes a step towards the practical deployment of IRS. Our examination shows that IRS can still improve the covert detection performance even when only the LoS channel components are known and the improvement increases as the LoS components become more dominant.

In this work, we consider a covert communication system with a random frequency diverse array (RFDA) at the transmitter Alice, while the warden Willie's location follows a binomial point process (BPP) around Alice. Specifically, we consider two scenarios, where Alice's location is known or unknown to Willie, for which Willie's detection performance is thoroughly anzlyzed in terms of deriving his optimal detection threshold and the minimum detection error rate. Our analysis explicitly reveals that a higher covert rate can be achieved when Willie does not know Alice's location and the performance gap decreases as the BPP radius increases. In addition, the conducted analysis also shows that the achievable covert rate increases with the number of antenna elements at Alice and the BPP radius for Willie, which is supported by the characterized best detection performance at Willie. Furthermore, we clearly show that a certain covert rate is still achievable from Alice to the receiver Bob even when Bob is further away from Alice relative to Willie's potential location, which shows one obvious benefit of using RFDA in covert communications.

In this letter, we investigate a covert communication system with a wireless-powered jammer, where the jammer firstly harvests energy from a receiver and then generates artificial noise (AN) to confuse the detection at a warden on the communication activity. To evaluate the communication covertness and quality, we analytically derive the sum of error probabilities and the effective covert rate. Then, we jointly optimize the time switching ratio at the wireless-powered jammer and the transmit power at the receiver to maximize the effective covert rate, which can be further improved by increasing the maximum transmit power at the receiver, loosing the covertness constraint, or adjusting cooperatively the information transmit power. Moreover, our proposed scheme with the adaptive time switching ratio at a wireless-powered jammer achieves a higher effective covert rate than the scheme with a fixed time switching ratio and the conventional scheme with a cable-powered jammer, which demonstrates the benefit of introducing the adaptive time switching ratio into the wireless-powered jammer.

Integrated Sensing and Communication (ISAC) Systems, which can support simultaneous information transmission and target detection, have been regarded as promising solutions for various emerging wireless network applications. In this work, a covert transmission system with the aid of the ISAC technique is proposed, where the radar intends to sense a target and send message to legitimate users covertly against the warden's surveillance with either perfect or imperfect channel state information (CSI) at the warden. We formulate a beamforming optimization problem for the dual-function signal used for sensing and communication to maximize the covert throughput, subject to a covertness constraint, a maximum transmit power constraint, and a radar detection constraint. We then determine the sufficient conditions under which the covert beamforming design problem can be solved by semidefinite relaxation (SDR). Numerical results show that the proposed covert ISAC system can guarantee covert transmission while ensuring a certain level of sensing performance, and there exists a performance trade-off between the considered radar detection and covert transmission.

In this work, we investigate short-packet covert communication in interweave cognitive radio networks (CRNs), where a secondary transmitter (ST) opportunistically accesses the occasionally idle spectrum under the supervision of a primary transmitter (PT). Considering that PT occupies the spectrum in a random manner, inevitable packet collisions occur when ST is transmitting while PT also starts to occupy its spectrum. To address this issue, we jointly optimize the block-length and the transmit power of ST's transmission to strike a balance between the packet collision and the decoding error subject to a covertness constraint. In particular, we first derive the packet collision probability, the covertness constraint, and the average packet error probability, characterizing the analytical expression for the average effective spectral efficiency (AESE). To further improve the system performance, an optimization problem is formulated to maximize AESE and the corresponding optimal block-length and transmit power are obtained for different kinds of channel information available at ST. Simulation results show that there is an optimal block-length to reduce the packet collision and decoding error in the interweave covert CRN.

In this work, we investigate the status packet covert communication with time uncertainty in time-critical wireless networks. We model the packet generation as a Poisson process that concatenates the information timeliness and communication covertness, since the prior transmission probability is highly correlated with the packet generation. To balance between the timeliness and covertness of the status packet transmission, we propose two schemes, named random sub-slot selection (RSS) scheme and random channel use selection (RCUS) scheme, by exploiting the random transmission time to confuse a warden Willie’s binary detection on the covert communication. Subsequently, the average age of information (AoI) subject to the covertness constraint is derived for the proposed RSS and RCUS schemes. It is demonstrated that the symbol-length of the status packet introduces a non-trivial tradeoff between the covertness and timeliness. Inspired by this, further designs of the symbol-length and the transmit power for the proposed two schemes are formulated as optimization problems and solved optimally. Our numerical results demonstrate the superiority of proposed schemes over the existing covert communication strategies. In addition, our examination reveals that the RCUS scheme outperforms the RSS scheme when the covertness constraint is extremely strict. Otherwise, the RSS scheme generally outperforms the RCUS scheme in terms of achieving a lower AoI.

