Modular extremely large-scale array (mXL-array) represents a novel architecture for the extremely large aperture array (ELAA), offering extended coverage and significantly enhanced spatial resolution in near-field communication scenarios due to its unique modular array structure and flexible deployment capabilities. This paper is the first study to explore the potential of mXL-array in enhancing covert performance in the near-field communication. We design a sub-module-connected hybrid beamforming architecture to maximize the covert transmission rate conditional on only imprecise location information of Willie is available. First, a computationally efficient algorithm incorporating alternating direction method of multipliers and majorization-minimization (ADMM-MM) is developed to derive a closed-form solution for the fully digital beamforming vector. Subsequently, we obtain the baseband digital beamforming vector and the analog beamforming matrix by employing an alternating optimization (AO) algorithm. Simulation results demonstrate the excellent spatial beamforming control capability of the proposed algorithm. Specifically, even when Willie is positioned between Alice and Bob, the proposed algorithm enables the beam to avoid the region occupied by Willie while focusing the main beam toward Bob, thereby significantly improving the covert transmission rate. In addition, the superior performance of the proposed schemes, compared to existing benchmark algorithms, is verified through numerical simulations.

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.

This paper proposes a novel diffusion-enabled pluggable encryption/decryption modules design against semantic eavesdropping, where the pluggable modules are optionally assembled into the semantic communication system for preventing eavesdropping. Inspired by the artificial noise (AN)-based security schemes in traditional wireless communication systems, in this paper, AN is introduced into semantic communication systems to prevent semantic eavesdropping. However, the introduction of AN also poses challenges for the legitimate receiver in extracting semantic information. Recently, denoising diffusion probabilistic models (DDPM) have demonstrated their powerful capabilities in generating multimedia content. Here, the paired pluggable modules are carefully designed using DDPM. Specifically, the pluggable encryption module generates AN and adds it to the output of the semantic transmitter, while the pluggable decryption module before semantic receiver uses DDPM to generate the detailed semantic information by removing both AN and the channel noise. In the scenario where the transmitter lacks eavesdropper’s knowledge, the artificial Gaussian noise (AGN) is used as AN. We first model a power allocation optimization problem to determine the power of AGN, in which the objective is to minimize the weighted sum of data reconstruction error of legal link, the mutual information of illegal link, and the channel input distortion. Then, a deep reinforcement learning framework using deep deterministic policy gradient is proposed to solve the optimization problem. In the scenario where the transmitter is aware of the eavesdropper’s knowledge, we propose an AN generation method based on adversarial residual networks (ARN). Unlike the previous scenario, the mutual information term in the objective function is replaced by the confidence of eavesdropper correctly retrieving private information. The adversarial residual network is then trained to minimize the modified objective function. Simulation results show that the diffusion-enabled pluggable encryption module prevents semantic eavesdropping with high covertness while the pluggable decryption module achieves the high-quality semantic communication.

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.

Existing reconfigurable intelligent surface (RIS)-enabled symbiotic systems generally rely on a dynamic adjustment of the amplitude of the RIS’s reflection coefficient to continuously change from 0 to 1 to support the underlying symbiotic trans-missions, which is however infeasible for practical RIS hardware. To address this, we propose a novel on-off digitalized symbiotic backscatter non-orthogonal multiple access (NOMA) system that employs an on-off mechanism of the RIS’s reflecting elements. In particular, the adjustment of the on-off state of reflecting elements in batches is adaptively invoked to control the power gain of the backscatter channel, thereby changing the amplitude of backscatter signals to establish symbiotic transmissions. In order to evaluate the practicability of the proposed on-off symbiotic mechanism, an analytical expression of coexistence outage probability at high SNR has been derived. Moreover, the adjustment of RIS’s elements on-off state is discussed and extended to the scenarios of one-shot and one-by-one activation modes. Finally, representative numerical results show that when a sufficient number of reflecting elements is deployed on the RIS, our proposed on-off mechanism can fully support the symbiotic transmissions of the underlying systems.

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.

In ground-air networks, dual-function radar and communication (DFRC) enables a base station (BS) to sense and send communication signals to unmanned aerial vehicles (UAVs) simultaneously, which has enormous potential. The emphasis of this letter is on the design of DFRC waveform for ground-air system under inaccurate target angle. The desired covariance matrix is considered to have the best radar performance in the presence of inaccurate target angle and the waveform design objective is to make the corresponding covariance matrix approximated to it. The constraints conclude multi-user interference (MUI) energy for high quality of communication and constant modulus for avoiding signal distortion. To address the problem that minimizes the covariance matrix difference under the constraints, we propose a block coordinate descent (BCD) method. Moreover, to ensure the problem feasible, we design an initial value acquisition strategy using bisection-based alternating direction method of multipliers (ADMM). Numerical results indicate that better communication and radar performance are achieved by the proposed method compared with existing methods.

