In this paper, we investigate an IRS-assisted hybrid active and passive jamming communication method in single-user scenarios. This paper aims to minimizing the signal-to-interference-plus-noise ratio (SINR) of users by optimizing intelligent reflecting surface (IRS) phase shifts to disrupt the communication between users. Due to the nonconvexity of the formulated problem, semidefinite relaxation (SDR) and Charnes-Cooper transformation (CCT) methods are explored to solve the problem. The simulation results demonstrate that the hybrid optimization jamming method significantly outperforms the individual active or passive jamming methods.

In extremely low signal-to-noise ratio (SNR) region, the useful features of the signal are weakened by higher-power noise, making it difficult for conventional direction-of-arrival (DOA) estimation methods to adequately exploit and extract the low-SNR signal features. Thus, a generative adversarial network (GAN) is presented to learn the underlying features and complex distributions of high-SNR covariance matrices. The introduced GAN establishes a mapping between low-SNR and high-SNR covariance matrices, thereby generating first-rate high-SNR covariance matrices that closely resemble real high-SNR matrices. Also, it effectively captures signal features that are overwhelmed by excessive noise power. Additionally, to improve the performance of convolutional neural network (CNN)-based DOA estimation models in medium-to-high SNR ranges, a deep residual attention network (DRANet) is designed to significantly enhance DOA estimation accuracy in such SNR region. By integrating residual and attention modules, the network effectively filters key features. This enhances feature learning and adaptability, allowing it to capture DOA-related features more proficiently. The experimental results indicate that the developed GAN-DRANet approach can approach the CRLB in the extremely low SNR range and improves the estimation resolution limits of the other two DL-based methods, DNN and CNN, in medium to high SNR conditions.

In this article, we investigate covert millimeter-wave (mmWave) communications with finite blocklength, where a multiantenna transmitter sends covert messages to a legitimate receiver in the presence of spatially random wardens. Both the phase array (PA) and linear frequency diverse array (LFDA) beamforming schemes, which are designed to maximize the antenna gain from the transmitter to the legitimate receiver, are investigated to improve the covert communication performance. First, the novel expressions of covert communication constraint and average effective covert throughput (AECT) are derived for both beamforming schemes. Then, taking into account the constraint of maximal available blocklength, the optimal transmit power and blocklength are determined for maximizing the AECT. Typically, comparing to the benchmark with fixed blocklength, the enhancement of AECT by utilizing the optimized blocklength enlarges as the density of wardens increases. In addition, it is observed that increasing the maximal available blocklength cannot always improve the maximum AECT due to the tradeoff between the transmit power and blocklength. Furthermore, it is shown that the maximum AECT varies for different directions of the legitimate receiver under both the beamforming schemes, and the transmitter can adaptively choose the PA or LFDA beamforming scheme to improve the covertness performance against spatially random wardens.

In this paper, we consider a novel covert communication scenario where the warden does not have prior knowledge of the transmitter’s power. In this scenario, the current widely adopted likelihood ratio test (LRT)-based detector is not optimal anymore. To address this, we formulate a detection framework based on the generalized likelihood ratio test (GLRT), which replaces the unknown parameter with maximum likelihood estimation (MLE) and is proven optimal in the scenario. Based on the GLRT-based framework, two different detection models are proposed, where one only utilizes observations of the current time and the other can exploit observations of past time slots. We analyze the estimation and fisher information of the transmit power, and even the detection and covert performance under two different detection models, respectively. Based on the analytical results, we further derive the maximum number of tolerable slots under which the transmitter can remain the same transmit power while the detection error probability of the warden is still larger than the regulated threshold. Moreover, the transmit power and the number of tolerable slots are jointly optimized to maximize the transmission throughput subjecting to covertness constraint. The numerical results demonstrate the correctness of the theoretical analysis. We show how the covert rate is influenced by the number of transmitting slots and the transmit power, which can guide the design of the covert transmission strategy.

This paper investigates the covert millimeter wave (mmWave) communication in the finite block-length regime, where spatially random wardens attempt to determine the presence of transmission. First, we derive a novel expression of covertness constraint by using the tools of stochastic geometry, based on which the expression of average effective covert throughput (AECT) is also presented. Then, considering the constraint of maximal available block-length, the optimization problem for maximizing the AECT is formulated, and the optimal transmit power and block-length are analytically determined. Our results show the superiority of our optimization in terms of AECT in contrast to the fixed block-length case, and the improvement is more significant when the density of wardens becomes large. Furthermore, the performance of covert mmWave communication can indeed be improved via increasing the number of antennas even there exist random distributed wardens.

