With the development of the sixth generation wireless communication, the increasingly scarce spectrum resources limit the further increase in data rate and exacerbate the interference problem among different users and applications. To address this issue, rate-splitting multiple access (RSMA) provides a flexible framework that unifies existing orthogonal and non-orthogonal multiple access schemes. In this work, we analyze the interference scenario of RSMA-based space-terrestrial transmission systems, with multiple satellite users independently and uniformly distributed in the coverage area of the serving satellite. Specifically, the outage performance at the satellite users (resp. terrestrial base station (BS) users) is assessed while considering the interference from the BS users (resp. the satellite and the BSs of other cells). Approximate analytical expressions of the outage probability at each satellite user/BS user are derived, numerically evaluated, and verified through simulation results. The impacts of RSMA power allocation factors, fading parameters, interference severity, and satellite altitude on outage performance are thoroughly analyzed, and the trade-off between outage performance and user fairness is also illustrated.

Integrated sensing and communication (ISAC) is a rapidly evolving and highly promising technology for wireless networks, enabling the concurrent execution of communication and sensing functions within constrained spectrum resources. This paper examines a secure ISAC system assisted by an uncrewed aerial vehicle (UAV), in which the extended Kalman filter is incorporated for accurate estimation and prediction of eavesdropper states. Radar signals are transmitted to track and jam potential eavesdroppers while communication services are simultaneously provided to ground users. Under practical constraints—including maximum UAV velocity, transmit power, and propulsion energy consumption—the achievable uplink rate is maximized through joint optimization of the transmit beamforming and UAV trajectory. To address the computationally complex and inherently non-convex optimization problem, an efficient iterative algorithm combining block coordinate descent with successive convex approximation is designed, yielding a high-quality suboptimal solution. Finally, comprehensive simulations are conducted to evaluate the effectiveness of the proposed scheme against benchmark methods and to elucidate the inherent trade-off between communication and sensing performance.

This paper tackles the challenge of enabling covert communication in a dual-hop space-air-ground integrated network (SAGIN) comprising a satellite, an aerial decode-and-forward relay, an aerial-friendly jammer, and a terrestrial receiver. The direct satellite-to-receiver link is often compromised by deep fading, necessitating the use of relays to forward signals. To counteract the warden’s detection of signals from the satellite and relay, a friendly jammer is deployed to emit artificial noise (AN) to confuse the warden’s decision with two jamming schemes: uninformed jamming (UJ) and cognitive jamming (CJ). In the UJ scheme, the jammer continuously sends AN, while in the CJ scheme, the jammer adjusts its AN emission based on its detection results. Considering finite block-length communication, we derive closed-form expressions for the system’s covert outage probability under both schemes, which are crucial for evaluating the warden’s detection performance and the quality of service of legitimate link messages. The correctness of the results is validated through Monte Carlo simulations, and the impact of relevant parameters on covert performance is analyzed. The findings can provide new insights and methods for enhancing covert communication in SAGIN systems.

In this paper, we investigate the outage probability of a multi-tier dual-hop terrestrial-low earth orbit (LEO) satellite-geostationary earth orbit (GEO) satellite hybrid wireless communication system. The system comprises multi-tier LEOs and one GEO act as relays in the uplink, which help the terrestrial ground station (S) transmit information to the terrestrial destination (D). In uplink transmission, we introduce a one-dimensional hardcore point process to model different altitudes of LEOs’ different tiers. We also use a generalized selection combining technique to achieve a trade-off between maximum ratio combining and selection combining. In downlink, GEO offers the maximum coverage to randomly distributed D. Moreover, the independent identically distributed Nakagami-m fading and shadowed Rician distribution are brought to model the different channels. Finally, Monte-Carlo simulations are presented to affirm the precision and accuracy of the derived analytical models and the proposed analysis. This framework offers crucial insights for system designers and network operators, enabling the optimization of resource allocation, relay strategies, and overall reliability in terrestrial-satellite hybrid networks.

This paper investigates the internal secrecy and external covertness of a mixed-trust autonomous aerial vehicle (AAV) communication system assisted by rate-splitting multiple access (RSMA). In this setting, a semi-trusted user with partial decoding capability poses an internal eavesdropping threat, while multiple distributed wardens attempt to detect the transmission from the AAV to the semi-trusted user, creating an external covertness challenge. To characterize these security aspects, a unified analytical framework is developed. First, the internal eavesdropping capability of the semi-trusted user is quantified by deriving a closed-form expression for its eavesdropping success probability. Based on the outcome of the eavesdropping attempt, tractable expressions for the secrecy outage probability of the legitimate user are obtained. Furthermore, the external covertness performance is analyzed by deriving closed-form false alarm probability, missed detection probability, and detection error probability (DEP) for an individual warden, together with the optimal detection threshold and the corresponding minimum DEP. The cooperative global detection performance with multiple wardens is further characterized under conservative fusion rules. Extensive Monte Carlo simulations validate the analytical results and, through a joint evaluation of secrecy, reliability, and covertness metrics, illustrate the feasible operating regions enabled by RSMA power allocation in comparison with a NOMA baseline. The results provide a comprehensive theoretical basis for the design of secure and covert AAV communication strategies in mixed-trust environments.

Integrated sensing and communication (ISAC), an emerging technology for wireless networks, enables efficient use of scarce spectrum resources by supporting simultaneous communication and sensing. This work investigates the secrecy performance of an uncrewed aerial vehicle (UAV)-assisted ISAC system, in which a UAV operates as an aerial base station to serve multiple ground users while simultaneously transmitting sensing signals to detect potential eavesdroppers. To maximize the average secrecy rate, we jointly optimize the user scheduling, transmit beamforming, receive filtering vector, and the UAV’s flight trajectory. The formulated optimization problem is decomposed into four subproblems, which are transformed into convex forms via successive convex approximation, and is solved iteratively using the block coordinate descent method. Simulation results demonstrate the correctness and effectiveness of the proposed scheme.

The Integrated Satellite-Ground Network (ISGN) is regarded as a promising technology for future communication systems, which can provide ubiquitous connectivity for various communication scenes, such as remote areas, maritime, and emergency communications. This paper presents a novel downlink transmission framework for the cell-free multi-input multi-output (CFmMIMO) network-based ISGN system. The satellite serves a group of earth stations through multicast technology, while the ground network embraces the cell-free network strategy to serve multiple terrestrial users. Furthermore, the CFmMIMO-based ISGN under consideration operates at millimeter-wave frequencies to satisfy the high data rate demand of the next-generation communication system. In order to enhance the spectral efficiency of the proposed CFmMIMO-based ISGN system, we formulate two joint beamforming design problems, which are denoted as the maximize sum-rate (MSR) problem and the max-min fairness (MMF) problem. Then, to solve this mathematically intractable problem, we initially applied Fractional programming, Alternating optimization, and successive convex approximation to convert the MSR-based joint precoding problem into an equivalent quadratically constrained quadratic program (QCQP) problem. After that, the QCQP optimization problem is solved using an alternating direction method of multipliers based algorithm, where we dramatically simplify the algorithm’s computational complexity with several straightforward variable substitutions. Furthermore, we extend our proposed algorithm to address the MMF-based joint beamforming design problem. At the last, the simulation result demonstrates the superiority of the proposed downlink transmission framework as well as the proposed algorithm.

The Internet of Things (IoT) supported by satellites is becoming indispensable for remote sensing in the forthcoming sixth-generation (6G) communication network. However, it is tempting and easy for unauthorized users to exploit the Direct Sequence Spread Spectrum (DSSS) technique to quietly complete their own transmissions due to the open nature of propagation and the publicly available satellite orbits and frequencies. Therefore, it is necessary to conduct a blind estimation of the DSSS signal to take a more proactive approach to protect satellites against illegal use. However, the modulation information is unknown, and the spreading code structure varies, making the blind estimation of spreading codes a significant challenge. Additionally, under data modulation, the dimensionality of the received spread spectrum sequence increases, greatly raising the complexity of spreading code estimation. Against this background, we propose a hybrid learning-based blind estimation algorithm for spreading codes, which combines K-means clustering and Convolutional Neural Networks (CNN). This algorithm achieves low-complexity blind estimation of spreading codes with unknown modulation information and low signal-to-noise ratio. Specifically, the K-means clustering algorithm uncouples the data modulation from the spreading code, reducing the dimensionality of the estimation process. On this basis, the CNN-based parallel convolution architecture is employed to achieve low-complexity and accurate estimation of the spreading code. Simulation results demonstrate that our proposed algorithm outperforms existing algorithms in both computational complexity and estimation performance.

The Non-Terrestrial Network (NTN) is a promising technology to achieve the ubiquitous, low latency, and high data rate next-generation communication system, however, since the broad application of the NTN, the security problem in the NTN is of paramount importance. In this regard, this paper investigates a novel secure communication strategy within the NTN, where a legitimate user of the LEO satellite-integrated unmanned aerial vehicles network (ISUAVN) is equipped with a high-gain antenna and attempts to cross-layer eavesdrop on the GEO satellite network.In the considered NTN system, the cell-free massive multi-input multi-output (CFmMIMO) network is utilized at the ISUAVN to enhance the spectral efficiency of the terrestrial users. Furthermore, the ISUAVN shares the millimeter wave spectrum with the GEO satellite multicast downlink communication link. We first formulated a multi-objective optimization (MOO) based joint beamforming design problem for the secure communication of the considered CFmMIMO NTN system to achieve a Pareto optimal trade-off between two conflicting and essential objectives: minimizing total transmitted power and maximizing the secret data rate of the satellite network. Thereafter, to address this mathematically complicated optimization problem, we utilized successive convex approximation methods to transform this intractable problem into an equivalent convex optimization problem. Then, the commercial optimization package is employed to achieve the optimal solution of the MOO problem, commencing from a feasible point that is identified by a semi-definite programming initialization algorithm. Ultimately, the numerical simulation results demonstrated that the Pareto optimal trade-offs for the formulated MOO problem were achieved using the proposed algorithm, and the effectiveness of our proposed algorithm was shown through comparisons with existing related methods.

This paper investigates the joint transmit and receive beamforming design for a bistatic integrated sensing and communication (ISAC) system, where a transmit base station (BS) and a receive BS are coordinated to simultaneously serve multiple downlink and uplink users as well as estimate target positions. The closed-form expression for the Cramér-Rao bound (CRB) for target positions and reflection coefficients is derived and minimized subject to constraints of the transmit power budget and communication requirements. To address the considered problem, the closed-form expression for the optimal receive beamforming vectors is derived, facilitating the development of a successive convex approximation (SCA)-based algorithm for optimizing the transmit information beamforming vectors and sensing covariance matrix. In addition, a learning-based approach named ISACNet, trained in an unsupervised manner, is proposed to handle the considered problem. The ISACNet incorporates multi-head self-attention and cross-attention mechanisms to significantly enhance its expressive capability. Simulations validate the effectiveness of the proposed SCA-based algorithm and ISACNet. It is observed that the sensing performance is predominantly affected by the downlink communication more than the uplink communication. Furthermore, our ISACNet generates an effective solution in millisecond-level response times with only a marginal performance degradation compared to the SCA-based algorithm.

The rapid expansion of Low Earth Orbit (LEO) satellite constellations has brought significant spectrum management challenges, including spectrum scarcity and complex interference issues. Traditional algorithms and prior Artificial Intelligence (AI) methods fail to meet LEO’s demands for managing extreme dynamics, massive scale, and multi-objective optimization. This paper introduces an innovative Large Language Model (LLM) framework for intelligent spectrum sharing and dynamic resource allocation in satellite-terrestrial down-link systems. First, we established a geometric model for satellite-terrestrial down-link communication, and accurately derived the statistical distribution function of satellites within the space enclosed by a specific orbital line by combining the stochastic geometry theory. Under this geometric model, a communication scenario was introduced, and an adaptive modulation transmission mechanism based on orthogonal frequency division multiplexing signals was designed. Then, the system combines the real-time spectrum sensing results with the natural language description of the quality of service of multi-service data using prompt engineering techniques, and delivers the comprehensive information to the LLM for resource allocation and generation of a transmission scheme. Finally, the resource allocation and transmission scheme determined by the LLM is applied to the established communication model, and the system performance is comprehensively evaluated by analyzing indicators such as outage probability, system throughput, and transmission and waiting delays. Primary contributions include novel dynamic service-to-strategy generation, an LLM-centric prompt-driven architecture, and a new paradigm that positions the LLM as an intelligent “spectrum orchestration brain” for complex global LEO resource management. Collectively, these advancements enhance spectrum utilization intelligence, adaptability, and efficiency, offering a transformative approach to overcome the limitations of prior methods in demanding LEO environments.

