Channel estimation in simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS)-assisted millimeter-wave (mmWave) systems faces significant challenges due to numerous cascaded channels and passive STAR-RIS elements. While least squares (LS) methods incur high pilot overhead and compressed sensing (CS) approaches require impractical prior sparsity knowledge, we propose a two-stage deep learning-aided CS (TSDL-CS) framework exploiting the double-structured sparsity of STAR-RIS mmWave channels. The first stage detects the common channel support structure shared across all users, while the second stage refines user-specific features on a reduced sub-matrix. Both stages incorporate specialized attention mechanisms and are followed by a small-scale LS refinement to accurately estimate channels with significantly reduced pilot overhead. To maintain performance in dynamic environments, we introduce hierarchical online adaptive learning enabling real-time updates via lightweight fine-tuning, retraining, and transfer learning. We validate our approach using both statistical channel models and realistic ray-tracing simulations via NVIDIA Sionna across urban, indoor, and suburban scenarios with hardware impairments. Simulation results demonstrate that TSDL-CS significantly outperforms conventional and deep learning baselines, achieving reliable performance with NMSE below -7 dB while reducing the required pilot overhead by 75% compared to conventional LS.

This paper investigates the covertness and reliability of unmanned aerial vehicle (UAV) relay communications assisted by a friendly jammer in the presence of co-channel interference (CCI). In the considered scenario, communication from a source to a destination is relayed by a UAV, while a malicious node attempts to monitor the transmission behaviors of both the source and the UAV. To enhance the system’s covertness, we propose a friendly jammer aided covert communication (FJACC) scheme, which leverages a friendly jammer to degrade the detection capability of the malicious monitoring node by sending artificial noise. We analyze the covertness and reliability of the FJACC scheme by deriving closed-form expressions of the outage probability and detection error probability under CCI, considering that the wireless channels are characterized by Nakagami-mfading for UAV-to-ground links and Rayleigh fading for terrestrial links. Numerical results confirm the superior covertness of the proposed FJACC scheme over traditional non-friendly jammer aided transmission scheme. Furthermore, increasing the number of co-channel interferers or their transmit power exhibits a dual effect: it enhances covertness at the expense of reliability.

This letter investigates blocked job offloading in a satellite-ground integrated network, which allows each low-earth orbit (LEO) satellite to execute more jobs on-board instead of sending the raw data to ground users. We propose an approximation offloading model to approximate the satellite-ground integrated network and analyze the average execution time, which is defined as when the user receives the raw data or processed result from the LEO satellite. Furthermore, we propose the maximum equivalent arrival rate and blocked job offloading algorithms. Numerical results validate the approximation model’s effectiveness and the proposed algorithms’ performance advantages and indicate a way to select the proper algorithm.

This paper studies the physical-layer security (PLS) for an unmanned aerial vehicle (UAV)-aided non-orthogonal multiple access (NOMA) network, where transmissions from a base station (BS) to a trusted near user (TNU) and an untrusted far user (UFU) are assisted by an UAV relay in the face of multiple non-colluding external eavesdroppers (Es). The traditional strategies such as artificial noise-aided friendly jammer (TAN-FJ), artificial noise with non-friendly jammer (TAN-NFJ), and reconfigurable intelligent surface (RIS)-aided schemes fail to simultaneously guarantee the secrecy for both TNU and UFU, thus how to guarantee the PLS for NOMA users becomes an urgent issue to be solved. In this context, we propose an artificial noise (PAN) scheme aided with digital network coding (DNC). Specifically, the artificial noise signals generated by UAV are resorted to encrypt confidential signals with the assistance of DNC, which realizes one-time pad, thus the PLS of TNU and UFU can be ensured. We resort security-reliability tradeoff (SRT) as a metric to evaluate the PLS for our proposed PAN scheme. Hence, the exact and asymptotic expressions of outage probability (OP) and intercept probability (IP) are derived to quantify the reliability and security. Moreover, numerical simulations validate the correctness of our theoretical results and reveal that: 1) the PAN scheme enhances the SRT performance of near user compared to the TAN-FJ, TAN-NFJ, and RIS-aided schemes, while far user by the PAN scheme achieves almost the same SRT performance as the TAN-FJ scheme and significantly better performance than the TAN-NFJ and RIS-aided schemes, which indicates that the PAN scheme realizes the fairness of secrecy transmissions for NOMA users; 2) the SRT performance of TNU benefits from multiple antennas at BS and UAV, which has ignorable effect on the SRT performance of UFU; 3) the antenna number at UAV has more obvious impact on the SRT of TUN than the antenna number at BS.

As a novel category of beyond-diagonal (BD)-reconfigurable intelligent surfaces (RISs), the fully-connected (FC)-RIS represents an unprecedented advancement in RIS technology for wireless networks. To unlock the full potential of FC-RISs for physical-layer surveillance, this paper investigates a wireless surveillance system assisted by an aerial FC-RIS. Specifically, a legitimate monitoring station exploits an unmanned aerial vehicle (UAV)-borne FC-RIS to enhance the channels related to the monitoring station for facilitating its monitoring of a signal transmitted from a suspicious source. Meanwhile, this signal is decoded at the suspicious destination. Capitalizing on the unparalleled configuration flexibility of FC-RISs, we reduce the implementation complexity associated with jointly optimizing both antenna selection and RIS configuration. In particular, we opportunistically select a antenna at the legitimate multi-antenna monitoring station for signal reception without requiring iterative RIS optimization. Accordingly, several schemes are proposed, each differentiated by their specific selection criteria: 1) round-robin antenna selection and FC-RIS-aided (RAS-FR), 2) antenna selection combined with known FC-RIS reflecting channels (ASC-FRRC), 3) threshold-based antenna selection with known FC-RIS reflecting channels (TAS-FRRC). Considering that successful monitoring can be achieved when the monitoring channel conditions surpass those of the suspicious channels, we derive closed-form expressions for monitoring success probabilities (MSPs) of the RAS-FR, ASC-FRRC, and TAS-FRRC schemes, respectively. Furthermore, when part of small-scale channel state information (CSI) is unavailable due to the inherent non-cooperative nature of suspicious party, the surveillance performance can be further improved by optimizing the UAV-borne FC-RIS location. Numerical results not only validate our closed-form MSP analysis, but also verify that the considered UAV-borne FC-RIS aided surveillance system outperforms the conventional diagonal-RIS or terrestrial-RIS-assisted surveillance systems in terms of MSP. Moreover, compared with non-channel-aware schemes or antenna selection with CSI of all sub-links in cascaded links, the proposed AS-FRRC framework dramatically reduces the computational complexity for selection and RIS optimization without introducing performance loss. Additionally, the TAS-FRRC scheme can achieve a more favourable performance-complexity tradeoff than the RAS-FR and ASC-FRRC schemes.

In this paper, we investigate the downlink non-orthogonal multiple access (NOMA) secure transmission systems, where a base station (BS) uses superposition coding to transmit information to users in the presence of multiple colluding eavesdroppers. To improve the information security, we propose a movable antenna (MA) and active beamforming (AB) assisted NOMA (MAABaN) secure transmission scheme, in which the BS is equipped with multiple MAs to enhance the physical layer security (PLS). To this end, we formulate a sum secrecy rate maximization optimization problem by jointly optimizing the AB vectors and the MA positions. To solve the complex non-convex problem, we employ the alternating optimization method to transform the original problem into two subproblems. Regarding the subproblem of MA positions, we analyze the coupling relationship between the position variables, and optimize one set of position variables while keeping others fixed. Then, we propose a low-complexity iterative algorithm for the MA positions based on multi-start gradient ascent (MSGA), and construct a joint optimization framework of MA and AB. Numerical results demonstrate that the proposed MAABaN scheme can effectively enhance PLS performance, and the proposed MSGA-based algorithm can also be applied to MA-assisted NOMA secure transmission systems.

This article studies the secrecy performance for a dual-hop nonorthogonal multiple access (NOMA)-aided unmanned aerial vehicle (UAV) network in the face of a passive untrusted far user (UFU). A new artificial noise (AN) scheme is proposed, where AN generated in the first hop can be used to encrypt confidential signals by XOR operation in the second hop. Based on the proposed AN (PAN) scheme, we analyze the exact and asymptotic outage probabilities (OPs) for both NOMA users and intercept probability (IP) for trusted near user (TNU). Simulation results verify the correctness of our theoretical analysis and demonstrate that the PAN scheme significantly improves the security for TNU at the cost of negligible reliability of UFU compared to the benchmark schemes.

This article investigates the physical-layer security (PLS) of a satellite-terrestrial integrated network (STIN) consisting of a satellite (S), a destination (D), and several friendly jammers in the presence of an eavesdropper (E). For improving PLS of the STIN, we propose three friendly jammer assisted communication schemes to help S securely send confidential information to D, i.e., the random jammer selection assisted communication (RJSC) scheme, the optimal jammer selection assisted communication (OJSC) scheme, and the multiple jammers assisted communication (MJC) scheme. For performance comparison, the nonjammer assisted communication (NJC) scheme is also presented. We analyze secrecy performance of the NJC, RJSC, OJSC, and MJC schemes by carrying out their closed-form secrecy outage probability (SOP) expressions, respectively. Numerical results unveil that the RJSC, OJSC, and MJC schemes exceed the NJC scheme with regard to their SOPs (e.g., as shown inFig. 2, when the signal-to-noise ratio is 25 dB, SOP of the STIN drops from 70% to 3%), and the OJSC scheme can get the best secrecy performance, showing the secrecy benefits of adopting friendly jammers assisted communication schemes against eavesdropping. Additionally, by increasing the number of friendly jammers, the secrecy performance of the OJSC and MJC schemes can be significantly enhanced, while that of the RJSC scheme remains unchanged.

This article investigates an uncrewed aerial vehicle (UAV) relay network in the urban Internet of Things (IoT) environment designed to facilitate communications for ground terminals (GTs) by transmitting their data to a remote base station (BS) using the decode and forward (DF) protocol, where the orthogonal frequency division multiple access (OFDMA) is adopted for the GTs’ transmissions. To address the challenges posed by building obstructions on radio propagation, we incorporate the multiple reconfigurable intelligent surfaces (RISs) to improve the quality of the GTs-UAV and UAV-BS links. The primary objective is to maximize the sum rate of all the GTs through the joint optimization of UAV trajectory, GTs transmit power, subchannel allocation, and multi-RIS phase shifts. To this end, we reformulate the optimization problem as a Markov decision process (MDP) and propose a deep reinforcement learning (DRL) approach for addressing our formulated problem, called the soft actor critic-based subchannel allocation and phase shift optimization with trajectory design (SAC-CAPSTD) algorithm. By continuously interacting with the environment, the proposed system refines its policy to determine the optimal UAV flight trajectory and the subchannel allocation strategy for GTs within their transmit power constraints. Concurrently, a discrete phase shift optimization method is implemented to adjust the phase shift for each RIS element. Finally, numerical results confirm that the proposed SAC-CATDPS algorithm can significantly achieve higher sum rate of all the served GTs and exhibit better convergence performance compared with the benchmark DRL-based algorithms.

This article explores the energy efficiency (EE) of a mobile-edge computing (MEC) architecture for Internet of Things (IoT) networks, in which multiple IoT devices (IoTDs) perform local computations while further offloading part of their tasks to the base station (BS)-enabled MEC server utilizing the time division multiple access (TDMA) protocol. To address the radio propagation issues caused by obstructions, we utilize an uncrewed aerial vehicle (UAV)-mounted active reconfigurable intelligent surface (RIS), referred to as a flying-RIS (FRIS), to reflect and even amplify the incident signal from IoTDs to the BS. Since tasks can be executed in parallel on both IoTDs and the BS, the assignment of service timeslots, the UAV trajectory, and the resource allocation for both IoTDs and FRIS should be jointly optimized to maximize system EE. To this end, we reformulate the optimization problem as a markov decision process (MDP) and introduce a deep reinforcement learning (DRL) approach for addressing the formulated problem, called the proximal policy optimization (PPO) based resource allocation with trajectory design and FRIS reflection matrix optimization (PPO-RATDFRO) algorithm. By continuously interacting within the constructed environment, the proposed system iteratively refines its policy to determine the FRIS trajectory and reflection matrix, along with the service timeslots and computation resources for each IoTD. Finally, simulation results demonstrate that the proposed PPO-RATDFRO algorithm significantly enhances EE for all served IoTDs, compared to various benchmark algorithms.

This paper studies the optimization of reconfigurable intelligent surface (RIS)-assisted unmanned aerial vehicle (UAV)-enabled Internet of Things (IoT) communication networks. In this framework, a rotary-wing UAV is dispatched to collect data form multiple ground IoT devices, leveraging the signal enhancement capabilities of an RIS to improve communication efficiency. We aim to enhance the overall energy efficiency (EE), taking into account both the system transmission rate and UAV propulsion power consumption, while satisfying the UAV initial/final position constraints, transmit power limits, and RIS unit-modules phase shift requirements. To achieve this goal, we first establish a closed-form analytical model for the EE of RIS-assisted UAV-enabled IoT wireless systems. Based on this model, we subsequently formulate the EE maximization (EEM) problem by jointly optimizing time allocation, transmit power control, RIS reflection coefficients, and UAV trajectory. Due to the non-convex nature of the proposed EEM problem, we decompose it into three subproblems: 1) RIS phase shift optimization with given resource allocation and UAV trajectory, 2) time and power allocation under fixed UAV trajectory and RIS configuration, and 3) UAV trajectory design based on provided resource allocation and RIS parameters. Particularly, the optimal phase shift solution is derived in a closed form to ensure phase alignment of signals from multiple transmission paths, while the non-convex resource allocation subproblem is solved through introducing relaxation variables. For UAV trajectory design, a hybrid optimization strategy combing Dinkelbach’s method and successive convex approximation approach is adopted to transform the non-convex EE optimization problem into a convex one. Numerical simulations demonstrate that the proposed EEM scheme obtains superior EE performance compared to benchmarks with fixed trajectory or without RIS deployment.

Edge computing is near end users and provides them with fast computing services. Compared to the cloud, edge provides services with low communication delays but can only service limited users due to constrained storage and computing resources. Thus, allocating more computing resources to some jobs will block the execution of others, and the edge sends them either to the cloud or back to the users. This article focuses on minimizing the average system time by exploring blocked job scheduling (BJS) and redundant computing resource allocation (RCRA) in the cloud–edge–user system. Since edge nodes often operate in highly unpredictable environments, we adopt the replication redundancy to use the resources to shorten the job execution time. First, we propose an approximate model to evaluate the average system time theoretically. Second, we analyze the optimal scheduling for the blocked jobs and the optimal resource allocation for the redundant computing resources. Finally, we propose an algorithm combining BJS and RCRA. Simulation results show that the proposed model approximates the cloud–edge–user system well, and the combined algorithm significantly outperforms the other algorithms under different service time distributions.

