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.

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.

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.

In multi-user adversarial scenarios involving external malicious jamming and internal co-channel interference, environmental instability and increased decision-making dimensions cause traditional deep reinforcement learning (DRL)-based anti-jamming schemes to suffer from insufficient exploration. Agents must choose policies from a large action set, leading to a significant decline in anti-jamming performance. To address these issues, this paper proposes a multi-agent discrete soft actorcritic (MA-DSAC) algorithm-based collaborative anti-jamming strategy, integrating frequency, power, and modulation-coding domains. This strategy first introduces a Markov game to model and analyze the multi-user anti-jamming problem. Next, the soft actor-critic (SAC) algorithm is discretized to handle the multi-dimensional discrete action space. Finally, through information exchange between communication transceivers and based on a centralized training with decentralized execution (CTDE) framework, it is extended to a multi-agent DRL algorithm to achieve efficient multi-user cooperative anti-jamming. Simulation results show that in various anti-jamming scenarios with both fixed-mode and intelligent jammers, the proposed anti-jamming strategy’s performance improves by more than 25% compared to traditional value-based DRL strategies, including independent deep Q-network (I-DQN) and multi-agent virtual exploration in deep Q-learning (MA-VEDQL). Furthermore, through information exchange between communication transceivers, the instability problem of multi-agent DRL is effectively alleviated, enabling the communication transceivers to balance competition and cooperation. Consequently, its anti-jamming performance improves by more than 6% compared to the independent DSAC (I-DSAC) strategy.

In complex jamming environments with multi-user spectrum sharing, existing distributed anti-jamming strategies are constrained by significant communication overhead, limited efficiency in knowledge dissemination, and low collaborative effectiveness. To address these challenges, a distributed anti-jamming strategy based on local knowledge diffusion and differential weighted fusion mechanisms (LKD-DWF-M) is proposed. In this strategy, a local knowledge diffusion mechanism is introduced to facilitate knowledge sharing among communication nodes, enabling each node to gain a comprehensive understanding of its neighbors’ behavior. Subsequently, a knowledge contribution measurement method based on mutual information is proposed, and a differential weighted fusion (DWF) mechanism is designed to effectively integrate the policy and value parameters of neighboring nodes. This integration enables accurate global value estimation while optimizing individual anti-jamming strategies. Additionally, the existence of the Nash equilibrium (NE) for each node’s policy and value parameters is theoretically established using Kakutani’s fixed-point theorem. Furthermore, through the construction of a Lyapunov function, it is demonstrated that the proposed strategy can stabilize and converge to the NE in the long-term jamming counteraction process. Simulation results indicate that, in comparison to anti-jamming strategies employing global knowledge diffusion and differential weighted fusion mechanism (GKD-DWF-M), global knowledge diffusion and average fusion (GKD-AF-M), and local knowledge diffusion and average fusion (LKD-AF-M), the proposed distributed anti-jamming strategy achieves respective improvements of 4%, 17%, and 20% in system normalized throughput under statistical jamming (SJ). Under dynamic sweeping jamming (DSJ), the system normalized throughput improves by 8%, 11%, and 11.5%, respectively; under intelligent comb jamming (ICJ), it increases by 10%, 10.5%, and 19%, respectively; and under intelligent block jamming (IBJ), it increases by 5%, 16%, and 21%, respectively. Moreover, the proposed strategy exhibits superior convergence speed compared to other strategies. When the jammer alternates between SJ, DSJ, ICJ, and IBJ, the proposed distributed anti-jamming strategy responds quickly, demonstrating robustness in dynamic jamming environments.

In this paper, we investigate covert communication in rate-splitting multiple access (RSMA)-assisted ambient backscatter communication (AmBC) systems. Specifically, in the presence of illegitimate users, a base station applies RSMA to the downlink transmission of covert users and common users in the the primary system of ambient backscatter. To evaluate the covertness of the system, we derive closed-form expressions for the detection error probability (DEP), outage probability (OP), and covert rate (CR), where the DEP is applied to evaluate the ability of the warden to detect the presence or absence of communication behaviors between the base station and the covert user. Based on these expressions, we also conduct an asymptotic analysis of the outage probability, an upper bound analysis of the covert rate, and an analysis of the impact of the power allocation factor on system performance. Numerical results show that the RSMA scheme has a higher covert rate and a higher minimum DEP value compared to the non-orthogonal multiple access (NOMA) scheme, but it may reduce its outage performance. Additionally, an appropriate use of the power allocation factor can enhance the covertness of the system. Furthermore, we find that the co-channel interference between the primary and secondary systems in the ambient backscatter system can degrade the covert performance of the primary system.