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.

This paper puts forth a proposal for a reconfigurable intelligent surface (RIS) enabled symbiotic backscatter system riding over a multiuser non-orthogonal multiple access (NOMA) carrier, where the RIS is employed as a substitute for the conventional backscatter device (BD) with the objective of enhancing both primary NOMA transmission and backscatter communications. In particular, a symbiotic mechanism is proposed to control the mutual interference by means of an adaptive adjustment of the reflection coefficient at the RIS. Moreover, a novel and effective multiuser decoding scheme is proposed, based on successive interference cancellation (SIC), with the objective of ensuring the successful transmission of the symbiotic system. To this end, analytical expressions re derived for the closed-form outage lower bounds, asymptotic outage behavior and outage error floors are derived. Furthermore, the upper bounds of the ergodic capacity and its asymptote at high signal-to-noise ratio (SNR) are developed in order to illustrate the spectral efficiency of the proposed symbiotic system. Theoretical analysis and numerical results demonstrate that a stronger backscatter channel with a greater number of elements deployed on a segmented RIS results in a reduction in transmission outage and an increase in system capacity. Additionally, numerical results illustrate the spectral efficiency advantage of the proposed symbiotic system over the conventional solutions.

Abstract Communication security has become particularly crucial with the rapid development of the Internet of Things (IoT). Frequency hopping spread spectrum (FHSS) technology, a prevalent method in wireless communication, has a wide range of applications in the Internet of Things. Enhancing the security of frequency hopping sequences is an essential means to improve the security of frequency hopping communication in the Internet of Things, as the performance of frequency hopping sequences plays a crucial role in frequency hopping systems. This paper proposes constructing secure adaptive frequency hopping sequence sets based on the advanced encryption standard (AES) algorithm. As a block cipher algorithm with superior security, the AES algorithm can provide a fundamental guarantee for the security of the proposed frequency hopping sequences. The mapping methods from ciphertext sequences to frequency hopping sequences proposed in this paper can achieve the construction of frequency hopping sequences of any frequency set size to meet the requirements of adaptive frequency hopping. In addition, we also model and analyse the problem of overlapping spectrum band of the IoT groups in the industrial, scientific, and medical (ISM) band, aiming to achieve better packet transmission performance by adjusting the frequency set size.

The orthogonal time frequency space (OTFS) modulation is resilient to the Doppler effect and thus is employed in high-mobility communications. In earlier work, the equivalent representations of the OTFS transmission were established, revealing the profound impact of fractional delay on channel sparsity. However, these representations tend to be distorted and cumbersome when accounting for digital reception and non-ideal filtering. In this letter, we propose an alternative representation with low distortion that uses the variable-fractional-delay filter to characterize the fractional delays in a conventional digital transceiver. In our proposed method, the approximation of the received signal is improved, and the error performance is enhanced compared to the original approaches. At last, the simulations show that our proposed representation is valid.

The interleaving technology improves the proximity between bits by dispersing the coded bits into different frames, so as to obtain better decoding performance, which is called time diversity. In the fast frequency hopping (FFH) scheme, each symbol is transmitted through one or more frequencies, so that the interference is averaged between several carrier frequencies, which is called frequency diversity. This paper introduces the idea of interleaving into the FFH scheme and proposes a joint design of fast frequency hopping and time hopping (FFH-TH), which can obtain the benefits of frequency diversity and time diversity simultaneously. In the presence of pulse full-band interference, the FFH-TH scheme can achieve better overall bit error rate performance under hard decision than the FFH scheme, which is strictly theoretically proven in the paper. In addition, random time hopping also brings anti-deciphering effect to demodulation.

We investigate the problem of network selection for the Internet of Things (IoT) to maximize the Quality of Experience (QoE) in a heterogeneous wireless environment. Different from the traditional network access approaches with the assumption that the network state information (NSI) is static and known a priori, a scenario where the NSI of networks is unknown and dynamic to IoT devices is considered. Due to hardware limitations in IoT, the device has a limited resource budget and consumes resources, e.g., the energy, during the process of network access. To maximize the cumulative QoE before resource exhausts, the device should learn and estimate the NSI of networks, and make appropriate decisions to balance the network selection and resource consumption. To address this issue, we formulate the problem as a combination of multiarmed bandit and optimization problems and propose two algorithms NCSA and network selection algorithm (NSA). Moreover, we consider the fact that the device has various traffic types and propose an algorithm MT-NSA. Theoretical analysis shows that the regret of all algorithms has a sublinear relationship with the resource budget. The effectiveness is validated by simulations.

