Millimeter-wave rate-splitting multiple access (mmWave-RSMA) technology combines the wideband characteristics of millimeter waves with the rate-splitting mechanism of RSMA, significantly improving the communication efficiency of the system. However, this feature also poses greater challenges to its secure transmission, especially in networks with dual-identity nodes which act as legitimate users while also being able to eavesdrop on information from other users. This behavior will simultaneously weaken the security of both public and private streams. In this regard, the active reconfigurable intelligent surface (RIS) offers a promising method by suppressing eavesdropper channels and enhancing the quality of legitimate links. Focusing on the physical layer security (PLS) challenge posed by dual-identity users in mmWave-RSMA networks, this paper proposes an active RIS assisted secure beamforming scheme integrated with resource allocation optimization. Specifically, while ensuring the quality of service (QoS) for dual-identity user and power amplification constraints of the active RIS, the sum secrecy rate is maximized by jointly optimizing the transmit beamforming vector, reflection coefficient matrix, common rate allocation, and power allocation coefficients. To solve this non convex problem, we divide it into three subproblems. Then, successive convex approximation (SCA) and semidefinite relaxation (SDR) are used to solve these subproblems. Simulation results validated the critical role of active RIS in reducing eavesdropping, and emphasized the importance of joint resource allocation for secure beamforming in mmWave-RSMA.

The open and vulnerable nature of wireless channels exacerbates security risks in integrated sensing and communication (ISAC) systems, especially when the sensing targets act as potential eavesdroppers (Eves), and these risks intensify with collusion among Eves. To address this challenge, this paper investigates a novel strategy for a robust reconfigurable intelligent surfaces (RIS)-assisted secure ISAC system, where an ISAC base station facilitates simultaneous secure communication with legitimate users and sensing of multiple targets that may serve as Eves. We examine two different interaction mechanisms among Eves, namely, non-colluding Eves (NCE) and colluding Eves (CE), under both perfect and imperfect channel state information (CSI) assumptions. For both mechanisms, we formulate the optimization problem of maximizing users’ sum secrecy rate by jointly designing the transmit beamforming and RIS phase-shifts. This optimization is subject to constraints on transmit power, sensing requirements, and unit-modulus RIS phase shifts. The resulting non-convex problems are solved via alternating optimization (AO) algorithms. Specifically, in order to handle the severely non-convex and coupled objective function and multi-link accumulated channel error constraints caused by CE as well as imperfect CSI, we employ the majorizationminimization algorithm and the S-procedure to convert these problems into tractable forms. Simulation results validate the effectiveness of our proposed algorithms. We highlight that, at the expense of a 15% reduction in the users’ sum rate, our proposed algorithm achieves up to a 185% increase in the sum secrecy rate. Furthermore, we quantify the sensing-security trade-off by analyzing the reduction of the sum secrecy rate induced by sensing requirements, and we reveal the impacts of various factors on the sum secrecy rate, such as RIS element number, channel estimation errors, and sensing thresholds.

Movable antenna (MA) is an emerging technology which can reconfigure wireless channels via adaptive antenna position adjustments at transceivers, thereby bringing additional spatial degrees of freedom for improving system performance. In this paper, from a security perspective, we exploit the MA-enabled legitimate jammer (MAJ) to subvert suspicious multiuser downlink communications consisting of one suspicious transmitter (ST) and multiple suspicious receivers (SRs). Specifically, our objective is to minimize the benefit (the sum rate of all SRs or the minimum rate among all SRs) of such suspicious communications, by jointly optimizing antenna positions and the jamming beamforming at the MAJ. However, the key challenge lies in that given the MAJ’s actions, the ST can reactively adjust its power allocations to instead maximize its benefit for mitigating the unfavorable interference. Such flexible behavior of the ST confuses the optimization design of the MAJ to a certain extent. Facing this difficulty, corresponding to the above two different benefits: i) we respectively determine the optimal behavior of the ST given the MAJ’s actions; ii) armed with these, we arrive at two simplified problems and then develop effective alternating optimization based algorithms to iteratively solve them. In addition to these, we also focus on the special case of two SRs, and reveal insightful conclusions about the deployment rule of antenna positions at the MAJ. Furthermore, we analyze the ideal antenna deployment scheme at the MAJ for achieving the globally performance lower bound. Numerical results demonstrate the effectiveness of our proposed schemes compared to conventional fixed-position antenna (FPA) and other competitive benchmarks.

This paper investigates transceiver design optimization strategies for frequency diverse array (FDA)-multiple-input multiple-output (MIMO) radar-communication coexistence (RCC) systems, focusing on both radar-centric and communication-centric modes. Specifically, the former formulates the design problem to maximize the signal-to-interference-plus-noise ratio (SINR) in mainlobe deceptive jammer scenarios, whereas the latter aims to maximize the communication rate while simultaneously satisfying a predefined radar SINR constraint. In this framework, practical constraints pertaining to the radar’s transmitted waveform, communication codebook, frequency increment, and receive filter are taken into account. To address the resultant non-convex and NP-hard optimization problems, a maximum block improvement (MBI) approach is employed, where the variables are alternately examined, which are achieved either by leveraging closed-form expressions and hidden convexities or by resorting to the minorization-maximization (MM) approach, while keeping the remaining parameters fixed. The convergence performance of the devised algorithm is thoroughly examined, alongside their computational complexity analyses. Numerical results are provided to validate the efficacy of our approach against mainlobe deceptive jammers, demonstrating superior SINR and communication rate performance compared to existing optimization strategies and benchmark system frameworks.

Task offloading among low-earth orbit (LEO) satellites with on-board computing (OBC) is important for real-time applications. However, OBC is constrained by the battery capacity of LEO, which fluctuates with orbital dynamics and available solar power. This paper addresses the problem of energy sustainability and timeliness in LEO-OBC systems by proposing a sustainable OBC-LEO framework that combines parallel offloading strategies with dynamic energy management. This problem is formulated as a Markov decision process aiming to minimize the overall delay while satisfying the LEO satellite energy constraints and achieving a high task success rate. To balance immediate computational demands and long-term energy stability, a Lyapunov optimization-based dynamic parallel offloading (LODPO) algorithm is designed to make decisions dynamically within each time slot, integrated with subtask allocation based on a low-cost (SABLC) algorithm that dynamically adjusts task allocations. Finally, simulation results demonstrate that the LODPO framework achieves a significant reduction in execution delay, incurring only 34.0% of the delay cost of binary offloading. Most critically, it ensures exceptional reliability, with a task drop rate that is only 8.5% of that seen in binary offloading and 12.0% of that in the DQN-based approach. This ensures high responsiveness and dependability for mission-critical, delay-sensitive applications.

Fluid antennas (FA) offer a revolutionary breakthrough in wireless communications by dynamically regulating antenna positions within confined spaces to significantly enhance system capacity and link reliability. In this paper, a FA integrated and reconfigurable intelligent surface empowered receive spatial modulation (FA-RIS-RSM) scheme is proposed. Specifically, due to the unique physical properties of FA, we propose two port selection strategies, namely the random port selection (RPS) and optimal port selection (OPS), to assist the transmission achieving with distinct extents of reliability. Furthermore, we derive closed-form expressions for the average bit error probability (ABEP), via utilizing the Gamma approximation analysis and correlated fading analogism analysis methods, respectively. Simulated and numerical performance results corroborate the correctness of the derivations, as well as demonstrate the superiority and effectiveness of the proposed FA-RIS-RSM in enhancing the wireless transmission reliability.