In this paper, we consider an integrated sensing, communications and computation (ISCC) energy-constrained system, where multiple users with limited energy budgets execute local computing and concurrently offload data to a dual-functional radar communication base station (DFRC-BS), while the DFRC-BS conducts target detection through radar sensing at the same time. We propose an active reconfigurable intelligent surface (ARIS)-assisted ISCC scheme, denoted by ARIS-ISCC, in which the ARIS is employed to facilitate the data offloading from the users to DFRC-BS. To enhance the data collection capability across radar sensing, communication offloading and local computation of the proposed ARIS-ISCC scheme, a weighted total computation bits (WTCB) maximization problem is formulated constrained by the users’ energy limits, power budgets constraints for the DFRC-BS and ARIS, and temporal restrictions. To tackle the high-coupling and non-convexity of the problem, we initially utilize the fractional programming (FP) to reframe the original objective function. Then, we employ the alternating optimization (AO) algorithm to decompose the reformulated problem into distinct sub-problems, which enables us to iteratively optimize one set of variables while keeping others fixed. We derive the closed-form solutions of each sub-problem by developing the Lagrange dual method and linear programming. Numerical results validate the advantages of the proposed ARIS-ISCC scheme compared to the conventional benchmarks regarding to the WTCB.

This paper focuses on a reconfigurable intelligent surface (RIS) enhanced wireless powered mobile edge computing network. With the assistance of an RIS, multiple passive devices (PDs) first capture energy from the radiated signals of an energy station (ES), and then use a portion of the captured energy for uplink task offloading following a hybrid time- and frequency- division multiple access (HTFDMA) protocol and another portion for local task computing. Considering that the ES and the PDs are not affiliated with the same service suppliers, we propose a partial offloading (PO) scheme that formulates an energy trading framework with price incentives through a Stackelberg game to replenish energy for PO. Specifically, the PDs as a leader receive the benefits of total computation bits with the charging payments as losses by adjusting the charging price, bandwidth and time allocation, task offloading phase shifts, PDs’ transmit power, and central processing unit frequency. Meanwhile, the ES as a follower receives the benefits of charging payments with the charging costs as losses by adapting the ES’s transmit power, power transfer phase shifts, and ES’s beamforming. Following backward induction method, the follower-level decision problem is first handled via an alternating optimization algorithm with majorization minimization. Then, based on the obtained follower-level solutions, the leader-level decision problem is tackled by introducing auxiliary variables and applying Lagrangian dual and block coordinate descent algorithm. Simulations exhibit that invoking RIS results in utility gains for both the PDs and the ES, especially when there are more RIS elements, ES antennas, and PDs.

For an uncrewed aerial vehicle (UAV)-aided energy-harvesting non-orthogonal multiple access (EH-NOMA) network, where an untrusted far user (UFU) overhears the confidential transmissions from base station (BS) to the trusted near user (TNU), how to ensure the security of TNU and achieve the reliability of UFU becomes an open issue. The traditional artificial noise (TAN) scheme just guarantees the security of TNU without focusing on the reliability of UFU, while the non-artificial noise (NAN) scheme only ensures the reliability of UFU but sacrifices the security of TNU. Thus, the TAN and NAN schemes are unable to guarantee the fairness between TNU and UFU. To this end, we employ digital network coding to encrypt confidential signals with artificial noise signals, and the proposed artificial noise (PAN) scheme not only protects the confidential transmissions of TNU but also guarantees the reliability of UFU. We use security-reliability tradeoff (SRT) and interference-reliability tradeoff (IRT) to evaluate the performance of TNU and UFU, respectively. Additionally, we derive the exact and asymptotic expressions of outage probabilities for both TNU and UFU, and the intercept probability for TNU. Numerical results verify the accuracy of our theoretical results and demonstrate that: 1) the SRT and IRT performance of the PAN scheme is better than that of the TAN and NAN schemes; 2) the PAN scheme achieves better secrecy for TNU compared to the TAN scheme. Meanwhile, the reliability of UFU in the PAN scheme is almost the same as the NAN scheme, indicating the PAN scheme simultaneously realizes the security of TNU and the reliability of UFU and the fairness of NOMA users is achieved by the PAN scheme.

In this article, we explore the application of hybrid automatic repeat request (HARQ) within the intelligent reflection surface-assisted nonorthogonal multiple access (IRS-NOMA) system. We investigate the closed-form expressions for the outage probability of multiple users in the HARQ-assisted IRS-NOMA system, considering scenarios with perfect successive interference cancellation (pSIC) and imperfect successive interference cancellation (ipSIC), respectively. A definite integral approximation method for multidimensional functions based on Gauss-Chebyshev quadrature (GCQ) is proposed to conduct the performance analysis of the proposed HARQ-assisted IRS-NOMA system. Based on the asymptotic outage probability, the diversity order of multiple users for HARQ-assisted IRS-NOMA is obtained. Under the analytical results, the diversity order of the mth$(m\gt 1)$user for the HARQ-assisted IRS-NOMA with ipSIC is zero, and that of the mth user for the HARQ-assisted IRS-NOMA with pSIC is in connection with the number of reflecting elements and the number of transmission rounds. The simulation results are presented to substantiate the accuracy of the analytical results. The results demonstrate that: 1) the HARQ-assisted IRS-NOMA systems can achieve a significant gain compared with the IRS-NOMA systems; 2) the HARQ-assisted IRS-NOMA outperforms the HARQ-assisted IRS-orthogonal multiple access in terms of outage probability and diversity order; and 3) the HARQ with incremental redundancy (HARQ-IR)-assisted IRS-NOMA system has a better performance than the HARQ with chase combining (HARQ-CC)-assisted IRS-NOMA system.

Intelligent reflecting surface (IRS)-assisted communication systems rely heavily on the precise estimation of channel state information (CSI) for their effective operation. However, obtaining the accurate CSI is challenging due to the substantial pilot overhead required in such systems. In this article, we propose a spatial attention (SA)-based method to estimate the cascaded downlink channel in an IRS-assisted millimeter-wave (mmWave) multiuser multiple input single output (MU-MISO) system that operates with the frequency division duplex (FDD) protocol. In order to estimate the channel information, we formulate it as a nonlinear prediction problem and propose a residual network-based channel estimation (RN-CE) framework that reduces the pilot overhead. To implement the RN-CE, we utilize the values of the least square (LS) estimators as coarse estimates for neural network training. Additionally, we develop a spatial attention-based RN-CE (SA-RN-CE) that integrates a lightweight SA module capable of extracting internal connections among spatial features. Simulation results demonstrate that both RN-CE and SA-RN-CE outperform the LS algorithm, minimum mean squared error (MMSE) algorithm, and convolutional neural network (CNN)-based ChannelNet algorithm in terms of normalized mean square error (NMSE). It has been found that by balancing the NMSE and pilot overhead, the length of the pilot signals used for training the networks in RN-CE and SA-RN-CE can be compressed to 1/8 of its original length, while still maintaining a minimal impact on NMSE, which is particularly advantageous in cascaded channels where pilot training overhead is typically extensive.

In this article, we analyze the security and reliability tradeoff (SRT) performance of a unmanned aerial vehicle (UAV) relays assisted cognitive network consisting of a cognitive source (S), multiple cognitive UAV relays (Rs), a cognitive destination (D) in the face of an eavesdropper (E). We consider a practical scenario where mutual interference exists between the primary users and cognitive users. In order to protect the wireless transmission against the eavesdropping attack, we propose two UAV relay selection schemes, namely, noninterference aware relay selection (NIARS) scheme and interference aware relay selection (IARS) scheme. Specifically, in the NIARS scheme, a relay having the best instantaneous (R-D) link (spanning from Rs to D) is selected to forward message. By contrast, a relay maximizing the main channel capacity is selected, which relies not only on the channel state informations (CSIs) of main links but also on the CSIs of the interference links. For the purpose of comparison, we present the round robin (RR) scheme as a baseline. To evaluate the SRT performance, we derive the closed-form intercept probability (IP) and outage probability (OP) expressions for RR, NIARS, and IARS schemes under the impact of hardware impairments (HIs) over Nakagami-$m$fading channels. Numerical results show that the proposed IARS and NIARS schemes perform better than RR scheme in terms of SRT performance. Additionally, the HI at D has a negative impact on SRT performance, while HI at E has a positive impact on SRT performance.

This article investigates physical-layer security (PLS) for an energy-harvesting cognitive network with nonorthogonal multiple access (EHCN-NOMA), where a cognitive base station (CBS) communicates with two cognitive users named cognitive near user (CNU) and cognitive far user (CFU) by adopting the NOMA principle under an active or passive eavesdropper (Eve) attack. By means of energy harvesters, the CBS collects the radio frequency energy from the primary source (PS) by a time-switching protocol; meanwhile, the cognitive transmit power is limited by the interference threshold of primary destination (PD) to guarantee the Quality of Service (QoS) of primary transmissions. To evaluate the impact of active and/or passive eavesdroppings on system performance, we derive closed-form expressions in terms of outage probabilities (OPs), intercept probabilities (IPs), as well as effective secrecy throughputs (ESTs). The numerical results verify the correctness of our theoretical derivations and confirm that there exists a tradeoff between security and reliability for the EHCN-NOMA transmissions. Moreover, the EST improvement depends on the competition between security and reliability. Furthermore, the maximum EST performance can be achieved for both eavesdropping scenarios by adjusting the time allocation between that of the energy transfer phase and the information transmission phase.

We explore a satellite internet of things (SIoT) system, wherein multiple IoT users are allowed to access the SIoT in a grant-free manner. Such a manner may incur the severe co-frequency interference (CFI), and decease the user capacity in terms of the number IoT users successfully accessing the SIoT. To this end, we first design two beam preprocessing oriented user grouping methods, called user distribution based user grouping (UDUG) and user position based user grouping (UPUG), respectively. With respect to the UDUG, we propose the regional statistical channel state information based multi-beam processing (RsCSI-MBP). For the UPUG, we conceive the user statistical channel state information with position error based MBP (UsCSIPe-MBP). Both the RsCSI-MBP and the UsCSIPe-MBP schemes can mitigate inter-beam CFI relying on MBP, wherein the optimized beamforming vectors are obtained by using generalized Rayleigh quotient. Furthermore, the user capacity, preamble collision probability and packet loss probability are analyzed for the RsCSI-MBP and UsCSIPe-MBP schemes. Numerical results demonstrate that the UsCSIPe-MBP scheme outperforms the RsCSI-MBP and the conventional seven-color frequency-reuse multibeam schemes in terms of a higher user capacity and a lower packet loss probability, even considering the satellite mobility, phase and amplitude inconsistence of the antenna array used.

This paper investigates the physical layer security for a distributed antenna system, where a base station (BS) equipped with multiple distributed antennas transmits signals to a legitimate user (U) through an opportunistic antenna. Meanwhile, multiple non-colluding eavesdroppers (Es) attempt to tap the information transmission between the BS and U. To enhance the physical layer security, a friendly jammer (J) is employed to create a potential interference to Es by emitting artificial noise. However, since U is not aware of the information sent by J, it can be affected by the interference. In this context, this paper proposes adaptive jamming (AJ) scheme and interference cancellation (IC) scheme to mitigate the interference’s impact on U and consequently improve secrecy performance. Specifically, the AJ scheme avoids interference from having a negative effect on the system security by setting an interference threshold, activating the jammer only when the interference to U is below the threshold value. By contrast, the IC scheme designs an advanced special signal to eliminate the interference from J to U. For comparison, we present conventional non-jammer (NJ) scheme as a benchmark. Exact and asymptotic secrecy outage probability expressions for NJ, AJ, and IC schemes are derived over Nakagami-mfading channels. Numerical results show that the proposed AJ and IC schemes perform better than NJ scheme in terms of secrecy outage probability. Additionally, the physical layer security of all schemes can be improved by increasing the number of distributed antennas.

This paper investigates a reconfigurable intelligent surface equipped unmanned aerial vehicle (RISeUAV)-enabled wireless powered communication network (WPCN) consisting of a ground hybrid access point (GHAP), a RISeUAV, and a ground wireless device (GWD). Specifically, the passive GWD first harvests energy through the downlink (DL) wireless power transfer (WPT) assisted by the RISeUAV, and then wireless information transfer (WIT) takes place in the RISeUAV-aided uplink (UL) communications. Due to the partial reciprocity of wireless channels, the unavailable DL channel state information (CSI) can be estimated by an outdated version of the UL CSI. Considering the negative effect of this outdated CSI and a non-linear energy-harvester (NLEH), we formulate a two-timescale active and passive beamforming (TTAPB) framework for maximizing the ergodic throughput (ET) and derive a closed-form expression of the ET, based on which the joint optimization of time allocation for WPT/WIT and UAV placement with two-timescale active and passive beamforming (JOTAUP-TTAPB) scheme is proposed. As for the solution of the JOTAUP-TTAPB scheme, although the objective function is non-convex with respect to the hovering altitude of the RISeUAV, it can be transformed into an equivalent exponential function of fractional forms that facilitates the utilization of a golden-section-search-aided Dinkelbach (GSS-D) algorithm to obtain a jointly optimal solution. Simulation results demonstrate the effectiveness of the TTAPB framework by validating our ET analysis. Furthermore, the JOTAUP-TTAPB scheme outperforms several benchmark schemes in spite of performance degradations of all schemes due to imperfect channel reciprocity, exhibiting remarkable robustness against outdated DL CSI. Moreover, the proposed JOTAUP-TTAPB scheme improves the performance bounds limited by NLEHs. Additionally, not only the GSS-D algorithm for the JOTAUP-TTAPB scheme shows a faster convergence than the golden-section-search-aided alternating optimization, but also the low-complexity joint optimization avoids optimality loss by exploiting semi-closed-form solutions.

This paper focuses on a reconfigurable intelligent surface (RIS) assisted multi-cluster wireless powered communication network (WPCN) in the presence of hardware impairments (HIs). With the aid of an RIS, passive devices (PDs) grouped into multiple clusters first jointly collect the energy radiated by an energy source (ES), and then transmit useful information to an access point through a hybrid time- and frequency- division multiple access (HTF) scheme. Specifically, the PDs from different clusters occupy different transmission time to avoid inter-cluster interference, while the PDs within the same cluster occupy different bandwidth to avoid intra-cluster interference. Due to non-ideal hardware, the impact of the phase shift (PS) error at the RIS and the HIs at the transceivers are revealed. Then, the closed-form expression for average system sum rate (ASSR) is derived. Assuming that the ES and the PDs are deployed by different service providers, a Stackelberg game (SG) is utilized to model energy trading based on price incentive. In the established SG, the PDs act as a leader to maximize the difference between the ASSR benefit and energy payment by optimizing energy price, RIS PSs in the information tansfer stage, as well as time and bandwidth allocation. Meanwhile, the ES acts as a follower to maximize the difference between energy payment and energy cost by optimizing transmit power, energy beamforming, and RIS PSs in the energy transfer stage. The follower and leader level problems are solved through algorithms such as majorization-minimization, alternating optimization, Lagrange dual approach, Karush-Kuhn-Tucker condition, block coordinate descent, and one-dimensional search. Simulation results verify that compared to the benchmark schemes, the proposed HTF scheme achieves higher PDs’ utility value and lower ES’s energy consumption, and strikes a balance between utility and overhead.