Achieving secure communication with a low probability of detection (covertness) is critical yet challenging, particularly when employing practical digital modulations that can compromise the statistical indistinguishability assumed in theoretical models. This paper introduces a novel end-to-end framework leveraging digitally modulated covert signal modeling, obfuscation, and deep learning to attain simultaneous covertness and reliability. Firstly, we propose a novel approach to covert performance evaluation for modulated covert signals against detection. To address the deteriorated covertness considering modulation schemes, we further propose generating Gaussianized camouflage signals via a multi-stage transmitter pipeline encompassing spreading, jitter, filtering, and non-linear transformations, designed to mimic noise statistics effectively. At the receiver, a specialized deep learning architecture, CovertSyncNet, performs robust joint dynamic synchronization and symbol recovery. This receiver incorporates dedicated components to precisely estimate time-varying chip offsets and invert the complex, nonlinear distortions inherent in the camouflaged signal, enabling accurate demodulation. Extensive simulations rigorously validate our approach, demonstrating that high reliability is maintained despite the heavy camouflage. Concurrently, enhanced covertness is confirmed through metrics indicating low statistical distinguishability from Gaussian noise. This work highlights the significant potential of deep learning to bridge the gap between theory and practice, realizing communication systems that are simultaneously reliable, secure, and highly covert, even under realistic operational conditions.

Next-generation satellite systems require efficient collaboration in terms of wide area coverage and signal augmentation, enabling intelligent allocation of available wireless resources to ensure the security of information. Meanwhile, machine learning (ML) is widely considered well-suited to massive, real-time data scenarios in satellite communication networks, and graph neural network (GNN) is a specific branch for processing the irregular data within such networks. In this paper, we propose physical layer security for a multi-satellite collaborative (MSC) system involving LEO satellites, users, and eavesdroppers. Specifically, the GNN-based security communication of the MSC (G-MSC-SC) architecture is designed to maximize the secrecy rate. Since heterogeneous and isomorphic methods can effectively solve multi-type node mapping and complex communication problems, the G-MSC-SC architecture is divided into two steps: A heterogeneous graph pruning attention coefficient network (HGPAN) and an isomorphic graph eavesdropper as an auxiliary node network (IGEAN). In the HGPAN architecture, different types of device nodes are embedded in the same dimensional space, addressing the challenge of matching LEO satellites to users. The IGEAN architecture maps user channel state information (CSI) to beamforming (BF) vectors through attention aggregation and an improved loss function. Moreover, the corresponding conventional optimization algorithms are designed as test and comparison baselines. Simulation results show that 1) the G-MSC-SC architecture outperforms neural networks and heuristic algorithms in terms of accuracy and efficiency; 2) as the numbers of users and virtual eavesdroppers increase, the directional alignment between the BF vectors and the LEO satellite-user channels shows an improvement; and 3) with imperfect CSI, the G-MSC-SC architecture still achieves an excellent balance between user secrecy rate and communication rate.

Low Earth Orbit (LEO) satellite communications provide uninterrupted coverage and seamless services, which are becoming a crucial element in the Sixth-Generation of wireless communications. However, LEO communications are subject to high path loss and Doppler frequency shift, which presents significant challenges for such large-distance and time-changeable satellite-terrestrial transmission links. Direct Sequence Spread Spectrum (DSSS) has been widely adopted as a robust modulation technique in LEO satellite systems. Traditional DSSS signal detection methods, which rely on 2D searching, fail to effectively acquire the weak uplink signals due to the extremely low signal-to-noise ratio and pronounced channel dynamics. In light of these considerations, this study proposes an algorithm designated as time synchronization-aided signal detection, with the objective of enhancing the signal detection probability in low signal-to-noise ratio and high-mobility communication scenarios. Furthermore, we develop a novel transmission system based on time synchronization, with the objective of reducing the signal detection threshold by minimizing the impact of the Doppler frequency shift and time delay searching ranges. We theoretically analyze performance at varying time synchronization precision, numerical simulations and hardware experiments under diverse conditions demonstrate that, in comparison to the conventional algorithm, time synchronization can enhance the probability of signal detection.

Satellite communications, characterized by their wide coverage, flexible deployment, and short construction cycles, play an indispensable role in modern communication systems. However, the inherent openness of satellite channels makes transmitted signals highly susceptible to detection and interception. To address these security challenges, this paper proposes an energy-dispersed random time-hopping (ED-RTH) covert communication scheme based on time-uncertain transmission, which effectively enhances system covertness. To overcome the multi-slot combining challenge in the highly dynamic and low-SNR environment of low Earth orbit (LEO) satellite communications, a cross-entropy-based joint reception (CE-JR) algorithm is developed, achieving minimal performance loss with significantly reduced computational complexity. Furthermore, the detection performance of an eavesdropping satellite against the proposed scheme is analyzed, and closed-form expressions for the miss detection probabilities under two typical detection methods are derived, providing valuable insights for the design and optimization of covert satellite communication systems. Finally, a simple implementation of the CE-JR algorithm was carried out on an FPGA development board, demonstrating the feasibility of the proposed algorithm.

With the rapid development of integrated air-space-ground networks, ensuring secure and covert access for unmanned aerial vehicles (UAVs) to low Earth orbit (LEO) satellite constellations has become a critical challenge. This paper proposes the novel adaptive redundancy contention resolution diversity slotted ALOHA (AR-CRDSA) scheme to provide reliable and covert communication for UAV users. The core of the scheme involves a dynamic pairing strategy that conceals a low-power covert user (CU) transmission under the signal of a high-power legitimate user (LU), thereby shielding it from detection by adversarial reconnaissance satellites. A key innovation is the departure from transmitting identical packet replicas; instead, the AR-CRDSA protocol sends two distinct yet complementary packet versions to adapt to fluctuating channel conditions. This adaptive mechanism, combined with multi-satellite selection diversity and an iterative interference cancellation process, is shown through extensive simulations to significantly enhance the effective covert throughput and overall system reliability when compared to traditional random access protocols.

In recent years, low Earth orbit (LEO) satellite communication has emerged as a focal area of extensive research due to its potential for global broadband connectivity. Multi-carrier direct sequence spread spectrum (MC-DSSS) technology, leveraging the inherent anti-jamming advantages of spread spectrum signals, has shown great promise in enhancing communication reliability. However, the acquisition of MC-DSSS signals in LEO transmission link presents significant challenges, primarily due to the coexistence of extremely low signal-to-noise ratio (SNR) and substantial Doppler effect.To address these issues, this paper proposes an optimal two-dimensional (2D) acquisition framework for MC-DSSS signals. Based on the maximum likelihood (ML) criterion, the proposed framework enables coherent combination of subcarriers with high time resolution, thereby improving the acquisition performance in harsh LEO environments. Furthermore, a two-step low-complexity coherent acquisition algorithm is developed. This algorithm significantly reduces the computational burden while maintaining the performance of fully coherent subcarrier combination, making it more suitable for real-time implementation in resource-constrained LEO communication terminals. Moreover, this paper derives closed-form solutions for the false alarm probability, detection probability, and mean squared error (MSE) in additive white Gaussian noise (AWGN) channel, which are validated through simulations. The results demonstrate that the proposed algorithm achieves 2 dB performance improvement in SNR compared to noncoherent combining method, with a 1024-fold reduction in MSE when the number of subcarriers is 16.

With the rapid development of global satellite Internet and the growing demand for seamless connectivity, the Satellite–Terrestrial Integrated Network (STIN) has become a key architecture for achieving ubiquitous communication coverage. However, STIN faces critical energy efficiency challenges, including the high peak-to-average power ratio (PAPR) that degrades high power amplifier efficiency at the physical layer and frequent satellite handovers that increase network overhead. To address these issues, this paper proposes a mobility-aware model of Multicarrier Direct-Sequence Spread Spectrum (MC-DSSS) STIN. At the physical layer, the Orthogonality-Based Generalized Multicarrier Constant Envelope Multiplexing (CEMIC) technique is adopted, and an energy efficiency maximization problem is formulated under constant-envelope constraints. A joint power allocation algorithm is developed based on the Dinkelbach method and the Alternating Direction Method of Multipliers (ADMM) to solve the non-convex problem efficiently. To overcome the limitations of single-layer optimization, a twolayer collaborative framework is further proposed. At the network layer, an improved binary particle swarm optimization (IBPSO-HO) algorithm is employed to optimize satellite handovers. This joint design enables two-layer energy efficiency optimization. Simulation results demonstrate that the proposed scheme significantly enhances overall energy efficiency across both layers, providing robust theoretical support for the large-scale green deployment of STIN.

The growth of communication technologies has led to a corresponding increase in the need for information security. The use of covert communication techniques has the potential to significantly reduce the likelihood of the transmitted signal being intercepted by unintended receivers, thereby significantly increasing the security of the information. In this paper, we propose an energy-dispersed random time-hopping (ED-RTH) covert communication scheme. Building on existing time uncertainty schemes, this scheme further enhances time uncertainty through random time-hopping, while dispersing signal energy to achieve stronger covertness. We present a model for analyzing the covertness of the ED-RTH scheme. Given the complexity of the likelihood ratio distribution, the Kullback-Leibler (KL) divergence is used to analyze the lower bound of Willie’s detection error probability. Through constructing behavioral-level simulations for validation, we investigated the impact of the number of dispersed time slotsKon both Willie’s detection error probability and the system’s covert throughput. Finally we compared the proposed ED-RTH scheme with existing time uncertainty scheme, and the results demonstrate that the ED-RTH scheme achieves significantly enhanced covertness.

Efficient and reliable neighbor discovery is critical for UAV networks equipped with directional antennas, particularly in dynamic and energy-constrained environments. We present a novel optimization framework that jointly minimizes delay and power consumption using the power-delay product as the optimization metric. The framework is formulated for both synchronous and asynchronous schemes, leveraging upper-bound metrics and a convex-concave procedure to achieve tractable convex formulations. Simulation results validate the theoretical models and show significant improvements over baseline methods. We also conducted real-world experiments, where our method reduced power-delay product by 11% compared to the baseline, though the dual-sector hardware configuration limited the achievable gains. This work provides a comprehensive solution for UAV neighbor discovery, with high potential for scalability in more complex and dynamic environments.

With the rapid advancement of unmanned aerial vehicle (UAV) technology in recent years, cooperative communication in UAV networks (UAV-Ns) has made significant strides. However, the effectiveness of UAV-Ns cooperative communication relies heavily on the accurate estimation of channel state information (CSI). Unlike terrestrial networks, the mobility of UAV introduces time-varying channel characteristics, which can substantially affect the overall system capacity. Therefore, this paper investigates the system capacity of UAV-Ns while accounting for the effect of jitter characteristics of UAV. Specifically, we propose a cooperative transmission model utilizing multiple UAV base stations (UAV-BSs) to enhance the signal quality received by ground users through coordinated multi-point (CoMP) transmission. Additionally, we present a jittering channel model, derive the channel autocorrelation function, and assess the capacity of the proposed system. To deal with the jitter, we introduce a jitter compensation scheme based on long short-term memory networks to counteract the effects of UAV jitter and improve the accuracy of channel precoding. Numerical results demonstrate that our approach significantly enhances the communication performance of UAV-Ns under the impact of jitter. Compared to the traditional method, our scheme improves the estimation accuracy of the channel state by up to 3.8%, highlighting the potential of distributed UAV-BSs with CoMP to strengthen UAV-Ns communication.