To address the physical layer security issues for the next generation wireless, we introduce and investigate a novel fluid antenna assisted secure joint transmitter-spatial and receiver-port index modulation (FA-SJIM) system. Specifically, we detail two schemes to enhance both the transmission efficiency and secrecy. The schemes are based on the continuous random phase and discrete random phase distortions at the transmitter and the constructive phase-aligning manipulations at the RIS, respectively. Furthermore, we derive the semi-closed-form secrecy outage probability (SOP) results of the proposed schemes with the maximum likelihood detection, so as to facilitate the evaluation of the system secrecy. Finally simulated and numerical performance results are characterized to reveal the high accuracy of SOP derivations, and the superiority of the FA-SJIM in terms of transmission efficiency and secrecy.

To address the increasing demand for transmission systems with enhanced efficiency and flexibility, two-way (TW) full-duplex (FD) systems have become a vital research focus in wireless communications. In this paper, a novel reconfigurable intelligent surfaces (RISs) assisted TW-FD spatial modulation (SM) model is proposed to improve the transmission reliability performance. With this proposal, we introduce the transmit-side SM to activate single antenna for fulfilling the transmission of amplitude and phase modulated symbols. The transmission is then reflected by the corresponding RIS to the antenna at the receiver end. Moreover, the prewired phase-offset (PPO) is included in the transmission process for attaining enhanced detecting reliability. To evaluate the performance boundaries of the proposed system, we consider the maximum likelihood detection based analytical derivations. More specifically, upon utilizing the moment generating function method, we derive the closed-form results for the system average bit error probability. Finally, with the simulation based comparative analysis, we verify the reliability and efficiency of the proposed TW-FD-RIS-PPOSM system.

Unmanned aerial vehicle (UAV) data collection problems in wireless sensor networks (WSNs) under random and uncertain environments are critical challenging, due to massive burden of real-time communication among UAVs for aligning with observation and state information, ect. For this sake, this work investigates the UAV cooperation problem of WSN data collection by jointly maximizing collected data amount while minimizing cooperation cost. We formulate the problem as a constrained partially observable Markov decision process (CPO-MDP), which stimulates the design of a novel safe reinforcement learning aided lightweight cooperation (SRL-LC) framework for multi-UAV data collection. Speficially, the self-conscious cooperative communication scheme is developed to assist the optimization of the UAV trajectory decision making. Additionally, a safety module embedded in the decision network integrates a relaxed artificial potential field (APF) algorithm, enabling UAVs to maintain safety distance constraints during training. Simulation results demonstrate that the proposed SRL-LC framework achieves data collection performance comparable to the full-cooperation scheme for various settings of prior information, while reducing communication cost by approximately 85%. Moreover, the SRL-LC framework ensures zero violations of safety constraints throughout the training process.

This paper investigates optimal antenna array configurations for multiple-input-multiple-output (MIMO) systems. The array configuration, particularly the element spacing, significantly affects the correlation between elements, which can lead to channel rank deficiency or deterioration in condition number. In practical sparse channels, deploying a massive number of antenna elements can become redundant. Therefore, exploring optimal array configurations is crucial for enhancing MIMO communication system. Although substantial theoretical analyses have been conducted for antenna configurations in line-of-sight (LOS) environments, analyses in non-line-of-sight (NLOS) environments remain limited. Through theoretical analysis, this paper demonstrates that in NLOS environments, the optimal condition number of MIMO channels is achieved when the antenna array configuration renders the antenna array matrix orthogonal, with the optimal condition number being determined by the wireless propagation environment. Building upon this finding, we show that there always exists an antenna spacing that makes the antenna array matrix orthogonal in a two-cluster environment, and we provide a closed-form solution for this spacing. For three-cluster environments, we derive the conditions that cluster angles must satisfy to achieve antenna array matrix orthogonality and provide a closed-form solution for optimal antenna spacing under these conditions. Furthermore, we propose a systematic solution based on Newton descent algorithm to calculate optimal antenna spacing, aimed at reducing analytical complexity in environments with more clusters. Finally, numerical simulations validate the theoretical analysis and provide recommendations for optimizing antenna placement in IoT devices operating in sparse channel environments.

This paper investigates a multi-user uplink mobile edge computing (MEC) network, where the users offload partial tasks securely to an access point under the non-orthogonal multiple access policy with the aid of a reconfigurable intelligent surface (RIS) against a multi-antenna eavesdropper. We formulate a non-convex optimization problem of minimizing the total energy consumption subject to secure offloading requirement, and we build an efficient block coordinate descent framework to iteratively optimize the number of local computation bits and transmit power at the users, the RIS phase shifts, and the multi-user detection matrix at the access point. Specifically, we successively adopt successive convex approximation, semi-definite programming, and semidefinite relaxation to solve the problem with perfect eavesdropper’s channel state information (CSI), and we then employ S-procedure and penalty convex-concave to achieve robust design for the imperfect CSI case. We provide extensive numerical results to validate the convergence and effectiveness of the proposed algorithms. We demonstrate that RIS plays a significant role in realizing a secure and energy-efficient MEC network, and deploying a well-designed RIS can save energy consumption by up to 60% compared to that without RIS. We further reveal impacts of various key factors on the secrecy energy efficiency, including RIS element number and deployment position, user number, task scale and duration, and CSI imperfection.

Despite narrow beams with strong anti-interception capabilities, terahertz communications still face eavesdropping risks in short-range indoor networks. This paper investigates physical-layer security of downlink terahertz communications for indoor three-dimensional (3D) networks comprised of a large number of access points (APs), users, human blockages, and eavesdroppers. We propose two different artificial noise (AN)-assisted terahertz secure transmission schemes, namely the full-AN (F-AN) scheme and partial-AN (P-AN) scheme, under the nearest line-of-sight association (NLA) strategy. The F-AN scheme involves full APs emitting AN to deteriorate the reception of eavesdroppers, and the P-AN scheme selects only those APs with blocked links to the typical user to emit AN based on the unique blocking feature of terahertz. We first obtain the expression for association probability. Then, we determine the eavesdropping region covered by the 3D beam on the ground. We derive the connection outage probability and secrecy outage probability for the two schemes by calculating the Laplace transform of aggregate interference. Our results provide interesting insights into how the secrecy performance is influenced by various system parameters, including the densities of APs and blockages. Moreover, we show that the P-AN scheme outperforms the F-AN scheme regarding the average number of perfect links per unit area.

Despite the enhanced spectral efficiency brought by the integrated radar and communication technique, it poses significant risks to communication security when confronted with malicious radar targets. To address this issue, a reconfigurable intelligent surface (RIS)-aided transmission scheme is proposed to improve secure communication in two systems, i.e., the radar and communication co-existing (RCCE) system, where a single transmitter is utilized for both radar sensing and communication, and the dual-functional radar and communication (DFRC) system. At the design stage, optimization problems are formulated to maximize the secrecy rate while satisfying the radar detection constraint via joint active beamforming at the base station and passive beamforming of RIS in both systems. Particularly, a zero-forcing-based block coordinate descent (BCD) algorithm is developed for the RCCE system. Besides, the Dinkelbach method combined with semidefinite relaxation is employed for the DFRC system, and to further reduce the computational complexity, a Riemannian conjugate gradient-based alternating optimization algorithm is proposed. Moreover, the RIS-aided robust secure communication in the DFRC system is investigated by considering the eavesdropper’s imperfect channel state information (CSI), where a bounded uncertainty model is adopted to capture the angle error and fading channel error of the eavesdropper, and a tractable bound for their joint uncertainty is derived. Simulation results confirm the effectiveness of the developed RIS-aided transmission scheme to improve the secrecy rate even with the eavesdropper’s imperfect CSI, and comparisons between both systems reveal that the RCCE system can provide a higher secrecy rate than the DFRC system.

