Terahertz (THz) communications are regarded as a pillar technology for the 6G systems, by offerin... more Terahertz (THz) communications are regarded as a pillar technology for the 6G systems, by offering multi-ten-GHz bandwidth. To overcome the huge propagation loss while reducing the hardware complexity, THz ultra-massive (UM) MIMO systems with hybrid beamforming are proposed to offer high array gain. Notably, the adjustable-phase-shifters considered in most existing hybrid beamforming studies are power-hungry and difficult to realize in the THz band. Moreover, due to the ultra-massive antennas, full channel-state-information (CSI) is challenging to obtain. To address these practical concerns, in this paper, an energy-efficient dynamic-subarray with fixed-phase-shifters (DS-FPS) architecture is proposed for THz hybrid beamforming. To compensate for the spectral efficiency loss caused by the fixed-phase of FPS, a switch network is inserted to enable dynamic connections. In addition, by considering the partial CSI, we propose a row-successive-decomposition (RSD) algorithm to design the hybrid beamforming matrices for DS-FPS. A row-by-row (RBR) algorithm is further proposed to reduce computational complexity. Extensive simulation results show that, the proposed DS-FPS architecture with the RSD and RBR algorithms achieves much higher energy efficiency than the existing architectures. Moreover, the DS-FPS architecture with partial CSI achieves 97% spectral efficiency of that with full CSI.
ICC 2022 - IEEE International Conference on Communications
In this paper, we analyze the impact of different encoding schemes on the age of information (AoI... more In this paper, we analyze the impact of different encoding schemes on the age of information (AoI) performance in a point-to-point system, where a source generates packets based on the status updates collected from multiple sensors and transmits the packets to a destination. In this system, we consider two encoding schemes, namely, the joint encoding scheme and the distributed encoding scheme. In the joint encoding scheme, the status updates from all the sensors are jointly encoded into a packet for transmission. In the distributed encoding scheme, the status update from each sensor is encoded individually and the sensors' packets are transmitted following the round robin policy. To ensure the freshness of packets, the zero-wait policy is adopted in both schemes, where a new packet is immediately generated once the source finishes the transmission of the current packet. We derive closed-form expressions for the average AoI achieved by these two encoding schemes and compare their performances. Simulation results show that the distributed encoding scheme is more appropriate for systems with a relatively large number of sensors, compared with the joint encoding scheme. Index Terms-Age of information, short packet communications, low latency communications, encoding scheme.
2020 IEEE Wireless Communications and Networking Conference (WCNC), 2020
We propose a novel analytical framework for evaluating the coverage performance of a millimeter w... more We propose a novel analytical framework for evaluating the coverage performance of a millimeter wave (mmWave) cellular network where idle user equipments (UEs) act as relays. In this network, the base station (BS) adopts either the direct mode to transmit to the destination UE, or the relay mode if the direct mode fails, where the BS transmits to the relay UE and then the relay UE transmits to the destination UE. To address the drastic rotational movements of destination UEs in practice, we propose to adopt selection combining at destination UEs. New expression is derived for the signal-to-interference-plusnoise ratio (SINR) coverage probability of the network. Using numerical results, we first demonstrate the accuracy of our new expression. Then we show that ignoring spatial correlation, which has been commonly adopted in the literature, leads to severe overestimation of the SINR coverage probability. Furthermore, we show that introducing relays into a mmWave cellular network vastly improves the coverage performance. In addition, we show that the optimal BS density maximizing the SINR coverage probability can be determined by using our analysis.
In this paper, we study the trajectory and resource allocation design for downlink energy-efficie... more In this paper, we study the trajectory and resource allocation design for downlink energy-efficient secure unmanned aerial vehicle (UAV) communication systems, where an information UAV assisted by a multi-antenna jammer UAV serves multiple ground users in the existence of multiple ground eavesdroppers. The resource allocation strategy and the trajectory of the information UAV, and the jamming policy of the jammer UAV are jointly optimized for maximizing the system energy efficiency. The joint design is formulated as a non-convex optimization problem taking into account the quality of service (QoS) requirement, the security constraint, and the imperfect channel state information (CSI) of the eavesdroppers. The formulated problem is generally intractable. As a compromise approach, the problem is divided into two subproblems which facilitates the design of a low-complexity suboptimal algorithm based on alternating optimization approach. Simulation results illustrate that the proposed algorithm converges within a small number of iterations and demonstrate some interesting insights: (1) the introduction of a jammer UAV facilitates a highly flexible trajectory design of the information UAV which is critical to improving the system energy efficiency; (2) by exploiting the spatial degrees of freedom brought by the multi-antenna jammer UAV, our proposed design can focus the artificial noise on eavesdroppers offering a strong security mean to the system.