This paper integrates a reconfigurable intelligent surface (RIS) and an unmanned aerial vehicle (UAV) into a wireless powered communication network. With the assistance of the RIS, multiple passive Internet of Things devices (IoTDs) gather energies from the signals of an energy station (ES) to power them to communicate with the UAV in time division multiple access mode. Given that the ES and the IoTDs are deployed by different service providers, an energy trading mechanism with price incentive is established through hierarchical Stackelberg game. By paying for the wireless charging service provided by the ES, the IoTDs aim to maximize the difference between achievable rate benefit and monetary payment, while the ES aims to maximize the difference between monetary payment and energy cost. To evaluate the impact of fairness, we propose sum-rate maximization (SRM) scheme and minimum-rate maximization (MRM) scheme, which take the sum-rate and minimum-rate of the IoTDs as achievable rate benefits, respectively. Specifically, in the follower game problem, an alternating optimization algorithm with majorization-minimization is utilized to alternately optimize energy beamforming and energy phase shifts (PSs) before optimal ES transmit power is achieved in closed form. In the leader game problem, a block coordinate descent algorithm with successive convex approximation is utilized to alternately optimize energy price, time allocation, and UAV trajectory after information PSs and transmit power of the IoTDs are achieved in closed form. Numerical results show that the combination of RIS and UAV significantly enhances the IoTD utility. Compared with the MRM scheme, the SRM scheme obtains higher IoTD utility at the cost of rate fairness and energy consumption.

We investigate the energy efficiency (EE) of cellular downlink transmissions with limited feedback, where the channel state information (CSI) assumed to be available at the destination is quantized and fed back to the multiantenna base station for transmitting precoding purposes. Typically, the employment of antennas with poor channel quality marginally enhances system performance but substantially consumes energy resources such as circuit power. Thus, we propose an adaptive transmit antenna selection (ATAS) scheme via setting a channel gain threshold to select transmit antennas with relatively good channel condition for reducing power resource consumption. Meanwhile, the conventional single transmit antenna (STAS) scheme is considered as a baseline. In order to improve EE while considering the impact of feedback overhead, we conduct the effective EE analysis for both ATAS and STAS schemes and show that a maximal effective EE of the proposed ATAS scheme can be achieved by jointly adjusting the channel gain threshold and transmit power. As a consequence, we propose the ergodic effective EE optimization of ATAS (eEEEO-ATAS) scheme to maximize the effective EE. Numerical simulation results demonstrate that the proposed ATAS scheme performs better than the STAS scheme in terms of effective EE.

Unmanned aerial vehicle (UAV)-enabled air-ground integrated communication systems are vulnerable to jamming attack, mainly due to the high probability with line-of-sight wireless channel conditions. Although a few UAV anti-jamming transmission schemes have been recently proposed, the fundamental performance limits by simultaneously considering the UAV and jammer’s mobility have not yet been reported. These observations motivate us to formulate an interactive reward region (IRR) characterization problem for the scenario that the UAV and the jammer compete with each other to maximize their respective rewards via power and mobility control. Then, we propose a transmit and jamming power optimization algorithm to characterize the IRR with power control, by leveraging successive convex approximation technique, where two transmit power design schemes are given in closed-form expressions from a game-theoretic perspective. Furthermore, the IRR with joint power and mobility control is characterized based on alternating optimization, in which both the UAV and jammer can simultaneously adjust their power and trajectories. Finally, extensive simulation results reveal that joint transmit power and trajectory optimization can enlarge the IRR, and increasing the UAV mobility and flight time length improves the UAV reward.

This paper integrates an intelligent reflecting surface (IRS) into a wireless powered Internet-of-Things (IoT) network, where IoT devices need to harvest energy from an energy station (ES) before transmitting their monitoring data to an access point. An IRS is invoked to improve energy and spectral efficiency by changing propagation environment. Considering that the ES and IoT devices come from different operators, IoT devices need to provide monetary payment in exchange for the ES’s charging before implementing nonlinear energy harvesting. We build this energy interaction via Stackelberg game under three multiple access schemes, i.e., IRS-assisted time division multiple access (IRS-TDMA), IRS-assisted non-orthogonal multiple access (IRS-NOMA), and IRS-assisted frequency division multiple access (IRS-FDMA). To solve the common follower game among the three schemes, we first employ an alternating optimization (AO) algorithm with Majorization-Minimization (MM) to alternately optimize the energy beamforming and energy phase shifts, and then derive the optimal ES transmit power. For the leader game of IRS-TDMA, the optimal time allocation and optimal information phase shifts are first derived in closed form through Lagrange dual method and triangular inequality, respectively. On this basis, an AO algorithm is developed to optimize energy price and energy transfer time alternately. Similar procedures are also used to solve the leader games for IRS-NOMA and IRS-FDMA. Simulation results show that IRS-NOMA and IRS-FDMA achieve the same utilities for the ES and IoT devices. Due to IRS time selectivity, IRS-TDMA is more energy and spectral efficient than IRS-NOMA and IRS-FDMA, and this advantage is more pronounced in higher numbers of IRS elements and IoT devices.

In this paper, we explore a satellite based spectrum-sensing system, where the spectrum -sensing data are used to assist spectrum sharing. However, these data may be outdated due to the long propagation delay of the satellite links. Such outdated data may incur wrong spectrum-sharing decisions, resulting in co-frequency interference. To avoid this negative effect caused by the long delay, we propose a joint long short-term memory and autoregressive moving average (LSTM-ARMA) aided spectrum-prediction scheme, where a LSTM-ARMA model is constructed and trained relying on a specially designed loss function, which can decrease prediction error by combining the LSTM and the ARMA. Furthermore, using the historical spectrum-sensing data, the well-trained LSTM-ARMA is used to predict the future spectrum occupancy in advance. The prediction performance of the proposed LSTM-ARMA is evaluated relying on an actually measured dataset captured from the Tiantong-1 satellite. Performance evaluations show that the proposed LSTM-ARMA scheme outperforms the conventional LSTM, the ARMA and the convolutional neural network and bidirectional long short-term memory schemes in terms of a lower mean absolute error (MAE). Moreover, the proposed LSTM-ARMA can simultaneously predict the spectrum situation of multiple transponders, whilst maintaining a low MAE.

In this paper, we consider a single-input single-output system consisting of a source, a destination, an energy harvesting (EH) decode-and-forward (DF) relay and two separate reconfigurable intelligent surfaces (RISs). A power-splitting ratio is employed at the EH DF relay to represent a tradeoff between the harvested energy partially for decoding the source signal received at the relay and the remaining energy for retransmitting the decode signal. By using Jensen Inequality, we derive a closed-form upper-bound expression of the ergodic capacity for the proposed RIS-aided EH relay transmission scheme with the optimal power-splitting ratio, which is shown to closely match Monte-Carlo simulations. We analyse the impact of the RIS and relay locations on the ergodic capacity performance. Additionally, our simulations show that the proposed transmission scheme achieves a better capacity than that without relay or just having a single RIS deployed between the source and relay.

Abstract In this paper, the secrecy performance in a MIMO cognitive radio (CR) relay network with energy harvesting (EH) and transmit antenna selection/maximal ratio combining (TAS/MRC) is invstigated, where the DF relaying protocol and multiple colluding passive eavesdroppers are considered. To improve the security of wireless transmission, two antenna selection schemes are proposed, namely, the optimal transmit antenna selection (OTAS) scheme and suboptimal transmit antenna selection (STAS) scheme. For the purpose of comparison, the space‐time transmission (STT) scheme is introduced as a baseline. The exact and asymptotic closed‐form secrecy outage probability (SOP) expressions for OTAS, STAS and STT schemes are derived over Rayleigh fading channels. An extension of the TAS framework to an artificial noise (AN) aided MIMO network is further presented and an AN aided transmit antenna selection (AN‐TAS) scheme is proposed, in which the unselected antennas at R are used to emit AN for interfering with Es. Numerical results show that the OTAS and STAS schemes perform better than STT scheme in terms of SOP. Meanwhile, the SOP of AN‐TAS scheme is much smaller than that of OTAS, STAS and STT schemes in the high SNR region, indicating the benefit of applying AN in MIMO network.

Timely status updates are significant for some critical Internet of Things applications. However, due to the transmission delay of status packets, the status information arriving at the destination is not fresh enough. Especially, when the transmission delay is large, the destination cannot know the current status of the target well. In this article, the prediction technology is introduced into status update systems to solve this problem. First, we study a prediction-based status update scheme for machine-type communications (MTCs) and use the recently proposed metric, Age of Information (AoI), to characterize the status update performance. With the scarce spectrum resources, an MTC device (MTCD), as an unlicensed user, adopts the overlay mode to reuse the spectrum resource of the primary user and sends status packets to the central controller (CC). Then, we consider prediction errors and packet decoding errors to characterize the reliability of MTC, analyze the status update process in the prediction-based scheme, and derive the closed-form expression of the average AoI. Finally, in the proposed prediction-based scheme, we analyze the tradeoff between the status update performance and energy consumption and design an optimization algorithm to improve the status update performance by adjusting the transmit power and prediction horizon of the MTCD. Simulation results verify the correctness of the theoretical analysis and show that the proposed scheme can help the MTCD effectively improve the status update performance.

This paper investigates the secrecy performance of a spectrum sharing network, where${N}$legitimate source-destination pairs orderly access the shared spectrum for communication, while an eavesdropper (E) attempts to tap the legitimate information transmission. To improve the physical layer security, we propose two jamming strategies that can intelligently switch between jamming and non-jamming, namely, suboptimal jammer selection (SJS) scheme and optimal jammer selection (OJS) scheme. Specifically, when a user pair is assigned to access the shared spectrum, another source is chosen as a friendly jammer in order to create intentional interference at E. For the purpose of comparison, we present the non-jammer selection (NJS) scheme as a benchmark. Analytical closed-form secrecy outage probability expressions of NJS, SJS and OJS schemes are derived over Nakagami-${m}$fading channels. We further present an asymptotic secrecy outage probability analysis to evaluate the secrecy diversity gain performance of NJS, SJS and OJS schemes. Numerical results show that the secrecy outage probability performance of OJS scheme is better than SJS and NJS schemes in the low average channel power gain${\mathop \Omega \nolimits _{D} }$region. Furthermore, the secrecy outage probabilities of NJS as well as SJS and OJS schemes converge to each other with the increase of${\mathop \Omega \nolimits _{D} }$, due to the fact that the OJS and SJS scheme will switch to NJS scheme when${\mathop \Omega \nolimits _{D} }$tends to infinity.

In this paper, we investigate the physical layer security for a cognitive multiuser network which is composed of multiple cognitive sources, a cognitive destination and an eavesdropper under the joint impact of hardware impairments (HIs) and channel estimation errors (CEEs). We consider a practical scenario where mutual interference exists between the primary users and cognitive users. To achieve high physical layer security with low implementation complexity, we propose three pure user scheduling schemes, namely, selection combining (SC) scheme, threshold-based switched diversity (tSD) scheme and switch-and-examine combining with post-selection (SECps) scheme. To further improve physical layer security, we present an extension of our SC framework to a jammer aided multiuser network and propose a jammer aided SC (JSC) scheme. We derive the closed-form intercept probability (IP), outage probability (OP) and effective secrecy throughput (EST) expressions for SC, tSD, SECps and JSC schemes over Nakagami-$m$channels to analyze the system performance. Numerical results show that among the three pure multiuser scheduling schemes, the SC scheme achieves the best secrecy performance with the highest complexity, the SECps scheme obtains the worst secrecy performance with the lowest complexity. In addition, the secrecy performance of JSC scheme is better than that of SC scheme in the high SNR region.

This paper investigates an intelligent reflecting surface (IRS) aided secure wireless powered communication network (WPCN), where a transmitter first harvests energy from a power station (PS), and then uses the collected energy to transmit information to multiple internet of things (IoT) devices in the form of multicast in the presence of multiple eavesdroppers. An IRS is deployed to enhance the efficiency of wireless energy transfer (WET) and secure wireless information transfer (WIT). Considering that the PS and transmitter belong to different service providers, we model this energy interaction through a Stackelberg game and propose an IRS-aided energy trading and secure communication (IRS-ETSC) scheme, in which the transmitter needs to pay an energy price as an incentive for the PS energy service. Specifically, the transmitter as the leader can control the energy price, WET time, two-stage phase shifts, and beamforming vector, while the PS as the follower can adjust the transmit power. To solve the non-convex leader game problem, we propose a two-step approach to decompose the original problem into two subproblems. The first subproblem can be solved independently by an efficient alternating optimization (AO) based algorithm, in which the closed-form optimal beamforming vector and energy phase shifts are alternately optimized. Then, the second subproblem is relaxed by semidefinite relaxation (SDR) and solved by an iterative algorithm based on block coordinate descent (BCD), where the optimal energy price, optimal WET time, and suboptimal information phase shifts can be obtained by golden section method and successive convex approximation (SCA) respectively. Both subproblems can converge to a stationary point. Numerical results show that compared with the traditional non-IRS scheme, the proposed IRS-ETSC scheme achieves utility improvement for both the PS and transmitter.

This paper investigates an intelligent reflecting surface (IRS) aided wireless powered communication network (WPCN) with a focus on multicast transmissions in a game-theoretic way, where a transmitter first harvests energy from a power station (PS) and then transmits the information to multiple Internet of Things (IoT) devices in a multicast form. An IRS is deployed to assist the wireless energy transfer (WET) and wireless information transfer (WIT) processes. Considering that the PS and the transmitter belong to different service providers, we propose two schemes based on Stackelberg game, namely IRS-aided transmitter dominant energy trading (IRS-TDET) with the transmitter as the leader and IRS-aided PS dominant energy trading (IRS-PDET) with the PS as the leader. To solve the non-convex optimization problem of the leader-level game in the IRS-TDET scheme, we first derive the closed-form optimal phase shifts in the WET stage, and then use the semidefinite relaxation approach to solve the max-min problem about the phase shifts of the WIT stage. Finally, a low-complexity alternating optimization algorithm is developed to solve the simplified non-convex optimization problem with regard to the energy price and WET time. Numerical results show that the performance of proposed IRS-TDET and IRS-PDET is superior to that of the corresponding non-IRS aided schemes, and the deployment of IRS can effectively improve the utilities of both the PS and the transmitter. In addition, it is preferred to deploy the IRS near the transmitter in WPCN, and the transmitter can be deployed near the PS or the IoT devices to obtain higher utility.