In this paper, we investigate a hybrid reconfigurable intelligent surface (RIS) enabled integrated sensing and commu-nication (ISAC) system, in which a hybrid RIS is employed to assist a base station (BS) to sense a specified target, while interacting with multiple communication users (CUs) simultaneously. In particular, a hybrid RIS is introduced such that each of its surface module is able to switch between active and passive modes, reducing the system's power consumption. Subsequently, an optimization problem is formulated with the aim of maximizing the radar output signal-to-noise ratio while satisfying communication requirement for each CU, transmit power constraint for BS and the active RIS elements, by jointly optimizing radar recieve filter, BS's transmit beamforming matrix, RIS reflection coefficients, and the selection matrix that determines the working modes for each unite of the hybrid RIS. Since this design problem is not convex, we propose an alternating optimization based method to solve this problem. Eventually, upon the simulation analysis, we demonstrate that the performance achievable by the proposed scheme is significantly better than the counterparts assisted solely by the active or passive RIS.

With the emergence of the fifth generation (5G) era, the development of the Internet of Things (IoT) network has been accelerated with a new impetus, making it imperative to strive for a more reliable and efficient network environment. To accomplish this, we introduce and investigate a novel proposal for the intelligent reflecting surface (IRS) enabled uplink secure receive spatial modulation (SM), named IRS-USRSM, to resolve the security issues arising from the open wireless transmission environment in the 5G IoT network. In the IRS-USRSM scheme, we assume that the passive eavesdropper is directly connected to the uplink user and occasionally connected to the IRS. To achieve enhanced secrecy with finite alphabet inputs, a joint transmitter perturbation and IRS reflection design for physical layer security is proposed to guarantee secure and reliable transmission of IRS-USRSM. Specifically, two categories of IRS-based random phase compensation strategies, namely, random perturbation compensation and random path synthesize, along with maximum likelihood detection and suboptimal detection are proposed to meet the variant design requirements between achieved performance and system cost. Furthermore, in order to evaluate the performance limits of the IRS-USRSM, the closed-form results of average bit error probabilities and discrete-input continuous-output memoryless channel capacities are derived using the method of moment generating function. Simulation results are presented to verify the correctness of our theoretical analyses, as well as to demonstrate the efficiency and superiority of the proposed IRS-USRSM scheme.

Multi-access edge computing (MEC) has been recognized as a promising technology for enhancing the computation capability for next generation wireless networks. This paper studies physical layer security for an MEC network, where multiple users desire to securely offload part of their computation tasks to a base station (BS) simultaneously using non-orthogonal multiple access (NOMA) subject to the potential overhearing of a malicious eavesdropper. The secrecy outage probability (SOP) is adopted as a secrecy performance metric of the computation offloading against eavesdropping attacks. We aim to minimize the total energy consumption of the MEC system subject to an individual SOP constraint for each user. To this end, we jointly design each user’s local computing bits, the transmit power, the secrecy code rates, as well as the successive interference cancellation decoding order at the BS side. As the formulated problem is highly non-convex and challenging to solve, we propose an efficient algorithm based on penalty dual decomposition (PDD) and sequential convex approximation methods to obtain an efficient suboptimal solution. To reduce the computational complexity, we further propose a reverse recursion (RR) algorithm and derive semi-closed-form solutions to the design problem. Numerical results are presented to validate the convergence and the effectiveness of our proposed algorithms. We show that the minimal total energy consumption obtained via either the PDD or RR method approaches the optimal performance of exhaustive search as the task duration increases. It is also demonstrated that the RR algorithm can achieve a comparable performance to that of the PDD algorithm while enjoying a much lower computational complexity.

This paper investigates the multi-antenna covert communications assisted by a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS). In particular, to shelter the existence of covert communications between a multi-antenna transmitter and a single-antenna receiver from a warden, a friendly full-duplex receiver with two antennas is leveraged to make contributions where one antenna is responsible for receiving the transmitted signals and the other one transmits the jamming signals with a varying power to confuse the warden. Considering the worst case, the closed-form expression of the minimum detection error probability (DEP) at the warden is derived and utilized in a covert constraint to guarantee the system performance. Then, we formulate an optimization problem maximizing the covert rate of the system under the covertness constraint and quality of service (QoS) constraint with communication outage analysis. To jointly design the active and passive beamforming of the transmitter and STAR-RIS, an iterative algorithm based on semi-definite relaxation (SDR) method and Dinkelbach’s algorithm is proposed to effectively solve the non-convex optimization problem. Simulation results show that the proposed STAR-RIS-assisted scheme highly outperforms the case with conventional RIS, which validates the effectiveness of the proposed algorithm as well as the superiority of STAR-RIS in guaranteeing the covertness of wireless communications.

This paper investigates the multi-antenna covert communications assisted by a simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS). In particular, to shelter the existence of communications between transmitter and receiver from a warden, a friendly full-duplex receiver with two antennas is leveraged to make contributions to confuse the warden. Considering the worst case, the closed-form expression of the minimum detection error probability (DEP) at the warden is derived and utilized as a covert constraint. Then, we formulate an optimization problem maximizing the covert rate of the system under the covertness constraint and quality of service (QoS) constraint with communication outage analysis. To jointly design the active and passive beamforming of the transmitter and STAR- RIS, an iterative algorithm based on globally convergent version of method of moving asymptotes (GCMMA) is proposed to effectively solve the non-convex optimization problem. Simu-lation results show that the proposed STAR-RIS-assisted scheme highly outperforms the case with conventional RIS.

In this paper, we address finite block-length covert wireless communication against an adversary that not only detects the signals but also localizes the source. Because of the probability of failed localization, the adversary’s detection strategy differs greatly from the former works. In the free space propagation model, we discuss the surveillance strategy of an adversary equipped with a uniform linear antenna array and the legitimate users’ resources optimization scheme for achieving covert communication. The theoretical analysis and the numerical results illustrate the properties of the adversary’s surveillance and the achievable covert communication performance, showing that the randomness in the source’s transmission time may also improve the covertness.

To address the security issues in wireless transmission communication systems with finite-alphabet inputs, a secure uplink reception scheme based on receiver spatial modulation is proposed to ensure high security of the wireless transmission system while achieving high spectrum efficiency. The proposed scheme introduces disturbance phase updates at the transmitter end and disturbance compensation at the intelligent reflecting surface (IRS) to ensure both the spectrum efficiency and physical layer security of the wireless transmission system. This enables the legitimate receiver to correctly receive the signal at maximum power while preventing eavesdroppers from accurately intercepting the signal. In this paper, we propose two phase compensation schemes, namely, element-wise random disturbance compensation (ERDC) and group-wise random disturbance compensation. Furthermore, two eavesdropping scenarios, referred to as ideal eavesdropping and jamming eavesdropping, are thoroughly investigated and considered. We introduce the maximum likelihood detection to reliably detecte the indices of the designed receive antenna and the based-band modulated signal, and the closed-form expressions of error performance with ERDC scheme are deduced. Finally, in the simulated and numercial results, the performance analysis verifies error performance discrepancy based security performance evaluation of the proposed scheme, demonstrating its effectiveness and superiority.