IEEE Transactions on Wireless Communications, 2019
In this work, we propose a novel beamforming design to enhance physical layer security of a non-o... more In this work, we propose a novel beamforming design to enhance physical layer security of a non-orthogonal multiple access (NOMA) system with the aid of artificial noise (AN). The proposed design uses two scalars to balance the useful signal strength and interference at the strong and weak users, which is a generalized version of the existing beamforming designs in the context of physical layer security for NOMA. We determine the optimal power allocation among useful signals and AN together with the two optimal beamforming scalars in order to maximize the secrecy sum rate (SSR). Our asymptotic analysis in the high signal-to-noise ratio regime provides an efficient and near-optimal solution to optimizing the beamforming scalars and power allocation coefficients. Our analysis indicates that it is not optimal to form a beam towards either the strong user or the weak user in NOMA systems for security enhancement. In addition, the asymptotically optimal power allocation informs that, as the transmit power increases, more power should be allocated to the weak user or AN signals, while the power allocated to the strong user keeps constant. Our examination shows that the proposed novel beamforming design can significantly outperform two benchmark schemes.
IEEE Transactions on Information Forensics and Security, 2018
This work proposes a new channel training (CT) scheme for a full-duplex receiver to enhance physi... more This work proposes a new channel training (CT) scheme for a full-duplex receiver to enhance physical layer security. Equipped with NB full-duplex antennas, the receiver simultaneously receives the information signal and transmits artificial noise (AN). In order to reduce the non-cancellable self-interference due to the transmitted AN, the receiver has to estimate the self-interference channel prior to the data communication phase. In the proposed CT scheme, the receiver transmits a limited number of pilot symbols which are known only to itself. Such a secret CT scheme prevents an eavesdropper from estimating the jamming channel from the receiver to the eavesdropper, hence effectively degrading the eavesdropping capability. We analytically examine the connection probability (i.e., the probability of the data being successfully decoded by the receiver) of the legitimate channel and the secrecy outage probability due to eavesdropping for the proposed secret CT scheme. Based on our analysis, the optimal power allocation between CT and data/AN transmission at the legitimate transmitter/receiver is determined. Our examination shows that the newly proposed secret CT scheme significantly outperforms the non-secret CT scheme that uses publicly known pilots when the number of antennas at the eavesdropper is larger than one.
IEEE Transactions on Information Forensics and Security, 2019
In this paper, we study decode-and-forward relaying networks in the presence of direct links, whe... more In this paper, we study decode-and-forward relaying networks in the presence of direct links, where they are used by the eavesdropper to overhear the confidential message from the source and relay. The secure data transmission can go through from either the direct or the relaying branch, and we focus on the practical communication scenarios, where the main and eavesdropper channels are correlated. Although traditional opportunistic selection techniques can choose one better branch to ensure the secure performance, it needs to continuously know the channel state information (CSI) of both branches and may result in a high branch switching rate. To overcome these limitations, we propose a distributed secure switch-and-stay combining (DSSSC) protocol, where only one between direct and relaying branches is activated to assist the secure data transmission, and the switching occurs when the branch cannot support the secure communication any longer. The DSSSC protocol uses either the instantaneous or the statistics of the eavesdropping CSI. For both cases, we quantify the impact of correlated fading on secure communication by deriving an analytical expression for the secrecy outage probability (SOP) as well as an asymptotic expression for the high main-to-eavesdropper ratio region. From the asymptotic SOP, we can conclude that the DSSSC can achieve the optimal secure performance of opportunistic selection with less implementation complexity, and the channel correlation can further enhance the transmission security.
In this paper, we study decode-and-forward (DF) relaying networks in the presence of direct links... more In this paper, we study decode-and-forward (DF) relaying networks in the presence of direct links, where they are used by the eavesdropper to overhear the confidential message from the source and relay. The secure data transmission can go through from either the direct or the relaying branch, and we focus on the practical communication scenarios, where the main and eavesdropper channels are correlated. Although traditional opportunistic selection techniques can choose one better branch to ensure the secure performance, it needs to continuously know the channel state information (CSI) of both branches, and may result in a high branch switching rate. To overcome these limitations, we propose a distributed secure switch-and-stay combining (DSSSC) protocol, where only one between direct and relaying branches is activated to assist the secure data transmission, and the switching occurs when the branch cannot support the secure communication any longer. The DSSSC protocol uses either the instantaneous or the statistics of the eavesdropping CSI. For both cases, we quantify the impact of correlated fading on secure communication by deriving an analytical expression for the secrecy outage probability (SOP) as well as an asymptotic expression for the high main-to-eavesdropper ratio (MER) region. From the asymptotic SOP, we can conclude that the DSSSC can achieve the optimal secure performance of opportunistic selection with less implementation complexity, and the channel correlation can further enhance the transmission security.