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.

This paper analyzes the block error rate (BLER) of hybrid automatic repeat request with incremental redundancy (HARQ-IR) assisted short packet communications. The small packet size leads to the occurrence of the time correlation among HARQ rounds. To accurately capture the effect of time correlation, a general correlated Nakagami-m fading channel model is first developed. However, the channel correlation, multiple transmissions, and finite blocklength information theory extremely challenge the analysis of BLER. To confront this, the average BLER is derived in a compact form by capitalizing on linearization approximation and conditional Laplace transform. To reveal more insights, the asymptotic BLER in high signal-to-noise ratio is obtained in a simple form. It is disclosed that full time diversity can be achieved by the proposed HARQ-IR-aided scheme. This justifies the feasibility of using retransmissions to offer reliable short packet communications. The numerical results finally corroborate the validity of our analysis.

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.

Cooperative nonorthogonal multiple access (NOMA) technology has the potential to greatly extend network coverage and elevate overall system performance. The NOMA-based coordinated direct and relay transmission (CDRT) scheme takes full advantage of successive interference cancellation results to effectively reduce interference from forward signals and improve the efficiency of parallel link transmission. However, the existing literature has overlooked an important premise of the CDRT system: the center user must successfully decode the signal for the edge user. In this work, new adaptive transmission schemes are proposed to enhance the performance of NOMA-based CDRT systems. Specifically, dynamic power allocation and maximum ratio transmission schemes are utilized to improve the transmission quality of the first hop, and directional antenna technology is utilized to enhance the transmission quality of the second hop. The closed-form expressions for the exact effective sum throughput are derived. Finally, simulation results are presented to validate the correctness of the theoretical analysis and these results demonstrate the effectiveness of the proposed scheme when compared to benchmark 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.

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.

Semi-grant-free (SGF) transmission scheme enables grant-free (GF) users to utilize resource blocks allocated for grant-based (GB) users while maintaining the quality of service of GB users. This work investigates the secrecy performance of non-orthogonal multiple access (NOMA)-aided SGF systems. First, analytical expressions for the exact and asymptotic secrecy outage probability (SOP) of NOMA-aided SGF systems with a single GF user are derived. Then, the SGF systems with multiple GF users and the best-user scheduling scheme is considered. By utilizing order statistics theory, analytical expressions for the exact and asymptotic SOP are derived. Monte Carlo simulation results are provided and compared with two benchmark schemes. The effects of system parameters on the SOP of the considered system are demonstrated and the accuracy of the developed analytical results is verified. The results indicate that both the outage target rate for GB and the secure target rate for GF are the main factors of the secrecy performance of SGF systems.

With the rapid proliferation of smart devices in wireless networks, more powerful technologies are expected to fulfill the network requirements of high throughput, massive connectivity, and diversify quality of service. To this end, rate splitting multiple access (RSMA) is proposed as a promising solution to improve spectral efficiency and provide better fairness for the next-generation mobile networks. In this paper, the outage performance of uplink RSMA transmission with randomly deployed users is investigated, taking both user scheduling schemes and power allocation strategies into consideration. Specifically, the greedy user scheduling (GUS) and cumulative distribution function (CDF) based user scheduling (CUS) schemes are considered, which could maximize the rate performance and guarantee scheduling fairness, respectively. Meanwhile, we re-investigate cognitive power allocation (CPA) strategy, and propose a new rate fairness-oriented power allocation (FPA) strategy to enhance the scheduled users’ rate fairness. By employing order statistics and stochastic geometry, an analytical expression of the outage probability for each scheduling scheme combining power allocation is derived to characterize the performance. To get more insights, the achieved diversity order of each scheme is also derived. Theoretical results demonstrate that both GUS and CUS schemes applying CPA or FPA strategy can achieve full diversity orders, and the application of CPA strategy in RSMA can effectively eliminate the secondary user’s diversity order constraint from the primary user. Simulation results corroborate the accuracy of the analytical expressions, and show that the proposed FPA strategy can achieve excellent rate fairness performance in high signal-to-noise ratio region.