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

In this work, we aim to maximize the timeliness of data collection subject to a covertness constraint in unmanned aerial vehicle (UAV)-aided Internet of Things (IoT) networks, where a UAV periodically conducts wireless power transfer (WPT) to charge an energy-constrained IoT device and then the IoT device opportunistically sends its collected data to the UAV. To this end, we first derive a lower bound on the covertness constraint and an analytical expression for Age of Information (AoI) to characterize timeliness. Then, we jointly optimize the UAV’s transmit power for WPT, the WPT duration, and the data transmission duration by considering two practical scenarios. For the fixed total duration scenario, our analytical optimal solutions indicate that the total harvested energy at the IoT device is independent of the covertness constraint, although both the UAV’s transmit power and the WPT duration are significantly affected by the covertness constraint. With the optimized total duration scenario, we prove that the UAV’s optimal transmit power is always attained at its maximum value regardless of the existence of the covertness constraint, but the WPT and data transmission durations are sensitive to the required covertness. Overall, there exists a nontrivial tradeoff between the timeliness and covertness for data collection in the considered system, which is determined by the WPT design. Furthermore, our numerical results show that the optimal prior probability of the IoT device’s opportunistic transmission is generally not 0.5 for the fixed total duration, but it is indeed 0.5 for the optimized total duration.

This article investigates secure transmission in an energy harvesting (EH) sensor network in the presence of noncolluding passive eavesdroppers, where the EH sensors communicate with a controller via a buffer-aided relay (sink node). Each sensor has a battery to accumulate the energy harvested from the radio-frequency (RF) signals transmitted by the relay, and purely relies on the harvested energy to perform transmission. Besides, equipping a data buffer at the relay for security improvement, each time slot can be allocated dynamically to the first hop or the second hop, but it also introduce a packet delay. To enhance the security, an adaptive link selection with user scheduling and cooperative jamming (ALS-USaCJ) scheme is proposed. For evaluating the secrecy performance, the coupled evolution of the batteries and data buffer should be dealt with. Specifically, discretizing the energy status of the batteries, the couple evolution is modeled as a Markov chain so that its steady-state distribution can be obtained. On this basis, the analytical expressions of the communication outage probability (COP), secrecy outage probability (SOP), secrecy throughput (ST), and average packet delay (APD) are derived. Finally, theoretical analysis and simulation results demonstrate that equipping a data buffer at the relay can significantly improve the secrecy performance, and the larger the buffer size, the better the performance, but the greater the APD. Moreover, the link selection factor in the proposed scheme, which is related to specific network configuration, can be properly set to achieve the optimal secrecy performance.

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

In this article, we investigate physical layer security enhancement methods for millimeter-wave (mmWave) simultaneous wireless information and power transfer (SWIPT) unmanned aerial vehicle (UAV) networks, where the power-limited destination decodes the information and harvests energy from the received radio-frequency signals. Considering the effect of beamforming design and actual 3-D antenna gain, the directional modulation (DM) technique based on random frequency diverse array (RFDA) is adopted to guarantee security. The closed-form expressions of the lower bound of average secrecy rates with uniform linear array (ULA) and uniform planar array (UPA) are derived, respectively. Furthermore, based on the theoretical analysis results, we formulate the secrecy rate maximization problem subject to the energy harvesting constraint at the destination. Then, a suboptimal iterative optimization algorithm is proposed to solve the secrecy rate maximization problem by optimizing the transmit power, power splitting ratio, and UAV trajectory jointly. The simulation results show that the average secrecy rate of RFDA scheme is much larger than the conventional phase array scheme, especially when using ULA. The proposed optimization algorithm can achieve a higher average secrecy rate than other benchmark algorithms.

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.

Future wireless networks have requirements of large connections, low power consumption, high reliability, and strong security. Considering the abundant available bandwidth of millimeter wave (mmWave) and the large line-of-sight (LOS) link probability of air-ground channel, in this article, we develop a framework to study security, reliability and energy coverage performance of the downlink mmWave simultaneous wireless information and power transfer (SWIPT) unmanned aerial vehicle (UAV) networks under nonorthogonal multiple access (NOMA) and orthogonal multiple access (OMA) schemes, where a UAV serves two types of authorized Internet of Things (IoT) devices in the presence of multiple passive eavesdroppers. Directional modulation (DM) is also used to improve the physical layer security (PLS) performance. First, the analytical expressions for connection outage probability (COP), secrecy outage probability (SOP), and effective secrecy throughput (EST) of the users with high rate security requirement (HRSR) under NOMA and OMA schemes are derived using stochastic geometry. Then, we optimize the active antenna selection of DM using adaptive genetic simulated annealing algorithm (AGSA) to further improve the secrecy performance based on the obtained analytical results. Finally, the closed-form expressions for the COP and energy-information coverage probability (EICP) of the energy-constrained users with low-rate requirement (ECLR) under NOMA and OMA schemes are obtained. The numerical and simulation results provide interesting insights into the influence of various parameters on the tradeoff between the reliability and security for the HRSR user, and the tradeoff between the reliability and energy coverage for the ECLR user. Moreover, the EST of the NOMA scheme outperforms OMA at low transmit power and high codeword transmission rate.