Millimeter-wave (mmWave) communications, coupled with directional antenna-based Flying Ad-Hoc Networks (FANETs), have received considerable attention for their potential to provide high-speed, low-latency communications for a variety of applications. However, the high mobility of Unmanned Aerial Vehicles (UAVs) in FANETs leads to dynamic changes in relative positions, resulting in frequent link failures. Effective link maintenance in such networks has become a critical challenge. This paper addresses this issue by developing mathematical models of link disconnections in directional antenna-based FANETs. Specifically, we derive the probability density functions for link disconnections due to distance and angular misalignment in closed-form expressions. Based on these prediction models, we propose the Adaptive Link Breakage Prediction with Directionality (ALBP-D) method, which exploits the high directional gain of directional antennas to extend link lifetime and improve network performance. We compare ALBP-D with two baseline methods, the Periodic Link Maintenance (PLM) method and the Residual Path Lifetime (RPL) method, through extensive simulations. The results show that ALBP-D achieves superior performance, with approximately a 10-fold improvement in both link lifetime and network connectivity duration compared to the baseline methods. In addition, ALBP-D exhibits significant improvements in maintenance overhead efficiency, especially at higher max range adjustment count, achieving a 5 to 7-fold improvement over baseline methods. These results highlight the effectiveness of ALBP-D in directional antenna-based FANETs. We also implemented a prototype system consisting of a directional antenna node and an omnidirectional antenna node using realistic UAV trajectory data. Experimental results show that the prediction models agree well with the real link disconnection data, confirming the practical feasibility and accuracy of the proposed method.

Low Earth Orbit (LEO) satellite systems with sensing capabilities are widely regarded as promoting reliable and efficient communication services globally. This paper proposes a Multi-Satellite Collaborative Security System with Integrated Sensing and Communication (ISAC-MSC). Considering the potential benefits of LEO satellites and ISAC, we exploit sensing performance in the MSC system by jointly optimizing LEO satellite assignments, communication Beamforming (BF) vectors, and sensing BF vectors. Specifically, we design improved Continuous Particle Swarm (CP) optimization and Discrete Particle Swarm (DP) optimization algorithms to maximize the target sensing Signal-to-Noise Ratio (SNR) for LEO satellite assignments. Additionally, with respect to the BF vector optimization, we develop Power Approximation (PA) optimization algorithm, Inner Approximation (IA) optimization algorithm, and Joint Sensing and Communication BF (JSC-BF) optimization algorithm. Multiple algorithms are tightly integrated and alternately iterated. Numerical results show that: 1) the JSC-BF algorithms outperform the PA and IA algorithms in terms of sensing performance and communication secrecy rate; 2) compared to the single satellite case, the ISAC-MSC system performance approximately linear growth, and has strong extensibility; 3) with imperfect MSC synchronization case, the communication secrecy rate appears inflection point and stabilization, but the JSC-BF algorithms still have excellent performance.

This study investigates covert communication for a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted system with non-orthogonal multiple access (NOMA). In particular, a source first transmits a NOMA signal consisting of both public and covert information to an unmanned aerial vehicle relay, which then forwards it to both the covert and the public users via an STAR-RIS in the presence of a warden, which attempts to detect whether the covert information is forwarded to the covert user or not. Under practical communication scenarios, the warden may locate at different positions in order to detect the covert transmission in the dual-hop systems, which may lead to several different detecting scenarios. Out of those possible scenarios, in this study, three typical ones are considered: 1) The warden can only detect the signal from the source in the first-hop transmission; 2) Both the signals from the source and the relay in the first-hop and second-hop transmissions could be detected; 3) The warden can detect the direct as well as the reflected transmission via the STAR-RIS from the relay in the second-hop transmission. Taking those three scenarios into consideration, the analytical expressions of detection error probability of the warden are derived and verified with Monte-Carlo simulations. In addition, the optimal detection threshold as well as the maximum effective covert rate is presented and discussed.

Integrated communication and sensing, which can make full use of the limited spectrum resources to perform communication and sensing tasks simultaneously, is an up-and-coming technology in wireless communication networks. In this work, we investigate the performance of an uncrewed aerial vehicle (UAV) -assisted secure integrated sensing, communication, and computing system, where the UAV sends radar signals to locate and suppress a potential eavesdropper while providing offload services to ground users (GUs). Considering the constraints of UAV maximum speed, transmit power, propulsion energy, data transmission, and computation time, the total energy consumption of GUs is minimized by jointly optimizing user offloading ratio, user scheduling strategy, transmit beamforming, and UAV trajectory. An efficient iterative optimization algorithm is proposed to solve the nonconvex optimization problem caused by tightly coupled dependent variables. In particular, the original optimization problem is decomposed into four suboptimization problems, and the nonconvex subproblems are transformed into approximately convex forms via successive convex approximation. Then, all subproblems are solved successively by using the block coordinate descent technique. Numerical results demonstrate the convergence and validate the effectiveness of the proposed algorithm.

Ensuring timely information delivery is of paramount importance for mission-critical applications in the Internet of Things (IoT). The age of information (AoI), which measures the time elapsed from data generation to reception, is critical for assessing information freshness in time-sensitive scenarios. This work focuses on the AoI analysis of cooperative IoT systems with decode-and-forward relaying mode, where automatic repeat request (ARQ) and truncated ARQ (TARQ) protocols are employed to enhance the reliability of short packet communications. The source and relay nodes have limited battery capacity, thereby harvesting energy from the charging towers. We derive analytical expressions for the average AoI and the average peak AoI for the proposed ARQ-assisted and TARQ-assisted protocols. Simulation results validate our theoretical analysis and explore the impact of system parameters on AoI performance. Our findings highlight the existence of the optimal packet length and retransmission time that minimize AoI, providing valuable insights for designing efficient IoT systems.

This work addresses the challenge of achieving covert communication in cooperative wireless systems, which consist of a source, a decode-and-forward relay, and a destination. The direct link between the source and the destination is often impaired by deep fading, necessitating the use of a relay to forward signals. The relay also attempts to transmit its covert signal to the destination over the same spectrum, complicating the source’s detection capabilities. To tackle this issue, we propose two power allocation schemes: 1) fixed power allocation (FPA) and 2) dynamic power allocation (DPA). The FPA scheme maintains a constant power allocation for the source’s message, while the DPA scheme adjusts power based on the channel quality from the relay to the destination. Considering noise uncertainty at the source, we derive closed-form expressions for false alarm probability, miss detection probability, and minimum detection error probability, which are crucial for evaluating the source’s detection performance while ensuring Quality of Service for both source and relay messages. Simulation results demonstrate that the DPA scheme outperforms the FPA scheme in terms of covert throughput, especially under conditions of higher transmission rate thresholds and improved channel conditions.

Due to some transmission delays and channel estimation errors, it is always difficult to acquire perfect channel state information (CSI) in practical communications. Therefore, in this study, taking the practical scenario of imperfect CSI into consideration, a simultaneously transmitting and reflecting-reconfigurable intelligent surface (STAR-RIS)-assisted satellite-autonomous aerial vehicle (AAV)-terrestrial non-orthogonal multiple access (NOMA) network is investigated, in which the AAV is assumed to be randomly located in a spherical cap space while the users are randomly located in an inner circular and an outer annular plane on the ground. Specifically, a satellite source first sends NOMA signals to a AAV relay, which then forwards them to both ground users via an STAR-RIS through reflection as well as transmission. In addition, considering that the satellite-AAV link is subject to the shadowed-Rician distribution while the other links are subject to the Nakagami-m distributions, the cumulative distribution functions and probability density functions of channel gains in the presence of imperfect CSI are derived. Furthermore, in practical application scenarios, the positions of the AAV and both users may be randomly located in some regions. Therefore, three randomly distributed scenarios are considered: 1) The users are randomly located while the AAV’s position is fixed; 2) The AAV is randomly located while both users are in fixed locations; and 3) Both are randomly located. Under those three scenarios, both analytical and asymptotic expressions of outage probability (OP) for two users as well as the system OP are derived using the stochastic geometry approach, and their accuracy is confirmed with Monte-Carlo simulations.

Both the space-air-ground integrated networks (SAGIN) and the integrated sensing and communication (ISAC) are promising technologies in future communication systems. This paper investigates the mobile user (MU) tracking and robust beamforming design by the unmanned aerial vehicle (UAV) in an SAGIN-ISAC system. Two schemes for acquiring the location information of MUs at the UAV are proposed, namely the space-assisted and ISAC-assisted schemes. The former requires the precise location information from the satellite by the space-air transmission, while the latter estimates the location information of MUs via a proposed extended Kalman filter based algorithm. The obtained location information is then utilized to predict the channel distribution of MUs, which can be used to formulate an outage-constrained energy efficiency (EE) maximization problem. The considered problem is first reformulated based on the Bernstein-type inequality to derive computationally tractable forms of the outage probability constraints. Then, the reformulated problem is solved via the semi-definite relaxation (SDR) and successive convex approximation methods, where the tightness of employing SDR is theoretically proved. Numerical results illustrate the trajectories of the UAV for tracking MUs under the space-assisted and ISAC-assisted schemes, and discuss the impact of the space-air transmission on the EE performance. It is observed that there exists a trade-off between space-air transmission overhead and location prediction precision of MUs. By integrating the ISAC in SAGIN, the information demand from the space is reduced compared with traditional SAGIN.

Millimeter wave (mmWave) communication is crucial for drones, leading to more being equipped with directional antennas. Consequently, the technology for fast detection of directional antenna neighbors is a major challenge in mmWave Flying Ad-Hoc Networks (FANETs). This paper proposes a fully directional Neighbor Discovery (ND) method called Directional ND with Multi-Token Passing (DNDMTP) Algorithm to work out the aforementioned issue. Specifically, a theoretical derivation of DNDMTP is first presented, followed by a verification of its correctness through simulations that match the theoretical results. Unlike existing methods, DNDMTP is designed to maximize the spatial reuse capability of directional antennas by forwarding tokens across multiple antenna coverage areas, accelerating neighbor discovery in multi-hop topologies. In DNDMTP, the token holder nodes can achieve bidirectional discovery with the neighbor nodes through multiple rounds of broadcasting. Furthermore, the optimal selection of parameters is proposed for different scenarios, e.g., the number of antennas, iterations, and the number of tokens passed. Finally, for comparison purposes, we chose the Scan-Based Algorithm-Deterministic (SBA-D) and Learning Automaton Based ND (LAND) algorithms, which represent deterministic and probabilistic algorithms, respectively. The numerical results from Python show that the proposed DNDMTP can reduce the neighbor discovery time by about 50% compared to existing methods, due to multiple token-holder nodes performing neighbor discovery simultaneously. As the number of nodes and antennas increases, DNDMTP performs better in terms of neighbor discovery time.

With the increasing demands of the Industrial Internet of Things (IIoT), highly dynamic multi-unmanned aerial vehicle (UAV) networks are becoming indispensable to IIoT due to their flexibility, cost-effectiveness, robust safety measures, and real-time data collection capabilities. Accurate time synchronization is crucial for coordinated missions of multi-UAV networks, yet the time-varying nature of clock parameters and the rapid movements of UAVs pose significant challenges to achieving precise synchronization. This article introduces new state and observation models for clock and velocity parameters and proposes a Doppler and timestamp-based distributed algorithm for tracking clock parameters using the Kalman filter. To evaluate the performance of the proposed algorithm, we derive the Bayesian Cramér–Rao lower bound and conduct numerical simulations. The results of the simulations demonstrate that our algorithm surpasses existing methods in terms of accuracy in tracking clock parameters.