In this work, we first propose a joint user and relay selection (JURS) scheme to enhance physical... more In this work, we first propose a joint user and relay selection (JURS) scheme to enhance physical layer security in a multiuser multi-relay network, where the best pair of the user and relay that maximizes the user-to-destination signal-tointerference-to-noise ratio (SINR) is jointly selected. We analytically examine the secrecy outage probability (SOP) of this scheme, based on which the optimal power allocation between the useful signal and artificial noise at the users is determined in order to minimize the corresponding SOP. To avoid the high complexity of the joint selection, we also propose a separate user and relay selection (SURS) scheme, where one relay is firstly selected to maximize the relay-to-destination SINR and a user is then selected to maximize the SINR from the user to the selected relay. We also derive the SOP of the SURS scheme and determine the corresponding optimal power allocation. As expected, the JURS scheme can outperform the SURS scheme. However, our examination also indicates that the low-complex SURS scheme can achieve similar secrecy performance as the high-complex JURS scheme under some specific conditions, e.g., when the number of users is much larger than that of the relays or when the average signal-to-noise ratio of user-to-relay channel is much higher than that of the relay-to-destination channel.
In this paper, we propose a novel physical layer strategy to improve the secrecy performance of m... more In this paper, we propose a novel physical layer strategy to improve the secrecy performance of multiuser multiple-input single-output networks. In this strategy, orthogonal space-time block code (OSTBC) is employed at an A Aantenna base station (BS) and artificial noise (AN) is employed at an AJ-antenna cooperative relay to enhance the security level of the network. Moreover, two opportunistic scheduling schemes, namely, selection combining (SC) and scan-and-wait combining (SWC), are leveraged to select one legitimate user for data transmission. To evaluate the secrecy performance of the proposed OSTBC-SCAN and OSTBC-SWC-AN schemes, we derive new exact closed-form expressions for the secrecy outage probability and the effective secrecy throughput. Using numerical results, we show that the OSTBC-SWC-AN scheme outperforms the OSTBC-SCAN scheme when the switching threshold is carefully chosen. We also show that increasing A A brings down the secrecy performance in the presence of a high switching threshold.
IEEE Transactions on Wireless Communications, 2016
We present two physical layer secure transmission schemes for multiuser multi-relay networks, whe... more We present two physical layer secure transmission schemes for multiuser multi-relay networks, where the communication from M users to the base station is assisted by direct links and by N decode-and-forward relays. In this network, we consider that a passive eavesdropper exists to overhear the transmitted information, which entails exploiting the advantages of both direct and relay links for physical layer security enhancement. To fulfill this requirement, we investigate two criteria for user and relay selection and examine the achievable secrecy performance. Criterion I performs a joint user and relay selection, while Criterion II performs separate user and relay selections, with a lower implementation complexity. We derive a tight lower bound on the secrecy outage probability for Criterion I and an accurate analytical expression for the secrecy outage probability for Criterion II. We further derive the asymptotic secrecy outage probabilities at high transmit signal-to-noise ratios and high main-to-eavesdropper ratios for both criteria. We demonstrate that the secrecy diversity order is min (M N, M + N) for Criterion I, and N for Criterion II. Finally, we present numerical and simulation results to validate the proposed analysis, and show the occurrence condition of the secrecy outage probability floor.
IEEE Transactions on Wireless Communications, 2017
We design a novel artificial-noise-aided secure onoff transmission scheme in a wiretap channel. W... more We design a novel artificial-noise-aided secure onoff 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 (SNRs) 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.
IEEE Transactions on Wireless Communications, 2016
Theoretical studies on physical layer security often adopt the secrecy outage probability as the ... more Theoretical studies on physical layer security often adopt the secrecy outage probability as the performance metric for wireless communications over quasi-static fading channels. The secrecy outage probability has two limitations from a practical point of view: a) it does not give any insight into the eavesdropper's decodability of confidential messages; b) it cannot characterize the amount of information leakage to the eavesdropper when an outage occurs. Motivated by the limitations of the secrecy outage probability, we propose three new secrecy metrics for secure transmissions over quasi-static fading channels. The first metric establishes a link between the concept of secrecy outage and the decodability of messages at the eavesdropper. The second metric provides an error-probabilitybased secrecy metric which is typically used for the practical implementation of secure wireless systems. The third metric characterizes how much or how fast the confidential information is leaked to the eavesdropper. We show that the proposed secrecy metrics collectively give a more comprehensive understanding of physical layer security over fading channels and enable one to appropriately design secure communication systems with different views on how secrecy is measured.
We investigate beamforming and artificial noise generation at the secondary transmitters to estab... more We investigate beamforming and artificial noise generation at the secondary transmitters to establish secure transmission in large scale spectrum sharing networks, where multiple non-colluding eavesdroppers attempt to intercept the secondary transmission. We develop a comprehensive analytical framework to accurately assess the secrecy performance under the primary users' quality of service constraint. Our aim is to characterize the impact of beamforming and artificial noise generation (BF&AN) on this complex large scale network. We first derive exact expressions for the average secrecy rate and the secrecy outage probability. We then derive an easy-to-evaluate asymptotic average secrecy rate and asymptotic secrecy outage probability when the number of antennas at the secondary transmitter goes to infinity. Our results show that the equal power allocation between the useful signal and artificial noise is not always the best strategy to achieve maximum average secrecy rate in large scale spectrum sharing networks. Another interesting observation is that the advantage of BF&AN over BF on the average secrecy rate is lost when the aggregate interference from the primary and secondary transmitters is strong, such that it overtakes the effect of the generated AN.