In this paper, we investigate the physical-layer security in an energy-harvesting (EH) multiuser network with the help of a friendly jammer (J), where multiple eavesdroppers are considered to tap the information transmission from users (Us) to base station (BS). In this system, a power beacon (PB) transmits radio frequency (RF) signals to Us for charging. In order to enhance the security of wireless transmission, we propose non-energy-aware multiuser scheduling (NEAMUS) scheme and energy-aware multiuser scheduling (EAMUS) scheme. For the purpose of comparison, we introduce conventional round robin multiuser scheduling (CRRMUS) scheme. The closed-form outage probability (OP) and intercept probability (IP) expressions of NEAMUS, EAMUS, and CRRMUS schemes are derived over Rayleigh fading channels. Additionally, we analyze the security-reliability tradeoff (SRT) of NEAMUS, EAMUS, and CRRMUS schemes in terms of OP and IP. Numerical results show that the proposed EAMUS scheme is superior to the CRRMUS scheme and NEAMUS scheme in terms of SRT, demonstrating the advantage of the proposed EAMUS scheme in improving the physical-layer security and reliability. Moreover, SRT performance of NEAMUS and EAMUS schemes can also be improved by increasing the number of users.

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

In this paper, we investigate the security and reliability trade-off (SRT) for a wireless sensor network (WSN) which consists of one cluster head (CH), multiple users, and one eavesdropper (E), where the eavesdropper attempts to tap the confidential transmission between multiple users and CH. We first present two multiuser scheduling schemes, namely the selection combining (SC) scheme and the switch-and-examine combining with post-selection (SECps) scheme to enhance physical-layer security (PLS) of the WSN system. For comparison purposes, the round-robin scheduling (RRS) scheme is also considered as a baseline. Then, we analyze the SRT of the RRS, SECps, and SC schemes by deriving their closed-form outage and intercept probability expressions under the joint impact of hardware impairment (HI) and co-channel interference (CCI). And, secrecy diversity analysis of the three schemes is also carried out. Numerical results show that the SC and SECps scheme outperform the RRS scheme in terms of the SRT performance, where the SC scheme can achieve the best SRT performance at the cost of an increased channel estimation complexity. Moreover, a trade-off between the security and reliability can be achieved for each of the RRS, SECps, and SC schemes by adjusting the overall data rate and signal-to-noise ratio (SNR).

Simultaneous wireless information and power transfer (SWIPT) is a promising solution for future wireless networks as it provides convenient and perpetual energy supplies to wireless users. This study proposes three new transceiver design schemes in two‐way relay networks with SWIPT. Firstly, a transmit power minimisation solution is proposed with an aim to minimise the transmit power while meeting an energy harvesting requirement. Then, a harvested energy maximisation solution that has the same complexity as the first solution but provides a much better sum harvested energy is proposed. Finally, a signal‐to‐interference‐plus‐noise ratio maximisation solution is developed, which gives the best sum‐rate performance for the two‐way relay networks while ensuring a desired level of energy harvesting. Furthermore, the complexity of the three new transceiver solutions is discussed. Simulation results have demonstrated the effectiveness of the proposed transceiver design schemes in two‐way relay networks with SWIPT.

In this6study, the channel estimation problem is investigated for a wireless communication system assisted by a reconfigurable intelligent surface (RIS). The RIS thus creates an assistant channel, which has the features of positivity and dominance. Owing to these features, the channel estimation problem is formulated as a constrained residual sum of squares minimisation problem, which differs radically from the traditional channel estimation issue. An efficient Lagrange multiplier and dual ascent‐based estimation scheme is then designed to obtain an iterative solution for the estimator. Moreover, the Cramér–Rao lower bounds are deduced as a performance benchmark. Simulation results show that the authors' designed scheme improves the estimation accuracy up to 33%, compared with the conventional least‐square method in the low signal‐to‐noise ratio regime.

We investigate an energy cost minimization problem for a smart home in the absence of a building thermal dynamics model with the consideration of a comfortable temperature range. Due to the existence of model uncertainty, parameter uncertainty (e.g., renewable generation output, nonshiftable power demand, outdoor temperature, and electricity price), and temporally coupled operational constraints, it is very challenging to design an optimal energy management algorithm for scheduling heating, ventilation, and air conditioning systems and energy storage systems in the smart home. To address the challenge, we first formulate the above problem as a Markov decision process, and then propose an energy management algorithm based on deep deterministic policy gradients. It is worth mentioning that the proposed algorithm does not require the prior knowledge of uncertain parameters and building the thermal dynamics model. The simulation results based on real-world traces demonstrate the effectiveness and robustness of the proposed algorithm.

In this article, we consider an energy-harvesting (EH) relay system consisting of a source, a destination, and multiple EH decode-and-forward (DF) relays that can harvest the energy from their received radio signals. A power-splitting ratio is employed at an EH DF relay to characterize a tradeoff between the energy used for decoding the source signal received at the relay and the remaining energy harvested for retransmitting the decode outcome. We propose an optimal power-splitting-based relay selection (OPS-RS) framework and also consider the conventional equal power-splitting-based relay selection (EPS-RS) for comparison purposes. We derive closed-form expressions of outage probability for both the OPS-RS and EPS-RS schemes and characterize their diversity gains through an asymptotic outage analysis in the high signal-to-noise ratio region. We further examine an extension of our OPS-RS framework to an EH battery (EHB)-enabled cooperative relay scenario, where the EH relays are equipped with batteries used to store their harvested energies. We propose an amplify-and-forward (AF)-based EHB-OPS-RS scheme and a DF-based EHB-OPS-RS scheme for AF and DF relay networks, respectively. Numerical results show that the proposed OPS-RS always outperforms EPS-RS in terms of outage probability. Moreover, the outage probabilities of the AF- and DF-based EHB-OPS-RS schemes are much smaller than that of the OPS-RS and EPS-RS methods, demonstrating the benefit of exploiting the batteries in EH relays. Additionally, the DF-based EHB-OPS-RS substantially outperforms the AF-based EHB-OPS-RS and such an outage advantage becomes more significant, as the number of EH relays increases.

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

The hardware impairments of transceiver nodes generate signal distortions and degrade the secrecy performance of wireless communications systems. In this paper, we investigate the effect of hardware impairments on the secrecy outage probability of hybrid satellite-terrestrial relay systems (HSTRS). The source satellite transmits its signal to the terrestrial destination with the help of decode-and-forward (DF) terrestrial relays, while the eavesdropper attempts to tap the transmission from the terrestrial relays to destination. An optimal relay selection (ORS) scheme is proposed for enhancing the physical-layer security of HSTRS against hardware impairments. The traditional round-robin scheduling (RRS) is also considered for comparison purposes. We derive closed-form expressions of the secrecy outage probability for both the RRS and ORS schemes. Numerical results show that given certain hardware impairments, the secrecy outage performance of ORS scheme is better than that of the RRS. Additionally, as the number of terrestrial relays increases, the secrecy outage probability of proposed ORS scheme decreases significantly, demonstrating the advantage of exploiting relay selection against hardware impairments for HSTRS.

In this study, the authors investigate the security and reliability trade‐off (SRT) performance of a multiuser wireless network, where an eavesdropper attempts to overhear the confidential transmissions between a base station and multiple users under the impact of co‐channel interference. To enhance the SRT performance of the network, the authors propose two joint user and jammer selection schemes, namely the joint optimal user and random jammer selection (JOU‐RJS) scheme and the joint optimal user and optimal jammer selection (JOU‐OJS) scheme. For SRT performance comparison purposes, the traditional multiuser scheduling (TMUS) scheme is also considered. The authors derive closed‐form outage and intercept probability expressions for the JOU‐RJS and JOUOJS schemes. Simulation results show that the SRTs of both JOU‐RJS and JOU‐OJS schemes are better than that of the TMUS scheme, where JOU‐OJS achieves the best SRT performance. Additionally, this study also shows that an expected outage or intercept probability of the TMUS, JOU‐RJS, and JOU‐OJS schemes can be achieved by giving an appropriate overall rate. Finally, the sum of the outage and intercept probability can be minimised through an optimal power allocation between the selected user and friendly jammer for the proposed JOU‐RJS and JOU‐OJS.

This paper investigates the physical-layer security (PLS) of wireless transmissions with the aid of multiple decode-and-forward one-way relays in the presence of both eavesdropping attacks and channel estimation errors (CEEs). To protect wireless transmission with CEE, two opportunistic relaying schemes are conceived to upgrade the PLS of the wireless communications with the aid of relays, namely, the CEE-oriented pure relay selection (CEE-PRS) and CEE-oriented jammer-aided relay selection (CEE-JRS), respectively. Moreover, the security-reliability tradeoff (SRT) is designed due to that increasing the transmit power may enhance the reliability, but the security may be sacrificed concurrently, where the security and reliability are characterized by the intercept probability (IP) and outage probability (OP), respectively. We then analyze the IP and OP of the CEE-PRS and CEE-JRS schemes. Furthermore, both the CEE-oriented direct transmission (CEE-DT) and the perfect channel estimation-oriented direct transmission (PCE-DT) schemes are also analyzed for comparison purposes. It is shown that although the SRT of the wireless communications can indeed be degraded by the eavesdropping attacks in low CEE regions, the proposed CEE-PRS and CEE-JRS schemes are capable of significant improving the SRT performance of wireless communications, demonstrating the advantage of the proposed CEE-PRS and CEE-JRS schemes against eavesdropping.

In this paper, we investigate the physical layer security (PLS) of cooperative wireless-powered networks where a source transmits confidential information to a destination with the aid of wireless-powered intermediate nodes equipped with multiple antennas in the presence of a passive eavesdropper. We consider two generalized joint relay and jammer selection (GJRJS) frameworks based on the power splitting (PS) and time switching (TS) techniques, respectively. Specifically, the intermediate nodes which cannot decode the source signal successfully are selected to act as friendly jammers to transmit artificial noise, and the remaining nodes are exploited as relays to simultaneously forward the source signal through cooperative beamforming. We further propose two cooperative secure beamforming (CSB) schemes for the PS-based GJRJS (PS-GJRJS) and TS-based GJRJS (TS-GJRJS) frameworks, respectively. To be specific, we investigate the optimization of the beamforming vector of our selected relays for maximizing the secrecy rate of the source–destination transmission. A closed-form solution is derived under the assumption of available instantaneous channel state information (CSI) of the main link and statistical CSI of the wiretap link. In addition, we also illustrate that the pure relay selection (PRS) scheme is a special case of our GJRJS framework at high signal-to-noise ratios (SNRs). The numerical results show that the proposed CSB scheme achieves a higher secrecy rate than the traditional maximal ratio transmission (MRT) method for both the PS-GJRJS and TS-GJRJS frameworks. Additionally, the GJRJS framework outperforms the PRS as well as the joint best relay and jammer selection (JBRJS) methods in terms of secrecy rate.

We study the physical-layer security of a cognitive radio system in the face of multiple eavesdroppers (EDs), which is composed of a secondary base station (SBS), multiple secondary users (SUs) as well as a pair of primary transmitter (PT) and primary receiver (PR), where the SUs first harvest energy from their received radio frequency signals transmitted by the PT and then communicate with the SBS relying on opportunistic scheduling. We consider two specific user scheduling schemes, namely, the channel-aware user scheduling (CaUS) and the energy-aware user scheduling (EaUS). In the CaUS scheme, an SU having the best instantaneous SU-SBS link (spanning from SUs to SBS) will be activated to communicate with the SBS. By contrast, the EaUS scheme takes into account both the amount of energy harvested from the PT and the instantaneous quality of the SU-SBS link. We analyze the security-reliability tradeoff (SRT) of both the CaUS and EaUS schemes in terms of their intercept versus outage probability. We also provide the SRT analysis of traditional round-robin user scheduling (RrUS) used as a benchmarker of the CaUS and EaUS schemes. We demonstrate that the EaUS scheme achieves the best outage and secrecy performance in the high main-to-eavesdropper ratio (MER) region, but a worse secrecy performance than the CaUS method in the low-MER region. Moreover, from a security versus reliability perspective, the CaUS outperforms both the EaUS and the RrUS in the low-MER region. Surprisingly, this also implies that although the user scheduling criterion of EaUS exploits the knowledge of both the amount of harvested power and instantaneous channel state information (CSI), it exhibits a degraded physical-layer security in the low-MER region due to the fact that the increased harvested energy is beneficial not only for the legitimate SBS receiver but also for the EDs.

A new joint best relay and jammer selection (JBRJS) scheme is conceived for enhancing the physical layer security (PLS) of cooperative networks relying on multiple energy harvesting (EH) aided intermediate nodes, which accumulate energy based on the power splitting (PS) protocol. Specifically, we select the best intermediate node as the relay, whilst exploiting all the remaining nodes as friendly jammers. Furthermore, we investigate the joint optimization of the PS ratio and the relay-aided beamforming for maximizing the system's secrecy rate and present a full channel state information (CSI) based joint PS and beamforming (fJPSB) scheme as the optimal solution by converting the optimization problem formulated into a singlevariable optimization problem. We also propose a partial-CSI based JPSB (pJPSB) method for the scenario where only the main link's CSI is available. Our numerical results show that the proposed JBRJS scheme beneficially enhances the PLS compared to the joint random relay and jammer selection (JRRJS) and to the pure best relay selection (PBRS) schemes. Moreover, the secrecy rate of the proposed fJPSB and pJPSB schemes is obviously higher than that of the fixed PS and beamforming (FPSB) method, and it further increases with the number of the relay's transmit antennas.

This paper investigates the problem of energy efficiency maximization (EEM) for the small cells that coexist with a macro cell in an underlay heterogeneous cellular network, where a macro base station and a number of small base stations transmit signals to a macro user and small users through their shared spectrum. We propose a joint power and bandwidth allocation (JPBA) scheme for the sake of maximizing the energy efficiency (EE) of small cells under the constraint of a guaranteed quality-of-service requirement for the macro cell. Considering that our formulated EEM-based JPBA (EEM-JPBA) problem is non-convex, we convert the original fractional problem into an equivalent subtractive form by adopting the Dinkelbach's method, which is addressed through the augmented Lagrange multiplier approach. Moreover, a new two-tier iterative algorithm is presented to obtain the optimal solution of our EEM-JPBA scheme. Simulation results demonstrate that the proposed two-tier iterative algorithm can quickly converge to the optimal EE solution. In addition, it is shown that the proposed EEM-JPBA scheme significantly outperforms the conventional power and bandwidth allocation methods in terms of their EE performance.

In this paper, we analyze the secrecy outage probability and computation overhead of a multi-user multi-eavesdropper wireless system which is composed of one cluster head (CH) transmitting to multiple users, while multiple eavesdroppers attempt to tap the transmission from CH to multiple users in the presence of co-channel interference (CCI). To achieve high security and low computation overhead, we present two multi-user scheduling schemes, namely, the switch-and-examine combining with post-selection user scheduling (SECpsUS) scheme and the selection combining-based user scheduling (SCbUS) scheme. For the purpose of comparison, the conventional round-robin scheduling (RRS) is also considered as a benchmark scheme. Furthermore, we derive closed-form secrecy outage probability expressions and analyze computation overheads for the conventional RRS, SCbUS, and SECpsUS schemes. Numerical results and Monte Carlo simulations are provided to verify the correctness of our theoretical secrecy outage analysis and to demonstrate the effect of CCI on the system performance. It is shown that the SECpsUS and SCbUS schemes outperform the conventional RRS scheme in terms of their secrecy outage probabilities, where SCbUS achieves the best secrecy outage performance but at cost of the highest computation overhead, while the conventional RRS scheme has no computation overhead with the worst secrecy performance. Moreover, a flexible tradeoff between the secrecy outage performance and computation overhead can be achieved by the proposed SECpsUS scheme by adjusting a so-called switching threshold. In addition, as the number of users increases, the secrecy outage performance of the SECpsUS and SCbUS is improved significantly, whereas no secrecy benefit is achieved by the conventional RRS scheme.