In the uplink cognitive radio-inspired nonorthogonal multiple access (CR-NOMA) systems, the successive interference cancellation (SIC), and power control (PC) schemes were designed to enhance the outage performance but they lack the capability of protecting signals from being eavesdropped. This work investigates the secrecy performance of uplink CR-NOMA systems with the hybrid SIC and PC (HSIC-PC) scheme in the presence of a passive eavesdropper. Furthermore, through transmitting artificial noise (AN), a new scheme is proposed to improve the security of the cognitive user with HSIC-PC without deteriorating the primary user’s quality of service. Specifically, the primary and secondary users transmit AN cooperatively to enhance the security of the secondary user. The secrecy performance of the proposed scheme is analyzed based on the relationship between the maximum interference power that the primary user can tolerate and the secondary user’s channel gain. The analytical expressions for the secrecy outage probability (SOP) are derived, and the effect of parameters on the SOP is discussed. We also derive the analytical expressions of the asymptotic SOP in the high-power region to obtain further insights. Monte Carlo simulation results are performed to verify the analytical results. The results demonstrate that the proposed scheme performs better than the benchmarks.

Unmanned aerial vehicles (UAVs) have been widely employed to enhance the end-to-end performance of wireless communications since the links between UAVs and terrestrial nodes are Line-of-Sight (LoS) with high probability. However, the broadcast characteristics of signal propagation in LoS links make them vulnerable to being wiretapped by malicious eavesdroppers, which poses a considerable challenge to the security of wireless communications. In this work, we investigate the security of aerial underlay Internet of Things (IoT) systems by jointly designing trajectory and user scheduling. An airborne base station transmits confidential messages to secondary users utilizing the same spectrum as the primary network. An aerial jammer transmits jamming signals to suppress the eavesdropper to enhance secrecy performance. The uncertainty of eavesdropping node locations is considered, and the average secrecy rate of the secondary user is maximized by optimizing multiple users’ scheduling, the UAVs’ trajectory, and transmit power. To solve the nonconvex optimization problem with a mixed multi-integer variable problem, we propose an iterative algorithm based on block coordinate descent and successive convex approximation. Numerical results verify the effectiveness of our proposed algorithm and demonstrate that our scheme is beneficial in improving the secrecy performance of aerial underlay IoT systems.

This paper proposes a new scheme to enhance the secrecy performance of non-orthogonal multiple access (NOMA)-based coordinated direct relay transmission (CDRT) systems with an untrusted relay. The physical-layer network coding (PNC) and the NOMA schemes are combined to improve spectrum efficiency. Furthermore, inter-user interference and friendly jamming signals are utilized to suppress the eavesdropping ability of the untrusted relay without compromising the acceptance quality of legitimate users. Specifically, the far user in the first slot and the near user in the second slot act as jammers that generate jamming signals to ensure secure transmissions of confidential messages. We investigate the secrecy performance of the NOMA-based CDRT systems with the PNC scheme and derive the closed-form expression for the ergodic secrecy sum rate. The asymptotic analysis at a high signal-to-noise ratio is performed to obtain more insights. Finally, simulation results are presented to demonstrate the proposed scheme’s effectiveness and the theoretical analysis’s correctness.

The time-reversal division multiple access (TRDMA) system employs multipath channels to realize multi-user transmission via time-reversal filtering. The performance of TRDMA is affected by the correlation among users’ channels. When the correlation is high, inter-user interference (IUI) in the TRDMA system will be serious and the system capacity will be severely restricted. In this paper, we propose an uplink multiple access scheme time-reversal non-orthogonal multiple access (TR-NOMA), wherein TRDMA and power-domain non-orthogonal multiple access are combined. The base station employs a TR filter for each user to suppress IUI and applies successive interference cancellations in the detection process of users’ signals. Users’ transmit power and TR filters are jointly optimized to promote system capacity. The original optimization problem is non-convex. We decompose the optimization problem into two sub-problems: the design of TR filters and the allocation of users’ transmit power, which is solved by utilizing generalized eigenvalue decomposition and successive lower bound convex approximation, respectively. The solution to the original optimization problem is obtained by the iterative solution between the two sub-problems. Simulation results demonstrate that the sum rate of the optimized TR-NOMA system is significantly higher than that of the TRDMA system.