Millimeter-wave (mmWave) can provide abundant spectrum resource in vehicular communication networks. Nevertheless, due to the high path-loss and blocking effects in mmWave propagation, and high mobility of vehicles, downlink services for vehicles would be seriously degraded. In this paper, we firstly propose a deep reinforcement learning-based joint beam allocation and relay selection (JoBARS) scheme to mitigate blocking effects and optimize the total transmission rate of the vehicular network, where the mmWave base station (mmBS) provides multi-user services. When downlinks are blocked, the mmBS can select appropriate idle vehicles as relay nodes to enhance service quality from a global perspective. We set the rate punishment restriction in JoBARS scheme to guarantee each vehicle can obtain high-quality service. Besides, a relaying incentive mechanism (RIM) is proposed to avoid vehicles being overly selected for relaying and ensure that relay vehicles have a higher chance of being served in the next round. We demonstrate that JoBARS scheme can effectively enhance the total transmission rate while alleviating transmission outages caused by severe propagation attenuation of mmWave signals. Compared with Greedy Selection scheme, the total rate and average connection probability of vehicles under JoBARS scheme are nearly 17% and 14% higher when blocking effects are severe.

This work investigates the covert information freshness in intelligent reflecting surface (IRS)-aided communications, where a public full-duplex user (Alice) and a private full-duplex user (Bob) exchange information in the presence of a watchful warden (Willie). In particular, with the help of Alice’s undisguised signal transmission, Bob can establish covert communications such that his transmission can be shielded from Willie. Considering both the non-retransmission protocol and the automatic repeat-request (ARQ) protocol for Bob’s transmission, we study the resource allocation design. By exploiting the channel statistics, the joint design of active beamforming at Alice and Bob, the passive beamforming at the IRS, and the packet length of the confidential data packet is formulated as a nonconvex optimization problem which minimizes the age of information (AoI) at Alice for the two considered protocols taking into account the quality of service in terms of the maximum tolerable AoI at Bob and communication covertness. To circumvent the non-convexity of the design problem, we propose alternating optimization algorithms to find effective solutions. Numerical results demonstrate the superiority of our proposed optimization algorithms over various benchmarks and unveil the decrease of the optimized packet length with the improved covert channel quality.

Reconfigurable intelligent surface (RIS) assisted spatial-domain pattern index modulations (PIMs) are recently emerged as promising transmission candidates for the next generation wireless. In this research, we are motivated to investigate a novel augmented PIM (APIM) framework empowered by RIS, such that the potentials of PIM and RIS can be merged for realizing energy efficient high data rate transmissions. In contrast to the spatial-domain PIM, the proposed APIM is implemented by utilizing the indices of activated patterns of the transmitter, receiver and the RIS to modulate information. To maximize the signal power observed with the desired receive pattern, the element-wise and subset-wise phase alignment based reflection designs are provided for the RIS, which may be imposed with different requirements regarding the reflection implementation complexity. Based on the principle of maximum-likelihood estimation, we introduce the optimal joint detection (OJD) to accomplish detecting of APIM signals. Furthermore, we analyze the error and capacity performance of the APIM systems employing the OJD, so as to evaluate the performance limits achievable by the proposed APIM. Finally, simulation results demonstrate the efficiency of our APIM scheme and verify the accuracy of the performance evaluation.

The full-duplex millimeter-wave communication has drawn significant attention for its rich spectrum resources and high spectrum efficiency characteristics. However, due to the information leakage during transmission, secure threats in the full-duplex systems still exist. In this paper, we propose a full-duplex jamming based secure transmission scheme, where the instantaneous channel state information of eavesdropping channel is unknown. We study the optimal design of hybrid beamforming and power allocation jointly with secrecy outage probability constraint. Our results reveal that joint optimization highly facilitates the secrecy performance of full-duplex mmWave communication, without any self-interference limitations in the multiple-antenna scenarios.

This paper investigates physical layer security for a large-scale WSN with random multiple access, where each fusion center in the network randomly schedules a number of sensors to upload their sensed data subject to the overhearing of randomly distributed eavesdroppers. We propose an uncoordinated random jamming scheme in which those unscheduled sensors send jamming signals with a certain probability to defeat the eavesdroppers. With the aid of stochastic geometry theory and order statistics, we derive analytical expressions for the connection outage probability and secrecy outage probability to characterize transmission reliability and secrecy, respectively. Based on the obtained analytical results, we formulate an optimization problem for maximizing the sum secrecy throughput subject to both reliability and secrecy constraints, considering a joint design of the wiretap code rates for each scheduled sensor and the jamming probability for the unscheduled sensors. We provide both optimal and low-complexity sub-optimal algorithms to tackle the above problem, and further reveal various properties on the optimal parameters which are useful to guide practical designs. In particular, we demonstrate that the proposed random jamming scheme is beneficial for improving the sum secrecy throughput, and the optimal jamming probability is the result of trade-off between secrecy and throughput. We also show that the throughput performance of the sub-optimal scheme approaches that of the optimal one when facing a stringent reliability constraint or a loose secrecy constraint.

In this paper, we investigate physical-layer security of the uplink millimeter wave communications for a cellular vehicle-to-everything (C-V2X) network comprised of a large number of base stations (BSs) and different categories of V2X nodes, including vehicles, pedestrians, and road side units. Considering the dynamic change and randomness of the topology of the C-V2X network, we model the roadways, the V2X nodes on each roadway, and the BSs by a Poisson line process, a 1D Poisson point process (PPP), and a 2D PPP, respectively. We propose two uplink association schemes for a typical vehicle, namely, the smallest-distance association (SDA) scheme and the largest-power association (LPA) scheme, and we establish a tractable analytical framework to comprehensively assess the security performance of the uplink transmission, by leveraging the stochastic geometry theory. Specifically, for each association scheme, we first obtain new expressions for the association probability of the typical vehicle, and then derive the overall connection outage probability and secrecy outage probability by calculating the Laplace transform of the aggregate interference power. Numerical results are presented to validate our theoretical analysis, and we also provide interesting insights into how the security performance is influenced by various system parameters, including the densities of V2X nodes and BSs. Moreover, we show that the LPA scheme outperforms the SDA scheme in terms of secrecy throughput.

This paper studies covert communications between a pair of legitimate transmitter-receiver against a watchful warden over fading channels. There coexist multiple friendly helper nodes who are willing to protect the covert communication from being detected by the warden. An uncoordinated jammer selection scheme is proposed where those helpers whose instantaneous channel gains to the legitimate receiver fall below a pre-established selection threshold will be chosen as jammers radiating jamming signals to defeat the warden. Afterwards, we jointly design the optimal selection threshold and transmission rate for maximizing covert throughput under the premise that the detection error of the warden exceeds a certain level. Numerical results demonstrate that the maximal covert throughput improves significantly as the total number of helpers increases.