In this paper, we study a multiuser uplink network consisting of one base station (BS), multiple users and one eavesdropper (E), where the users are intended to transmit their confidential messages to BS, while the eavesdropper attempts to tap their transmissions. To improve the transmission secrecy, we propose two friendly jammer selection-aided multiuser scheduling schemes, namely, the random jammer selection-aided multiuser scheduling (RJS-MUS) without knowing the eavesdropper's channel state information (CSI) and the optimal jammer selection-aided multiuser scheduling (OJS-MUS), where the CSIs of eavesdropper are available. For comparison purposes, the conventional non-jammer selection-aided multiuser scheduling (NJS-MUS) scheme is considered as a benchmark. We derive exact and asymptotic closed-form secrecy outage probability expressions for the conventional NJS-MUS as well as proposed RJS-MUS and OJS-MUS schemes. The numerical results show that the proposed RJS-MUS and OJS-MUS schemes with equal power allocation between the selected friendly jammer and scheduled user perform worse than the conventional NJS-MUS approach in terms of the secrecy outage probability in the low signal-to-noise ratio (SNR) region. As the SNR increases, the secrecy outage performance of RJS-MUS and OJS-MUS schemes substantially improves, which is, in turn, better than that of conventional NJS-MUS approach. Moreover, the secrecy advantage of RJS-MUS and OJS-MUS over NJS-MUS becomes more significant with an increasing SNR. Also, it is shown that for both the RJS-MUS and OJS-MUS schemes, a better secrecy performance can be achieved through an optimal power allocation (OPA) between the scheduled user and friendly jammer. In addition, the proposed RJS-MUS and OJS-MUS schemes with OPA strictly outperform the conventional NJS-MUS approach in terms of the secrecy outage probability.

In this paper, we investigate physical-layer security for a heterogeneous cellular network consisting of a macro cell and a small cell in the presence of a common eavesdropper that intends to tap both the macro cell and small cell. The orthogonal spectrum sharing (OSS) and non-orthogonal spectrum sharing (NOSS) are considered for the heterogeneous cellular network. The OSS allows the macro cell and small cell to access a given spectrum band in an orthogonal manner, whereas the NOSS enables them to access the same spectrum simultaneously and mutual interference exists. We present two NOSS schemes, namely, the interference-limited NOSS (IL-NOSS) and interference-canceled NOSS (IC-NOSS), where the mutual interference is constrained below a tolerable level through power control in the IL-NOSS, and the IC-NOSS exploits a specially designed signal for canceling out the interference received a legitimate user while confusing the eavesdropper. We derive closed-form expressions for an overall secrecy outage probability of the OSS, IL-NOSS, and IC-NOSS schemes by jointly considering the secrecy of both the macro cell and small cell. Furthermore, we characterize the secrecy diversity of the OSS, IL-NOSS, and IC-NOSS schemes through the asymptotic secrecy outage analysis in the high signal-to-noise ratio (SNR) region. It is proved that the OSS and IL-NOSS schemes obtain the same secrecy diversity gain of zero only, and a higher secrecy diversity gain of one is achieved by the IC-NOSS scheme. In addition, numerical results demonstrate that secrecy outage probabilities of the IL-NOSS and IC-NOSS can be minimized through an optimization of the ratio of the SNR of small cell to the SNR of macro cell, called the small-to-macro ratio (SMR). Moreover, with an optimized SMR, the proposed IC-NOSS scheme significantly outperforms the OSS and IL-NOSS schemes in terms of individual secrecy outage probabilities of both the macro cell and small cell.

In this paper, we consider a dual-hop decode-and-forward (DF) relay network which consists of a source, a destination and multiple DF relays in the presence of an eavesdropper, where the source transmits its data to the destination with the help of DF relays while the eavesdropper attempts to tap the relay transmissions. We examine the impact of channel estimation errors on the secrecy performance of source-destination transmissions and propose an optimal relay selection scheme for improving the transmission security against eavesdropping. To be specific, the proposed relay selection is designed by considering the fact that the channel state information (CSI) of any link from the source via DF relays to destination is inaccurate with channel estimation errors. The traditional round-robin relay selection is also considered for comparison purposes. We derive close-form expressions of the secrecy outage probability for both the proposed optimal relay selection scheme and traditional round-robin relay selection schemes. Numerical results demonstrate an adverse effect of the channel estimation errors on the secrecy outage probability of cooperative relay systems. Additionally, it is shown that the proposed optimal relay selection scheme substantially outperforms the traditional round-robin selection approach in terms of the secrecy outage probability.

In this paper, we consider an energy-harvesting (EH) decode-and-forward (DF) relay system consisting of a source, a destination and multiple EH relays each equipped with multiple antennas. We first propose an optimal power splitting and relay selection (OPS-RS) scheme for our considered EH multi-antenna multi-relay system, where a closed-form solution to the optimal power-splitting ratio is obtained for maximizing an instantaneous channel capacity from the source via an EH relay to the destination. For the purpose of comparison, we also take into account the equal power splitting and relay selection (EPS-RS) scheme as a benchmark. Then, we derive closed-form expressions of the outage probability for both the OPS-RS and EPS-RS schemes over Rayleigh fading channels. Numerical results demonstrate that as the number of EH multi-antenna relays increases, the outage probabilities of both OPS-RS and EPS-RS decrease significantly. In addition, the proposed OPS-RS scheme outperforms the traditional EPS-RS method in terms of outage probability.

In this paper, we investigate the problem of minimizing the long-term total cost (i.e., the sum of energy cost and thermal discomfort cost) associated with a heating, ventilation, and air conditioning (HVAC) system of a multizone commercial building under smart grid environment. To be specific, we first formulate a stochastic program to minimize the time average expected total cost with the consideration of uncertainties in electricity price, outdoor temperature, the most comfortable temperature level, and external thermal disturbance. Due to the existence of temporally and spatially coupled constraints as well as unknown information about the future system parameters, it is very challenging to solve the formulated problem. To this end, we propose a real-time HVAC control algorithm based on the framework of Lyapunov optimization techniques without the need to predict any system parameters and know their stochastic information. The key idea of the proposed algorithm is to construct and stabilize virtual queues associated with indoor temperatures of all zones. Moreover, we provide a distributed implementation of the proposed real-time algorithm with the aim of protecting user privacy and enhancing algorithmic scalability. Extensive simulation results based on real-world traces show that the proposed algorithm could reduce energy cost effectively with small sacrifice in thermal comfort.

This paper explores the co-existence of a macro cell and a small cell for heterogeneous cellular networks, where a macro base station (MBS) and small base station (SBS) transmit to respective macro user (MU) and small user (SU) through their shared spectrum in the face of a common eavesdropper. We consider two spectrum sharing mechanisms, namely the overlay spectrum sharing (OSS) and underlay spectrum sharing (USS). In the OSS, the MBS and the SBS take turns to access their shared spectrum. By contrast, the USS allows the MBS and SBS to simultaneously transmit over the shared spectrum with the aid of power control for limiting their mutual interference, thus called interference-limited USS (IL-USS). In order to take advantage of mutual interference in confusing the eavesdropper without causing adverse effect on the MU, we propose an interference-canceled USS (IC-USS) scheme, where a sophisticatedly designed signal is emitted at the MBS to cancel out the interference received at the MU, which is also beneficial in terms of defending the common eavesdropper. Closed-form expressions of overall outage probability and intercept probability are derived for OSS, IL-USS, and IC-USS schemes by taking into account both MBS-MU and SBS-SU transmissions. The secrecy diversity analysis is also carried out by characterizing an asymptotic behavior of the overall outage probability with a given intercept probability in the high signal-to-noise ratio region. It is shown that the secrecy diversity gains of conventional OSS and IL-USS are zero, whereas the proposed IC-USS achieves a higher secrecy diversity gain of one. This implies that with an arbitrarily low overall intercept probability, the conventional OSS and IL-USS methods converge to their respective outage probability floors; however, the proposed IC-USS scheme can make the overall outage probability asymptotically decrease to zero by simply increasing the transmit power. In addition, numerical results demonstrate an obvious advantage of the proposed IC-USS over OSS and IL-USS against eavesdropping.

In this paper, we investigate the problem of developing a geographical load balancing (GLB) scheme for distributed Internet data centers (IDCs) when they are price-makers in the deregulated electricity markets, i.e., GLB may impose impact on electricity prices in IDC locations due to the large power consumption of IDCs. Taking into account the dynamic characteristics (e.g., time-varying power demands and generation outputs) and actual physical constraints (e.g., active/reactive power flow balancing, transmission congestion, and network loss) of smart grids, it is difficult to obtain price impact models with analytical expressions accurately. As a result, it is challenging to design an efficient GLB scheme without requiring price impact models. To overcome the above challenge, we propose a price-sensitivity aware GLB scheme. The key idea of the proposed scheme is to impose proper limits on the workloads allocated to the IDC locations with high price-sensitivity coefficients by exploiting a number of interactive information between IDCs and main grids, so that the sudden increase in the total cost could be avoided. Here, the price-sensitivity coefficient is defined as the ratio of the percentage change in price to the percentage change in IDC power demand. Extensive simulation results show the effectiveness of the proposed GLB scheme.

This paper analyzes the secrecy outage probability of a cellular downlink system which consists of multiple users, one base station (BS), and one eavesdropper in the presence of co-channel interferers, where the BS transmits its signals to users which are distributed according to a Poisson point process with a fixed density, while the eavesdropper attempts to tap the transmission from the BS to the users. To enhance the transmission security against eavesdropping, we propose two multiuser scheduling schemes, namely, the partial channel state information (CSI) aided user scheduling (PCSI-US) scheme where only the CSIs of users are available without knowing the co-channel interferers' CSIs and the full CSI aided user scheduling (FCSI-US) scheme which requires the CSIs of both users and interferers. For comparison purposes, the conventional round-robin scheduling scheme is also considered as a baseline. We derive closed-form expressions of the secrecy outage probability for the proposed PCSI-US and FCSI-US as well as conventional round-robin scheduling schemes in co-channel interference (CCI) environments. Numerical results show that the proposed FCSI-US and PCSI-US schemes outperform the conventional round-robin scheduling in terms of their secrecy outage probabilities, where FCSI-US achieves the best secrecy outage performance in CCI environments. It is also demonstrated that increasing the mean number of users has a marginal effect on the secrecy performance of conventional round-robin scheduling, which can significantly decrease the secrecy outage probabilities of PCSI-US and FCSI-US schemes. This further validates the secrecy effectiveness of the proposed user scheduling methods in terms of combating the CCI.

In this paper, we investigate physical-layer security for a spectrum-sharing heterogeneous cellular network comprised of a macro cell and a small cell, where a passive eavesdropper is assumed to tap the transmissions of both the macro and small cells. In the macro cell, a macro base station (MBS) equipped with multiple distributed antennas sends its confidential information to a macro user (MU) through an opportunistic transmit antenna. Meanwhile, in the small cell, a small base station (SBS) transmits to a small user (SU) over the same spectrum used by MBS. We propose an interference-canceled opportunistic antenna selection (IC-OAS) scheme to enhance the physical-layer security for the heterogeneous network. To be specific, when MBS sends its confidential message to MU through an opportunistic distributed antenna, a special signal is artificially designed and emitted at the MBS to ensure that the received interference at MU from SBS is canceled out. For comparison, the conventional interference-limited opportunistic antenna selection (IL-OAS) is considered as a benchmark. We characterize the security-reliability tradeoff (SRT) for the proposed IC-OAS and conventional IL-OAS schemes in terms of deriving their closed-form expressions of intercept probability and outage probability. Numerical results show that compared with the conventional IL-OAS, the proposed IC-OAS scheme not only brings SRT benefits to the macro cell but also has the potential of improving the SRT of small cell by increasing the number of distributed antennas. Additionally, by jointly taking into account the macro cell and small cell, an overall SRT of the proposed IC-OAS scheme is shown to be significantly better than that of the conventional IL-OAS approach in terms of a sum intercept probability versus sum outage probability.

In this paper, we consider a hybrid satellite- terrestrial relay network (HSTRN) composed of a geostationary satellite, a terrestrial destination and multiple terrestrial relays in the presence of an eavesdropper, where the satellite transmits its data to the terrestrial destination via decode-and- forward (DF) relays while the secure transmission from relay to destination may leak to the eavesdropper. We explore the physical-layer security against eavesdropping attacks in HSTRN and propose the optimal relay selection scheme for improving the transmission security. Additionally, the classic round-robin scheduling is also considered to a benchmark scheme. Furthermore, the closed-form expressions of secrecy outage probability for both the round-robin scheduling and optimal relay selection schemes are derived. Finally, numerical results show that our proposed scheme generally outperforms the round-robin scheduling in terms of the secrecy outage probability.

In this paper, we investigate the problem of energy management for an HVAC (Heating, Ventilation, and Air Conditioning) system of a residential building in smart grid environment without violating user thermal comfort limits. Specifically, we intend to minimize the long-term total cost (i.e., the sum of energy cost and thermal discomfort cost) associated with the HVAC system by taking into account uncertainties of outdoor temperature and electricity price. Due to the time coupling incurred by indoor temperature dynamics, it is very challenging to solve the formulated minimization problem. To address the challenge, we propose an online HVAC control algorithm based on Lyapunov optimization techniques without requiring any parameter predictions. Simulation results based on real-world traces show that the proposed algorithm can reduce energy cost significantly with small sacrifice in thermal comfort.

In this paper, we examine the physical layer security for cooperative wireless networks with multiple intermediate nodes, where the decode-and-forward (DF) protocol is considered. We propose a new joint cooperative beamforming and jamming (JCBJ) scheme to protect wireless communication against eavesdropping, where the intermediate nodes which successfully decode the source signal are selected as the relays for simultaneously forwarding the source signal by employing a weight vector, and meanwhile, the others are used to act as the friendly jammers for broadcasting artificial noise to disturb the eavesdropper. We further investigate the optimal power allocation among the source, relays and friendly jammers for maximizing the secrecy rate of the proposed JCBJ scheme and derive a closed-form sub-optimal solution. Simulation results show that the proposed JCBJ scheme achieves obviously higher secrecy rate than the conventional generalized singular-value-decomposition (GSVD) based beamforming and pure jamming schemes. Additionally, the proposed optimal power allocation scheme performs much better than the equal power allocation scheme in terms of secrecy rate.