Space-air-ground integration has become paramount in the next generation of wireless communication systems in the era marked by the seamless integration of terrestrial and celestial domains. On the other hand, the age of information (AoI) has recently emerged as a vital metric for evaluating the timeliness and freshness of data in these multi-hop communication systems. This paper focuses on investigating the information freshness of multi-hop satellite IoT systems while considering several automatic repeat request (ARQ) and hybrid ARQ (HARQ) schemes over different hops to promise the reliability of data transmissions. Specifically, a group of remote sensors transmits their data to a terrestrial base station (B) via the code-division multiple access (CDMA) strategy to exploit CDMA’s natural merits, e.g., anti-jamming and simultaneous transmissions. Then, B sends these received data to a data destination (D) via a satellite (R) under the transparent forwarding strategy. We derive the closed form of the outage probability for the CDMA protocol considering multi-user interference (MUI) and the closed form of the end-to-end outage probability for the three kinds of ARQ and HARQ schemes on the dual-hopB-R-Dtransmission, and then finally derive the expression of the corresponding AoI. Finally, numerical results show that simulations match well with the theoretical results, proving the analysis’s correctness and the pros and cons of the different ARQ/HARQ schemes.

Low Earth orbit (LEO) Mega-constellations have emerged as a transformative approach to realize enhanced system capacity and improved coverage to satisfy the ever-increasing global demand for data services. Subsequently, the high density of satellites in a confined orbital region poses challenges, including potential interference among neighboring satellites. Further, it is vital to adequately address the impacts of safety distances in satellite communication systems on ensuring proper operation, collision avoidance, and interference management. Inspired by these observations, this work proposes a novel analysis tool, the Poisson arc hardcore point process (PAHPP), by extending the traditional Poisson line hardcore point process to characterize the unique orbiting properties of the satellites in LEO mega-constellations, accounting for factors such as the orbit, the satellite density, and spatial distribution. Specifically, this paper presents the PAHPP by enforcing a minimum separation between satellites operating in the same circular orbit to reflect the practical LEO mega-constellations. The imposed minimum inter-satellite separation in the proposed PAHPP model has also been applied to multi-orbit multi-satellite communication cases. Moreover, the discretization approximation technique is employed to analyze system performance, focusing on serving distance and outage probability. Numerical results provide valuable insights and conclusions for uncovering and recognizing LEO mega-constellations.

This study investigates the secrecy performance of a rate-splitting multiple-access system inspired by cognitive radio principles, where the primary user takes on the role of a potential eavesdropper seeking to intercept the secondary user's message. The on-off scheme is utilized to guarantee the transmission of the primary user. We provide closed-form expressions for critical metrics, including the transmission probability of the system under consideration, the secrecy outage probability, and the reliable outage probability of the cognitive user. Monte Carlo simulation results are presented to substantiate our findings and explore the impact of system parameters, such as transmit power and the number of antennas at the base station, on system performance.

Wireless relays can effectively extend the transmission range of information. However, if relay technology is utilized unlawfully, it can amplify potential harm. Effectively surveilling illegitimate relay links poses a challenging problem. Unmanned aerial vehicles (UAVs) can proactively surveil wireless relay systems due to their flexible mobility. This work focuses on maximizing the eavesdropping rate (ER) of UAVs by jointly optimizing the trajectory and jamming power. To address this challenge, we propose a new iterative algorithm based on block coordinate descent and successive convex approximation technologies. Simulation results demonstrate that the proposed algorithm significantly enhances the ER through trajectory and jamming power optimization.

Satellite-to-ground communication systems typically operate in environments with high interference levels, complex topologies, and stringent platform constraints. Therefore, intelligent, anti-interference, and low-power systems are required to achieve the desired transmission performance. This paper proposes a system for optimizing high-frequency satellite-to-ground communications using computer vision (CV) technology, like millimeter-wave (mmWave) satellite communication systems. The system uniquely combines CV-based target localization with adaptive beamforming and power control to optimize communication links with ground targets such as base stations, ships, and aircraft. This approach significantly outperforms traditional radio frequency-based methods in accuracy and efficiency, particularly in dynamic mmWave scenarios. Simulation results confirm the superiority of our system in terms of sum rate and energy efficiency, demonstrating its potential to revolutionize high-frequency satellite communications by providing reliable, high-quality service to terrestrial targets. Finally, simulation results are presented to demonstrate the efficiency of the proposed schemes.

Millimeter-wave (mmWave) communication is one of the effective technologies for the next generation of wireless communications due to the enormous amount of available spectrum resources. Rate splitting multiple access (RSMA) is a powerful multiple access, interference management, and multiuser strategy for designing future wireless networks. In this work, a multiple-input-single-output mmWave RSMA system is considered wherein a base station serves two users in the presence of a passive eavesdropper. Different eavesdropping scenarios are considered corresponding to the overlapped resolvable paths between the main and the wiretap channels under the considered transmission schemes. The analytical expressions for the secrecy outage probability (SOP) are derived respectively through the Gaussian–Chebyshev quadrature method. Monte Carlo simulation results are presented to validate the correctness of the derived analytical expressions and demonstrate the effects of system parameters on the SOP of the considered mmWave RSMA systems.

In this work, the physical-layer security for a maritime relay-assisted hybrid satellite and underwater wireless optical communication system consisting of a satellite source, a maritime relay (R), an underwater destination, and an eavesdropper (E) is investigated. In addition, both R and E are with multiple antennas and randomly located. In real application scenarios, E may be different carriers, i.e., an aircraft in the air or a ship on the sea, with various scopes of activity. Therefore, two different randomly located scenarios are considered, such as randomly distributed in a three dimensional space and a two dimensional plane. Furthermore, the beam-angle information of the source is assumed to be unavailable to E, and E may be located outside of the beam coverage area. Therefore, the beam coverage probability (BCP) for E is also presented. Employing the stochastic geometry theory and the maximum ratio combining scheme, the analytical and asymptotic expressions of secrecy outage probability (SOP) are obtained. In addition, the overall system SOP is defined and presented by taking both SOP and BCP into account. Finally, Monte-Carlo simulations verify the accuracy of our analysis.

To address the stringent demands of next-generation networks for massive connectivity, ultralow latency, and ultrahigh spectral efficiency, the win–win integration of reconfigurable intelligent surface (RIS) and nonorthogonal multiple access (NOMA) is considered as a promising solution. In this article, we investigate a downlink RIS-empowered NOMA system with short packet communications (SPCs) in the presence of hardware impairments at the transceiver nodes. To characterize the performance of the proposed network, the approximate and asymptotic closed-form expressions of average block error rate (BLER) at far and near users are derived and analyzed. Based on asymptotic average BLER in high signal-to-noise ratio regime, the diversity order, minimum blocklength, and optimal power allocation are examined. RIS exhibits an improvement in diversity order and minimum blocklength. The achieved results show that users with poorer channel conditions have less sensitivity to hardware impairments whether for near users or RIS-assisted far users. Furthermore, our results manifest the benefits of NOMA and RIS in SPC over the benchmark orthogonal multiple access (OMA) scheme.

As air-to-ground links tend to exhibit a high probability of being line-of-sight (LoS), unmanned aerial vehicles (UAVs) are widely used to improve the performance of wireless communications. The design of the UAV flight path plays a pivotal role in determining the effectiveness of UAV communication systems. However, the air-to-ground links with a high probability of LoS introduce a heightened risk of eavesdropping, posing a significant security challenge. In this work, we investigate the problem of ensuring secure data acquisition for quadrotor UAV-based communication systems in the presence of multiple location-uncertain terrestrial eavesdroppers. The bandwidth allocation and the three-dimensional trajectory of the UAV are jointly designed to maximize the system’s overall fair secrecy rate. This design also considers the UAV’s energy consumption during flight and aims to ensure fairness among users. Solving this problem poses a challenge since it is a non-convex and involves multiple variables, making it difficult to address using conventional optimization methods. Therefore, a deep reinforcement learning algorithm is developed based on the deep deterministic policy gradient algorithm. Simulation results are given to verify the effectiveness of the proposed algorithm in improving the security of aerial Internet of Things systems.

Underwater exploration, inspection, and surveillance have become more popular thanks to recent advancements in underwater wireless communication technologies, of which underwater wireless optical communication (UWOC) boasts increased bandwidth, reduced latency, and heightened security. Normally, once underwater sensors collect the data, it is transmitted to remote offices through a relay node stationed on the sea surface. In this study, our focus is on establishing and analyzing the performance of secrecy outage in an uplink hybrid radio frequency (RF)-UWOC space-ocean system that comprises a collection of underwater sensors, a relay floating on the ocean surface, a legitimate satellite receiver, and an eavesdropping satellite. In the considered system, the transmission performance from end-to-end is influenced by several factors. These include path loss, small-scale fading, position randomness of the satellites in RF links, propagation loss, turbulence-induced fading, and random location distribution of the underwater sensors in UWOC links. To enhance our understanding of system design, geometric probability theory is employed to investigate the end-to-end secrecy outage performance of the dual-hop RF-UWOC space-ocean uplink transmission while comprehensively considering the impacts of these factors. Finally, the accuracy of the proposed analysis models is verified through numerical results.

Underwater acoustic (UWA) channels present diverse sparsity, which is challenging for UWA communications and underwater networked applications. Conventional methods fail to adapt to the uncertain sparsity due to inflexible adjustment rules or improper assumptions. This paper investigates the estimation of channels with uncertain sparsity and model-driven deep learning (DL) based estimation networks are proposed. We construct the channel estimation as sparse signal recovery model and exploit the channel sparsity based on the approximate message passing (AMP) method. Then the learned AMP network with soft-thresholding shrinkage function (ST-LAMP) is designed as a generalized estimator and suitable to random sparse UWA channels. Moreover, to achieve the Bayesian optimal solution, Gaussian mixture (GM) prior is imposed for UWA channels and the GM-LAMP network is formulated with the derived shrinkage function based on the minimum mean squared error criterion. The proposed networks incorporate the learning ability of DL methods by exploiting marine data and the domain knowledge from the classical model to realize parameter configuration and adaptive optimization. Experimental results verify that the proposed networks outperform benchmarks over UWA channels with diverse sparsity in terms of adaptability and effectiveness. Especially, real tests present the feasibility of proposed networks for on-line deployments.

Due to the unique merits of unmanned aerial vehicle (UAV) systems, they have already been harnessed for military, public, and civil applications. Time synchronization is a significant premise of formatting and applying UAV networks. However, the irregular high-speed mobile UAVs pose new challenges to time synchronization, especially when external time references are unavailable in some rigid scenarios. Therefore, in these harsh cases, self-time-synchronization (STS) without any external assistance should be concerned to overcome the relative velocity between UAVs caused by irregular high-speed motion. In this paper, a realistic timestamps model for the multi-UAV networks is established, and then a dynamic topology-based maximum likelihood estimator will be developed to carry out the STS. Furthermore, by introducing the information on frequency offset, a new estimator with the closed-form expression is proposed based on a two-way message exchange framework. After that, a tracking algorithm with the assistance of estimation results will be introduced to compensate for the time-varying change of the clock parameters for the dynamic topology UAV networks. To evaluate the performance of the estimator, both the estimation error and Cramér-Rao lower bound are analyzed. Numerical results show that the proposed algorithm exhibits its superiority in STS performance and computational complexity compared to the existing two-way message exchange algorithm using timestamps only.

Unmanned aerial vehicles (UAVs) have found extensive applications across diverse domains owing to their cost-effectiveness, uncomplicated structure, and adaptable takeoff and landing functionalities. Multi-UAV network systems can be coordinated to improve system performance significantly. Network time synchronization is a crucial prerequisite for establishing and operating multi-UAV networks, whereas the high-speed movement of UAVs presents challenges for time synchronization within multi-UAV networks. In this paper, we develop a practical information exchange model with high relative radial velocity and Gaussian distribution random transfer delay. By introducing Doppler information and a novel clock skew and clock offset consensus model, a Doppler and timestamp joint (DATJ) network time synchronization algorithm has been developed. Assessing the performance of the algorithms, we provide rigorous theoretical proof of network time synchronization convergence. Simulation results further validate the theoretical analysis and demonstrate that the proposed algorithm outperforms similar approaches in terms of synchronization performance.