We propose a secure transmission scheme for a relay wiretap channel, where a source communicates ... more We propose a secure transmission scheme for a relay wiretap channel, where a source communicates with a destination via a decode-and-forward relay in the presence of spatially random-distributed eavesdroppers. We assume that the source is equipped with multiple antennas, whereas the relay, the destination, and the eavesdroppers are equipped with a single antenna each. In the proposed scheme, in addition to information signals, the source transmits artificial noise signals in order to confuse the eavesdroppers. With the target of maximizing the secrecy throughput of the relay wiretap channel, we derive a closed-form expression for the transmission outage probability and an easy-to-compute expression for the secrecy outage probability. Using these expressions, we determine the optimal power allocation factor and wiretap code rates that guarantee the maximum secrecy throughput, while satisfying a secrecy outage probability constraint. Furthermore, we examine the impact of source antenna number on the secrecy throughput, showing that adding extra transmit antennas at the source brings about a significant increase in the secrecy throughput.
2015 International Conference on Wireless Communications & Signal Processing (WCSP), 2015
This work reveals some fundamental properties of an on-off transmission (OOT) scheme, in which a ... more This work reveals some fundamental properties of an on-off transmission (OOT) scheme, in which a transmitter sends signals occasionally as per the capacity of the main channel in order to achieve physical layer security. To this end, we first identify the widely used hybrid secrecy outage probability as a function of the transmission probability and the conditional secrecy outage probability of the OOT scheme. This indicates, for the first time, that the hybrid secrecy outage probability can be achieved by the OOT scheme. We then derive a lower bound on the conditional secrecy outage probability of the OOT scheme in case of transmission, which is solely determined by the average signal-to-noise ratios (SNRs) of the main channel and eavesdropper's channel. Finally, we re-investigate the OOT scheme within an absolutely completely passive eavesdropping scenario, in which even the average SNR of the eavesdropper's channel is not required. Specifically, we derive an easy-evaluated expression for the average conditional secrecy outage probability of the OOT scheme by adopting an annulus threat model.
IEEE Transactions on Wireless Communications, 2015
We propose a new framework for determining the wiretap code rates of single-input single-output m... more We propose a new framework for determining the wiretap code rates of single-input single-output multi-antenna eavesdropper (SISOME) wiretap channels when the capacity of the eavesdropper's channel is not available at the transmitter. In our framework we introduce the effective secrecy throughput (EST) as a new performance metric that explicitly captures the two key features of wiretap channels, namely, reliability and secrecy. Notably, the EST measures the average rate of the confidential information transmitted from the transmitter to the intended receiver without being eavesdropped on. We provide easy-to-implement methods to determine the wiretap code rates for two transmission schemes: 1) adaptive transmission scheme in which the capacity of the main channel is available at the transmitter and 2) fixed-rate transmission scheme in which the capacity of the main channel is not available at the transmitter. Such determinations are further extended into an absolutepassive eavesdropping scenario where even the average signalto-noise ratio of the eavesdropper's channel is not available at the transmitter. Notably, our solutions for the wiretap code rates do not require us to set reliability or secrecy constraints for the transmission within wiretap channels.
We design new secure on-off transmission schemes in wiretap channels with outdated channel state ... more We design new secure on-off transmission schemes in wiretap channels with outdated channel state information (CSI). In our design we consider not only the outdated CSI from the legitimate receiver but two distinct scenarios, depending on whether or not the outdated CSI from the eavesdropper is known at the transmitter. Under this consideration our schemes exploit the useful knowledge contained in the available outdated CSI, based on which the transmitter decides whether to transmit or not. We derive new closed-form expressions for the transmission probability, the connection outage probability, the secrecy outage probability, and the reliable and secure transmission probability to characterize the achievable performance. Based on these results, we present the optimal solutions that maximize the secrecy throughput under dual connection and secrecy outage constraints. Our analytical and numerical results offer detailed insights into the design of the wiretap coding parameters and the imposed outage constraints. We further show that allowing more freedom on the codeword transmission rate enables a larger feasible region of the dual outage constraints by exploiting the trade-off between reliability and security.
2015 IEEE International Conference on Communications (ICC), 2015
We design a linear precoder based on the principles of the generalized regularized channel invers... more We design a linear precoder based on the principles of the generalized regularized channel inversion (RCI) precoder that achieves confidential broadcasting in a two-cell network. In each cell of the network, an N-antenna base station (BS) communicates with K single-antenna users. We consider coordinated beamforming where the BSs in the two cells do not share messages but the users in the two cells feed back their channel state information to both BSs. In the precoder design, we determine the optimal regularization parameter that maximizes the secrecy sum rate. To this end, we derive new channel-independent expressions for the secrecy sum rate in the large-system regime, where K and N approach infinity with a fixed ratio µ = K/N. Moreover, we propose a power-reduction strategy that significantly improves the secrecy sum rate at high transmit signal-to-noise ratios when µ is higher than 0.5.