In this letter, we develop a new framework to design optimal waveform in a radar-communication system for maximizing its communication performance subject to a constraint on its detection performance. The novelty of our newly developed framework lies in that our detection constraint is expressed as the radar detection probability being greater than or equal to a specific value for a given false alarm probability, while this constraint was directly put on the waveform. Our framework can significantly outperform two benchmark schemes in terms of achieving a higher average sum rate subject to guaranteeing the same detection performance.

This letter considers covert communications in the context of unmanned aerial vehicle (UAV) networks, where a UAV is employed as a base station to transmit covert data to a legitimate ground user, while ensuring that the data transmission cannot be detected by a warden. Aiming at maximizing the legitimate user’s average effective covert throughput (AECT), the UAV’s trajectory and transmit power are jointly optimized. Taking advantage of deep reinforcement learning (DRL) on solving dynamic and unpredictable problems, we develop a twin-delayed deep deterministic policy gradient aided covert transmission algorithm (TD3-CT), to determine the UAV’s optimal trajectory and transmit power. Furthermore, by introducing a reward shaping mechanism, the convergence of the algorithm is guaranteed. The experiment results show that the developed TD3-CT algorithm not only enables the covert transmission but also significantly improves its performance in termed of achieving a higher AECT, compared with the benchmark schemes.

In this letter, we propose a novel energy-efficient covert wireless communication scheme through probabilistic jamming, where a jammer probabilistically transmits artificial noise (AN) with variable power against a warden’s detection. For the proposed scheme, the closed-form expressions for the warden’s detection error probability, the transmission outage probability, and the system energy efficiency are derived to reveal the impacts of the prior AN transmission probability on communication covertness, reliability, and system energy efficiency. To further improve the system energy efficiency, an optimization problem is formulated to determine the optimal prior AN transmission probability and the optimal transmit power of private information subject to a covertness constraint, which shows that continuous jamming is generally not optimal to maximize the energy efficiency in covert communication systems. Simulation results verify our analysis and show the superiority of proposed scheme comparing to a widely adopted scheme where the jammer always transmits AN with random transmit power.

This work employs an unmanned aerial vehicle (UAV) as a jammer to aid a covert communication from a transmitter Alice to a receiver Bob, where the UAV transmits artificial noise (AN) with random power to deliberately create interference to a warden Willie. In the considered system, the UAV’s trajectory is critical to the covert communication performance, since the AN transmitted by the UAV also generates interference to Bob. To maximize the system performance, we formulate an optimization problem to jointly design the UAV’s trajectory and Alice’s transmit power. The formulated optimization problem is non-convex and is normally solved by a conventional iterative (CI) method, which requires multiple approximations based on Taylor expansions and an initialization on the UAV’s trajectory. In order to eliminate these requirements, this work, for the first time, develops a geometric (GM) method to solve the optimization problem. By analyzing the covertness constraint, the GM method decouples the joint optimization into optimizing the UAV’s trajectory and Alice’s transmit power separately. Our examination shows that the GM method can significantly outperform the CI method in terms of achieving a higher average covert rate and the complexity of the GM method is lower than that of the CI method.

In this paper, we investigate the optimal design of a wireless-powered covert communication (WP-CC) system, where a full-duplex (FD) receiver transmits artificial noise (AN) to simultaneously charge an energy-constrained transmitter and to confuse a warden’s detection on the transmitter’s communication activity. In order to achieve a higher level of covertness, we propose a probabilistic accumulate-then-transmit (ATT) protocol, where the transmitter is able to adjust the prior probability conditioned on the available energy being sufficient, i.e., $p$ , rather than setting $p=1$ as in the traditional ATT protocol to maximize the system throughput. Then, we derive the warden’s minimum detection error probability and characterize the effective covert rate from the transmitter to the receiver to quantify the communication covertness and quality, respectively. The derived analytical results facilitate the joint optimization of the probability $p$ and the information transmit power to maximize the communication covertness subject to a quality-of-service (QoS) requirement on communication. We further present the optimal design of a cable-powered covert communication (CP-CC) system as a benchmark for comparison. Our simulation shows that the proposed probabilistic ATT protocol (with a varying $p$ ) can achieve the covertness upper bound determined by the CP-CC system, while the traditional ATT protocol (with $p=1$ ) cannot, which confirms the benefits brought by the proposed probabilistic ATT protocol in covert communications.