Hybrid precoding architecture, as a cost-effective approach for millimeter-wave (mmWave) communications, can achieve an excellent tradeoff between spectrum efficiency and hardware implementation complexity. However, the design of a robust hybrid precoding for improving the physical layer security (PLS), which is insensitive to the uncertainty of eavesdropper's channel state information (CSI), has not been well studied. This article for the first time designs a probabilistically robust hybrid precoding scheme for securing broadcast communications in Internet of Things (IoT) with eavesdropper's imperfect CSI. Specifically, considering the Gaussian CSI error model, we maximize the minimum secrecy rate of multiple IoT devices (IoDs) by jointly designing analog and digital precoders under the constraints in terms of secrecy outage probability and per IoD's information rate. The optimization problem is challenging due to the coupling of the analog and digital precoders, and the secrecy outage constraint. To handle these challenges, we first employ a conservative probability inequality to transform the secrecy outage probability constraint into a deterministic one. Then, by employing the penalty dual decomposition (PDD) method, we develop a novel iterative algorithm to convert the resultant nonconvex problem into a sequence of convex problems, which can guarantee the convergence to its Karush-Kuhn-Tucker (KKT) solution. Simulation results show that the proposed algorithm can achieve significant secrecy performance gains compared with the benchmark algorithm.

In this paper, we investigate secure content delivery in a two-tier cache-enabled millimeter wave (mmWave) heterogeneous network composed of a macro base station (MBS) and K small base stations (SBSs) with caching capabilities. We allocate finite cache units at the SBSs and MBS to pre-store files with high popularities, where the SBSs store the most popular files, and the MBS stores the less popular ones. To deliver the file requested by a legitimate user securely, two secure transmission schemes, namely, distributed beamforming and direct transmission, are employed at the SBSs and MBS, respectively. Moreover, artificial noise (AN) is combined with the above two transmission schemes to further improve transmission security. The connection outage probability, secrecy outage probability, and secrecy throughput for the proposed mmWave transmission schemes are obtained. Based on these results, we jointly design the transmission rates and the cache resource allocation between the SBSs and MBS to maximize the overall secrecy throughput. We also provide insights into how the overall secrecy throughput is influenced by various parameters, including transmission rates, power allocation ratio of the AN scheme, and cache allocation factor. Numerical results are eventually presented to validate our theoretical analysis and demonstrate the effectiveness of the proposed transmission schemes and cache resource allocation strategy.

In this paper, we study covert communications between a pair of legitimate transmitter-receiver against a watchful warden over slow fading channels. There coexist multiple friendly helper nodes who are willing to protect the covert communication from being detected by the warden. We propose an uncoordinated jammer selection scheme where those helpers whose instantaneous channel gains to the legitimate receiver fall below a pre-established selection threshold will be chosen as jammers radiating jamming signals to defeat the warden. By doing so, the detection accuracy of the warden is expected to be severely degraded while the desired covert communication is rarely affected. We then jointly design the optimal selection threshold and message transmission rate for maximizing covert throughput under the premise that the detection error of the warden exceeds a certain level. Numerical results are presented to validate our theoretical analyses. It is shown that the multi-jammer assisted covert communication outperforms the conventional single-jammer method in terms of covert throughput, and the maximal covert throughput improves significantly as the total number of helpers increases, which demonstrates the validity and superiority of our proposed scheme.

In this paper, we study millimeter wave (mmWave) communications of an uplink cellular vehicle-to-everything (C-V2X) network consisting of vehicles, pedestrians, road side units (RSUs), and cellular base stations (BSs). We propose an association scheme that a vehicle delivers messages to either the V2X nodes, including vehicles, pedestrians, and RSUs, or the BSs, based on the distance and the bias factor. Subsequently, we provide a tractable analytical framework to comprehensively assess the reliability performance of the considered uplink transmission, in terms of success probability. By leveraging the stochastic geometry theory, we model the locations of vehicles, pedestrians, and RSUs as independent cox process and model the locations of BSs as a Poisson point process (PPP), and derive new expressions for the association probability, the success probability of different associating types, and the overall success probability of the C-V2X network. Numerical results are presented to validate the theoretical analyses and provide interesting insights into how the success probability is influenced by various parameters, including the signal-to-interference-plus-noise ratio (SINR) threshold, the densities of V2X nodes and BSs, the blockage density, and the bias factor.

This paper studies physical-layer secure transmissions from a transmitter to a legitimate receiver against an eavesdropper over slow fading channels, taking into account the impact of finite blocklength secrecy coding. A comprehensive analysis and optimization framework is established to investigate secrecy throughput for both single- and multi-antenna transmitter scenarios. Both adaptive and non-adaptive design schemes are devised, in which the secrecy throughput is maximized by exploiting the instantaneous and statistical channel state information of the legitimate receiver, respectively. Specifically, optimal transmission policy, blocklength, and code rates are jointly designed to maximize the secrecy throughput. Additionally, null-space artificial noise is employed to improve the secrecy throughput for the multi-antenna setup with the optimal power allocation derived. Various important insights are developed. In particular, 1) increasing blocklength benefits both reliability and secrecy under the proposed transmission policy; 2) secrecy throughput monotonically increases with blocklength; 3) secrecy throughput initially increases but then decreases as secrecy rate increases, and the optimal secrecy rate maximizing the secrecy throughput should be carefully chosen in order to strike a good balance between rate and decoding correctness. Numerical results are eventually presented to verify theoretical findings.

Recently, device-to-device (D2D) communication has emerged as a promising solution to meet rapidly growing demands for data services. This paper studies the physical layer security of D2D underlay cellular network, where the D2D and cellular communications coexist in the network, in the presence of randomly distributed eavesdroppers (Eves). We establish an analytical framework to assess the security performance of the network of interest. To be specific, by modeling the D2D underlay cellular network as a Poisson cluster process (PCP), we derive exact expressions for the coverage outage probabilities (COP) and secrecy outage probabilities (SOP), respectively, for both the cellular users (CU) and D2D users (DU). The numerical results are finally presented to verify our theoratical derivations and reveal some interesting insights into the effects of various parameters on the system performance.

This paper studies multi-antenna covert communications coexisting with randomly located wardens and interferers. We analyze and optimize the covert throughput under a stochastic geometry framework. We first introduce covert outage probability and connectivity probability to respectively characterize covertness and reliability, and derive analytically tractable expressions for them. We then consider a worst-case covert communication, where the wardens can invariably maximize the covert outage probability by adjusting the detection thresholds of their detectors. Afterwards, we jointly design the optimal transmit power and transmission rate to maximize the covert throughput while satisfying the covertness requirement. Interestingly, it is found that the maximal covert throughput is invariant to either the density of interferers or the interfering power, regardless of the number of transmit antennas.

Device-to-device (D2D) communication raises new transmission secrecy protection challenges, since conventional physical layer security approaches, such as multiple antennas and cooperation techniques, are invalid due to its resource/size constraints. The full-duplex (FD) jamming receiver, which radiates jamming signals to confuse eavesdroppers when receiving the desired signal simultaneously, is a promising candidate. Unlike existing endeavors that assume the FD jamming receiver always improves the secrecy performance compared with the half-duplex (HD) receiver, we show that this assumption highly depends on the instantaneous residual self-interference cancellation level and may be invalid. We propose an adaptive jamming receiver operating in a switched FD/HD mode for a D2D link in random networks. Subject to the secrecy outage probability constraint, we optimize the transceiver parameters, such as signal/jamming powers, secrecy rates, and mode switch criteria, to maximize the secrecy throughput. Most of the optimization operations are taken off-line and only very limited on-line calculations are required to make the scheme with low complexity. Furthermore, some interesting insights are provided, such as the secrecy throughput is a quasi-concave function. Numerical results are demonstrated to verify our theoretical findings, and to show its superiority compared with the receiver operating in the FD or HD mode only.