Terahertz (THz) communications are regarded as a pillar technology for the 6G systems, by offerin... more Terahertz (THz) communications are regarded as a pillar technology for the 6G systems, by offering multi-ten-GHz bandwidth. To overcome the huge propagation loss while reducing the hardware complexity, THz ultra-massive (UM) MIMO systems with hybrid beamforming are proposed to offer high array gain. Notably, the adjustable-phase-shifters considered in most existing hybrid beamforming studies are power-hungry and difficult to realize in the THz band. Moreover, due to the ultra-massive antennas, full channel-state-information (CSI) is challenging to obtain. To address these practical concerns, in this paper, an energy-efficient dynamic-subarray with fixed-phase-shifters (DS-FPS) architecture is proposed for THz hybrid beamforming. To compensate for the spectral efficiency loss caused by the fixed-phase of FPS, a switch network is inserted to enable dynamic connections. In addition, by considering the partial CSI, we propose a row-successive-decomposition (RSD) algorithm to design the hybrid beamforming matrices for DS-FPS. A row-by-row (RBR) algorithm is further proposed to reduce computational complexity. Extensive simulation results show that, the proposed DS-FPS architecture with the RSD and RBR algorithms achieves much higher energy efficiency than the existing architectures. Moreover, the DS-FPS architecture with partial CSI achieves 97% spectral efficiency of that with full CSI.
ICC 2022 - IEEE International Conference on Communications
In this paper, we analyze the impact of different encoding schemes on the age of information (AoI... more In this paper, we analyze the impact of different encoding schemes on the age of information (AoI) performance in a point-to-point system, where a source generates packets based on the status updates collected from multiple sensors and transmits the packets to a destination. In this system, we consider two encoding schemes, namely, the joint encoding scheme and the distributed encoding scheme. In the joint encoding scheme, the status updates from all the sensors are jointly encoded into a packet for transmission. In the distributed encoding scheme, the status update from each sensor is encoded individually and the sensors' packets are transmitted following the round robin policy. To ensure the freshness of packets, the zero-wait policy is adopted in both schemes, where a new packet is immediately generated once the source finishes the transmission of the current packet. We derive closed-form expressions for the average AoI achieved by these two encoding schemes and compare their performances. Simulation results show that the distributed encoding scheme is more appropriate for systems with a relatively large number of sensors, compared with the joint encoding scheme. Index Terms-Age of information, short packet communications, low latency communications, encoding scheme.
2020 IEEE Wireless Communications and Networking Conference (WCNC), 2020
We propose a novel analytical framework for evaluating the coverage performance of a millimeter w... more We propose a novel analytical framework for evaluating the coverage performance of a millimeter wave (mmWave) cellular network where idle user equipments (UEs) act as relays. In this network, the base station (BS) adopts either the direct mode to transmit to the destination UE, or the relay mode if the direct mode fails, where the BS transmits to the relay UE and then the relay UE transmits to the destination UE. To address the drastic rotational movements of destination UEs in practice, we propose to adopt selection combining at destination UEs. New expression is derived for the signal-to-interference-plusnoise ratio (SINR) coverage probability of the network. Using numerical results, we first demonstrate the accuracy of our new expression. Then we show that ignoring spatial correlation, which has been commonly adopted in the literature, leads to severe overestimation of the SINR coverage probability. Furthermore, we show that introducing relays into a mmWave cellular network vastly improves the coverage performance. In addition, we show that the optimal BS density maximizing the SINR coverage probability can be determined by using our analysis.
In this paper, we study the trajectory and resource allocation design for downlink energy-efficie... more In this paper, we study the trajectory and resource allocation design for downlink energy-efficient secure unmanned aerial vehicle (UAV) communication systems, where an information UAV assisted by a multi-antenna jammer UAV serves multiple ground users in the existence of multiple ground eavesdroppers. The resource allocation strategy and the trajectory of the information UAV, and the jamming policy of the jammer UAV are jointly optimized for maximizing the system energy efficiency. The joint design is formulated as a non-convex optimization problem taking into account the quality of service (QoS) requirement, the security constraint, and the imperfect channel state information (CSI) of the eavesdroppers. The formulated problem is generally intractable. As a compromise approach, the problem is divided into two subproblems which facilitates the design of a low-complexity suboptimal algorithm based on alternating optimization approach. Simulation results illustrate that the proposed algorithm converges within a small number of iterations and demonstrate some interesting insights: (1) the introduction of a jammer UAV facilitates a highly flexible trajectory design of the information UAV which is critical to improving the system energy efficiency; (2) by exploiting the spatial degrees of freedom brought by the multi-antenna jammer UAV, our proposed design can focus the artificial noise on eavesdroppers offering a strong security mean to the system.