Unmanned aerial vehicle (UAV) communication systems usually operate in harsh scenarios, which require accurate information about the topology and wireless channel to achieve the desired transmission performance. Therefore, when millimeter-wave (mmWave) communication with its intrinsic Line-of-Sight (LoS) condition is adopted, accurate target localization is essential to determine the spatial relationship between the UAV and the grounded receivers (Rxs). In this article, a computer-vision (CV)-aided jointly optimization scheme of flight trajectory and power allocation is designed for mmWave UAV communication systems by utilizing the visual information captured via cameras equipped at the UAV. Compared with traditional schemes, the implementation cost and overhead can be greatly saved as no radio frequency transmissions are required in the proposed localization scheme. In addition, the transmit power at the UAV is jointly optimized with its flight trajectory in two different cases. Finally, simulation results are presented to demonstrate the efficiency of the proposed schemes.

With the development of the sixth-generation wireless communication technology, the difficulty of covert communication in the satellite communications system (SCS) is further increased. In this paper, a satellite covert communication system based on reconfigurable intelligent surface (RIS) is studied, which consists of a satellite, a terrestrial receiver, RIS, and a warden. Specifically, the ground terminal simultaneously receives the signal transmitted by the satellite and the signal reflected by RIS, while the warden wants to overhear the transmitted signal from the satellite and that reflected by the RIS. The main target of this wort is to maximize the minimum covert rate to improve signal quality while considering the limited onboard resources, the hardware constraints of RIS, and the requirement to implement covert communication. To address the non-convex problem, we propose an effective alternating optimization scheme by jointly optimizing transmitter precoding and RIS to improve the covert communication performance of the considered system. In addition to ideal RIS, we also consider the non-ideal RIS, the angle of which is discrete in the actual case. In addition, several simplified methods are proposed to decrease the computational complexity of the covert communication optimal problem under multiple terminals. Finally, simulation results are provided to verify the superiority of the proposed scheme compared with the benchmark scheme, which suggests that the considered RIS-based satellite-terrestrial covert communication system can significantly improve the achievable masking.

In this work, a reconfigurable intelligent surface (RIS)-based multi-input–multi-output (MIMO) covert communication system is considered and studied for Internet of Things (IoT) networks, while the joint optimization of precoder and RIS reflection phase is carried out to improve the covert communication performance. To solve this problem, we first derive the optimal signal-interference-to-noise ratio under covert communication and transmit power constraints. Then, we simplify the optimized function and use an iterative optimization algorithm to determine the optimal phase shift and precoding in continuous and discrete cases. Simulation results show that the RIS-aided MIMO covert communication system proposed in this article can significantly improve the invisibility of the implementation from Willie. In addition, for covert communication, the performance of the continuous phase shift case outperforms that of the discrete phase shift and fixed phase.

In this study, we investigate a non-orthogonal multiple access (NOMA)-assisted dual-hop underwater wireless optical communication-radio frequency (UWOC-RF) network. Specifically, an optical source forwards a NOMA signal to a marine relay, which first deploys either the heterodyne detection or intensity modulation direct detection to process the received signal, and then encodes and re-transmits a superimposed signal to two terrestrial destinations. Assuming the UWOC channel is subject to a unified mixture Exponential-Generalized Gamma (mEGG) distribution and the RF channels experience Nakagami-${m}$fading, the analytical as well as asymptotic expression for outage probability is obtained in the form of the Meijer’s$G$-function. Moreover, the average achievable rate is also investigated by utilizing the bivariate Fox’s$H$-function. In addition, the accuracy of those expressions is verified with Monte-Carlo simulations.

In this article, we investigate the outage performance for a dual-hop free space optical-radio frequency communication system, in which a source node communicates with two users on a non-orthogonal multiple access basis via a single-aperture and multi-antenna relay, which can harvest energy from a power beacon under a nonlinear energy harvesting mode. In particular, two transmitting antenna selection (AS) criteria, namely, the random-maximum AS (RA-AS) and the minimum-maximum AS (IA-AS) schemes, are considered to improve the outage performance of the system. In addition, an adaptive power allocation algorithm, which allocates the power to each user according to the instantaneous channel power gain, is adopted for the RA-AS scheme. Considering the Gamma–Gamma distribution and the Rayleigh fading, the closed-form expressions of the outage probability for each user under both the RA-AS and IA-AS schemes are derived. Moreover, the Monte-Carlo simulation verifies the correctness of the theoretical derivations.

Energy harvesting (EH) is an effective approach that can not only extend the service life for those energy-constraint devices but also reduce the carbon footprint for wireless communication. Unlike the commonly used single EH scheme adopted in existing works, in this considered free-space optical-radio frequency (FSO-RF) system, a dual EH mode is considered, in which an EH relay is capable of harvesting energy from both the ambient RF signal in the environment and the direct-current component separated from the FSO signal simultaneously. In addition, in this considered system, a source node plans to communicate with two users equipped with multiple antennas via an EH relay on a non-orthogonal multiple access (NOMA) basis, and all the harvested energy of the relay is used to transmit the signal to both NOMA users. Moreover, assuming a selection-combining scheme is adopted at the user side, both the analytical and asymptotic expressions of outage probability for each NOMA user are presented, and the system performance with various atmospheric turbulence, detection technique, as well as pointing error is investigated. Finally, numerical results show that our proposed dual EH mode can lead to better outage performance than that with the single EH one.

Large low earth orbit (LEO) mega-constellation systems have been designed and deployed as a global backbone to provide ubiquitous connectivity across the world. However, due to the high traffic load/congestion arisen from the required numerous information relay and forwarding, it is a challenge for LEO mega-constellation systems to set up long-distance connections between two remote terrestrial users for real-time communications, which requires strict low latency. Other than inter-LEO satellite links (ILSL), introducing third-party relays, such as terrestrial, aerial, and satellite relays, is an alternative way to improve the latency performance for wide-area deliveries of real-time traffic. However, the reliability of the transmitted signal will unavoidably degrade, because of the increased path-loss attributed to long-distance relaying transmissions. Then, to reveal the principle that the third-party relays affect the latency and reliability, in this work, an LEO satellite-terrestrial communication scenario is considered, in which two remote terrestrial users communicate with each other via an LEO mega-constellation system. Analysis models are built up to investigate the end-to-end time delay and outage performance while considering different ILSL, terrestrial, aerial, and satellite relay assisted transmission scenarios. More specifically, by applying geometrical probability theory, exact/approximated closed-form analytical expressions have been derived for average time delay

Nowadays, satellite and aerial platforms are playing an important role in realizing global seamless wireless coverage. In this paper, a cooperative satellite-aerial-terrestrial network (SATN) is considered, in which two kinds of relaying links, satellite and aerial relaying links, are used to assist a group of aerial terminals to forward their information to a remote terrestrial destination (D). Specifically, we model these aerial platforms sharing the same frequency band as a Matérn hard-core point process type-II. Also, a group of aerial jammers at D’s side is modeled as a Poisson point process. To demonstrate the end-to-end (e2e) performance of the two relaying links, the statistical characteristics of the received signal-to-interference are characterized and then a closed-form expression for the outage probability (OP) over the uplink from the aerial source to the satellite/the aerial relay, the downlink from the satellite/the aerial relay to D, and the inter-aerial relay link are derived. Numerical results are presented to verify the proposed analysis models and compare the outage performance of the considered cooperative SATN with the two relay links under numerous scenarios.

Satellite-aerial communication (SAC) is envisioned as a fundamental component of the sixth-generation (6G) wireless networks. Motivated by its importance, we investigate a SAC system including a geostationary satellite (S), a target aircraft (TA), and a set of interfering aircraft (IA). Specifically, TA sends signals to S in the presence of IA interference. Considering the trajectory, hierarchy, and safety distance of the aircraft’s flight routes, we propose a novel three-dimensional stacked Poisson line hardcore point process. That is, we introduce safety distances to the stacked Poisson line Cox process in order to describe the locations of IA in the sky. We also propose two approximations, namely, the equi-dense model and the discretization model, to maintain the tractability of the analysis. To this end, the uplink coverage probability is studied by using the two proposed mathematical models. Moreover, we investigate the coverage probability of the aviation use case with predefined flight altitudes. Finally, numerical results and Monte Carlo simulations are presented to validate the accuracy of the proposed analysis.

This article investigates the secrecy performance for an unmanned aerial vehicle (UAV)-aided nonorthogonal multiple access (NOMA) network in the context of an underlay cognitive radio. Specifically, a multiantenna secondary source ($S$) transmits a mixed signal incorporated with NOMA to two multiantenna secondary destinations ($D_{i}$) via a UAV-enabled decode-and-forward relay in the presence of an eavesdropper ($E$). In addition, both a transmit antenna selection scheme at$S$and a maximal ratio combining protocol at${D_{i}}$and$E$are taken into account. Considering all the links undergo Nakagami-$m$fading with path loss, we analyze the secrecy performance in terms of secrecy outage probability for both users. Moreover, we also use Monte-Carlo simulations to verify the accuracy of those derived expressions.

This work investigates the secrecy outage performance of the uplink transmission of a radio-frequency (RF)-free-space optical (FSO) hybrid cooperative satellite-aerial-terrestrial network (SATN). Specifically, in the considered cooperative SATN, a terrestrial source (S) transmits its information to a satellite receiver (D) via the help of a cache-enabled aerial relay (R) terminal with the most popular content caching scheme, while a group of eavesdropping aerial terminals (Eves) trying to overhear the transmitted confidential information. Moreover, RF and FSO transmissions are employed over S-R and R-D links, respectively. Considering the randomness of R, D, and Eves, and employing a stochastic geometry framework, the secrecy outage performance of the cooperative uplink transmission in the considered SATN is investigated and a closed-form analytical expression for the end-to-end secrecy outage probability is derived. Finally, Monte-Carlo simulations are shown to verify the accuracy of our analysis.

Satellite-aerial communication (SAC) is the most necessary and reliable way to bridge the aerial terminals with their remote command and control center when terrestrial relay infrastructure is unavailable. However, as an inescapable obstacle, adjacent space interference from neighboring satellites unavoidably leads to the loss of the performance of SACs. To uncover the impacts of space interference, in this paper, a SAC system including a satellite (S), an aerial platform (D), and a space interference (I) is considered. Specifically, S operates in a circular orbit and I, which is randomly distributed in the neighboring space around S, produces a jamming signal to interfere with the information delivery between S and D. Considering the randomness of S and I and employing geometric probability theory, an approximate and asymptotic outage probabilities of the considered SAC system are studied. Moreover, the multi-interference scenario is investigated accordingly. Finally, numerical results are presented to validate our proposed analysis and some insightful conclusions are achieved to reveal the effects of spatially random space interference on SAC.

Due to the considerable fading arising from the long-distance transmission between satellites and terrestrial terminals as well as the complex geographic environments on the earth, aerial relays have been designed to facilitate the information delivery between the satellites and terrestrial terminals when reliable direct data transmissions cannot be achieved. In this paper, a cooperative satellite-aerial-terrestrial network (CSATN) system including a satellite (S), an aerial relay (R), and a terrestrial receiver (D) is considered, while decode-and-forward relay scheme is adopted. Specifically, inspired by the beam shape of the transmission signal from S, we model the operation space of R as a circular truncated cone to accurately reflect the practical large-scale transmission scenarios, the height of which is decided by the upper and lower limitations of the operation height of R. Thus, considering the randomness of R and D and employing geometric probability theory, the exact and asymptotic outage probability (OP) of S-R, S-D, and R-D links is studied, and then the end-to-end outage performance of CSATN is investigated accordingly by using approximation method. Finally, numerical results are presented to validate our proposed analysis.

This paper considers a dual-hop cooperative satellite-unmanned aerial vehicle (UAV) communication system including a satellite (S) and a group of cluster headers (CHs) which are respectively with a group of uniformly distributed UAVs. Specifically, the CHs serve as aerial decode-and-forward relays to forward the information transmitted by the S to the UAVs. Moreover, free-space optical (FSO) and radio frequency (RF) technologies are respectively adopted over S-CH and CH-UAV links to exploit the high directivity of FSO over long-distance transmission and the omnidirectional coverage ability of RF. The positions of the CHs in the three-dimensional space follow the Matérn hard-core point processes type-II, in which each CH cannot be closer to another by less than a predefined distance. Three different cases over CH-UAV links are considered during the performance modeling: interference-free, interference-dominated, and interference-and-noise. Then, the coverage performance of the S-CH link and the CH-UAV link in the aforementioned three cases is studied and the closed-form analytical expressions of the coverage probability (CP) over both two links are derived. Additionally, the asymptotic expressions for the CP over S-CH link and CH-UAV link in the interference-free case are derived. Finally, numerical results are provided to validate our proposed analytical models and thus some meaningful conclusions are achieved.