This paper studies multi-antenna-aided covert communications coexisting with randomly located wardens and interferers, considering both centralized and distributed antenna systems (CAS/DAS). The throughput performance of the covert communication is analyzed and optimized under a stochastic geometry framework, where the joint impact of the small-scale channel fading and the large-scale path loss is examined. To be specific, two probabilistic metrics, namely, the covert outage probability and the connectivity probability, are adopted to characterize the covertness and reliability of the transmission, respectively, and analytically tractable expressions for the two metrics are derived. The worst-case covert communication scenario is then investigated, where the wardens invariably can maximize the covert outage probability by adjusting the detection thresholds for their detectors. Afterward, the optimal transmit power and transmission rate are jointly designed to maximize the covert throughput subject to a covertness constraint. Interestingly, it is found that the maximal covert throughput for both the CAS and DAS is invariant to the density of interferers and the interfering power, regardless of the number of transmit antennas. The numerical results demonstrate that the CAS outperforms the DAS in terms of the covert throughput for the random network of interest, and the throughput gap between the two systems increases dramatically when the number of transmit antennas becomes higher.

This paper explores physical-layer security in a small cell network with cooperative cache-enabled small base stations (SBSs) in the presence of randomly distributed eavesdroppers. We put forward a hybrid caching placement strategy where a proportion of the cache space in each SBS is assigned to store the most popular files (MPFs), while the remaining is used to cache the disjoint subfiles (DSFs) of less popular files in different SBSs as a means to improve secrecy and content diversity. We then propose two coordinated multi-point techniques, namely, joint transmission and orthogonal transmission, to deliver the MPFs and DSFs, respectively. We jointly design the optimal transmission rate and caching assignment proportion to maximize the secure content delivery probability, and provide various insights into the optimal results. Numerical results are also presented to verify the theoretical findings and to demonstrate the superiority of our caching and transmission strategies.

In the fifth generation era, the pervasive applications of Internet of Things and massive machine-type communications have initiated increasing research interests on the backscatter wireless powered communication (B-WPC) technique due to its ultrahigh energy efficiency and low cost. The ubiquitous B-WPC network is characterized by nodes with dynamic spatial positions and sporadic short packets, of which the performance has not been fully investigated. In this paper, we give a comprehensive analysis of a multiantenna B-WPC network with sporadic short packets under a stochastic geometry framework. By exploiting a time-space Poisson point process model, the behavior of the network is well captured in a decentralized and asynchronous transmission way. We then analyze the energy and information outage performance in the energy harvest and backscatter modulation phases of the backscatter network, respectively. The optimal transmission slot length and division are obtained by maximizing the network-wide spatial throughput. Moreover, we find an interesting result that there exists the optimal tradeoff between the durations of the energy harvest and backscatter modulation phases for spatial throughput maximization. Numerical results are demonstrated to verify our analytical findings and show that this tradeoff region gets shrunk when the outage constraints become more stringent.

This paper studies physical-layer security for a cache-enabled heterogeneous cellular network comprised of a macro base station and multiple small base stations (SBSs). We investigate a joint design on caching placement and file delivery for realizing secure and energy-efficient transmissions against randomly distributed eavesdroppers. We propose a novel hybrid “most popular content” and “largest content diversity” caching placement policy to distribute the files of different popularities. Depending on the availability and placement of the requested file, we employ three cooperative transmission schemes, namely, distributed beamforming, frequency-domain orthogonal transmission, and best SBS relaying. We derive analytical expressions for the connection outage probability and secrecy outage probability for each transmission scheme. Afterward, we design the optimal transmission rates and caching allocation successively to achieve a maximal overall secrecy throughput and secrecy energy efficiency, respectively. Numerical results verify the theoretical analyses and demonstrate the superiority of the proposed hybrid caching policy.

Mm-wave offers a sensible solution to the capacity crunch faced by 5G wireless communications. This paper comprehensively studies physical layer security in a multi-input single-output mm-wave system, where multiple single-antenna eavesdroppers are randomly located. Concerning the specific propagation characteristics of mm-wave, we investigate two secure transmission schemes, namely maximum ratio transmitting beamforming and artificial noise (AN) beamforming. Specifically, we first derive closed-form expressions of the connection probability for both schemes. We then analyze the secrecy outage probability in both non-colluding eavesdroppers and colluding eavesdroppers scenarios. Also, we maximize the secrecy throughput under a secrecy outage probability constraint, and obtain optimal transmission parameters, especially the power allocation between AN and the information signal for AN beamforming. Numerical results are provided to verify our theoretical analysis. We observe that the density of eavesdroppers, the spatially resolvable paths of the destination and eavesdroppers all contribute to the secrecy performance and the parameter design of mm-wave systems.

This paper explores the physical-layer security in a small cell network with cooperative cache-enabled small base stations (SBSs) in the presence of randomly distributed eavesdroppers. We propose a joint design on the caching placement and the physical-layer transmission to improve the secure content delivery probability (SCDP). We first put forward a hybrid caching placement strategy in which a proportion of the cache unit in each SBS is assigned to store the most popular files (MPFs), while the remaining is used to cache the disjoint subfiles (DSFs) of the less popular files in different SBSs as a means to enhance transmission secrecy and content diversity. We then introduce two coordinated multi-point techniques, namely, joint transmission and orthogonal transmission, to deliver the MPFs and DSFs, respectively. We derive analytical expressions for the SCDP in each transmission scheme, considering both non-colluding and colluding eavesdropping scenarios. Based on the obtained analytical results, we jointly design the optimal transmission rates and the optimal caching assignment for maximizing the overall SCDP. Various insights into the optimal transmission and caching designs are further provided. Numerical results are also presented to verify our theoretical findings and to demonstrate the superiority of the proposed caching and transmission strategies.

This paper proposes a new nonadaptive (NADP) transmission scheme with adjustable power allocation based on the instantaneous channel state information (CSI) of the main channel for the multiple-input-single-output-single-eavesdropper (MISOSE) system. With the assistance of the adjustable power allocation ratio, we can improve the secure transmission performance under the constraints of secrecy outage probability (SOP) and upper bound rate (UBR). To verify the superiority of our new approach, we consider the multi-antenna communication systems with NADP scheme and give the optimal solution for achieving the maximum secrecy throughput based on a two-dimensional (2-D) searching method. In addition, a good suboptimal solution is also provided to reduce the time complexity of the optimal solution. Simulation results are provided and confirm that the proposed NADP scheme has significant advantages over the existing work especially when the transmit power is high.

In this paper, the outage performance of nonorthogonal multiple access (NOMA) is investigated in a downlink space division multiple access (SDMA) network with a multi-antenna base station and randomly deployed users. The NOMA technology is concurrently exploited with SDMA to further improve spectral efficiency. With limited channel state information (CSI) feedback taken into account, an analytical framework is proposed to evaluate outage performance for a given user. An expression for the outage probability is derived in closed form. Moreover, the diversity order and the effect of the number of feedback bits on the outage performance of NOMA are analyzed. Numerical results are demonstrated to verify our analytical findings and show that different from the perfect CSI case there always exists performance floor in the considered network due to limited feedback.