IEEE Transactions on Wireless Communications, 2019
In this work, we propose a novel beamforming design to enhance physical layer security of a non-o... more In this work, we propose a novel beamforming design to enhance physical layer security of a non-orthogonal multiple access (NOMA) system with the aid of artificial noise (AN). The proposed design uses two scalars to balance the useful signal strength and interference at the strong and weak users, which is a generalized version of the existing beamforming designs in the context of physical layer security for NOMA. We determine the optimal power allocation among useful signals and AN together with the two optimal beamforming scalars in order to maximize the secrecy sum rate (SSR). Our asymptotic analysis in the high signal-to-noise ratio regime provides an efficient and near-optimal solution to optimizing the beamforming scalars and power allocation coefficients. Our analysis indicates that it is not optimal to form a beam towards either the strong user or the weak user in NOMA systems for security enhancement. In addition, the asymptotically optimal power allocation informs that, as the transmit power increases, more power should be allocated to the weak user or AN signals, while the power allocated to the strong user keeps constant. Our examination shows that the proposed novel beamforming design can significantly outperform two benchmark schemes.
IEEE Transactions on Information Forensics and Security, 2018
This work proposes a new channel training (CT) scheme for a full-duplex receiver to enhance physi... more This work proposes a new channel training (CT) scheme for a full-duplex receiver to enhance physical layer security. Equipped with NB full-duplex antennas, the receiver simultaneously receives the information signal and transmits artificial noise (AN). In order to reduce the non-cancellable self-interference due to the transmitted AN, the receiver has to estimate the self-interference channel prior to the data communication phase. In the proposed CT scheme, the receiver transmits a limited number of pilot symbols which are known only to itself. Such a secret CT scheme prevents an eavesdropper from estimating the jamming channel from the receiver to the eavesdropper, hence effectively degrading the eavesdropping capability. We analytically examine the connection probability (i.e., the probability of the data being successfully decoded by the receiver) of the legitimate channel and the secrecy outage probability due to eavesdropping for the proposed secret CT scheme. Based on our analysis, the optimal power allocation between CT and data/AN transmission at the legitimate transmitter/receiver is determined. Our examination shows that the newly proposed secret CT scheme significantly outperforms the non-secret CT scheme that uses publicly known pilots when the number of antennas at the eavesdropper is larger than one.
IEEE Transactions on Information Forensics and Security, 2019
In this paper, we study decode-and-forward relaying networks in the presence of direct links, whe... more In this paper, we study decode-and-forward relaying networks in the presence of direct links, where they are used by the eavesdropper to overhear the confidential message from the source and relay. The secure data transmission can go through from either the direct or the relaying branch, and we focus on the practical communication scenarios, where the main and eavesdropper channels are correlated. Although traditional opportunistic selection techniques can choose one better branch to ensure the secure performance, it needs to continuously know the channel state information (CSI) of both branches and may result in a high branch switching rate. To overcome these limitations, we propose a distributed secure switch-and-stay combining (DSSSC) protocol, where only one between direct and relaying branches is activated to assist the secure data transmission, and the switching occurs when the branch cannot support the secure communication any longer. The DSSSC protocol uses either the instantaneous or the statistics of the eavesdropping CSI. For both cases, we quantify the impact of correlated fading on secure communication by deriving an analytical expression for the secrecy outage probability (SOP) as well as an asymptotic expression for the high main-to-eavesdropper ratio region. From the asymptotic SOP, we can conclude that the DSSSC can achieve the optimal secure performance of opportunistic selection with less implementation complexity, and the channel correlation can further enhance the transmission security.
In this paper, we study decode-and-forward (DF) relaying networks in the presence of direct links... more In this paper, we study decode-and-forward (DF) relaying networks in the presence of direct links, where they are used by the eavesdropper to overhear the confidential message from the source and relay. The secure data transmission can go through from either the direct or the relaying branch, and we focus on the practical communication scenarios, where the main and eavesdropper channels are correlated. Although traditional opportunistic selection techniques can choose one better branch to ensure the secure performance, it needs to continuously know the channel state information (CSI) of both branches, and may result in a high branch switching rate. To overcome these limitations, we propose a distributed secure switch-and-stay combining (DSSSC) protocol, where only one between direct and relaying branches is activated to assist the secure data transmission, and the switching occurs when the branch cannot support the secure communication any longer. The DSSSC protocol uses either the instantaneous or the statistics of the eavesdropping CSI. For both cases, we quantify the impact of correlated fading on secure communication by deriving an analytical expression for the secrecy outage probability (SOP) as well as an asymptotic expression for the high main-to-eavesdropper ratio (MER) region. From the asymptotic SOP, we can conclude that the DSSSC can achieve the optimal secure performance of opportunistic selection with less implementation complexity, and the channel correlation can further enhance the transmission security.