BACKGROUND: Protein subcellular localization prediction plays an important role in biology research. Since traditional methods are laborious and time-consuming, many machine learning-based prediction methods have been proposed. However, most of the proposed methods ignore the evolution information of proteins. In order to improve the prediction accuracy, we present a deep learning-based method to predict protein subcellular locations. RESULTS: Our method utilizes not only amino acid compositions sequence but also evolution matrices of proteins. Our method uses a bidirectional long short-term memory network that processes the entire protein sequence and a convolutional neural network that extracts features from protein sequences. The position specific scoring matrix is used as a supplement to protein sequences. Our method was trained and tested on two benchmark datasets. The experiment results show that our method yields accurate results on the two datasets with an average precision of 0.7901, ranking loss of 0.0758 and coverage of 1.2848. CONCLUSION: The experiment results show that our method outperforms five methods currently available. According to those experiments, we can see that our method is an acceptable alternative to predict protein subcellular location.

The high spectrum efficiency of nonorthogonal multiple access (NOMA) is attractive to solve the massive number of connections in the Internet of Things (IoT). This article investigates a buffer-aided cooperative NOMA (C-NOMA) system in the IoT, where the intended users are equipped with buffers for cooperation. The direct transmission from the access point to the users and the buffer-aided cooperative transmission between the intended users are coordinated. In particular, a novel buffer-aided C-NOMA scheme is proposed to adaptively select a direct or cooperative transmission mode, based on the instantaneous channel state information and the buffer state. Then, the performance of the proposed scheme, in terms of the system outage probability and average delay, is theoretically derived with closed-form expressions. Furthermore, the full diversity order of three is demonstrated to be achieved for each user pair if the buffer size is not less than three, which is larger than conventional nonbuffer-aided C-NOMA schemes whose diversity order is only two in the considered C-NOMA system in the IoT.

Unmanned aerial vehicles (UAVs) communications have been widely exploited in our daily life, which leads to rising concerns about the security issue. This work investigates the secrecy performance of a UAV-to-vehicle (UAV-2-V) communication system, where the information delivered over both downlink and uplink between a UAV ($S$) acting as a temporary aerial base-station and a legitimate vehicle ($D$) moving along a road which is overheard by an eavesdropping vehicle ($E$) on the same road. The location of$S$is assumed to be uniformly distributed in the sky, while the locations of$D$and$E$are uniformly distributed on the highway. The statistical characteristics, including the cumulative distribution function and probability density function of the received signal-to-noise ratio over both downlink and uplink, are characterized respectively. Closed-form expressions for the approximate and asymptotic secrecy outage probability (SOP) of the downlink experiencing Rician fading channels have been derived accordingly. Moreover, the secrecy outage performance of the uplink is investigated by deriving the closed-form expression of the exact and asymptotic SOP in two cases: the eavesdropping channel suffers Rician and Weibull fading, respectively. Finally, Monte-Carlo simulations are shown to verify our proposed analytical models.

This paper employs hybrid automatic repeat request (HARQ) to assist the cognitive non-orthogonal multiple access (NOMA) scheme for secure transmission in Internet of Things networks where the security-required (SR) users with high-security requirements and quality of services (QoS)-sensitive (QS) users with real-time process requirements are paired to perform NOMA for increasing the network connectivity. In addition, the imperfect successive interference cancellation (SIC) is considered at both legitimate users and eavesdropper for providing a realistic analysis. The closed-form expressions for the connection outage probability (COP), the secrecy transmission probability (RSP), and effective secrecy throughput (EST) of the SR user are derived in the proposed lightweight but efficient randomized retransmission scheme (RRS). As benchmarks, the secrecy performances of the fixed retransmission scheme (FRS) and orthogonal multiple access (OMA) scheme are also investigated. Besides, the asymptotic secrecy analysis is given to gain a better insight into secrecy performance. The analytical results developed in the paper demonstrate that HARQ is beneficial to secrecy transmission and the RRS outperforms or equals to the FRS and OMA scheme when the transmit power and secrecy coding redundancy is sufficiently high.

In this paper, we propose a non-stationary satellite communication system model considering the impacts of Earth rotation by adopting the Earth-centered inertial (ECI), and the Earth-centered Earth-fixed (ECEF) coordinates. The position variations of a satellite (S) and a ground user (U) via coordinate transformations are demonstrated. Considering the variations of the distance between S and U, the instantaneous outage probability (OP) and channel capacity are calculated, as well as the system throughput within finite communication time. A simplified case is considered and analyzed while ignoring the Earth's rotation. Furthermore, the asymptotic expressions for the OP, capacity, and throughput are developed in the high signal-to-noise ratio (SNR) regime to obtain some insights. We also provide new definitions for throughput and OP within a short communication duration. The application and future research directions based on the derived results, including resource allocation, satellite handover, communication scenarios with multiple satellites and mobile users, are also discussed. Finally, some selected numerical results are provided to validate our proposed analysis models.

This work studies the event-triggered control problem for networked control systems. The plant is controlled directly by a dynamic local controller, which receives the reference control signal from the remote controller. The measurement signal of the plant and the reference control signal of the remote controller has their separate event trigger, and thus the remote controller and the local controller can decide when to transmit signals on their own. It is proved that with the proposed control approach and the dual event trigger, the closed loop system is globally asymptotically stable, which has been illustrated by simulation results.

BACKGROUND: Reconstructing ancestral genomes is one of the central problems presented in genome rearrangement analysis since finding the most likely true ancestor is of significant importance in phylogenetic reconstruction. Large scale genome rearrangements can provide essential insights into evolutionary processes. However, when the genomes are large and distant, classical median solvers have failed to adequately address these challenges due to the exponential increase of the search space. Consequently, solving ancestral genome inference problems constitutes a task of paramount importance that continues to challenge the current methods used in this area, whose difficulty is further increased by the ongoing rapid accumulation of whole-genome data. RESULTS: In response to these challenges, we provide two contributions for ancestral genome inference. First, an improved discrete quantum-behaved particle swarm optimization algorithm (IDQPSO) by averaging two of the fitness values is proposed to address the discrete search space. Second, we incorporate DCJ sorting into the IDQPSO (IDQPSO-Median). In comparison with the other methods, when the genomes are large and distant, IDQPSO-Median has the lowest median score, the highest adjacency accuracy, and the closest distance to the true ancestor. In addition, we have integrated our IDQPSO-Median approach with the GRAPPA framework. Our experiments show that this new phylogenetic method is very accurate and effective by using IDQPSO-Median. CONCLUSIONS: Our experimental results demonstrate the advantages of IDQPSO-Median approach over the other methods when the genomes are large and distant. When our experimental results are evaluated in a comprehensive manner, it is clear that the IDQPSO-Median approach we propose achieves better scalability compared to existing algorithms. Moreover, our experimental results by using simulated and real datasets confirm that the IDQPSO-Median, when integrated with the GRAPPA framework, outperforms other heuristics in terms of accuracy, while also continuing to infer phylogenies that were equivalent or close to the true trees within 5 days of computation, which is far beyond the difficulty level that can be handled by GRAPPA.

Unmanned aerial vehicles (UAVs) will be extensively utilized in various Internet of Things (IoT) scenarios due to their flexible mobility and rapid on-demand deployment. It becomes necessary and urgent to investigate the secrecy performance of the UAV IoT communication systems because of the open characteristics of wireless channels. In this article, the physical layer security of UAV IoT communication systems is studied, in which a ground IoT device transmits some confidential messages to a UAV hovering in the air, while a random number of eavesdroppers are randomly positioned around the ground source. The ground-to-air channel is assumed to experience Rician fading for the case of line-of-sight propagation and Rayleigh fading for the case of nonline-of-sight propagation, respectively. In addition, in order to improve the security of the system, we also investigate the secrecy performance of the UAV IoT system with a friendly UAV that generates jamming signals to distract the eavesdroppers. Utilizing the stochastic geometry theory, the cumulative distribution functions for the signal-to-interference-plus-noise ratio of the main and eavesdropping links are derived, and then the analytical expressions of the secrecy outage probability and the average secrecy rate are obtained. Finally, the accuracy of the analytical results is verified by Monte Carlo simulation.

In this work, we analyze the secrecy outage performance of a dual-hop relay system composed of multipleinput- multiple-output radio-frequency (RF) links and a freespace optical (FSO) link while a multiple-antenna eavesdropper wiretaps the confidential information by decoding the received signals from the source node. The channel state information (CSI) of the RF and FSO links is considered to be outdated and imprecise, respectively. We propose four transmit antenna selection (TAS) schemes to enhance the secrecy performance of the considered systems. The secrecy outage performance with different TAS schemes is analyzed and the effects of misalignment and detection technology on the secrecy outage performance of mixed systems are studied. We derive the closed-form expressions for probability density function (PDF) and cumulative distribution function (CDF) over M´alaga channel with imperfect CSI. Then the closed-form expressions for the CDF and PDF of the equivalent signal-to-noise ratio (SNR) at the legitimate receiver over Nakagami-m and M´alaga channels are derived. Furthermore, the bound of the effective secrecy throughput (EST) with different TAS schemes are derived. Besides, the asymptotic results for EST are investigated by exploiting the unfolding of Meijer’s G-function when the electrical SNR of FSO link approaches infinity. Finally, Monte-Carlo simulation results are presented to testify the correctness of the proposed analysis. The results illustrate that outdated CSI shows a strong effect on the secrecy performance of the mixed RF-FSO systems. In addition, increasing the number of antennas at the source cannot significantly enhance the secrecy performance of the considered systems

This paper investigates the secure transmission in hybrid automatic repeat request (HARQ)-assisted non-orthogonal multiple access (NOMA) networks, where a security-required (SR) user is paired with an opportunity-served (OS) user to perform NOMA. Different from the previous works where NOMA and HARQ are separately considered for secure transmissions, in this paper, both NOMA and HARQ are utilized to enhance the security of the SR user. We derive the closed-form expressions for connection outage probability (COP), secrecy outage probability (SOP), reliable and secure transmission probability (RSP) and effective secrecy throughput (EST) of the SR user in both maximum ratio combining (MRC) and selection combining (SC) schemes. The security-reliability trade-off (SRT) results of the SR user in both MRC and SC schemes are also provided. Analysis and simulation results show that HARQ improves secrecy performance in MRC and SC schemes in terms of RSP, especially in low transmit power region. Besides, since every retransmission is utilized in the MRC scheme, it strictly outperforms the SC scheme in terms of the SRT metric. However, the SC scheme has the potential to achieve higher EST than the MRC scheme by optimizing the transmit power or the power allocation factor. Significantly, the quality of service (QoS) of the OS user can be guaranteed by pairing an SR user who is far away from the transmitter and the secrecy performance of the SR user can be enhanced by pairing an OS user.

By observing the fact that moving in a straight line is a common flying behavior of unmanned aerial vehicles (UAVs) in normal applications, e.g., power line inspections, and air patrols along with highway/streets/borders, in this paper we investigate the secrecy outage performance of a UAV system with linear trajectory, where a UAV (S) flies in a straight line and transmits its information over the downlink to a legitimate receiver (D) on the ground while an eavesdropping UAV (E) trying to overhear the information delivery between S and D. Meanwhile, some information is delivered to S over the uplink from D, such as commanding messages to control S's detecting operations, which can also be eavesdropped by E. The locations of S, D, and E are randomly distributed. We first characterize the statistical characteristics (including cumulative distribution functions and probability density function) of the received signal-to-noise ratio over both downlink and uplink, and then the closed form analytical expressions for the lower boundary of the secrecy outage probability of both downlink and uplink have also been derived accordingly. Finally, Monte-Carlo simulations are given to testify our proposed analytical models.