In this letter, a reconfigurable intelligent surface (RIS) is employed to enhance the secure performance of non-orthogonal unicast-multicast (NOUM) transmissions, where the propagation environment can be manipulated to achieve communication security while maintaining the quality of service (QoS) requirements of the NOUM system. Our objective is to maximize the secrecy rate of the unicast users under the QoS constraints on the multicast transmissions, where the multicast users are eavesdropping on the unicast messages. The joint optimization of the non-orthogonal power splitting and the RIS reflection matrix leads to a non-convex problem, to which two alternating optimization (AO) based solutions are developed based on the semi-definite programming (SDP) and penalty-based successive convex approximation (SCA). Our results demonstrate that secure transmission can be guaranteed in the RIS-aided NOUM system. Furthermore, the developed efficient solutions together with the RIS significantly improve the achievable security.

In this letter, we develop a new framework to jointly characterize covertness and timeliness of short-packet communications, in which a new metric named covert age of information (CAoI) is first proposed and then a closed-form expression for the average CAoI is derived. Our examination explicitly reveals the tradeoff between communication covertness and timeliness affected by the block-length, transmit power and prior transmission probability. Multiple transmission designs are tackled in order to minimize the average CAoI subject to covertness constraint, where the resultant differences relative to the designs with traditional metrics (e.g., effective covert rate, AoI) as objective functions are clarified. Our examination demonstrates that the optimal block-length is not the largest one, which is optimal in delay-constrained covert communication without considering the communication timeliness, and the optimal prior transmission probability may not be 1/2, which has been widely assumed in the literature of covert communications.

In this letter, we consider the requirement of information freshness in covert communications for the first time. With artificial noise (AN) generated from a full-duplex (FD) receiver, we formulate a covertness maximization problem under the average age of information (AoI) constraint to optimize the transmit probability of information signal. In particular, the transmit probability not only represents the generation rate of information signal but also represents the prior probability of the alternative hypothesis in covert communications, which builds up a bridge between information freshness and communication covertness. Our analysis shows that the best transmit probability is not always 0.5, which differs from the equal prior probabilities assumption in most related works on covert communications. Furthermore, the limitation of average AoI enlarges the transmit probability at the cost of the covertness reduction and leads to a positive lower bound on the information transmit power for non-zero covertness.

The composite-based biomass-derived carbon materials have attracted numerous researchers’ interests for their advantages in supercapacitors. Herein, biomass-derived carbon from vegetable sponge (VSCs) was combined with Mn3O4through a facile strategy, in which the residual base at activating VSC process was reused. The composites (VSCs/Mn3O4) were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and Brunauer–Emmett–Teller surface area measurements, respectively. Cyclic voltammetry and galvanostatic charge/discharge technique were performed for the composites in 1 M Na2SO4aqueous solutions. The obtained VSCs/Mn3O4-2 exhibited a promising capacity of 162.8 F·g−1at 0.5 A·g−1with good rate capability of 89.5% at high current density of 5 A·g−1, which was better than pure Mn3O4and VSCs for special structure of composites and introduction of Mn3O4with various valence states of Mn. The present work provided an economical and simple way to prepare biomass carbon/metallic oxide composites.

This letter tackles whether and how repetition coding enables reliable and covert communications by considering binary phase shift keying. Communication reliability and covertness are analyzed in terms of the bit error rate $P_{e}$ and the minimum detection error probability $\xi ^{\ast }$ . Our examination shows that, when a warden Willie knows the codeword structure, both $P_{e}$ and $\xi ^{\ast }$ are determined by the total power nP and do not depend on the codeword length ${n}$ or the transmit power ${P}$ individually. However, this conclusion does not hold for the unknown codeword structure, where increasing ${n}$ can significantly improve the communication reliability under the same covertness constraint, and vice versa.