This article investigates secure communication between a source and a destination via multiple radio frequency (RF) energy harvesting (EH) relays, in which the RF-EH relays are untrusted and apply the amplify-and-forward policy. On the one hand, to prevent the untrusted relays from eavesdropping the confidential information, the destination-aided jamming is employed, in which the destination emits jamming signal to interfere the relays while the source transmits information signal. On the other hand, the power splitting (PS) policy is adopted at the relays to harvest energy and process information, in which every relay has a finite energy storage to accumulate the energy harvested from the source’s information signal and destination’s jamming signal. To achieve energy-efficient and fully distributed implementation, we propose an energy-aware distributed beamforming (EADB) scheme, in which each relay only needs local information to decide whether to assist the source-destination communication. To evaluate the secrecy performance of the EADB scheme, the charging and discharging behaviors of the relays’ energy storage are first tracked using the Markov chain. On this basis, we derive analytical expressions for the hybrid outage probability (HOP) and secure energy efficiency (SEE) of the considered network. Finally, numerical results show that the EADB scheme has a better secrecy performance than the existing scheme. In addition, despite the curiousness of the untrusted relays, deploying more relays and increasing their energy storage capacity can enhance the security of the considered network.

Considering the diversified communication requirements in the Internet-of-Things (IoT) networks, this article proposes a nonorthogonal multiple access (NOMA) scheme which pairs two types of typical users, i.e., delay-sensitive user (DSU) and security-required user (SRU) to share the same nonorthogonal communication resources. We focus on the investigation of the secure transmission for the SRU while guaranteeing the communication requirements of the DSUs at the high priority. Specifically, the delay requirement of the DSUs and the security demand of the SRU are guaranteed by using short-packet communications and the maximal ratio transmission (MRT) scheme, respectively. Since the packets cannot always be detected correctly in the short-packet communications, a novel power allocation strategy is designed sophisticatedly to avoid the outage performance floor at the SRU caused by the short-packet communications. A set of closed-form expressions of the connection outage probability (COP), secrecy outage probability, and effective secrecy throughput (EST) of the SRU are derived over Nakagami-m channels in the proposed NOMA scheme and also the benchmark OMA scheme. Besides, we provide the security- reliability tradeoff (SRT) and security-efficiency tradeoff (SET) results for obtaining more insights into system performance. Results demonstrate the superiority of the proposed NOMA scheme over the benchmark OMA scheme in terms of the COP, EST, SRT, and SET when the transmitter is equipped with multiple antennas.

In this paper, we design a secure transmission scheme in hybrid automatic repeat request (HARQ) assisted cognitive non-orthogonal multiple access (NOMA) networks, where a security-required user (SRU) is paired with a quality of service (QoS)-sensitive user (QSU) to perform NOMA. To elaborate, the QoS requirement of the QSU is guaranteed by a cognitive power allocation scheme, while the HARQ technique is employed to mitigate the successive interference cancellation (SIC) errors and improve the secrecy performance of the SRU. For reducing information leakage, a randomized retransmission NOMA (RR-NOMA) scheme is designed, where the retransmitted signals are generated from independent randomized codebooks. In this scheme, the closed-form expressions for the connection outage probability (COP), the average number of transmission (ANT), the secrecy outage probability (SOP), and effective secrecy throughput (EST) of the SRU are derived. In addition, as benchmarks, the performance analyses for the fixed retransmission (FR-NOMA) scheme and the randomized retransmission orthogonal multiple access (RR-OMA) scheme are also provided. Results show a trade-off between SOP and COP or EST, which is denoted by security-reliability trade-off (SRT) or security-efficiency trade-off (SET). Furthermore, simulation results show that the HARQ technique improves SRT and the RR-NOMA scheme achieves better SET than the FR-NOMA scheme in the low SOP region. We further conduct asymptotic analysis in the RR-NOMA, FR-NOMA and RR-OMA schemes. Asymptotic results demonstrate that the three schemes achieve the same ANT and the RR-NOMA scheme obtains better secrecy performance than the RR-OMA scheme and equal secrecy performance to the FR-NOMA scheme in terms of both EST and SOP.

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.

In this paper, we investigate secure unmanned aerial vehicle (UAV) communication in the presence of multiple UAV adaptive eavesdroppers (AEs), where each AE can conduct eavesdropping or jamming adaptively by learning others’ actions for degrading the secrecy rate more seriously. The one-leader and multi-follower Stackelberg game is adopted to analyze the mutual interference among multiple AEs, and the optimal transmit powers are proven to exist under the existing conditions. Following that, a mixed-strategy Stackelberg Equilibrium based on finite and discretized power set is also derived and a hierarchical Q-learning based power allocation algorithm (HQLA) is proposed to obtain the optimal power allocation strategy of the transmitter. Numerical results show that secrecy performance can be degraded severely by multiple AEs and verify the availability of the optimal power allocation strategy. Finally, the effect of the eavesdropping cost on the AE’s attack mode strategies is also revealed.

The millimeter-wave (mmWave) ultra-dense networks are more suitable for wireless power transfer, since the short-distance transmissions experience less pathloss and the base station (BS) packed with large-scale antenna arrays can achieve significant array gains. However, the secrecy performance of the simultaneous wireless information and power transfer (SWIPT) mmWave ultra-dense networks has not been investigated so far. In this paper, we consider the secure communications in downlink SWIPT mmWave ultra-dense networks, where the energy-constrained users extract energy and information from the mmWave signals in the presence of multiple eavesdroppers. First, the analytical expressions of the energy-information coverage probability are derived for both power splitting and time switching policies using stochastic geometry. Then, we derive the closed-form expressions of the secrecy probability in the presence of multiple independent or colluding eavesdroppers. Finally, the effective secrecy throughput (EST), which can measure the network energy coverage, secure, and reliable transmission performance in a unified manner, is derived. Theoretical analysis and simulation results reveal that the EST first increases and then decreases with the increasing of the transmit power, power/time splitting ratio, codeword transmission rate, and confidential information rate. Furthermore, reducing the beamwidth of the signal at BSs can decrease the information leakage and improve the EST.