In this work, we first propose a joint user and relay selection (JURS) scheme to enhance physical... more In this work, we first propose a joint user and relay selection (JURS) scheme to enhance physical layer security in a multiuser multi-relay network, where the best pair of the user and relay that maximizes the user-to-destination signal-tointerference-to-noise ratio (SINR) is jointly selected. We analytically examine the secrecy outage probability (SOP) of this scheme, based on which the optimal power allocation between the useful signal and artificial noise at the users is determined in order to minimize the corresponding SOP. To avoid the high complexity of the joint selection, we also propose a separate user and relay selection (SURS) scheme, where one relay is firstly selected to maximize the relay-to-destination SINR and a user is then selected to maximize the SINR from the user to the selected relay. We also derive the SOP of the SURS scheme and determine the corresponding optimal power allocation. As expected, the JURS scheme can outperform the SURS scheme. However, our examination also indicates that the low-complex SURS scheme can achieve similar secrecy performance as the high-complex JURS scheme under some specific conditions, e.g., when the number of users is much larger than that of the relays or when the average signal-to-noise ratio of user-to-relay channel is much higher than that of the relay-to-destination channel.
In this paper, we propose a novel physical layer strategy to improve the secrecy performance of m... more In this paper, we propose a novel physical layer strategy to improve the secrecy performance of multiuser multiple-input single-output networks. In this strategy, orthogonal space-time block code (OSTBC) is employed at an A Aantenna base station (BS) and artificial noise (AN) is employed at an AJ-antenna cooperative relay to enhance the security level of the network. Moreover, two opportunistic scheduling schemes, namely, selection combining (SC) and scan-and-wait combining (SWC), are leveraged to select one legitimate user for data transmission. To evaluate the secrecy performance of the proposed OSTBC-SCAN and OSTBC-SWC-AN schemes, we derive new exact closed-form expressions for the secrecy outage probability and the effective secrecy throughput. Using numerical results, we show that the OSTBC-SWC-AN scheme outperforms the OSTBC-SCAN scheme when the switching threshold is carefully chosen. We also show that increasing A A brings down the secrecy performance in the presence of a high switching threshold.
IEEE Transactions on Wireless Communications, 2016
We present two physical layer secure transmission schemes for multiuser multi-relay networks, whe... more We present two physical layer secure transmission schemes for multiuser multi-relay networks, where the communication from M users to the base station is assisted by direct links and by N decode-and-forward relays. In this network, we consider that a passive eavesdropper exists to overhear the transmitted information, which entails exploiting the advantages of both direct and relay links for physical layer security enhancement. To fulfill this requirement, we investigate two criteria for user and relay selection and examine the achievable secrecy performance. Criterion I performs a joint user and relay selection, while Criterion II performs separate user and relay selections, with a lower implementation complexity. We derive a tight lower bound on the secrecy outage probability for Criterion I and an accurate analytical expression for the secrecy outage probability for Criterion II. We further derive the asymptotic secrecy outage probabilities at high transmit signal-to-noise ratios and high main-to-eavesdropper ratios for both criteria. We demonstrate that the secrecy diversity order is min (M N, M + N) for Criterion I, and N for Criterion II. Finally, we present numerical and simulation results to validate the proposed analysis, and show the occurrence condition of the secrecy outage probability floor.
IEEE Transactions on Wireless Communications, 2017
We design a novel artificial-noise-aided secure onoff transmission scheme in a wiretap channel. W... more We design a novel artificial-noise-aided secure onoff 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 (SNRs) 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.
IEEE Transactions on Wireless Communications, 2016
Theoretical studies on physical layer security often adopt the secrecy outage probability as the ... more Theoretical studies on physical layer security often adopt the secrecy outage probability as the performance metric for wireless communications over quasi-static fading channels. The secrecy outage probability has two limitations from a practical point of view: a) it does not give any insight into the eavesdropper's decodability of confidential messages; b) it cannot characterize the amount of information leakage to the eavesdropper when an outage occurs. Motivated by the limitations of the secrecy outage probability, we propose three new secrecy metrics for secure transmissions over quasi-static fading channels. The first metric establishes a link between the concept of secrecy outage and the decodability of messages at the eavesdropper. The second metric provides an error-probabilitybased secrecy metric which is typically used for the practical implementation of secure wireless systems. The third metric characterizes how much or how fast the confidential information is leaked to the eavesdropper. We show that the proposed secrecy metrics collectively give a more comprehensive understanding of physical layer security over fading channels and enable one to appropriately design secure communication systems with different views on how secrecy is measured.
We investigate beamforming and artificial noise generation at the secondary transmitters to estab... more We investigate beamforming and artificial noise generation at the secondary transmitters to establish secure transmission in large scale spectrum sharing networks, where multiple non-colluding eavesdroppers attempt to intercept the secondary transmission. We develop a comprehensive analytical framework to accurately assess the secrecy performance under the primary users' quality of service constraint. Our aim is to characterize the impact of beamforming and artificial noise generation (BF&AN) on this complex large scale network. We first derive exact expressions for the average secrecy rate and the secrecy outage probability. We then derive an easy-to-evaluate asymptotic average secrecy rate and asymptotic secrecy outage probability when the number of antennas at the secondary transmitter goes to infinity. Our results show that the equal power allocation between the useful signal and artificial noise is not always the best strategy to achieve maximum average secrecy rate in large scale spectrum sharing networks. Another interesting observation is that the advantage of BF&AN over BF on the average secrecy rate is lost when the aggregate interference from the primary and secondary transmitters is strong, such that it overtakes the effect of the generated AN.