Millimeter-wave (mmWave) communication has attracted considerable attention from academia and industry, providing multi-gigabits per second rates due to the substantial bandwidth. Rate splitting multiple access (RSMA) is an effective technology that provides a generalized multiple access framework. Regarding the new propagation characteristics of the mmWave, we investigate the outage performance of the mmWave RSMA multiple-input-single-output system with a fixed-located user and a randomly-located user. Based on the spatial correlation of the paired users, the user’s paths are divided into overlapped and non-overlapped paths. Two beamforming schemes are proposed to improve the reliability of the mmWave RSMA system. The common stream is transmitted on the overlapped paths or all the paths. By utilizing stochastic geometry theory, the closed-form expressions of the outage probability (OP) with proposed schemes are derived. To obtain more insights, the expressions for the asymptotic OP are derived. Monte Carlo simulation results are presented to validate the analysis and the effects of the system parameters, such as power allocation coefficients and the number of resolvable paths, on the outage performance are investigated.

Non-orthogonal multiple access (NOMA) assisted semi-grant-free (SGF) transmission has recently received significant research attention due to its outstanding ability of serving grant-free (GF) users with grant-based (GB) users’ spectrum, which greatly improves the spectrum efficiency and effectively relieves the massive access problem of 5G and beyond networks. In this paper, we first study the outage performance of the greedy best user scheduling SGF scheme (BU-SGF) by considering the impacts of Rayleigh fading, path loss, and random user locations. In order to tackle the admission fairness problem of the BU-SGF scheme, we propose a fair SGF scheme by applying cumulative distribution function (CDF)-based scheduling (CS-SGF), in which the GF user with the best channel relative to its own statistics will be admitted. Moreover, by employing the theories of order statistics and stochastic geometry, the outage performances of both BU-SGF and CS-SGF schemes are analyzed. Theoretical results show that both schemes can achieve full diversity orders only when the served users’ data rate is capped, which severely limits the rate performance of SGF schemes. To further address this issue, we propose a distributed power control strategy to relax such data rate constraint, and derive analytical expressions of the two schemes’ outage performances under this strategy. Finally, simulation results validate the fairness performance of the proposed CS-SGF scheme, the effectiveness of the power control strategy, and the accuracy of the theoretical analyses.

In this article, we study the secure communication performance of nonorthogonal multiple access (NOMA) communication systems with aerial eavesdroppers. First, the ground-to-ground NOMA communications systems are considered wherein the near user is uniformly distributed in an inner disc and the far user is uniformly distributed in an outer ring within cell coverage of a base station (BS) at the center. A single eavesdropper is uniformly distributed outside the protected zone or multiple eavesdroppers are randomly dispersed and modeled as a 3-D homogeneous Poisson point process. Subsequently, the secrecy performance of the ground-to-air NOMA communications systems is investigated wherein the near user is uniformly distributed in an internal 3-D spherical cap and the far user is uniformly distributed in an external 3-D spherical cap. To enhance secrecy performance, a protected zone is introduced around the ground BS. Novel exact expressions for the secrecy outage probability (SOP) for both scenarios are derived. Finally, Monte Carlo simulation results are presented to validate the correctness of the derived analytical expressions and demonstrate the effects of system parameters on SOP of the considered system.

In this paper, we study the secure communication performance of nonorthogonal multiple access (NOMA) communication systems in presence of aerial eavesdroppers. The security of ground-to-air NOMA communication systems is investigated wherein, utilizing stochastic geometry, the near user is uniformly distributed in an internal three-dimensional (3D) spherical cap and the far user is uniformly distributed in an external 3D spherical cap. To enhance secrecy performance, a protection zone and directional antenna are introduced. The scenarios of both a single eavesdropper and multiple eavesdroppers are considered and new exact expressions for the secrecy outage probability (SOP) under both scenarios are derived. Monte Carlo simulation results are presented to verify the correctness of the derived analytical expressions and the effects of system parameters on SOP of the considered system are investigated.