In this paper, we investigate the secrecy performance of energy-constrained wireless-powered networks with considering the passive eavesdropping scenario, where the simultaneous wireless information and power transfer-based full-duplex self-jamming (SWIPT-FDSJ) scheme is developed. The maximal ratio transmission protocol is applied at the multi-antenna source such that the wireless signals are designated to the destination directly. Besides, the energy harvesting and full-duplex self-jamming operations are adopted at the energy-constrained destination to prolong its lifetime as well as to confuse the eavesdropper. Specifically, the exact and asymptotic closed-form expressions of the connection outage probability (COP), the secrecy outage probability (SOP), and the secrecy throughput of the proposed system are obtained, based on which we optimize the time-switching ratio to maximize the secrecy throughput. We also degenerate the proposed SWIPT-FDSJ scheme to the reduced half-duplex with no self-jamming (HDNSJ) scheme. The finds suggest that in the HDNSJ scheme, adding the antenna number of the source only benefits the COP performance, but has no impact on the SOP performance. By contrast, it will promote the COP and SOP performance at the same time in the SWIPT-FDSJ scheme, which eventually results in the great improvement of secrecy throughput. In addition, we present the practical application condition of the SWIPT-FDSJ scheme. It is demonstrated that the secrecy throughput performance of the SWIPT-FDSJ scheme is much superior to the HDNSJ scheme on condition that the application condition is satisfied.

This paper studies the physical layer security performance of a Simultaneous Wireless Information and Power Transfer (SWIPT) millimeter wave (mmWave) ultra-dense network under a stochastic geometry framework. Specifically, we first derive the energy-information coverage probability and secrecy probability in the considered system under time switching policies. Then the effective secrecy throughput (EST) which can characterize the trade-off between the energy coverage, secure and reliable transmission performance is derived. Theoretical analyses and simulation results reveal the design insights into the effects of various network parameters like, transmit power, time switching factor, transmission rate, confidential information rate, etc, on the secrecy performance. Specifically, it is impossible to realize the effective secrecy throughput improvement just by increasing the transmit power.

In this paper we study on a massive MIMO relay system with linear precoding under the conditions of imperfect channel state information at the transmitter (CSIT) and per-user channel transmit correlation. In our system the source-relay channels are massive multiple-input multiple-output (MIMO) ones and the relay-destination channels are massive multiple-input single-output (MISO) ones. Large random matrix theory (RMT) is used to derive a deterministic equivalent of the signal-to-interference-plus-noise ratio (SINR) at each user in massive MIMO amplify-forward and decode-forward (M-MIMO-ADF) relaying with regularized zero-forcing (RZF) precoding, as the number of transmit antennas and users M,K→∞ and M≫K. Simulation results show that the deterministic equivalent of the SINR at each user in M-MIMO-ADF relaying and the results of Theorem 1, Theorem 2 are accurate.

This paper considers secure communication for amplify-and-forward (AF) relay networks with finite alphabet input. The joint optimization of power selection and beamforming design for improving the physical layer security of AF relay networks with single and multiple eavesdroppers is investigated. For the case with one eavesdropper, we transform the problem of multi-variable beamforming design into a single-variable optimization problem through semi-definite programming and solve it with one-dimensional optimization techniques. Moreover, the corresponding source power is obtained by utilizing the relation between the mutual information and minimum mean square error. Then, an iterative two-step algorithm is proposed to maximize the achievable secrecy rate. In the presence of multiple eavesdroppers, a zero-forcing beamforming scheme, where the confidential signal is nulled out in the direction of all eavesdroppers, is proposed to enhance the physical layer security. We decouple the source power and the beamforming vector by transforming the achievable secrecy rate into a single-variable function of the source power. Then, the suboptimal source power and the corresponding beamforming vector with low-complexity are derived. Numerical examples show that the proposed schemes significantly enhance the secrecy performance of the AF relay networks.

We investigate the secrecy performance in large‐scale cellular networks, where both Base Stations (BSs) and eavesdroppers follow independent and different homogeneous Poisson point processes (PPPs). Based on the distances between the BS and user, the intended user selects the nearest BS as serving BS to transmit the confidential information. We first derive closed‐formed expressions of secrecy outage probability and average secrecy rate of a single‐antenna system for both noncooperative and cooperative eavesdroppers scenarios. Then, to further improve the secrecy performance through additional spatial degrees of freedom, the above analyses generalize to the multiantenna scenario, where BSs employ the transmit antenna selection (TAS) scheme. Finally, the results show the small‐scale fading has a considerable effect on the secrecy performance in certain density of eavesdroppers and small path loss exponent environment, and when the interference caused by BS is considered, the secrecy performance will be reduced. Moreover, the gap of secrecy performance between noncooperative and cooperative eavesdroppers cases is nearly invariable as the number of antennas increases.