In this study, the authors investigate the employment of transmit antenna selection ( TAS ) against eavesdropping in cognitive radio systems, where a cognitive source (CS) node transmits to a cognitive destination (CD) node in the face of an eavesdropper, each equipped with multiple antennas. The authors derive a closed‐form expression of secrecy outage probability for the proposed TAS scheme. For the purpose of comparison, the authors also study the random antenna selection (RAS) as a baseline. Moreover, the secrecy diversity orders of TAS and RAS are analysed to provide an intuitive insight into the effect of the number of antennas on the secrecy outage performance. It is of interest to observe that the secrecy diversity order of TAS is the product of number of transmit antennas at CS and receive antennas at CD and the secrecy diversity order achieved by RAS scheme is the number of receive antennas at CD, which both have nothing to do with the number of antennas at eavesdropper. Finally, the simulation results validate the authors’ theoretical results and the proposed TAS outperforms the conventional RAS in terms of secrecy outage probability.

In this study, the authors propose a secure transmission scheme using polar codes and artificial noise (AN) for the wiretap system without assuming the channel quality advantage of main channel over wiretap channel. They first derive lower and upper bounds on the symmetric capacity of the polarised bit‐channels , which depend on the signal‐to‐noise ratio of each use of physical channel. According to the bounds, they prove existence of the bit‐channels that are beneficial to the signal reception of the main channels but hostile to the wiretap channel, and a method based on injecting the AN at the transmitter is introduced to achieve those bit‐channels. Through theoretical analysis, the security of the proposed AN method is proven. Thereafter, they elaborate two AN power allocation schemes denoted by AN‐PA‐I and AN‐PA‐II for each use of physical channel. The former proves the existence of the minimum AN power to achieve the secrecy rate that is equal to the capacity of the main channel, referred to as the maximum secrecy rate, and the latter introduces how to obtain it with an optimisation method. Numerical results show that both the two schemes can achieve the maximum secrecy rate.

Welcome to the third one in the sequel of three IEEE JSAC special issues on Spectrum Sharing and Aggregation for Future Wireless Networks. In recognition of the fact that a substantial number of submissions have been received in response to the call for papers, the decision has been made to publish three issues on the cutting-edge advances in spectrum sharing and aggregation. The first two issues were published in October 2016 with 20 papers and November 2016 with 19 papers, respectively. This is the third issue with 17 papers, covering a feast of hot research topics as follows.

In this paper, we examine the physical layer security for cooperative wireless networks with multiple intermediate nodes, where the decode-and-forward protocol is considered. We propose a new joint relay and jammer selection (JRJS) scheme for protecting wireless communications against eavesdropping, where an intermediate node is selected as the relay for the sake of forwarding the source signal to the destination and meanwhile, the remaining intermediate nodes are employed to act as friendly jammers, which broadcast the artificial noise for disturbing the eavesdropper. We further investigate the power allocation among the source, relay and friendly jammers for maximizing the secrecy rate of proposed JRJS scheme, and derive a closed-form sub-optimal solution. Specifically, all the intermediate nodes, which successfully decode the source signal, are considered as relay candidates. For each candidate, we derive the sub-optimal closed-form power allocation solution and obtain the secrecy rate result of the corresponding JRJS scheme. Then, the candidate, which is capable of achieving the highest secrecy rate, is selected as the relay. Two assumptions about the channel state information (CSI), namely the full CSI (FCSI) and partial CSI (PCSI), are considered. Simulation results show that the proposed JRJS scheme outperforms the conventional pure relay selection, pure jamming, and generalized singular-value-decomposition-based beamforming schemes in terms of secrecy rate. Additionally, the proposed FCSI-based power allocation and PCSI-based power allocation schemes both achieve higher secrecy rates than the equal power allocation scheme.

In this paper, we examine the physical-layer security for a spectrum sharing system consisting of multiple source-destination pairs, which dynamically access their shared spectrum for data transmissions in the presence of an eavesdropper. We propose a source cooperation (SC) aided opportunistic jamming framework for protecting the transmission confidentiality of the spectrum sharing system against eavesdropping. Specifically, when a source node is allowed to access the shared spectrum for data transmissions, another source is opportunistically selected in the spectrum sharing system to transmit an artificial noise for disrupting the eavesdropper without affecting the legitimate transmissions. We present two specific SC aided opportunistic jamming schemes, namely the SC aided random jammer selection (RJS) and optimal jammer selection (OJS), which are referred to as the SC-RJS and SC-OJS, respectively. We also consider the conventional non-cooperation as a baseline. We derive closed-form intercept probability expressions for the non-cooperation, SC-RJS and SC-OJS schemes, based on which their secrecy diversity gains are determined through an asymptotic intercept probability analysis in the high signal-to-noise ratio (SNR) region. It is proved that the conventional non-cooperation exhibits a secrecy diversity of zero, whereas the proposed SC-RJS and SC-OJS achieve a higher secrecy diversity of one. This also surprisingly means that no additional secrecy diversity gain is achieved by the optimal jammer selection compared to the random selection strategy. In addition, numerical results show that the intercept probability performance of the SC-OJS is always better than that of the SC-RJS and non-cooperation, even when the legitimate channel is worse than the eavesdropping channel.

We explore the physical-layer security for a multisource multi-relay wireless network in the presence of an eavesdropper, in which one of multiple sources is scheduled to send its message to a destination with the assistance of multiple decode-and-forward (DF) relays. One eavesdropper is considered at the receiver side, and assumed to be capable of overhearing the transmission from relays to destination. In this paper, the round-robin scheduling and the joint source-relay selection schemes are presented to improve the physical-layer security of the multi-source multi-relay system, and their closedform intercept probability expressions are derived over Rayleigh fadingchannels.Numericalresultsdemonstratethattheproposed joint source-relay selection scheme achieves a better performance than the traditional round-robin scheduling in terms of the intercept probability.

In this paper, we propose a new joint relay and jammer selection (JRJS) scheme to enhance the physical layer security for cooperative wireless networks with multiple intermediate nodes, where the decode-and-forward (DF) protocol is considered. In the proposed JRJS scheme, an intermediate node is selected as the relay for data transmission, while the others are used to act as friendly jammers for disrupting the eavesdropper by broadcasting artificial noise. We focus on the power allocation between the relay and jammers to maximize the secrecy rate of proposed JRJS scheme under a total transmit power constraint, and derive a closed-form optimal power allocation solution. For the purpose of comparision, the conventional pure relay selection and pure jammer selection schemes are considered as benchmark schemes. Simulation results show the advantage of the proposed JRJS scheme over conventional pure relay selection and pure jammer selection methods in terms of secrecy rate. Additionally, the derived optimal power allocation solution leads to a significantly higher secrecy rate than the equal power allocation scheme.

In this paper, we investigate the physical-layer security for a wireless communications network comprised of multiple users and a common base station (BS) in the presence of an eavesdropper. We propose a joint user-jammer selection scheme for preventing the eavesdropper from intercepting the confidential user-BS transmission. Specifically, a user is first selected to transmit its message to the BS, while another user is chosen to act as a friendly jammer for emitting the artificial noise to interfere with the eavesdropper. We also consider the conventional multiuser scheduling as a benchmark. Closed-form intercept probability expressions are derived for the conventional multiuser scheduling and proposed joint user-jammer selection schemes over Rayleigh fading channels. Numerical results show that the proposed joint user-jammer selection significantly outperforms the conventional multiuser scheduling in terms of the intercept probability, especially with an increasing number of users.

In this paper, we investigate the physical-layer security for a wireless two-hop relay network in the presence of channel estimation errors when estimating the channel state information (CSI) of the main links (from the source via relays to the destination) and wiretap links (from the source and relays to the eavesdropper). The eavesdropper can not only overhear the confidential signals transmitted by the relays, but also tap the secret messages transmitted by the source. We present an channel estimation error oriented relay selection (CEEoRS) scheme to improve the security-reliability tradeoff (SRT). We also analyze the intercept probability and outage probability of the CEEoRS scheme, where the intercept probability and outage probability can quantify the security and reliability, respectively. For comparison purposes, the traditional direct transmission with channel estimation errors (TDTwCEE) is also analyzed. It is shown that the proposed CEEoRS scheme outperforms the TDTwCEE scheme in terms of its SRT. More specifically, the proposed CEEoRS scheme is capable of significantly improving the SRT performance of wireless communications through increasing the number of relays.

To avoid an additional demand charge incurred by the adoption of electric vehicles (EVs), we study an online cost minimization problem for geo-distributed data centers and EVs of their employees without violating heterogeneous EV charging delays and the given peak power limits in all geographical locations, where the total cost consists of the electricity bill, revenue loss, and the penalty imposed on unfinished EV demands. We first formulate a stochastic programming problem with the considerations of uncertainties in data center workloads, electricity prices, and EV energy demands. Since the future parameters are uncertain, an online algorithm is proposed to solve the formulated problem. Moreover, the performance analysis of the online algorithm is conducted. Simulation results indicate the advantages of the proposed algorithm over other baselines.

Time-varying channel estimation is a well-known challenge for wireless communications on high-speed railways. Existing estimation algorithms include interpolation, basis expansion model (BEM) and autoregressive models. These methods remain unchanged for every scenario on high speed railways (HSRs), such as viaducts, tunnels, cutting and open areas. Thus their estimation performance are not consistent and also not satisfactory in many cases. Noting that every train on one high-speed railway follows the same track and therefore the channel parameters are correlated at each fixed position. Based on this observation, we propose a new BEM that explores the correlation between current channels parameters and past ones. More importantly, we analyze its channel estimation performance and justify our suggestions. Finally, simulation results are provided to corroborate our proposed studies.

In this paper, we consider a cooperative relay network consisting of a source, a destination, and multiple decode-and-forward (DF) relays in the presence of multiple eavesdroppers, which intend to tap confidential messages transmitted by both the source and the relays. We propose a so-called secrecy maximization oriented relay selection (SMORS) scheme to improve the physical-layer security of wireless communications. In the SMORS scheme, a relay with the maximal secrecy rate is selected among all the DF relays to forward the source signal. We analyze the intercept probability of the proposed SMORS scheme as well as the traditional max-min relay selection scheme. Numerical results show that the proposed SMORS scheme outperforms the conventional max-min relay selection scheme in terms of the intercept probability. Additionally, it is shown that with an increasing number of eavesdroppers, the intercept performance of wireless communications degrades, which can be well addressed using the proposed SMORS scheme through increasing the number of relays. By contrast, increasing the number of relays has little impact on the intercept probability for the conventional max-min relay selection scheme, especially when the number of relays is sufficiently high (e.g., exceeding 20 relays).

In this study, the authors investigate the physical‐layer security in a cognitive amplify‐and‐forward relay network consisting of a secondary transmitter (ST) and a secondary destination (SD) with the aid of multiple secondary relays (SRs) in the face of multiple eavesdroppers. In cognitive radio networks, increasing transmit power may not always be beneficial in terms of improving the channel capacity of cognitive transmissions, which would not only cause an extra interference to primary user, but also enhance the possibility of successfully intercepting the cognitive transmissions at an eavesdropper because an improved signal strength is received in this case. The authors propose two relay selection schemes to improve the physical‐layer security of cognitive transmissions against eavesdropping attacks, which are referred to as the global and partial channel state information based relay selection, denoted by GCSIbRS and PCSIbRS, respectively. The authors analyse the intercept probability of the proposed GCSIbRS and PCSIbRS, as well as the traditional round‐robin and all‐relay transmission schemes. It is shown that the proposed GCSIbRS and PCSIbRS schemes both outperform the conventional round‐robin and all‐relay schemes in terms of their intercept probability performance.

To avoid an additional large demand charge introduced by the adoption of electric vehicles (EVs), we investigate a joint energy management problem for geographically distributed data centers (DCs) and EVs of the employees in this paper. Specifically, we intend to minimize the long-term total cost of DCs and EVs by jointly scheduling DC workloads and EV charging demands, without violating heterogeneous EV charging requirements and the given power limits in all geographical locations. We first formulate a stochastic programming problem with the considerations of uncertainties in DC workloads, electricity prices, and EV energy demands. Since these uncertain system parameters are time-varying and the size of the formulated problem is large, we then propose a distributed online algorithm to solve the formulated problem based on Lyapunov optimization technique and a variant of alternating direction method of multipliers. Finally, extensive simulation results show the effectiveness of the proposed algorithm.

Welcome to the IEEE JSAC special issue on Spectrum Sharing and Aggregation for Future Wireless Networks. The conception of this special issue is motivated by the following observations: the ever-increasing penetration of both the mobile Internet and of the Internet-of-things is gradually clogging up the most valuable spectral bands available in the sub-2 GHz frequency range for future wireless networks. Hence there is an urgent need for improved spectrum exploitation to satisfy this demand. It is expected that the wireless tele-traffic will continue to grow quite dramatically in the ensuing years, hence further widening the spectrum-supply versus demand gap. In order to mitigate this gap, spectrum sharing and aggregation have been well recognized as promising approaches, which led to rapid advances by harnessing a large cross-section of the research community. Nonetheless, there are numerous unsolved technical challenges. This special issue aims for reporting on some of these cutting-edge advances in spectrum sharing and aggregation, whilst opening new avenues of research in this area.

The papers in this special issue represent the second one in the sequel of three special issues on spectrum sharing and aggregation for future wirelessn networks.

Due to the broadcast nature of radio propagation, the wireless air interface is open and accessible to both authorized and illegitimate users. This completely differs from a wired network, where communicating devices are physically connected through cables and a node without direct association is unable to access the network for illicit activities. The open communications environment makes wireless transmissions more vulnerable than wired communications to malicious attacks, including both the passive eavesdropping for data interception and the active jamming for disrupting legitimate transmissions. Therefore, this paper is motivated to examine the security vulnerabilities and threats imposed by the inherent open nature of wireless communications and to devise efficient defense mechanisms for improving the wireless network security. We first summarize the security requirements of wireless networks, including their authenticity, confidentiality, integrity, and availability issues. Next, a comprehensive overview of security attacks encountered in wireless networks is presented in view of the network protocol architecture, where the potential security threats are discussed at each protocol layer. We also provide a survey of the existing security protocols and algorithms that are adopted in the existing wireless network standards, such as the Bluetooth, Wi-Fi, WiMAX, and the long-term evolution (LTE) systems. Then, we discuss the state of the art in physical-layer security, which is an emerging technique of securing the open communications environment against eavesdropping attacks at the physical layer. Several physical-layer security techniques are reviewed and compared, including information-theoretic security, artificial-noise-aided security, security-oriented beamforming, diversity-assisted security, and physical-layer key generation approaches. Since a jammer emitting radio signals can readily interfere with the legitimate wireless users, we also introduce the family of various jamming attacks and their countermeasures, including the constant jammer, intermittent jammer, reactive jammer, adaptive jammer, and intelligent jammer. Additionally, we discuss the integration of physical-layer security into existing authentication and cryptography mechanisms for further securing wireless networks. Finally, some technical challenges which remain unresolved at the time of writing are summarized and the future trends in wireless security are discussed.