This work considers covert communications with the aid of an intelligent reflecting surface (IRS). We jointly design the transmit power and IRS’s reflection phase shifts and amplitudes to enhance covert communication performance. To this end, we first derive the total detection error rate at a warden. We then determine the IRS’s optimal phase shifts and develop an effective one-dimensional search method to identify the IRS’s optimal amplitudes. We show that the IRS can significantly improve the achievable covertness. In addition, optimizing the IRS’s amplitudes on top of its phase shifts significantly outperforms fixing them as unit in covert communications.

This letter considers covert communication in downlink NOMA systems, where a public action (communication from Alice to the NOMA-weak user U1) is used to provide a cover for a secret action (communication from Alice to the NOMA-strong user U2). To aid system design, we first derive the detection error probability (DEP) at Willie and the connect outage probabilities (COPs) at U1 and U2. Then, considering the covertness and outage constraints, the optimization of power allocation factor ratio between U1 and U2 is tackled in order to maximize the effective covert rate. The results demonstrate that the minimum DEP is the power allocation factor for U1. In addition, for a given covert constraint, relaxing the outage constraint will effectively increase the maximum effective covert rate within a certain range of the maximum transmit power of U1.

URLLC is one category of service to be provided by next-generation wireless networks. Motivated by increasing security concerns in such networks, this article focuses on physical layer security in the context of URLLC. The physical layer security technique mainly uses transmission designs based on the intrinsic randomness of the wireless medium to achieve secrecy. As such, physical layer security is of a lower complexity and incurs less latency than traditional cryptography. In this article, we first introduce appropriate performance metrics for evaluating physical layer security in URLLC and investigate the trade-off between latency, reliability, and security. We then identify the key challenging problems for achieving physical layer security in URLLC, and discuss the role that channel state information can have in providing potential solutions to these problems. Finally, we present our recommendations on future research directions in this emerging area.

LPD communication has recently emerged as a new transmission technology to address privacy and security in wireless networks. Recent studies have established the fundamental limits of LPD communication in terms of the amount of information that can be conveyed from a transmitter to a receiver subject to a constraint on a warden's detection error probability. The established information- theoretic metric enables analytical studies on the design and performance of LPD communication under various channel conditions. In this article, we present the key features of LPD communication and discuss various important design considerations. First, we clarify the differences between LPD communication and well-known physical-layer security. Then, from an information-theoretic point of view, we discuss the optimal signaling strategies for transmitting the message-carrying signal and artificial noise signal for LPD communication. Finally, we identify the key challenges in the design of practical LPD communication systems and point out future research directions in this context. This article provides guidelines for designing practical LPD communication strategies in wireless systems and networks.

In this letter, we consider delay-constrained covert communications with the aid of a full-duplex (FD) receiver. Without delay constraints, it has been shown that the transmit power of artificial noise (AN) at the FD receiver should be random in order to enhance covert communications. In this letter, we show that transmitting AN with a fixed power indeed improve covert communications with delay constraints, since in a limited time period the warden cannot exactly learn its received power. This explicitly shows one benefit of considering practical delay constraints in the context of covert communications. We analyze the optimal transmit power of AN for either fixed or globally optimized transmit power of covert information, based on which we also determine the specific condition under which transmitting AN by the FD receiver can aid covert communications and a larger transmit power of AN always leads to better covert communication performance.

Mobile data traffic has dramatically increased by a factor of around 100 over the last five years, while it is still expected to grow exponentially in the near future. Evidence indicates that the repetitive downloading of popular contents accounts for the increase in wireless traffic. This has consequently led to the development of edge caching technology for efficiently mitigating redundant transmissions from backbone networks by pre-caching frequently requested data at radio access networks. Among a variety of research topics in this field, the commercialization of edge caching has attracted augmented attention. In this article, we focus on the design of incentive mechanisms for commercial edge caching in 5G cellular networks, where the edge device providers (EDPs) and content providers (CPs) are the two counter-parties competing to maximize their own welfare. We first propose the architecture of a commercial caching system and analyze the revenue of the entities involved. Due to selfishness, each entity tries to achieve its highest benefit by squeezing profits from others, causing contentions of interest. Game theoretic approaches are then introduced to balance the conflicts. Four incentive frameworks are developed based on different categories of games in the context of the caching system. Afterward, we conduct case studies for three application scenarios to demonstrate the superiority of our proposed game models. Numerical results are also provided to validate the effectiveness of our proposed game frameworks.