This paper exploits nonorthogonal multiple access (NOMA) to enhance the uplink secure transmission in Internet of Things (IoT) networks. Considering the different intercept ability of eavesdroppers (Eve), secrecy performances of both strong and weak Eve wiretap scenarios have been investigated. In strong Eve wiretap scenario (SWS), Eve is assumed to be powerful enough to decode message without interference and, in weak Eve wiretap scenario (WWS), Eve is assumed to have significant demodulation capability constraint. The new closed‐form expressions of joint connection outage probability (JCOP), joint secrecy outage probability (JSOP), and sum secrecy throughput (SST) are derived in these two scenarios to indicate the impact of parameters, i.e., transmit power, codeword rate, and the placement of devices, on security performance. In order to demonstrate the superiority of NOMA, we also investigate the secrecy performance of orthogonal multiple access (OMA) system as a benchmark. Analysis results show that the performance in WWS is always better than that in SWS and, in low signal‐to‐noise ratio (SNR) or high codeword rate region, the performances of these two scenarios are close. In addition, we present the condition that NOMA outperforms OMA in terms of SST. Moreover, the placements of devices are significant to the SST performance of NOMA system. The suboptimal device placement scheme has been designed to maximize SST. Analysis results demonstrate that when Eve is far away from legal users, the suboptimal results tend to optimal.

This work studies the secrecy outage performance of buffer-aided dual-hop multi-antenna relay systems without eavesdropper's channel state information (CSI). By modeling the dynamic buffer state transitions with the Markov chain, the secrecy outage probability at each state is investigated and the stationary distribution probabilities of all states are subsequently derived. Using the total probability theorem, the closed-form expression of the secrecy outage probability of the system is finally obtained. It demonstrates that due to the fully exploitation of the available channels, the buffer-aided relay selection yields to better performance than Best Relay Selection (BRS), even when less relays and antennas are utilized. It is also shown that the buffer-aided relaying only results in a small performance degradation when the buffers are constrained to finite size, thus can be well applied to practical relaying cooperative networks. Simulation results are given to verify the theoretical analysis.

This study considers linear precoding for secure transmission in a multiuser peer‐to‐peer relay network with finite alphabet input. Under the assumption that the global channel‐state‐information is available, the achievable secrecy rate is derived. However, the computational complexity to evaluate the achievable secrecy rate grows exponentially with respect to the number of pair users. To reduce the computational complexity caused by the multiuser interference, an accurate approximation of the achievable secrecy rate is derived. Based on Karush–Kuhn–Tucker analysis, necessary conditions for the optimal precoder which maximises the approximated achievable secrecy rate are presented. In light of this, an iterative gradient method is developed to find the optimal precoder. Numerical examples demonstrate that the proposed scheme achieves significant gains in terms of the secrecy rate over schemes designed for Gaussian input.

This study investigates the physical‐layer security in random cognitive radio network (CRN) consisting of randomly distributed primary nodes, secondary nodes, relay nodes and eavesdroppers, where the secondary nodes share the spectrum of the primary nodes and the eavesdroppers attempt to intercept the secondary transmissions. The authors develop a framework to analysis the reliability–reliability tradeoff (RRT) and security–reliability tradeoff (SRT) in the random CRNs, where the security and reliability are quantified in terms of secrecy outage probability and connection outage probability. The RRT evaluates performance tradeoff between the primary and the secondary networks and the SRT evaluates the performance tradeoff inside the secondary network. Furthermore, they propose an opportunistic relay selection (ORS) scheme to enhance the secondary confidential transmission. It is demonstrated that the ORS scheme significantly improves the RRT and SRT as the density of relays increases, and outperforms the conventional direct transmission when the density of relays is larger than a certain value. This actually promotes the understandings of the performance tradeoffs and provides direct insights on system design for the practical CRNs.

In this paper, we investigate the secrecy performance of full-duplex multi-antenna spectrum-sharing wiretap networks in which a jamming signal is simultaneously transmitted by the full-duplex secondary receiver (Bob) based on the zero forcing beamforming (ZFB) algorithm. For the security enhancement, we propose the two antenna reception schemes: 1) random selection combining (RSC) where Bob selects LB antennas at random to combine the received signals and 2) generalized selection combining (GSC) where Bob selects LB strongest antennas to combine the received signals. We derive the exact closed-form expressions for the secrecy outage probability of full-duplex multi-antenna spectrum-sharing wiretap networks with ZFB algorithm. In order to explore a new design of the proposed schemes, we provide tractable asymptotic approximations for the secrecy outage probability in high signal-to-noise ratio regime under two distinct scenarios. From the analysis, we demonstrate that: 1) when the main channel is much better than the eavesdropper's channel, GSC/ZFB scheme achieves full diversity NB, while RSC/ZFB scheme only achieves partial diversity LB and 2) GSC/ZFB scheme achieves better secrecy performance than RSC/ZFB with different antenna numbers at Bob.