As an efficient supplement for terrestrial communications, e.g., fifth-generation communication networks, satellite communication has been proved as a promising choice to realize seamless coverage, due to its inherent superiority over common wireless communication systems, like large-scale coverage area and flexibility of deployment. In this article, a satellite-terrestrial transmission (STT) system including a satellite transmitter (S) and a group of terrestrial receivers (D) is focused on. Considering the randomness of D and employing stochastic geometry, the outage performance and diversity gain of three hybrid automatic repeat request (HARQ) schemes, generalized slotted ALOHA (GSA), repetition time diversity (RTD) and general incremental redundancy (IR), are respectively studied. In detail, the exact closed-form analytical expression for the outage probability (OP) is derived for the GSA scheme, while the approximated expression for the OP of RTD scheme is presented and a useful expression for the OP of IR scheme is derived. Furthermore, the diversity gains of the three considered HARQ schemes are respectively investigated in STT scenarios. Finally, some numerical results are provided to validate the proposed analysis models.

Aerial relays have been regarded as an alternative and promising solution to extend and improve satellite-terrestrial communications, as the probability of line-of-sight transmissions increases compared with adopting terrestrial relays. In this paper, a cooperative satellite-aerial-terrestrial system including a satellite transmitter (S), a group of terrestrial receivers (D), and an aerial relay (R) is considered. Specifically, considering the randomness of S and D and employing stochastic geometry, the coverage probability of R-D links in non-interference and interference scenarios is studied, and the outage performance of S-R link is investigated by deriving an approximated expression for the outage probability. Moreover, an optimization problem in terms of the transmit power and the transmission time over S-R and R-D links is formulated and solved to obtain the optimal end-to-end energy efficiency for the considered system. Finally, some numerical results are provided to validate our proposed analysis models, as well as to study the optimal energy efficiency performance of the considered system.

In this paper, a cooperative hybrid visible light communication (VLC)-radio frequency (RF) system with spatially random terminals is considered. Especially, a source node ($S$) with a group of light-emitting diode (LED) lamps transmits information bits to a destination ($D$), which is located out of$S$’s coverage area, via a relay node ($R$).$R$is equipped with a photodetector and an antenna to set up a VLC link between$S$and$R$, and a radio frequency (RF) link between$R$and$D$, respectively. Meanwhile, an eavesdropper ($E$) equipped with a photodetector and an antenna tries to overhear the information delivery over VLC and RF links. Also, diversity receiving scheme is considered at$E$, while the same modulation scheme is considered over both VLC and RF links. Furthermore, decode-and-forward scheme is adopted at$R$to process and forward the received VLC signals. By employing stochastic geometry, we first characterize the probability density function and cumulative distribution function of the received signal-to-noise-ratio over both VLC and RF links, while considering the randomness of the locations of both$R$,$E$, and$D$. Then, the secrecy performance of the target system is studied by deriving the approximated expressions for the secrecy outage probability under various cases. Finally, the proposed analytical models are verified via Monte-Carlo simulations.

Motivated by the move towards the next generation of green communications, we study a power beacon (PB) assisted wireless-powered cooperative communication network (PBWPCCN). A new system model is proposed where a source and a number of relay terminals are not directly connected to the power grid. These terminals, including the source, need to harvests energy from radio frequency (RF) signals radiated from PB and works cooperatively to deliver the information to the target device successfully. The system executes this process in a two-stage relay selection decode-then-forward (2SRS-DF). First, the system selects a group of relay candidates that can successfully decode the received signal from the source. Second, from the first relays group that successfully decodes the source signal, the best relay with the maximum capability to forward the decoded message to the destination is selected. The closed-form expression for the average outage probability and the throughput of the proposed system is derived and presented. We validate the theoretical results by numerical simulations. The impact of position and number of relay terminals along with time allocation parameters are also studied.

In a downlink non-orthogonal multiple access (NOMA) system, the reliable transmission of cell-edge users cannot be guaranteed due to severe channel fading. On the other hand, the presence of eavesdroppers can severely threaten the secure transmission due to the open nature of wireless channel. Thus, a two-user NOMA system assisted by a multi-antenna decode-and-forward relay is considered in this paper, and a two-stage jamming scheme, full-duplex-jamming (FDJam), is proposed to ensure the secure transmission of NOMA users. In the FDJam scheme, using full-duplex, the relay transmits the jamming signal to the eavesdropper while receiving confidential messages in the first stage, and the base station generates the jamming signal in the second stage. Furthermore, we eliminate the self-interference and the jamming signal at the relay and the legitimate node, respectively, through relay beamforming. To measure the secrecy performance, analytical expressions for secrecy outage probability (SOP) are derived for both the cell-center and cell-edge users, and the asymptotic SOP analysis at high transmit power is presented as well. Moreover, two benchmark schemes, half-duplex-jamming and full-duplex-no-jamming, are also considered. Simulation results are presented to show the accuracy of the analytical expressions and the effectiveness of the proposed scheme.

This paper considers the secrecy outage performance of a multiple-relay assisted non-orthogonal multiple access (NOMA) network over Nakagami-m fading channels. Two time slots are utilized to transmit signals from the base station to destination. At the first time slot, the base station broadcasts the superposition signal of the two users to all decode-and-forward relays by message mapping strategy. Subsequently, the selected relay transmits superposition signal to the two users via power-domain NOMA technology. Three relay selection schemes, i.e., optimal single relay selection (OSRS) scheme, two-step single relay selection (TSRS) scheme, and optimal dual relay selection (ODRS) scheme are proposed and the secrecy outage performance is analyzed. As a benchmark, we also examine the secrecy outage performance of the NOMA systems with traditional multiple relay forwarding (TMRF) scheme in which all the relay that successfully decode signals from the source forward signals to the NOMA users with equal power. Considering the correlation between the secrecy capacity of two users and different secrecy requirement for two NOMA users, the analytical expressions for the security outage probability (SOP) of the proposed OSRS, TSRS, and ODRS schemes along with the TMRF scheme are derived and validated via simulations. To get more insights, we also derive the analytical expressions for the asymptotic SOP for all the schemes with fixed and dynamic power allocations. Furthermore, the secrecy diversity order (SDO) and secrecy array gain of cooperative NOMA systems are obtained. The results demonstrate that our proposed schemes can significantly enhance the secrecy performance compared to the TMRF scheme and that all the schemes with fixed power allocation obtain zero SDO and the OSRS scheme with dynamic power allocation obtains the same SDO as TMRF.

In this study, the secrecy performance of the classic Wyner's wiretap model over shadowed fading channels is studied. More specifically, the authors derive two analytical expressions for the lower bound of secure outage probability at high signal‐to‐noise ratio regime and the probability of strictly positive secrecy capacity over shadowed fading channels, respectively. As there exist infinite series in the two derived expressions, they further obtain two simple and explicit approximate expressions for the lower bound of secure outage probability and the probability of strictly positive secrecy capacity with the aid of the moment matching method. It is shown that the match between the analytical results and simulations is very excellent for all parameters under considerations.

We analyze a secure dual-hop mixed radio frequency-free space optical (RF-FSO) downlink simultaneous wireless information and power transfer system. The FSO link and all RF links experience Gamma-Gamma, independent, and identical Nakagami-m fading, respectively. We analyze the effects of atmospheric turbulence, pointing error, detection technology, path loss, and energy harvesting on secrecy performance. Signalto-noise ratios at both the legitimate and illegitimate receivers are not independent since they are both simultaneously influenced by the FSO link. We derive the closed-form expression of the secrecy outage probability (SOP) as well as the asymptotic result for SOP when signal-to-noise ratios at relay and legitimate destinations tend to infinity. Monte-Carlo simulations are performed to verify the accuracy of our analysis. The results show that the secrecy diversity order (SDO) depends on the fading parameter of the relay-destination link and the number of the destination's antennas. In addition, the SDO also depends on the fading parameters, the pointing error parameter, and the detection type of the FSO link.

In this paper, a cooperative hybrid visible light communication (VLC)-radio frequency (RF) relay system, which includes a transmitter (S) with a cluster of light emitting diodes (LED) lamps, a relay (R), and a destination (D), is taken into consideration. Moreover, decode-forward (DF) and amplify-forward (AF) schemes are considered at R to aid the data transmissions between S and D. Specially, R is randomly located in the coverage of the LED lamps, D is randomly distributed out of the coverage of the LED lamps. Then, VLC is employed for S-R link, and a traditional RF communication technology is adopted over the R-D link. Considering Nakagami-$m$fading over the R-D link, and the randomness of the locations of R and D, the statistical characteristics of the received signal-to-noise ratio at D under AF scheme, and the ones at R and D under the DF scheme are characterized, respectively. Subsequently, approximated analytical and asymptotic expressions for the outage probability of the considered system are derived, as well as the approximated expression for the average symbol error rate of M-ary quadrature amplitude modulation scheme under the DF scheme. Finally, simulation and numerical analysis are conducted to verify the correctness of our proposed models.

In this paper, an indoor 3-dimensional (3-D) hybrid visible light communication (VLC)-radio frequency (RF) system with spatially random terminals is considered. Specifically, VLC and RF communications are employed over downlink and uplink, respectively. Meanwhile, the devices, like sensor nodes, are designed to harvest the energy from the light emitted by the light-emitting diode over the downlink, which is used for the transmissions over the uplink. The light energy harvesting model is proposed after introducing the line of sight propagation model for VLC. Then, the outage performance for orthogonal multiple access (OMA) scheme over the uplink has been studied by using stochastic geometry theory, while considering all devices are spatially random distributed in the 3-D room and all uplinks follow independent/correlated Rician fading. Finally, the analytical expressions for the outage probability with OMA scheme are derived and verified through Monte Carlo simulations.

In this paper, the secrecy outage performance of an underlay cognitive decode-and-forward (DF) relay network over independent and non-identical distributed (i.n.i.d) Nakagami-m fading channels is investigated, in which the secondary user transmitter communicates with the secondary destination via relays, and an eavesdropper attempts to overhear the information. We assume that the channel state information (CSI) of all links is available. The exact and asymptotic closed-form expressions for the secrecy outage probability with optimal relay selection are derived, and verified by Monte-Carlo simulations. Through asymptotic analysis, we find that the secrecy diversity order is determined by the number of relays and the fading parameters between relay and destination.

This study investigates the secrecy outage performance of a single‐input multiple‐output underlay cognitive radio network (CRN) with outdated channel state information (CSI). The confidential messages are transmitted from transmitter to the destination, while a multi‐antenna eavesdropper exists. The maximal ratio combining and selection combining schemes are utilised at the receivers to improve the quality of the received signal‐to‐noise ratio. The exact and asymptotic closed‐form expressions of secrecy outage probability are derived, and simulation results are provided to verify the authors' proposed analytical results. The results reveal that imperfect CSI of main channels deteriorates the secrecy outage performance while that of eavesdropping and interfering channels has contrary effect, and only a unity diversity order can be obtained in underlay CRNs with imperfect CSI.

In this study, a wireless powered dual‐hop relay system, which consists of a source, a destination and a relay that is equipped with ( ) receiving antennas and finite energy storage, is considered. Antenna selection scheme is employed for the relay to harvest energy from the received signals, under which the ( ) strongest received signals are combined for information processing and the other copies of signals are used for energy harvesting. Considering Nakagami‐ m fading scenarios, the authors first derive the closed‐form expressions for the probability density function and the cumulative distribution function of the signal‐to‐noise ratio with generalised selection combining (GSC), and then the exact and asymptotic analytical expressions for outage probability are derived, respectively. Finally, simulation results are presented to verify the proposed analysis model.

We consider a cooperative system consisting of a source node, a destination node, N ( N >1) wireless-powered relays, and an eavesdropper. Each relay is assumed to be with a nonlinear energy harvester, in which there exists a saturation threshold, limiting the level of the harvested power. For decode-and-forward and power splitting protocols, the K th best relay is selected to assist the source-relay-destination transmission. An analytical expression for the secrecy outage probability is derived, and also verified by simulation.