This paper investigates the limit covert rate of the backscatter communication systems based on undecodable ambient radio frequency (RF) signals over additive white Gaussian noise channel. In the considered network, source node Alice expects to transmit information to receiver Bob without being detected by warden Willie, by backscattering undecodable ambient RF signals. In this case, it is challenging for Bob to recover the information from Alice, since the undecodable signals result in both unknown additive and multiplicative interference. A subtle communication strategy is proposed to alleviate the influences of the interference, by which we prove that no matter Willie can or cannot decode the information from the RF source and then eliminate its interference, the limit covert rate is subject to the well-known square root law (i.e.,O(√n) bits pernchannel uses). This interesting result means that even if the RF source is a cooperative node of Willie (e.g., a jammer deployed by Willie to restrict Alice from active transmission), Alice can still covertly and reliably transmit information to Bob by backscattering the ambient RF signals (e.g., the malicious jamming signals).

In this paper, a backscatter cooperation (BC) scheme is proposed for non-orthogonal multiple access (NOMA) downlink transmission. The key idea is to backscatter the surplus power of the received downlink signals at one user to enhance the reception of the user who cannot recover its information. To evaluate the performance of the proposed BC-NOMA scheme, three benchmark schemes are introduced, which include the non-cooperation (NC)-NOMA scheme, the conventional relaying (CR)-NOMA scheme, and the incremental relaying (IR)-NOMA scheme. For all these schemes, the outage performance, the expected rate, and the diversity-multiplexing trade-off (DMT) performance are analyzed. Additionally, the optimal power allocation to maximize the expected rate of the BC-NOMA scheme in the high signal-to-noise ratio region is derived. The theoretical results show that the proposed BC-NOMA can enhance both transmission reliability (outage performance) and transmission effectiveness (expected rate) compared with NC-NOMA. Furthermore, representative numerical results are presented to validate the theoretical results. It is shown that unlike the two relaying-based cooperative schemes (CR/IR-NOMA), which improve the outage performance at the cost of transmission rate, the proposed BC-NOMA scheme can enhance the transmission reliability without impairing the spectrum efficiency, which makes backscattering an appealing solution to cooperative NOMA downlinks.

The frequency diverse array (FDA)-based directional modulation (DM) technology plays an important role in the physical-layer security (PLS) transmission of 5G and beyond communications. In order to meet the tremendous increase in mobile data traffic, a new memory-efficient design for the FDA-DM-based PLS transmission is urgently demanded. In this article, an analytical symmetrical multi-carrier FDA model is proposed in three dimensions, namely, range, azimuth angle, and elevation angle, differing from the conventional analytical approach with only range and azimuth angle considered. Then, a single-point (SP) artificial noise (AN) aided FDA-DM scheme is proposed, which reduces the memory consumption significantly compared with the conventional zero-forcing (ZF) and singular value decomposition (SVD) approaches. Moreover, the PLS performance of the proposed FDA-DM scheme is analyzed in fluctuating two-ray (FTR) fading channels for the first time, including the average secrecy capacity (ASC) and the secrecy outage probability (SOP). More importantly, the closed-form expressions for the lower bound on ASC and the upper bound on SOP are derived, respectively. The effectiveness of the analytical expressions is verified by numerical simulations. This work opens a way to lower the memory requirements for DM-based PLS transmission of 5G and beyond communications.

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.

A backscatter cooperation (BC) mechanism is pro-posed for power-domain non-orthogonal multiple access (NOMA) downlink transmission. The key idea is to enable one user to split and then backscatter part of its received signals to improve the reception at another user. To evaluate its performance, three benchmark schemes are introduced. They are non-cooperation (NC)-NOMA, conventional relaying (CR)-NOMA, and incremental relaying (IR)-NOMA. For all these schemes, the analytical expressions of the minimum total power to avoid information outage are derived, based on which their respective outage performance, expected rates, and diversity-multiplexing trade-off (DMT) are investigated. Analytical results show that the proposed BC-NOMA strictly outperforms NC-NOMA in terms of all the three metrics. Furthermore, theoretical analyses are validated via Monte-Carlo simulations. It is shown that compared with CR/IR-NOMA, only the proposed BC-NOMA scheme can enhance the transmission reliability without impairing the transmission rate.