We design a novel artificial-noise-aided secure ON-OFF transmission scheme in a wiretap channel. We consider a practical scenario, where the multi-antenna transmitter only obtains partial channel knowledge from the single-antenna receiver through limited training and feedback but has no channel knowledge about the single-antenna eavesdropper. In the design, we first propose a three-period block transmission protocol to capture the practical training and quantization features. We then characterize the statistics of the received signal-to-noise ratios at the receiver and the eavesdropper. Under the secrecy outage constraint, we exploit the ON-OFF scheme to perform secure transmission and derive a closed-form expression for the secrecy throughput. Moreover, we investigate the optimization problem of maximizing the secrecy throughput by proposing an iterative algorithm to determine the optimal power allocation between the information signal and artificial noise, as well as the optimal codeword transmission rate. Furthermore, we define the net secrecy throughput (NST), which takes the signaling overhead into account and address the problem of optimally allocating the block resource to the training and feedback overhead. Numerical results clearly demonstrate how the optimal signaling overhead changes with the number of transmit antennas, and there exists an optimal number of antennas that maximizes the NST.

In this paper, we study secure transmission designs for underlay cognitive radio networks in the present of randomly distributed eavesdroppers. We consider the scenario where a secondary transmitter sends confidential messages to a secondary receiver subject to an interference constraint set by the primary user. We design two transmission protocols under different channel knowledge assumptions at the transmitter. For each protocol, we first give a comprehensive performance analysis to investigate the transmission delay, secrecy, and reliability performance. We then optimize the transmission design for maximizing the secrecy throughput subject to both secrecy and reliability constraints. Finally, we numerically compare the performance of the two transmission protocols.

In this paper, we analyze the secrecy performance of multi-antenna cognitive wiretap network, where the secondary transmitter (Alice) communicates with the secondary receiver (Bob) in the presence of an eavesdropper (Eve). Specifically, we investigate the cases of maximal-ratio combining (MRC) with half-duplex (HD) and selection combining/Zero forcing beamforming (SC/ZFB) scheme with full-duplex (FD) operation, respectively. Assuming the Rayleigh fading, closed-form expressions for the secrecy outage probability of cognitive wiretap channels with MRC and SC/ZFB are derived. Furthermore, we provide simple asymptotic approximations for the secrecy outage probability and find that both schemes achieve full diversity. In addition, simulation results reveal that MRC outperforms SC/ZBF in the low interference threshold regime, while the opposite holds in the high interference threshold regime.

The authors study the secure transmission design in random cognitive radio networks where the primary users, the secondary users and the eavesdroppers are randomly distributed according to Poisson point processes. Centring on this scenario, the authors propose a simple and decentralised secure transmission scheme by jointly incorporating the secrecy guard zone and artificial noise. In particular, this transmission scheme helps to enhance secrecy performance via differentiating between secondary transmitters in accordance with the eavesdropping environment. They then analyse the connection outage and secrecy outage performance of the secondary network, based on which they obtain the closed‐form expression of the secrecy throughput. They further determine the optimal transmission power of the secondary transmitters and the optimal power allocation between the information‐bearing signal and artificial noise to maximise the secrecy throughput of the secondary network under primary and secondary outage constraints. The authors’ analysis highlights that introducing secrecy guard zone provides better security performance, and artificial noise performs as additional interference to insert a control between the reliability and security. Numerical results show how the system parameters affect the achievable maximum secrecy throughput, the optimal transmission power and the optimal power allocation between the information‐bearing signal and the artificial noise.

In this study, the authors investigate the physical layer security for safeguarding secure transmission in multi‐antenna relaying networks with imperfect channel state information (CSI). Different from traditional strategies in multi‐antenna relaying systems, which would terminate the communication if the relay could not decode the signals successfully, the authors propose robust secure transmission strategy which switches between cooperative jamming (CJ) and decode‐and‐forward (DF) beamforming. Specifically, if the received signal‐to‐noise ratio at the relay is lower than a predefined threshold, it will impose CJ signals on potential eavesdropper, otherwise the relay would operate DF beamforming. The authors’ objective is to maximise secrecy rate and minimise secrecy outage probability in different practical communication scenarios with relaxed or strict time‐delay constraint. Thus, given the unavailability of the perfect eavesdropping CSI, the authors propose a worst‐case robust design and a statistical‐approach robust design based on secrecy rate and secrecy outage criterion, respectively. In addition, a traditional DF robust scheme and a non‐robust scheme are also considered as benchmarks. Simulation results verify that the proposed scheme significantly outperforms the traditional DF robust scheme and the non‐robust scheme.

This paper investigates the secure transmission in simultaneous wireless information and power transfer (SWIPT) system, where a source node communicates with a wireless‐powered full duplex destination node in the presence of a passive eavesdropping node. To take advantage of the benefits from SWIPT and full duplex techniques, we propose a two‐phase time‐sharing protocol: the destination node harvests energy form the source node in the first phase, while the information‐bearing signal is transmitted under the protection of artificial noise sent from the destination node in the second phase. The transmit power of the artificial noise is converted from the harvested energy and self‐interference exists at the destination. We derive the closed‐form expressions for the connection outage probability (COP), the secrecy outage probability (SOP) and the transmission outage probability (TOP), based on which the secrecy energy efficiency (SEE) is formulated. Furthermore, we determine the optimal time allocation factor and transmission power that maximize the SEE while satisfying the COP and SOP constraints. Finally, numerical results verify our analytical results and show that there are tradeoffs among security, reliability and energy efficiency. When the residual loopback interference is mitigated under some certain level, our proposed optimization scheme can significantly boost the SEE.