This paper studies the intercept behavior of an industrial wireless sensor network (WSN) consisting of a sink node and multiple sensors in the presence of an eavesdropping attacker, where the sensors transmit their sensed information to the sink node through wireless links. Due to the broadcast nature of radio wave propagation, the wireless transmission from the sensors to the sink can be readily overheard by the eavesdropper for interception purposes. In an information-theoretic sense, the secrecy capacity of the wireless transmission is the difference between the channel capacity of the main link (from sensor to sink) and that of the wiretap link (from sensor to eavesdropper). If the secrecy capacity becomes nonpositive due to the wireless fading effect, the sensor's data transmission could be successfully intercepted by the eavesdropper and an intercept event occurs in this case. However, in industrial environments, the presence of machinery obstacles, metallic frictions, and engine vibrations makes the wireless fading fluctuate drastically, resulting in the degradation of the secrecy capacity. As a consequence, an optimal sensor scheduling scheme is proposed in this paper to protect the legitimate wireless transmission against the eavesdropping attack, where a sensor with the highest secrecy capacity is scheduled to transmit its sensed information to the sink. Closed-form expressions of the probability of occurrence of an intercept event (called intercept probability) are derived for the conventional round-robin scheduling and the proposed optimal scheduling schemes. Also, an asymptotic intercept probability analysis is conducted to provide an insight into the impact of the sensor scheduling on the wireless security. Numerical results demonstrate that the proposed sensor scheduling scheme outperforms the conventional round-robin scheduling in terms of the intercept probability.

In this paper, we propose a cognitive transmission scheme for Amplify-and-Forward (AF) two-way relay networks (TWRNs) and investigate its joint sensing and transmission performance. Specifically, we derive the overall false alarm probability, the overall detection probability, the outage probability of the cognitive TWRN over Rayleigh fading channels. Furthermore, based on these probabilities, the spectrum hole utilization efficiency of the cognitive TWRN is defined and evaluated. It is shown that smaller individual or overall false alarm probability can result in less outage probability and thus larger spectrum hole utilization efficiency for cognitive TWRN, and however produce more interference to the primary users. Interestingly, it is found that given data rate, more transmission power for the cognitive TWRN does not necessarily obtain higher spectrum hole utilization efficiency. Moreover, our results show that a maximum spectrum hole utilization efficiency can be achieved through an optimal allocation of the time slots between the spectrum sensing and data transmission phases. Finally, simulation results are provided to corroborate our proposed studies. Copyright © 2016 John Wiley & Sons, Ltd.

This paper addresses the problem of secure amplify-and-forward (AF) relaying for multiple-input multiple output (MIMO) relaying networks in the presence of multiple eavesdroppers. Assuming practical imperfect eavesdroppers' channel state information (ECSI), we propose a robust approach to optimize the relay AF matrix, subject to power constraint, in order to maximize the received signal-to-interference-plus-noise ratio (SINR) at the destination while satisfying a set of secrecy constraints. The ECSI errors are assumed to fall within some predefined bounded sets. Since the resultant optimization problem is non-convex and semi-infinite, we transform it into a form constituted by the differences of convex functions (DC) using suitable matrix transformation techniques. Then an algorithmic solution with proven convergence is proposed by resorting to the penalty-DC algorithm (P-DCA). Experimental results show the security of the proposed transceiver design against eavesdropping and the robustness against the channel uncertainties.

We consider a cognitive radio (CR) network consisting of a secondary transmitter (ST), a secondary destination (SD) and multiple secondary relays (SRs) in the presence of an eavesdropper, where the ST transmits to the SD with the assistance of SRs, while the eavesdropper attempts to intercept the secondary transmission. We rely on careful relay selection for protecting the ST-SD transmission against the eavesdropper with the aid of both single-relay and multi-relay selection. To be specific, only the “best” SR is chosen in the single-relay selection for assisting the secondary transmission, whereas the multi-relay selection invokes multiple SRs for simultaneously forwarding the ST's transmission to the SD. We analyze both the intercept probability and outage probability of the proposed single-relay and multi-relay selection schemes for the secondary transmission relying on realistic spectrum sensing. We also evaluate the performance of classic direct transmission and artificial noise based methods for the purpose of comparison with the proposed relay selection schemes. It is shown that as the intercept probability requirement is relaxed, the outage performance of the direct transmission, the artificial noise based and the relay selection schemes improves, and vice versa. This implies a trade-off between the security and reliability of the secondary transmission in the presence of eavesdropping attacks, which is referred to as the security-reliability trade-off (SRT). Furthermore, we demonstrate that the SRTs of the single-relay and multi-relay selection schemes are generally better than that of classic direct transmission, explicitly demonstrating the advantage of the proposed relay selection in terms of protecting the secondary transmissions against eavesdropping attacks. Moreover, as the number of SRs increases, the SRTs of the proposed single-relay and multi-relay selection approaches significantly improve. Finally, our numerical results show that as expected, the multi-relay selection scheme achieves a better SRT performance than the single-relay selection.

In this paper, we propose a cognitive transmission scheme for Amplify-and-Forward (AF) two-way relay networks (TWRNs) and investigate its joint sensing and transmission performance. Specifically, we derive the overall false alarm probability, the overall detection probability, the outage probability of the cognitive TWRN over Rayleigh fading channels. Furthermore, based on these probabilities, the spectrum hole utilization efficiency of the cognitive TWRN is defined and evaluated. It is shown that smaller individual or overall false alarm probability can result in less outage probability and thus larger spectrum hole utilization efficiency for cognitive TWRN, and also produce more interference to the primary users. Interestingly, it is found that given data rate, more transmission power for the cognitive TWRN does not necessarily obtain higher spectrum hole utilization efficiency. Moreover, our results show that a maximum spectrum hole utilization efficiency can be achieved through an optimal allocation of the time slots between the spectrum sensing and data transmission phases. Finally, simulation results are provided to corroborate our proposed studies.

This paper investigates the issue of cooperative spectrum sensing with a crowd of low-end personal spectrum sensors (such as smartphones, tablets, and in-vehicle sensors), where one critical challenge is the uncertainty of the quality of sensing data from crowd sensors that may be unreliable, untrustworthy, or even malicious. Moreover, due to either unexpected equipment failures or malicious behaviors, every crowd sensor could sporadically and randomly contribute abnormal data, which makes the existing defense schemes ineffective. To tackle these unique challenges, we propose a robust spectrum sensing scheme by developing a data cleansing framework, where the underutilization of licensed spectrum bands and the sparsity of nonzero abnormal data are jointly exploited to robustly cleanse out the potential nonzero abnormal data component from the original corrupted sensing data. Simulation results demonstrate that the proposed robust sensing scheme outperforms the state-of-art schemes under various abnormal data parameter configurations.

In this paper, we investigate the physical-layer security of a multi-user multi-eavesdropper cognitive radio system, which is composed of multiple cognitive users (CUs) transmitting to a common cognitive base station (CBS), {while multiple eavesdroppers may collaborate with each other or perform independently in intercepting the CUs-CBS transmissions, which are called the coordinated and uncoordinated eavesdroppers, respectively}. Considering multiple CUs available, we propose the round-robin scheduling as well as the optimal and suboptimal user scheduling schemes for improving the security of CUs-CBS transmissions against eavesdropping attacks. Specifically, the optimal user scheduling is designed by assuming that the channel state information (CSI) of all links from CUs to CBS, to primary user (PU) and to eavesdroppers are available. By contrast, the suboptimal user scheduling only requires the CSI of CUs-CBS links without the PU's and eavesdroppers' CSI. We derive closed-form expressions of the secrecy outage probability of these three scheduling schemes in the presence of {the coordinated and uncoordinated eavesdroppers}. We also carry out the secrecy diversity analysis and show that the round-robin scheduling achieves the diversity order of only one, whereas the optimal and suboptimal scheduling schemes obtain the full secrecy diversity, {no matter whether the eavesdroppers collaborate or not. In addition, numerical secrecy outage results demonstrate that for both the coordinated and uncoordinated eavesdroppers, the optimal user scheduling achieves the best security performance and the round-robin scheduling performs the worst.} Finally, upon increasing the number of CUs, the secrecy outage probabilities of the optimal and suboptimal user scheduling schemes both improve significantly.

This paper investigates the issue of cooperative spectrum sensing with a crowd of low-end personal spectrum sensors (such as smartphones, tablets, and in-vehicle sensors), where the sensing data from crowd sensors that may be unreliable, untrustworthy, or even malicious. Moreover, due to either unexpected equipment failures or malicious behaviors, every crowd sensor could sporadically and randomly contribute with abnormal data, which makes the existing cooperative sensing schemes ineffective. To tackle these challenges, we first propose a generalized modeling approach for sensing data with an arbitrary abnormal component. Under this model, we then analyze the impact of general abnormal data on the performance of the cooperative sensing, by deriving closed-form expressions of the probabilities of global false alarm and global detection. To improve sensing data quality and enhance cooperative sensing performance, we further formulate an optimization problem as stable principal component pursuit, and develop a data cleansing-based robust spectrum sensing algorithm to solve it, where the under-utilization of licensed spectrum bands and the sparsity of nonzero abnormal data are jointly exploited to robustly cleanse out the potential nonzero abnormal data component from the original corrupted sensing data. Extensive simulation results demonstrate that the proposed robust sensing scheme performs well under various abnormal data parameter configurations.

In this paper, we investigate security issues in a collaborative wireless network in the presence of eavesdropping attacks, where multiple amplify-and-forward (AF) relays are exploited to secure the message transmission between legitimate users. We first consider the multiple AF relays all participating in assisting the transmission from source to destination, which is called all-relay based collaborative transmission scheme as denoted by all-relay scheme for notational convenience. We also propose the best-relay transmission scheme in which only the single “best” relay is selected to help the source transmit messages to destination. We then analyze the intercept behavior in wireless networks and evaluate intercept probabilities of the proposed all-relay and best-relay schemes as well as the conventional direct transmission without relay in a Rayleigh fading environment. Numerical results show that the best-relay transmission scheme always outperforms the all-relay and direct transmission schemes in terms of intercept probability. It is also shown that as the number of eavesdroppers increases, the intercept probabilities of both all-relay and best-relay schemes increase. Moreover, the intercept probability performance of all-relay and best-relay schemes significantly improves with an increasing number of relays, implying the advantage of exploiting multiple relays against eavesdropping attacks.

In cooperative amplify-and-forward (AF) relay networks, the system performance over multipath channels is impacted by both frequency-selective fading and delay spread. However, most relay selection (RS) schemes choose the best relay node only based on the channel gain while ignoring the delay spread effect on the performance. If orthogonal frequency division multiplexing (OFDM) is used in AF relay networks, the cyclic prefix (CP) length has to be extended to tolerate the accumulated delay spread from source via relay to destination due to the lack of the channel compensation at relay nodes. A long CP, which is the transmission overhead, redeces both the effective data transmission throughput and the overall system transmission efficiency. Therefore, an appropriate RS scheme in cooperation multiple-relay networks should not only enhance the overall transmission reliability but also minimize the relay overhead. To this end, we propose a variable-CP based RS scheme for AF relay networks to maximize the transmission efficiency by dynamically choosing the most suitable relay node. In the proposed scheme, a normalized effective throughput is defined as the selection criterion which depends on both the end-to-end channel gain and the accumulated delay spread. Based on this criterion, the best relay link is selected by achieving the tradeoff between the transmission reliability and overhead. Both the theoretical analysis and simulation results show that when the channel delay spread varies, the proposed scheme can dramatically improve the the effective data transmission throughput compared to the maximum signal-to-noise-ratio schemes with variable/fixed CP.

The number of active users in a network is crucial for understanding the security level of wireless operating environments, since any node in a network could perform malicious attacks and be a potential threat. In this paper, we propose a novel estimation technique for the number of active users by exploiting a typical device fingerprint - I/Q imbalance, which has been identified as a device-specific hardware impairment and can be utilized to distinguish different wireless devices. In the design, I/Q imbalance of a transmitter is first estimated from its transmitting signals. The estimate is then compared with the observed I/Q imbalances of previously identified users through a hypothesis testing, where the distances between the new estimate and previous observations are adopted as the test metric. If all the distances are larger than a properly selected threshold, a new active user is claimed. Finally, the number of active users is determined by counting all the distinct I/Q imbalances. Simulation results are provided to validate the proposed estimation scheme.

Due to the broadcast nature of wireless medium, wireless communication is extremely vulnerable to eavesdropping attack. Physical-layer security is emerging as a new paradigm to prevent the eavesdropper from interception by exploiting the physical characteristics of wireless channels, which has recently attracted a lot of research attentions. In this paper, we consider the physical-layer security in cooperative wireless networks with multiple decode-and-forward (DF) relays and investigate the best relay selection in the presence of eavesdropping attack. For the comparison purpose, we also examine the conventional direct transmission without relay and traditional max-min relay selection. We derive closed-form intercept probability expressions of the direct transmission, traditional max-min relay selection, and proposed best relay selection schemes in Rayleigh fading channels. Numerical results show that the proposed best relay selection scheme strictly outperforms the traditional direct transmission and max-min relay selection schemes in terms of intercept probability. In addition, as the number of relays increases, the intercept probabilities of both traditional max-min relay selection and proposed best relay selection schemes decrease significantly, showing the advantage of exploiting multiple relays against eavesdropping attack.

There is a growing interest in energy efficient or so-called “green” wireless communication to reduce the energy consumption in cellular networks. Since today's wireless terminals are typically equipped with multiple network access interfaces such as Bluetooth, Wi-Fi, and cellular networks, this paper investigates user terminals cooperating with each other in transmitting their data packets to a base station (BS) by exploiting the multiple network access interfaces, referred to as inter-network cooperation, to improve the energy efficiency in cellular uplink transmission. Given target outage probability and data rate requirements, we develop a closed-form expression of energy efficiency in Bits-per-Joule for the inter-network cooperation by taking into account the path loss, fading, and thermal noise effects. Numerical results show that when the cooperating users move towards to each other, the proposed internetwork cooperation significantly improves the energy efficiency as compared with the traditional non-cooperation and intranetwork cooperation. This implies that given a certain amount of bits to be transmitted, the inter-network cooperation requires less energy than the traditional non-cooperation and intra-network cooperation, showing the energy saving benefit of inter-network cooperation.

In amplify-and-forward (AF) relay systems, multi-hop transmissions over multiple frequency selective fading channels result in an accumulation of delay spread, which could lead to strong intersymbol interference (ISI) at the destination since there is no proper channel compensation at the relay nodes. If orthogonal frequency division multiplexing (OFDM) with fixed-length guard interval (GI) is used in these systems, then the length of the GI has to be extended to the longest possible channel delay spread to avoid ISI. However, using such a long GI lowers the effective data transmission throughput. To this end, we propose a novel OFDM system with adaptive GI for AF relay networks to reduce the overall transmission overhead by dynamically choosing the suitable GI length. More specifically, the GI is adaptively selected at the transmitter from a variable-length orthogonal code set, which provides a GI sequence with proper length, depending on the accumulated channel duration. By exploiting the orthogonal property between different GI sequences, the proposed adaptive GI scheme can be implemented without any extra control signal transmitted by the source to notify the destination about the GI used. This differs from existing adaptive GI studies which require such an additional control signal and thus introduce extra system overhead. Numerical results show that the proposed adaptive scheme (without additional control signal) can achieve the same symbol error rate (SER) performance as the conventional adaptive GI approaches (with control signal). This implies that the proposed scheme can further save the control signaling overhead without any SER performance loss.