In this paper, we investigate physical-layer security in a wireless ad hoc network in which a large number of legitimate transmitter-receiver pairs coexist with randomly distributed eavesdroppers; each legitimate receiver works in either half-duplex (HD) or full-duplex (FD) mode. We aim to increase the number of secure links per unit area in the network of interest by determining the allocation between HD- and FD-mode links. We use tools from stochastic geometry theory and analyze the connection outage probability and the secrecy outage probability for an arbitrary legitimate link by deriving analytical expressions, based on which we then figure out the optimal fraction of FD-mode links that maximizes the area secure link number given a pair of wiretap code rates. We further develop insights into this optimal fraction and derive a closed-form expression for it assuming perfect self-interference cancellation. Numerical results are demonstrated to validate our theoretical findings.

Multi-region geocast, an efficient technique for routing protocols at the network layer, realizes the delivery of different information to different user groups that are differentiated from each other by their geographical locations. This paper studies multi-region geocast at the physical layer for the first time by exploiting non- orthogonal multiple access (NOMA) which is one of the promising technologies for future 5G systems. We investigate physical-layer multi-region geocast by studying its beamforming design in a multiple-input single-output (MISO) NOMA system. The objective is to minimize the total transmit power subject to a prescribed data rate for each user group. To this end, we propose an efficient algorithm based on the sequential convex approximation (SCA) method. Our simulation results show that multi-region geocast using NOMA achieves better performance than conventional multi-group multicast when the disparity level among the geographical locations of user groups is remarkable. Further, NOMA is more favorable for limited transmit antennas and massive connectivity in future 5G systems.

Exploiting millimeter wave is an effective way to meet the data traffic demand in the 5G wireless communication system. In this paper, we study secure transmissions under slow fading channels with multipath propagation in millimeter wave systems. Concerning the new propagation characteristics of millimeter wave, we investigate three transmission schemes, namely, maximum ratio transmitting (MRT) beamforming, artificial noise (AN) beamforming, and partial MRT (PMRT) beamforming. We evaluate the secrecy performance by analyzing both the secrecy outage probability (SOP) and the secrecy throughput for each scheme. Particularly, for the AN scheme, we derive a closed-form expression for the optimal power allocation ratio of the information signal power to the total transmit power that minimizes the SOP, as well as obtain an explicit solution on the optimal transmission parameters that maximize the secrecy throughput. By comparing the secrecy performances achieved by different strategies, we demonstrate that the secrecy performance of the millimeter wave system is significantly influenced by the relationship between the legitimate user's and the eavesdropper's spatially resolvable paths, which is different from the wireless systems with statistically independent channel models. In the absence of the common path between the legitimate user and the eavesdropper, MRT beamforming is the best scheme. In the presence of common paths, AN beamforming and PMRT beamforming show their respective superiorities depending on the transmit power and the number of common paths. Numerical results are provided to verify our theoretical analysis.

With the recent emergence of 5G era, heterogeneous cellular networks (HCNs) have invoked a popular research interest. In this paper, we provide a comprehensive analysis for multi-antenna transmissions in a multi-tier downlink HCN. We first propose a reliability-oriented threshold-based mobile association policy, where each user connects to the strongest base station from which this user can obtain the largest truncated long-term received power. Under our mobile association policy, we derive analytical expressions for the exact outage probability of an arbitrary randomly located user, along with computationally convenient lower and upper bounds. Asymptotic analysis on the outage probability shows that introducing a large access threshold into mobile association significantly decreases the outage probability. We further investigate the spectrum efficiency and the energy efficiency of the HCN. Our theoretic analysis and numerical validations show that both the spectrum and energy efficiencies can be improved by properly choosing the access threshold.

This paper studies physical layer security in a wireless ad hoc network with numerous legitimate transmitter-receiver pairs and eavesdroppers. A hybrid full-duplex (FD)/half-duplex receiver deployment strategy is proposed to secure legitimate transmissions, by letting a fraction of legitimate receivers work in the FD mode sending jamming signals to confuse eavesdroppers upon their information receptions, and letting the other receivers work in the half-duplex mode just receiving their desired signals. The objective of this paper is to choose properly the fraction of FD receivers for achieving the optimal network security performance. Both accurate expressions and tractable approximations for the connection outage probability and the secrecy outage probability of an arbitrary legitimate link are derived, based on which the area secure link number, network-wide secrecy throughput, and network-wide secrecy energy efficiency are optimized, respectively. Various insights into the optimal fraction are further developed, and its closed-form expressions are also derived under perfect self-interference cancellation or in a dense network. It is concluded that the fraction of FD receivers triggers a non-trivial tradeoff between reliability and secrecy, and the proposed strategy can significantly enhance the network security performance.

In this paper, we study the benefits of full-duplex (FD) receiver jamming in enhancing the physical-layer security of a two-tier decentralized wireless network with each tier deployed with a large number of pairs of a single-antenna transmitter and a multi-antenna receiver. In the underlying tier, the transmitter sends unclassified information and the receiver works in the half-duplex (HD) mode receiving the desired signal. In the overlaid tier, the transmitter delivers confidential information in the presence of randomly located eavesdroppers, and the receiver works in the FD mode radiating jamming signals to confuse eavesdroppers and receiving the desired signal simultaneously. We provide a comprehensive performance analysis and network design under a stochastic geometry framework. Specifically, we consider the scenarios where each FD receiver uses single- and multi-antenna jamming, and analyze the connection probability and the secrecy outage probability of a typical FD receiver by deriving accurate expressions and more tractable approximations for the two probabilities. We also determine the optimal deployment density of the FD-mode tier to maximize the network-wide secrecy throughput subject to constraints including the given dual probabilities and the network-wide throughput of the HD-mode tier. Numerical results are demonstrated to verify our theoretical findings, and show that the network-wide secrecy throughput is significantly improved by properly deploying the FD-mode tier.

In this paper, we provide a comprehensive analysis of the multi-antenna transmission in a K-tier downlink heterogeneous cellular network (HCN). We first propose a reliability-oriented mobile access policy with an access threshold, in which each user connects to the strongest base station in terms of the truncated long-term received power. Under this policy, we derive for a random user explicit analytical expressions of the exact out-age probability along with its computational convenient lower and upper bounds. Asymptotic analysis on the outage probabilities shows that introducing the access threshold efficiently improves outage performance. We further investigate the area network throughput from the perspective of outage. Our theoretic analysis and numerical validations show that throughput performance can be enhanced by properly designing the access threshold.

In this paper, we study the secrecy throughput in millimeter wave systems under slow fading channels considering multipath propagation. For the specific propagation characteristics of millimeter wave, we provide transmission scheme designs and a comprehensive secrecy performance analysis. Specifically, we maximize the secrecy throughput under a secrecy outage probability (SOP) constraint through a dynamic parameter transmission scheme, and provide the optimal solution to transmission parameters, including the codeword rate and the power allocation ratio of the information signal power to the total transmit power. We find that the secrecy performance of the millimeter wave system under investigation is significantly influenced by the relationship between spatially resolvable paths of the legitimate user and those of the eavesdropper, which differs from those wireless systems with statistically independent channel. Numerical results are provided to verify our theoretical analysis.