This paper investigates the secrecy performance of maximal ratio combining (MRC) and selection combining (SC) with imperfect channel state information (CSI) in the physical layer. In a single-input multipleoutput (SIMO) wiretap channel, a source transmits confidential messages to the destination equipped with M antennas using the MRC/SC scheme to process the received multiple signals. An eavesdropper equipped with N antennas also adopts the MRC/SC scheme to promote successful eavesdropping. We derive the exact and asymptotic closed-form expressions for the ergodic secrecy capacity (ESC) in two cases: (1) MRC with weighting errors, and (2) SC with outdated CSI. Moreover, two important indicators, namely high signal-to-noise ratio (SNR) slope and high SNR power offset, which govern ESC at the high SNR region, are derived. Finally, simulations are conducted to validate the accuracy of our proposed analytical models. Results indicate that ESC rises with the increase of the number of antennas and the received SNR at the destination, and fades with the increase of those at the eavesdropper. Another finding is that the high SNR slope is constant, while the high SNR power offset is correlated with the number of antennas at both the destination and the eavesdropper.

This paper investigates and optimizes the performance of simultaneous wireless information and power transfer (SWIPT) in wireless sensor networks over Nakagami-m fading channels. In the considered system, there is one mobile reader (R), which is equipped with one transmit antenna and one receive antenna, and a group of passive sensors. The information delivery includes two stages: (1) R broadcasts a command with radio-frequency energy to the sensors, which adopt time splitting (TS)/power splitting (PS) schemes to harvest energy and (2) sensors deliver their information to R over orthogonal channels by using the harvested energy. In this paper, we propose a unified framework to study and optimize the impact of SWIPT on the system performance with both TS and PS schemes. First, we characterize the probability density function and cumulative distribution function of the signal-to-interference-plus-noise-ratio in high signal-to-noise ratio region, then we study the outage and ergodic capacity performance of the backward links. The approximated closed-form expressions for the outage probability and ergodic capacity are derived and validated through Monte Carlo simulations. Finally, we also evaluate the energy efficiency of the target system, and propose an optimal splitting scheme for TS and PS to maximize the throughput of the target system.

In this paper, a hybrid visible light communication-radio frequency (RF) system, including a legitimate receiver (R) and an eavesdropper (E) is considered. R can harvest energy from the light emitted by light emitting diodes (LEDs), which is used for the information transmission between R and the RF receiver which is close to the LED. It is assumed that E tries to eavesdrop the information delivered from R to the RF receiver and R is with finite energy storage. Considering the randomness of the locations of R and E, the statistical characteristics of the received signal-to-noise ratio at the RF receiver and E are characterized; then, we derive the analytical expressions for exact and asymptotic secrecy outage probability by using the stochastic geometry method. Finally, simulations are carried out to verify our proposed analytical models.

In this paper, a dual-hop decode-to-forward cooperative system is considered where multiple relays are with finite energy storage and can harvest energy from the destination. In our analysis, the relays are spatially randomly located with invoking stochastic geometry. In an effort to improve spectral efficiency, an optimal source-relay link scheme is employed. Assuming Nakagami-m fading, two different scenarios are considered: 1) the single-antenna source with perfect channel state information (CSI) and 2) the multiple-antenna source with transmit antenna selection and imperfect CSI. In both scenarios, the destination node is equipped with a single transmit antenna to forward power via frequency radio signal to the relay candidates. For improving the system performance, multiple antennas at the destination are considered to process the multiple copies of the received signal from the best relay. For characterizing the performance of the proposed scenarios, exact closed-form analytical expressions for the outage probability are derived. To obtain further insights, we carry out diversity gain analysis by adopting asymptotic relative diversity. We also derive the exact closed-form analytical expression for the system throughput. Finally, simulation results are presented to corroborate the proposed analysis and to show that: 1) the system performance is improved by enlarging the area of the circle and the density of the relays and 2) the energy storage size has impacts on the performance of considered networks, which determines the maximal transmit power at relays.

In this work, we consider the transmission of confidential message through independent and identically distributed generalized-K fading single-input multiple-output (SIMO) channels in the presence of an eavesdropper. The security performance of SIMO generalized-K systems are investigated using a mixture gamma (MG) distribution, and the analytical expressions for the average secrecy capacity (ASC) and the secure outage probability (SOP) of SIMO systems are derived. Numerical results are presented and verified via Monte-Carlo simulations.

In this study, the secrecy performance of the classic Wyner's wiretap model over generalised‐ K fading channels is studied. The closed‐form expressions for the average secrecy capacity, the lower bound of secure outage probability, and the probability of strictly positive secrecy capacity are derived. The new expressions provide a unified form, which can handle several of the well‐known composite fading environments as special or limiting cases. Moreover, all the derived expressions are given in terms of Meijer's G ‐function, which can be simply realised in MATLAB and MATHEMATICA. Monte‐Carlo simulations are performed to verify the proposed analysis models.

In this paper, the transmission of confidential messages through single-input multiple-output (SIMO) independent and identically generalized- K ( K G ) fading channels is considered, where the eavesdropper overhears the transmission from the transmitter to the receiver. Both the receiver and the eavesdropper are equipped with multiple antennas, and both active and passive eavesdroppings are considered where the channel state information of the eavesdropper’s channel is or is not available at the transmitter. The secrecy performance of SIMO K G systems is investigated. Analytical expressions for secrecy outage probability and average secrecy capacity of SIMO systems are derived via two different methods, in which K G distribution is approximated by the Gamma and mixture Gamma distributions, respectively. Numerical results are presented and verified via the Monte-Carlo simulation.

We investigate the secrecy outage performance of maximal ratio combining (MRC) in cognitive radio networks over Rayleigh fading channels. In a single-input multiple-output wiretap system, we consider a secondary user (SU-TX) that transmits confidential messages to another secondary user (SU-RX) equipped with M ( M ≥ 1) antennas where the MRC technique is adopted to improve its received signal-to-noise ratio. Meanwhile, an eavesdropper equipped with N ( N ≥ 1) antennas adopts the MRC scheme to overhear the information between SU-TX and SU-RX. SU-TX adopts the underlay strategy to guarantee the service quality of the primary user without spectrum sensing. We derive the closed-form expressions for an exact and asymptotic secrecy outage probability.

In this letter, we consider a single-input multiple-output cognitive wiretap system over generalized-K channels, where the eavesdropper overhears the transmission from the secondary transmitter (ST) to the legitimate receiver. Both the primary user and the ST are equipped with a single antenna, whereas the legitimate and the eavesdropper receivers are equipped with multiple antennas. The close-form expression for the secrecy outage probability is derived. Simulations are presented to validate the accuracy of our proposed analytical results.

In this paper, a multiple-input single-output (MISO) simultaneous wireless information and power transfer (SWIPT) system, including one base station (BS) equipped with multiple antennas, one desired single-antenna information receiver (IR), and N (N > 1) single-antenna energy-harvesting receivers (ERs) is considered. Assuming that the information signal to the desired IR may be eavesdropped by ERs if ERs are malicious, we investigate the secrecy performance of the target MISO SWIPT system when imperfect channel state information (CSI) is available and adopted for transmit antenna selection at the BS. Considering that each eavesdropping link experiences independent but not necessarily identically distributed Rayleigh fading, the closed-form expressions for the exact and the asymptotic secrecy outage probability, and the average secrecy capacity are derived and verified by simulations. Furthermore, the optimal power splitting factor is derived for each ER to realize the tradeoff between the energy harvesting and the information eavesdropping. Our results reveal the impact of the imperfect CSI on the secrecy performance of MISO SWIPT systems in the presence of multiple wiretap channels.

In this study, secrecy outage performance is studied for partial relay selection (PRS) schemes in cooperative systems over Rayleigh fading channels. In the considered systems, there are one source (S), one destination (D), one eavesdropper, one jammer that can send interference to the eavesdropper, and N ( N > 1) relay candidates, one of which is selected as the relay (R). By considering the practical scenarios and to protect the transmitted information over R–D link from being eavesdropped, the authors propose a new transmission scheme: let D send random information to R while S sending information to R. Then, R processes the messages from S and D by XOR operation in order to prevent the eavesdropper's interception while the information is delivered between R and D. They study the secrecy outage performance for two PRS schemes: (i) the optimal source–relay link (OSRL) scheme; and (ii) the optimal source–relay–destination link (OSRDL) scheme. The closed‐form expressions for the secrecy outage probability (SOP) of the two PRS schemes are derived, as well as the closed‐form upper bound for SOP under OSRL scheme and the lower bound for SOP under OSRDL scheme. Finally, simulation results are given to verify the presented analytical models and the proposed transmission scheme.

In this paper, a single-input multiple-output (SIMO) simultaneous wireless information and power transfer system consisting of one base station, one desired information receiver and N (N > 1) energy-harvesting receivers is considered. Each link in the considered system suffers not only from small-scale fading but also path-loss. In particular, we focus on the physical layer security performance including secrecy outage and secrecy capacity in the presence of eavesdroppers. We derive the closed-form analytical expression for the exact and the asymptotic secrecy outage probability while considering each eavesdropping link experiences independent but not necessarily identical Rayleigh fading. The closed-form analytical expression for the average secrecy capacity has also been derived for the case that all eavesdropping links experience independent and identical Rayleigh fading. Numerical and Monte Carlo simulation results are presented to verify our derived analytical models.

Closed‐form expressions of the upper and lower bounds of the outage probability are derived for interference‐limited dual‐hop decode‐and‐forward and channel‐state‐information‐assisted amplify‐and‐forward relay systems using three relay selection (RS) schemes [optimal source–relay link (OSRL) scheme, optimal relay–destination link (ORDL) scheme and optimal source–relay–destination link (OSRDL) scheme] over independent log‐normal fading channels. Multiple interferers at both relay and destination are considered. The diversity order of the targeted cooperative system under the three different RS schemes is analysed. The analysis shows that the asymptotic relative diversity orders (ARDO) for OSRL, ORDL and OSRDL schemes are 2, 2 and ( N + 1) (where N is the number of the relay candidates), respectively. Finally, the ARDO is verified by simulation results.

In this paper, analytical models are proposed to study the performance of amplify-and-forward (AF) multi-branch multi-hop (MBMH) wireless relay systems (WRS) over log-normal fading channels. The presented analytical models are used to estimate the end-to-end performance of the MBMH WRS with maximal ratio combining (MRC) and selection combining (SC) schemes. In particular, we derive closed-form expressions for the parameters of the end-to-end signal-to-noise ratio (SNR) of an AF multi-hop chain relay link. Further, we characterize the distribution of the end-to-end SNR and derive closed-form expressions for the mean and standard deviation of the end-to-end SNR's natural logarithm for AF MBMH WRS with MRC scheme, and the probability density function (pdf) for the end-to-end SNR of AF MBMH WRS with SC scheme. Then, under MRC and SC schemes, we analyze the diversity gain by adopting asymptotical relative diversity order (ARDO) and develop accurate analytical expressions for average bit error probability, outage probability, and ergodic capacity, respectively. Our analysis reveals that the computational complexity of the proposed models is acceptably low for practical applications. Numerical and Monte Carlo simulation results are presented to substantiate the accuracy of the proposed analytical models.

Multi-hop time synchronization in wireless sensor networks (WSNs) is often time-consuming and error-prone due to random time-stamp delays for MAC layer access and unstable clocks of intermediate nodes. Constructive interference (CI), a recently discovered physical layer phenomenon, allows multiple nodes transmit and forward an identical packet simultaneously. By leveraging CI, we propose direct multi-hop (DMH) time synchronization by directly utilizing the time-stamps from the sink node instead of intermediate nodes, which avoids the error caused by the unstable clock of intermediate nodes. DMH doesn't need decode the flooding time synchronization beacons. Moreover, DMH explores the linear regression technique in CI based time synchronization to counterbalance the clock drifts due to clock skews.

In this letter, we propose an analytical model to study the performance of cooperative links for time hopping ultra wideband (UWB) systems under two relay models: amplify-and-forward (AF) and decode-and-forward (DF). The effects of multi-user interference (MUI) are considered when deriving closed-form expressions for outage probability and average bit error probability (BEP) over single-path model. Simulation results show that the presented analytical model can accurately predict the link performance.