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.

In this paper, a new transmit diversity scheme is proposed for cooperative pseudo-non-orthogonal multiple access (Pseudo-NOMA) without assuming full channel state information at the transmitter (CSIT). To support two users under the near-far user pairing constraint, a distributed cyclic delay diversity (dCDD) scheme is adapted into NOMA by dividing a set of remote radio heads (RRHs) into two groups for multiple cyclic-prefixed single carrier transmissions. To maximize a far user's rate and two users' sum rate over independently but non-identically distributed frequency selective fading channels and under a near-far user pairing constraint, we first derive closed-form expressions for the rates of the two users with full CSIT. Considering that only partial CSIT is available, a new RRH assignment and power allocation scheme is proposed for dCDD-Pseudo-NOMA. For various simulation scenarios, the provided link-level simulations verify that higher rates can be achieved by dCDD-Pseudo-NOMA compared with the traditional orthogonal multiple access with dCDD and dCDD-Conventional-NOMA that uses the superimposed signals. Furthermore, the proposed RRH assignment and power allocation scheme makes dCDD-Pseudo-NOMA achieve almost the same rate as that of ideal dCDD-Pseudo-NOMA which requires full CSIT.

In this paper, a new transmit diversity scheme for cooperative non-orthogonal multiple access (NOMA) is proposed without perfect channel state information at the transmitter (CSIT). To support two users under a near-far user pairing constraint, a distributed cyclic delay diversity (dCDD) scheme is adjusted into NOMA by dividing a set of remote radio heads (RRHs) into two groups for multiple cyclic-prefixed single carrier transmissions. Using only a limited channel relevant information needed to make dCDD work, a new RRH assignment and power allocation mechanism is proposed. After then, closed-form expressions for the rates of two users achieved by the proposed RRH assignment and power allocation mechanism are derived. For various scenarios, link-level simulations verify that superior rates can be achieved by NOMA with dCDD over the traditional orthogonal multiple access with dCDD.

Thanks to its characteristics of temporal compression and spatial focusing, the time reversal (TR) transmission system has the intrinsic capability of anti‐eavesdropping. This study analyses the physical layer security performance of the TR transmission system. The probability density functions and cumulative distribution functions of the signal‐to‐noise ratio of the legitimate user and eavesdropper are given. On this basis, the theoretical expressions of the ergodic capacities of the legitimate channel and the wiretap channel, and the achievable ergodic secrecy rate are derived. The bit error rates of the legitimate user and eavesdropper for binary phase‐shift keying modulation are given too. The correctness of the theoretical derivation is verified by simulation. For the legitimate user, the signal power increases as the number of paths increases, which indicates a higher diversity gain can be obtained, and the inter‐symbol interference power decreases as the up‐sampling factor increases. So, the signal‐to‐interference‐plus‐noise ratio (SINR) of the received signal can be significantly improved by appropriately reducing the spectral efficiency. However, the received signal's SINR of the eavesdropper cannot be promoted obviously. That is to say, compared with the eavesdropper, the legitimate user can achieve a higher rate and a lower BER, so secure transmission can be realised.

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 investigate the relationship between the reliability and security of a typical two-user downlink non-orthogonal multiple access (NOMA) communication system. The level of successive interference cancellation on NOMA user is considered. Firstly, the impact of various key parameters on transmit signal-to-noise ratio (SNR) of the NOMA users with the reliability outage probability (ROP) constraint is discussed. Taking the minimum of transmit SNR for ROP into account, the secrecy outage performance of the downlink NOMA systems is studied and the analytical expressions of the secrecy outage probability of the NOMA system are derived under two cases of eavesdropping capability. Furthermore, the sum effective secrecy throughput of the NOMA system under two scenarios with considering ROP constraint is derived. Monte Carlo simulations are provided to verify the accuracy of our analysis.

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

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.

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.

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.

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, 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.

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 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.

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.