In this paper, an impartial simultaneous wireless information and power transfer (SWIPT)-assisted cooperation mechanism is proposed for a non-orthogonal multiple access (NOMA) downlink scenario. The key idea of the proposed impartial user cooperation mechanism (IUCM) is that not only the cell-center user but also the cell-edge user applies the power-splitting architecture and utilizes the harvested energy to forward the other user's information. Analytical results show that the proposed IUCM outperforms the traditional partial cooperation mechanism in terms of outage probability, diversity order and diversity-multiplexing trade-off (DMT). For comparison, the proposed IUCM is further incorporated into an orthogonal frequency-division multiple access (OFDMA) framework, which is shown to preserve the same diversity order, while has a worse but more flexible DMT performance compared with the IUCM in the NOMA framework. Furthermore, a classic non-linear energy harvesting model is introduced, based on which the outage probability, diversity order and DMT performance of the proposed IUCM and the conventional mechanism are analyzed. Finally, representative numerical results are given to validate our theoretical analysis.

A two-ray multipath model for frequency diverse array (FDA)-based directional modulation (DM) is proposed in multi-input single-output multi-eavesdropper (MISOME) wiretap channels for the first time. The excitation factors of the FDA and the weighting coefficients of the inserted artificial noise (AN) are jointly designed in a way which imposes no impact on the desired receiver while simultaneously distorting the received signals of eavesdroppers. Secrecy rate is analyzed for the proposed two-ray multipath FDA-based DM model. Numerical simulations verify the capability of physical layer secure (PLS) transmissions of the proposed FDA-DM model in two-ray multipath MISOME wiretap channels.

Hybrid beamforming in transmitter and receiver can improve the spectral efficiency of millimeterwave (mmWave) multiple-input multiple-output (MIMO) communication system significantly, and can reduce the complexity of communication system. However, when the number of transmitted data streams is large, the existing designing schemes of hybrid beamforming, such as sparse approximation by orthogonal matching pursuit(OMP) algorithm, suffer from performance degradation. In this paper, we propose dictionary learning (DL) algorithm to perform the design of hybrid beamforming for mmWave MIMO system. Simulation result shows that the spectral efficiency of hybrid beamforming based on DL approaches that of optimal beamforming without constraint, which is far superior to the spectral efficiency of hybrid beamforming based on OMP. Moreover, the convergence property of the proposed algorithm is also verified.

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.

The artificial noise (AN) aided multi-beam directional modulation (DM) technology is capable of wireless physical layer secure (PLS) transmissions for multiple desired receivers in free space. The application of AN, however, makes it less power-efficient for such a DM system. To address this problem, the weighted fractional Fourier transform (WFRFT) technology is employed in this paper to achieve power-efficient multi-beam DM transmissions. Specifically, a power-efficient multi-beam WFRFT-DM scheme with cooperative receivers and a power-efficient multi-beam WFRFT-DM scheme with independent receivers are proposed based on frequency diverse array (FDA), respectively. The bit error rate (BER), secrecy rate, and robustness of the proposed multi-beam WFRFT-DM schemes are analyzed. Simulations demonstrate that 1) the proposed multi-beam WFRFT-DM schemes are more power-efficient than the conventional multi-beam AN-DM scheme; 2) the transmission security can also be guaranteed even if the eavesdroppers are located close to or the same as the desired receivers; and 3) the proposed multi-beam WFRFT-DM schemes are capable of independent transmissions for different desired receivers with different modulations.

This letter considers the decentralized detection of a noncooperative moving target by employing a wireless sensor network. Suppose that, if present, the target moves along a direction with a constant velocity, and it emits an unknown signal experiencing distance-dependent attenuation that is periodically sampled by sensors. The sensor observations are quantized into one-bit data individually and then sequentially transmitted to a fusion center, which is in charge of making a global decision. We first derive the generalized Rao test statistic as a more computationally efficient alternative when compared to the typical generalized likelihood ratio test statistic. Then, we propose a truncated one-sided sequential (TOS) test rule by imposing a finite maximum stopping time (namely the deadline) on typical one-sided sequential tests. With a deadline slightly larger than the sample size of a benchmarked fixed-sample-size (FSS) test, the proposed TOS test rule provides the same detection performance and significantly accelerates the target-detection process on average, which is corroborated by simulation results.