Spectrum efficiency (SE) and energy efficiency (EE) in secure communications have attracted much attention recently due to the fact that future wireless networks need to address the issues of high throughput, low power consumption, and high level of security. However, maximizing EE and SE are conflicting objectives, which can hardly be achieved simultaneously. In this paper, we develop a framework to study the SE and EE for secure transmission in underlay random cognitive radio (CR) networks where the primary, secondary, and eavesdropper nodes are randomly distributed according to Poisson point processes. We first analyze the connection outage probability and the secrecy outage probability of the typical links in the random CR network. Then, we evaluate the secure SE and EE in the secondary network based on the outage probability analysis. It is demonstrated that the scheduling scheme of the secondary network (i.e., the transmission power and the intensity of secondary transmitters arouses a tradeoff between the secure SE and EE. Furthermore, applying a unified secure SE-EE tradeoff metric, which condenses the secure SE and EE into a single utility function with a tradeoff factor, we formulate the joint secure SE and EE optimization problem with respect to the scheduling scheme of the secondary network. An iterative algorithm is proposed based on the separation optimization of the transmission power and the intensity. The analytical and simulation results show the effects of system parameters on the optimal solutions and secure SE-EE tradeoff. We also verify that the optimal scheduling scheme can improve the secure SE-EE tradeoff. Using these results, one can easily understand and make the secure SE-EE tradeoff in random CR networks.

In this paper, we study physical layer security in an underlay cognitive radio (CR) network. We consider the problem of secure communication between a secondary transmitter-receiver pair in the presence of randomly distributed eavesdroppers under an interference constraint set by the primary user. For different channel knowledge assumptions at the transmitter, we design four transmission protocols to achieve the secure transmission in the CR network. We give a comprehensive performance analysis for each protocol in terms of transmission delay, security, reliability, and the overall secrecy throughput. Furthermore, we determine the optimal design parameter for each transmission protocol by solving the optimization problem of maximizing the secrecy throughput subject to both security and reliability constraints. Numerical results illustrate the performance comparison between different transmission protocols.

With the emerging demands for local area services of popular content downloading, D2D communication is recognized as a promising technical support for cellular networks. In this work, we propose a social-aware content downloading mode selection scheme, which involves a novel mode, named MD2D, by collaborating multiple available D2D links. In particular, we construct a social-aware evaluation framework, which understands the interplay between physical transmission property and social networking characteristics for defining the performance metric with respect to the downloading mode selection. Accordingly, we formulate the social-aware content downloading mode selection problem based on the combinatorial auctions. By exploring the submodular approximation of this problem, we design a social-aware mode selection algorithm. We demonstrate that the solution achieves incentives for content contribution and resistance to misbehaving potential content providers, and also derive a theoretic upper bound of the corresponding performance loss.

We propose a new secure transmission scheme in the multi-input multi-output multi-eavesdropper wiretap channel. In this channel, the NA-antenna transmitter adopts transmit antenna selection (TAS) to choose the antenna that maximizes the instantaneous signal-to-noise ratio (SNR) at the receiver to transmit, while the NB-antenna receiver and the NE-antenna eavesdropper adopt maximal-ratio combining (MRC) to combine the received signals. We focus on the practical scenario where the channel state information (CSI) during the TAS process is outdated. In this scenario, we propose a new transmission scheme to prevent the detrimental effect of the outdated CSI on the wiretap codes design at the transmitter. To thoroughly assess the secrecy performance achieved by the proposed scheme, we derive new closed-form expressions for the exact secrecy outage probability and the probability of non-zero secrecy capacity for arbitrary SNRs. We also derive new compact expressions for the asymptotic secrecy outage probability at high SNRs. Notably, in the analysis, we take spatial correlation at the receiver into consideration. Apart from the advantage of our scheme over the conventional TAS/MRC scheme, we demonstrate that the outdated TAS reduces the secrecy diversity order from NANBto NB. We also demonstrate that antenna correlation improves the secrecy performance at low SNR but deteriorates the secrecy performance at medium and high SNRs, by affecting the secrecy array gain only.

In this paper, we investigate the secrecy outage probability (SOP) for the cooperative decode-and-forward (DF) underlay cognitive radio networks (CRNs). Duo to the spectrum sharing approach in underlay CRNs, the secondary transmitters (secondary source (SUS) and secondary relay (SUR)) are subjected to interference power constraint at primary user (PU), and the SU receivers (SURand secondary destination (SUD)) suffer from the co-channel interferences (CCIs) from PU. We focus on the practical scenario where the SU transmitter has no channel state information (CSI) of the eavesdropper's channel. Interference-power-confined selection scheme is proposed for the cooperative DF CRNs in the presence of an eavesdropper. The proposed scheme selects the “best” terminal (SUSor SUR) to enhance the secure performance which must firstly satisfy the interference power constraint. What's more, the effect of feedback delay on the selection scheme is studied. We derive an exact closed-form expression for the SOP. Simulation results validate our theoretical analysis and show that the proposed selection scheme determinately enhance the performance, and feedback delay decreases the SOP seriously, specially, the interference power constraint hugely confine the SOP.