We propose a secure transmission scheme for a relay wiretap channel, where a source communicates ... more We propose a secure transmission scheme for a relay wiretap channel, where a source communicates with a destination via a decode-and-forward relay in the presence of spatially random-distributed eavesdroppers. We assume that the source is equipped with multiple antennas, whereas the relay, the destination, and the eavesdroppers are equipped with a single antenna each. In the proposed scheme, in addition to information signals, the source transmits artificial noise signals in order to confuse the eavesdroppers. With the target of maximizing the secrecy throughput of the relay wiretap channel, we derive a closed-form expression for the transmission outage probability and an easy-to-compute expression for the secrecy outage probability. Using these expressions, we determine the optimal power allocation factor and wiretap code rates that guarantee the maximum secrecy throughput, while satisfying a secrecy outage probability constraint. Furthermore, we examine the impact of source antenna number on the secrecy throughput, showing that adding extra transmit antennas at the source brings about a significant increase in the secrecy throughput.
2015 International Conference on Wireless Communications & Signal Processing (WCSP), 2015
This work reveals some fundamental properties of an on-off transmission (OOT) scheme, in which a ... more This work reveals some fundamental properties of an on-off transmission (OOT) scheme, in which a transmitter sends signals occasionally as per the capacity of the main channel in order to achieve physical layer security. To this end, we first identify the widely used hybrid secrecy outage probability as a function of the transmission probability and the conditional secrecy outage probability of the OOT scheme. This indicates, for the first time, that the hybrid secrecy outage probability can be achieved by the OOT scheme. We then derive a lower bound on the conditional secrecy outage probability of the OOT scheme in case of transmission, which is solely determined by the average signal-to-noise ratios (SNRs) of the main channel and eavesdropper's channel. Finally, we re-investigate the OOT scheme within an absolutely completely passive eavesdropping scenario, in which even the average SNR of the eavesdropper's channel is not required. Specifically, we derive an easy-evaluated expression for the average conditional secrecy outage probability of the OOT scheme by adopting an annulus threat model.
IEEE Transactions on Wireless Communications, 2015
We propose a new framework for determining the wiretap code rates of single-input single-output m... more We propose a new framework for determining the wiretap code rates of single-input single-output multi-antenna eavesdropper (SISOME) wiretap channels when the capacity of the eavesdropper's channel is not available at the transmitter. In our framework we introduce the effective secrecy throughput (EST) as a new performance metric that explicitly captures the two key features of wiretap channels, namely, reliability and secrecy. Notably, the EST measures the average rate of the confidential information transmitted from the transmitter to the intended receiver without being eavesdropped on. We provide easy-to-implement methods to determine the wiretap code rates for two transmission schemes: 1) adaptive transmission scheme in which the capacity of the main channel is available at the transmitter and 2) fixed-rate transmission scheme in which the capacity of the main channel is not available at the transmitter. Such determinations are further extended into an absolutepassive eavesdropping scenario where even the average signalto-noise ratio of the eavesdropper's channel is not available at the transmitter. Notably, our solutions for the wiretap code rates do not require us to set reliability or secrecy constraints for the transmission within wiretap channels.
We design new secure on-off transmission schemes in wiretap channels with outdated channel state ... more We design new secure on-off transmission schemes in wiretap channels with outdated channel state information (CSI). In our design we consider not only the outdated CSI from the legitimate receiver but two distinct scenarios, depending on whether or not the outdated CSI from the eavesdropper is known at the transmitter. Under this consideration our schemes exploit the useful knowledge contained in the available outdated CSI, based on which the transmitter decides whether to transmit or not. We derive new closed-form expressions for the transmission probability, the connection outage probability, the secrecy outage probability, and the reliable and secure transmission probability to characterize the achievable performance. Based on these results, we present the optimal solutions that maximize the secrecy throughput under dual connection and secrecy outage constraints. Our analytical and numerical results offer detailed insights into the design of the wiretap coding parameters and the imposed outage constraints. We further show that allowing more freedom on the codeword transmission rate enables a larger feasible region of the dual outage constraints by exploiting the trade-off between reliability and security.
2015 IEEE International Conference on Communications (ICC), 2015
We design a linear precoder based on the principles of the generalized regularized channel invers... more We design a linear precoder based on the principles of the generalized regularized channel inversion (RCI) precoder that achieves confidential broadcasting in a two-cell network. In each cell of the network, an N-antenna base station (BS) communicates with K single-antenna users. We consider coordinated beamforming where the BSs in the two cells do not share messages but the users in the two cells feed back their channel state information to both BSs. In the precoder design, we determine the optimal regularization parameter that maximizes the secrecy sum rate. To this end, we derive new channel-independent expressions for the secrecy sum rate in the large-system regime, where K and N approach infinity with a fixed ratio µ = K/N. Moreover, we propose a power-reduction strategy that significantly improves the secrecy sum rate at high transmit signal-to-noise ratios when µ is higher than 0.5.
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