In this paper, we propose a simple and effective eavesdropping-resilient OFDM system achieved by dynamic subcarrier allocation, exploiting the independent frequency selectivities of different wireless channels. The transmitter utilizes the channel state information (CSI) between the legitimate receiver and itself for the OFDM subcarrier allocation. The highly faded subcarriers are dropped for the data transmission and the constellation size of subcarriers with excellent channel conditions is increased in order to retain the overall throughput. Based on channel reciprocity, the channel behaves in the same manner at each pair of users. The subcarrier allocation scheme is thus shared by the transmitter and the legitimate receiver without additional signaling. In contrast, with an independent multipath channel, the eavesdropper at a separate location cannot derive an identical subcarrier allocation scheme. Consequently, mismatched demodulation is carried out at the eavesdropper so that disrupts the information recovery for eavesdropping. Moreover, due to the time-varying nature of wireless channels, the subcarrier allocation is frequently updated which further enhances the security. Theoretical analysis and simulation results are provided to evaluate the performance of the proposed secure OFDM system. It is validated that the proposed system is much more resilient to eavesdropping compared to the conventional OFDM system.

In this paper, we propose an opportunistic network coding (ONC) scheme in cellular relay networks, which operates depending on whether the relay decodes source messages successfully or not. A fully distributed method is presented to implement the proposed opportunistic network coding scheme without the need of any feedback between two network nodes. We consider the use of proposed ONC for cellular downlink transmissions and derive its closed-form outage probability expression considering cochannel interference in a Rayleigh fading environment. Numerical results show that the proposed ONC scheme outperforms the traditional non-cooperation in terms of outage probability. We also develop the diversity-multiplexing tradeoff (DMT) of proposed ONC and show that the ONC scheme obtains the full diversity and an increased multiplexing gain as compared with the conventional cooperation protocols.

In this paper, we explore the physical-layer security in cooperative wireless networks with multiple relays where both amplify-and-forward (AF) and decode-and-forward (DF) protocols are considered. We propose the AF and DF based optimal relay selection (i.e., AFbORS and DFbORS) schemes to improve the wireless security against eavesdropping attack. For the purpose of comparison, we examine the traditional AFbORS and DFbORS schemes, denoted by T-AFbORS and T-DFbORS, respectively. We also investigate a so-called multiple relay combining (MRC) framework and present the traditional AF and DF based MRC schemes, called T-AFbMRC and T-DFbMRC, where multiple relays participate in forwarding the source signal to destination which then combines its received signals from the multiple relays. We derive closed-form intercept probability expressions of the proposed AFbORS and DFbORS (i.e., P-AFbORS and P-DFbORS) as well as the T-AFbORS, T-DFbORS, T-AFbMRC and T-DFbMRC schemes in the presence of eavesdropping attack. We further conduct an asymptotic intercept probability analysis to evaluate the diversity order performance of relay selection schemes and show that no matter which relaying protocol is considered (i.e., AF and DF), the traditional and proposed optimal relay selection approaches both achieve the diversity order M where M represents the number of relays. In addition, numerical results show that for both AF and DF protocols, the intercept probability performance of proposed optimal relay selection is strictly better than that of the traditional relay selection and multiple relay combining methods.

In this paper, we consider a cognitive radio network that consists of one cognitive base station (CBS) and multiple cognitive users (CUs) in the presence of multiple eavesdroppers, where CUs transmit their data packets to CBS under a primary user's quality of service (QoS) constraint while the eavesdroppers attempt to intercept the cognitive transmissions from CUs to CBS. We investigate the physical-layer security against eavesdropping attacks in the cognitive radio network and propose the user scheduling scheme to achieve multiuser diversity for improving the security level of cognitive transmissions with a primary QoS constraint. Specifically, a cognitive user (CU) that satisfies the primary QoS requirement and maximizes the achievable secrecy rate of cognitive transmissions is scheduled to transmit its data packet. For the comparison purpose, we also examine the traditional multiuser scheduling and the artificial noise schemes. We analyze the achievable secrecy rate and intercept probability of the traditional and proposed multiuser scheduling schemes as well as the artificial noise scheme in Rayleigh fading environments. Numerical results show that given a primary QoS constraint, the proposed multiuser scheduling scheme generally outperforms the traditional multiuser scheduling and the artificial noise schemes in terms of the achievable secrecy rate and intercept probability. In addition, we derive the diversity order of the proposed multiuser scheduling scheme through an asymptotic intercept probability analysis and prove that the full diversity is obtained by using the proposed multiuser scheduling.

There is a growing interest in energy efficient or so-called "green" wireless communication to reduce the energy consumption in cellular networks. Since today's wireless terminals are typically equipped with multiple network access interfaces such as Bluetooth, Wi-Fi, and cellular networks, this paper investigates user terminals cooperating with each other in transmitting their data packets to the base station (BS), by exploiting the multiple network access interfaces, called inter-network cooperation. We also examine the conventional schemes without user cooperation and with intra-network cooperation for comparison. Given target outage probability and data rate requirements, we analyze the energy consumption of conventional schemes as compared to the proposed inter-network cooperation by taking into account both physical-layer channel impairments and upper-layer protocol overheads. It is shown that distances between different network entities (i.e., user terminals and BS) have a significant influence on the energy efficiency of proposed inter-network cooperation scheme. Specifically, when the cooperating users are close to BS or the users are far away from each other, the inter-network cooperation may consume more energy than conventional schemes without user cooperation or with intra-network cooperation. However, as the cooperating users move away from BS and the inter-user distance is not too large, the inter-network cooperation significantly reduces the energy consumption over conventional schemes.

In this paper, we first explore a selective cooperation framework for secondary user transmissions in a cognitive radio network with single relay. In the selective cooperation framework, two transmission modes (i.e., relay diversity transmission and non-relay direct transmission) are considered. We study two specific selective cooperation schemes with and without an acknowledgement (ACK) from a cognitive destination as to if it succeeds in decoding or not, called ACK and non-ACK based selective cooperation, respectively. We derive closed-form outage probability expressions for the two schemes with imperfect spectrum sensing, showing that an outage probability floor occurs in high signal-to-noise ratio (SNR) regions due to mutual interference between primary and secondary users. We consider the use of the outage probability floor to generalize the traditional diversity-multiplexing tradeoff (DMT) definition, based on which a DMT analysis is conducted for the non-ACK and ACK based selective cooperation schemes. We then extend the selective cooperation framework to a multiple-relay cognitive radio network considering the best cognitive relay only to participate in assisting secondary transmissions, referred to as the selective best-relay cooperation. We also consider the non-ACK and ACK based selective best-relay cooperation schemes and develop their DMTs by using the generalized DMT definition.

In cognitive radio networks, cooperative spectrum sensing typically requires two essential phases: the phase of primary user's signal detection by cognitive users and the phase of initial detection result reporting from the cognitive users to a fusion center, which are referred to as detection and reporting phases, respectively. Common control channels (also called dedicated reporting channels) from the cognitive users to fusion center are assumed in previous research to avoid interfering with the primary user in the reporting phase. This, however, requires additional channel resources and increases implementation complexity due to the dedicated reporting channels management. In this paper, we propose an alternative cooperative spectrum sensing framework without dedicated reporting channels and present an interference analysis of its impact on primary users. We show that the interference caused by the proposed scheme is controllable and can be constrained to satisfy a given primary user's quality-of-service (QoS) requirement. By jointly considering the detection and reporting phases, we further examine the receiver operating characteristics (ROC) performance of the proposed cooperative spectrum sensing scheme in Rayleigh fading environment. Numerical results illustrate that, with a guaranteed detection probability constraint, a minimized false alarm probability can be achieved through an optimization of the time durations between the detection and reporting phases.

In cognitive radio networks, each cognitive transmission process typically requires two phases: the spectrum sensing phase and data transmission phase. In this paper, we investigate cognitive transmissions with multiple relays by jointly considering the two phases over Rayleigh fading channels. We study a selective fusion spectrum sensing and best relay data transmission (SFSS-BRDT) scheme in multiple-relay cognitive radio networks. Specifically, in the spectrum sensing phase, only the initial spectrum sensing results, which are received from the cognitive relays and decoded correctly at a cognitive source, are selected and used for fusion. In the data transmission phase, only the best relay is utilized to assist the cognitive source for data transmissions. Under the constraint of satisfying a required probability of false alarm of spectrum holes (for the protection of the primary user), we derive an exact closed-form expression of the spectrum hole utilization efficiency for the SFSS-BRDT scheme, which is used as a measure to quantify the percentage of spectrum holes utilized by the cognitive source for its successful data transmissions. For the comparison purpose, we also examine the spectrum hole utilization efficiency for a fixed fusion spectrum sensing and best relay data transmission (FFSS-BRDT) scheme, where all the initial spectrum sensing results are used for fusion without any refined selection. Numerical results show that, under a target probability of false alarm of spectrum holes, the SFSS-BRDT scheme outperforms the FFSS-BRDT scheme in terms of the spectrum hole utilization efficiency. Moreover, the spectrum hole utilization efficiency of the SFSS-BRDT scheme always improves as the number of cognitive relays increases, whereas the FFSS-BRDT scheme's performance improves initially and degrades eventually after a critical number of cognitive relays. It is also shown that a maximum spectrum hole utilization efficiency can be achieved through an optimal allocation of the time durations between the spectrum sensing and data transmission phases for both the FFSS-BRDT and SFSS-BRDT schemes.

Typically, each cooperative spectrum sensing process requires two phases: the primary user's signal detection phase, in which all cognitive users attempt to detect the presence of the primary user within a certain observation window (called signal detection overhead); and the initial detection result reporting phase, in which the cognitive users forward their detection results to a fusion center. To avoid interfering with the primary user in the reporting phase, previous research assumed that there is a common control channel (also known as dedicated reporting channel) between the cognitive users and fusion center, which, however, requires extra channel resources and introduces an additional complexity due to the dedicated channel resource management. In this paper, we propose a selective-relay based cooperative spectrum sensing scheme, which is able to control and reduce the interference from cognitive reporting users to primary user without the dedicated channel. We analyze the interference impact on the primary user and show that the interference induced by the reporting users is controllable and can be reduced to satisfy a given outage probability requirement of the primary transmissions. In addition, we investigate the receiver operating characteristics (ROC) of the traditional cooperative sensing scheme (with dedicated reporting channel) and the proposed scheme (without dedicated reporting channel) by jointly considering the signal detection and reporting phases. It is proven that, given a target detection probability, a unique optimal signal detection overhead exists to minimize an asymptotic overall false alarm probability in high SNR regions. We illustrate that, compared to the traditional scheme, the selective-relay based cooperative sensing scheme can save the dedicated channel resources without sacrificing ROC performance. Numerical results also show that, under a guaranteed overall detection probability, an overall false alarm probability can be minimized through an optimization of the signal detection overhead.

In this paper, we propose a cognitive transmission scheme with the best relay selection in a multiple-relay cognitive radio network to improve the secondary transmission performance while guaranteeing the quality-of-service (QoS) of the primary transmissions. We derive the closed-form expressions of outage probability for the secondary transmissions, called secondary outage probability, with the constraint of ensuring a target outage probability of primary transmissions (primary outage probability) for both the traditional non-relay and proposed best-relay selection based cognitive transmission schemes. Numerical results illustrate that, under a primary outage probability constraint, a secondary outage probability floor of the cognitive transmission occurs in high signal-to-noise ratio regions. Besides, the secondary outage probability floor of the proposed scheme is lower than that of the non-relay transmission, which is further reduced with an increasing number of cognitive relays.

In cognitive radio networks, a cognitive source node requires two essential phases to complete a cognitive transmission process: the phase of spectrum sensing with a certain time duration (also referred to as spectrum sensing overhead) to detect a spectrum hole and the phase of data transmission through the detected spectrum hole. In this paper, we focus on the outage probability analysis of cognitive transmissions by considering the two phases jointly to examine the impact of spectrum sensing overhead on system performance. A closed-form expression of an overall outage probability that accounts for both the probability of no spectrum hole detected and the probability of a channel outage is derived for cognitive transmissions over Rayleigh fading channels. We further conduct an asymptotic outage analysis in high signal-to-noise ratio regions and obtain an optimal spectrum sensing overhead solution to minimize the asymptotic outage probability. Besides, numerical results show that a minimized overall outage probability can be achieved through a tradeoff in determining the time durations for the spectrum hole detection and data transmission phases. In this paper, we also investigate the use of cognitive relay to improve the outage performance of cognitive transmissions. We show that a significant improvement is achieved by the proposed cognitive relay scheme in terms of the overall outage probability.

Cooperation is emerging as a promising technology to combat the wireless fading and enhance the channel capacity. However,cooperative transmission can not be guaranteed to perform better always than direct transmission, since the statistics of both direct and relay channels are varied due to large scale fading. Therefore, we design an opportunistic criterion to automatically determine which transmission mode should be adopted depending on the channel quality from the source to the cooperative relay, as opposed to the deterministic cooperation regardless of relay channel conditions. According to the proposed criterion, we further present two opportunistic cooperation schemes, namely, the SDC (selection diversity combining) -based and the MRC (maximum ratio combining) -based opportunistic cooperation, and develop the closed-form expressions of outage probability for the proposed schemes over Rayleigh fading channels. For the purpose of comparison, we also analyze the outage probability for the known deterministic cooperation and coded cooperation. Numerical results show that our schemes outperform the traditional schemes in terms of outage probability, especially when the relay channel conditions worsen. In addition, we compare the MRC-based opportunistic cooperation with the SDC-based opportunistic cooperation, showing the superiority of the former scheme.

The cooperative communication technology proposed in recent years enables network nodes to share their antennas to achieve diversity gain. In this paper, an efficient variation scheme on opportunistic cooperation is proposed by using an outage criterion, in which the cooperation mode will be adopted only when the channel from source to relay does not occur outage event. We derive a closed-form BER (bit error rate) expression for the proposed scheme over Rayleigh fading channels, showing that the full diversity is achieved by the new scheme. Also, the BER performance of the known coded cooperation is presented for the purpose of comparison with our scheme. Numerical results illustrate the superiority of the proposed scheme over the coded cooperation in terms of BER performance. It is pointed out that the corresponding BER advantage of the proposed scheme comes at the expense of increasing system overhead since the new scheme needs some feedback from relay to both source and destination.

The traditional cooperative diversity could improve the system performance greatly through sharing some single-antenna users' antennas of the network, but can't achieve the diversity gain and coding gain at the same time. Accordingly, a novel cooperative diversity scheme based on concatenating channel code and distributed space-time block code (DSTBC) is proposed in this paper to further obtain much more performance gain. And the analytic expressions and numerical results of the single-user capacity, outage probability and bit error rate (BER) for the cooperative transmission over the Rayleigh fading channel are also presented by the theoretical derivation and system simulation. Numerical results show that, even though the interuser channel is noisy, our proposed scheme outgoes the traditional cooperative diversity in terms of system capacity, outage probability and BER.