Relay transmission can enhance coverage and throughput, whereas it can be vulnerable to eavesdropping attacks due to the additional transmission of the source message at the relay. Thus, whether or not one should use relay transmission for secure communication is an interesting and important problem. In this paper, we consider the transmission of a confidential message from a source to a destination in a decentralized wireless network in the presence of randomly distributed eavesdroppers. The source-destination pair can be potentially assisted by randomly distributed relays. For an arbitrary relay, we derive exact expressions of secure connection probability for both colluding and noncolluding eavesdroppers. We further obtain lower bound expressions on the secure connection probability, which are accurate when the eavesdropper density is small. Using these lower bound expressions, we propose a relay selection strategy to improve the secure connection probability. By analytically comparing the secure connection probability for direct transmission and relay transmission, we address the important problem of whether or not to relay and discuss the condition for relay transmission in terms of the relay density and source-destination distance. These analytical results are accurate in the small eavesdropper density regime.

In this paper we analyze the effect of co-channel interference on the performance of cooperative decode-and-forward (DF) automatic-repeat-request (ARQ) scheme under Rayleigh fading channels. We assume that the relay node is affected by one interferer and the destination node is affected by another one. The outage probability of the cooperative DF ARQ scheme is derived based on the maximal ratio combining (MRC) technique. The closed-form expression clearly shows the outage probability has much to do with the maximum number of ARQ transmissions. Finally, Monte Carlo simulation results verify our theoretical analysis.

In this paper, the authors propose two cognitive relaying schemes based on overlay and underlay, which exploit the cooperation opportunities inherent in primary retransmission to improve secondary throughput. If a primary signal is not decoded by the primary receiver (PR), a secondary user (SU) can be selected to relay it invisibly along with the primary retransmission. For overlay cognitive relaying, SUs intend to reduce primary retransmission time by relaying primary message, so that more access opportunities are available. While in underlay cognitive relaying, SU allocates part of its power to help primary user (PU) and the remaining power is used to transmit secondary message simultaneously. By controlling the phase of the relay signal, signals retransmitted from primary transmitter (PT) and SU can constructively combine at the PR. In both relaying schemes, we consider the best-relay selection as well. We define some novel metrics to evaluate the performance of PU and SU. For PU, we study the improvements in outage performance and average transmitting time per packet, while for SU, we consider the cooperation gain and cooperation efficiency, respectively. Theoretical analysis and numerical results verify the validity of both schemes, and a comparison is made between them.

In this paper, we consider a single-antenna OFDM-based relaying system over frequency selective fading channels employing subcarrier grouping and linear constellation precoding at the source node, and linear detection at the destination node. We derive the closed-form expressions of the diversitymultiplexing tradeoff (DMT) relation, and develop the optimal symbol loading strategy (the optimal number of symbols assigned to each group), and the optimal grouping strategy (the optimal partition of the subcarriers) to achieve the optimum DMT performance. Simulation results are presented to verify our theoretical analysis.

In this paper, we propose a hybrid cooperative scheme to improve the secrecy rate for a cooperative network in presence of multiple relays. Each relay node transmits a mixed signal consisting of weighted source signal and intentional noise. The problem of power allocation and joint design of beamforming and jamming weights are investigated, and an iterative solution for the secrecy rate maximization problem is presented. The numerical results demonstrate that the proposed hybrid scheme further improves secrecy rate, as compared to traditional cooperative schemes.

In this paper, we adopt the DMT framework to study a cooperative downlink OFDMA system with multiple relays. We derive the closed-form expression of the DMT curve for the cooperative OFDMA system employing linear constellation precoding at the base station, and linear detection at the mobile users, and develop the optimal symbol loading, relay selection and subcarrier allocation strategy. Simulation results are presented to verify the theoretical analysis and show that our proposed resource allocation strategy can improve the system performance significantly.

It is conventionally supposed that the periodicity of superimposed training (ST) sequence designed for channel estimation has no impact on the ST system's performance, as long as the channel identification condition is satisfied. Accordingly the shortest sequence period equal to channel length is always preferred in ST-OFDM systems for its computational tractability. However, with additional consideration on extraction of the transmitted information, not only focusing on the accuracy of channel estimation, analysis and simulation indicate that the period length of ST sequence may have an unignorable influence on BER performance of ST-OFDM systems. As a special case, we show that ST with training sequence period equal to channel length can bring OFDM no benefit but BER performance degradation compared with its frequency-divided pilot counterpart.

In this paper, we propose a semi-blind channel estimation method based on first-order statistics of OFDM systems. The transmitter superimposes periodic pilot sequences on information sequences, where there is no loss in information rate but a controllable increase in transmission power. Employing the first-order statistics, it enables a closed-form channel estimation method to estimate the frequency-selective fading channel. Then, a mean square error (MSE) bound is derived to evaluate the proposed channel estimator. Computer simulations have been provided to compare the MSE performances of the proposed method with the LS estimation method based PSAM and the subspace method.