Non-orthogonal multiple access (NOMA) has been recognized as a potential technique to achieve higher spectral efficiency (SE) for future 5G systems. However, the anticipated thousand-fold increase in wireless data traffic urgently calls for new designs of power-efficient communication systems. In this paper, we investigate NOMA from the perspective of energy efficiency (EE) in a multiuser downlink system. Firstly, we generalize an existing cognitive radio (CR) inspired NOMA scheme by introducing multiple antenna techniques and extending the number of primary users to an arbitrary number. Then, we aim at optimizing the EE of this generalized CR inspired NOMA scheme subject to an individual quality of service (QoS) constraint for each primary user, which leads to a non-convex fractional programming problem. For this challenging problem, we propose an efficient algorithm based on the sequential convex approximation (SCA) method. Our numerical results show that NOMA has superior EE performance in comparison with conventional orthogonal multiple access (OMA).

The use of millimeter wave is an effective way to meet the data traffic demand of the 5G wireless communication system. For the new propagation characteristics of millimeter wave, in this paper, we study the secure transmission with artificial noise under slow fading channels considering multipath propagation in millimeter wave systems. Firstly, when partial eavesdropper's channel state information is known at the transmitter, an artificial noise transmission strategy which depends on directions of the destination's and the eavesdropper's propagation paths is proposed. Then we analyze the secrecy outage probability (SOP) through an on-off transmission scheme. Furthermore, we minimize the SOP subject to a secrecy rate constraint and derive a closed-form optimal power allocation between the information bearing signal and artificial noise. Numerical results are provided to show the superiority of the proposed scheme.

The heterogeneous cellular network (HCN) is a promising approach to the deployment of 5G cellular networks. This paper comprehensively studies physical layer security in a multitier HCN where base stations (BSs), authorized users, and eavesdroppers are all randomly located. We first propose an access threshold-based secrecy mobile association policy that associates each user with the BS providing the maximum truncated average received signal power beyond a threshold. Under the proposed policy, we investigate the connection probability and secrecy probability of a randomly located user and provide tractable expressions for the two metrics. Asymptotic analysis reveals that setting a larger access threshold increases the connection probability while decreases the secrecy probability. We further evaluate the network-wide secrecy throughput and the minimum secrecy throughput per user with both connection and secrecy probability constraints. We show that introducing a properly chosen access threshold significantly enhances the secrecy throughput performance of a HCN.

This paper studies the impact of artificial noise (AN) on the secrecy performance of a target cell in multi-cell cellular networks. Although AN turns out to be an efficient approach for securing a point-to-point/single-cell confidential transmission, it would increase the inter-cell interference in a multi-cell cellular network, which may degrade the network reliability and secrecy performance. For analyzing the average secrecy performance of the target cell which is of significant interest, we employ a hybrid cellular deployment model, where the target cell is a circle of fixed size, and the base stations outside the target cell are modeled as a homogeneous Poisson point process. We investigate the impact of AN on the reliability and security of users in the target cell in the presence of pilot contamination using a stochastic geometry approach. The analytical results of the average connection outage and the secrecy outage of its cellular user (CU) in the target cell are given, which facilitates the evaluation of the average secrecy throughput of a randomly chosen CU in the target cell. It shows that with an optimized power allocation between the desired signals and AN, the AN scheme is an efficient solution for securing the communications in a multi-cell cellular network.

In this paper, we provide a comprehensive study of secrecy transmission in decode-and-forward (DF) relay networks subjected to slow fading. With only channel distribution information (CDI) of the wiretap channels, we aim at maximizing secrecy throughput of the two-hop transmission under a secrecy outage constraint through optimizing transmission region, rate parameters of the wiretap codes and power allocation between the source and relay. We propose fixed transmission parameter scheme (FTPS) and variable transmission parameter scheme (VTPS), which are based on the CDI and instantaneous channel state information of the main channels, respectively. In both schemes, source and relay use the same codeword, and the eavesdropper can use maximum ratio combining (MRC) reception. To improve the secrecy throughput, we further propose VTPS-D1 and VTPS-D2 schemes, where the source and relay either use independent codewords with identical code rates, or different codebooks with different code rates so that the eavesdropper can only decode the two-hop signals individually rather than using MRC. We provide explicit results on the design for all proposed schemes. Numerical results and comparisons on the secrecy throughput of these schemes are presented to reveal their respective superiorities and give some insights into the choice of design scheme.

In this paper, we study the secure multi-antenna transmission with artificial noise (AN) under slow fading channels coexisting with randomly located eavesdroppers. We provide a comprehensive secrecy performance analysis and system design/optimization under a stochastic geometry framework. Specifically, we first evaluate the secrecy outage performance, and derive a closed-form expression for the optimal power allocation ratio of the information signal power to the total transmit power that minimizes the secrecy outage probability (SOP). Subject to a SOP constraint, we then propose a dynamic parameter transmission scheme (DPTS) and a static parameter transmission scheme (SPTS) to maximize secrecy throughput, and provide explicit solutions on the optimal transmission parameters, including the wiretap code rates, the on-off transmission threshold and the power allocation ratio. Our results give new insight into secure transmission designs. For example, secrecy rate is a concave function of the power allocation ratio in DPTS, and AN plays a significant role under SOP constraints and in dense eavesdropper scenarios. In SPTS, transmission probability is a concave function of the power allocation ratio, and secrecy throughput is a quasi-concave function of the secrecy rate. Numerical results are demonstrated to validate our theoretical analysis.

The artificial noise (AN) scheme is an efficient strategy for enhancing the secrecy rate of a multiple-input-single-output channel in the presence of a passive eavesdropper, whose channel state information is unavailable. Recently, a randomized beamforming scheme has been proposed for deteriorating the eavesdropper's bit-error-rate performance via corrupting its receiving signal by time-varying multiplicative noise. However, the secrecy rate of such a scheme has not been well addressed yet. In this paper, we name it the artificial fast fading (AFF) scheme and provide a comprehensive secrecy rate analysis for it. We show that with this scheme, the eavesdropper will face a noncoherent Ricean fading single-input-multiple-output channel. Although the closed-form secrecy rate is difficult to obtain, we derive an exact expression for the single-antenna-eavesdropper case and a lower bound for the multiantenna-eavesdropper case, both of which can be numerically calculated conveniently. Furthermore, we compare the AFF scheme with the AN scheme and show that their respective superiorities to each other depend on the number of antennas that the transmitter and the eavesdropper possessed, i.e., when the eavesdropper has more antennas than the transmitter does, the AFF scheme achieves a larger secrecy rate; otherwise, the AN scheme outperforms. Motivated by this observation, we propose a hybrid AN-AFF scheme and investigate the power allocation problem, which achieves better secrecy performance further.

We investigate the physical layer security of a multiple-input single-output (MISO) channel in the presence of a multi-antenna passive eavesdropper. Traditionally, artificial noise (AN) scheme is an efficient security strategy without the channel state information (CSI) of the eavesdropper. In this paper, we propose and analyze an efficient security scheme called the artificial fast fading (AFF) scheme. The basic idea is to randomly weight the information symbols at different transmit antennas in a special manner so that the channel of the intended receiver is an AWGN channel while that of the eavesdropper is a fast fading channel. This prevents the eavesdropper obtaining the CSI using the blind channel estimation techniques so that it can only detect the information symbols in a non-coherent way, which greatly reduces the amount of information intercepted. More importantly, we derive the achievable secrecy rate of the sheme and provide a low bound of it, which can be numerically calculated. Compared with the exact secrecy rate of the AN scheme, even the low bound of the AFF scheme is larger, when the eavesdropper has more antennas than the transmitter.