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 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.

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.

It is anticipated that integrated sensing and communications (ISAC) would be one of the key enablers of next-generation wireless networks (such as beyond 5G (B5G) and 6G) for supporting a variety of emerging applications. In this paper, we provide a comprehensive review of the recent advances in ISAC systems, with a particular focus on their foundations, physical-layer system design, networking aspects and ISAC applications. Furthermore, we discuss the corresponding open questions of the above that emerged in each issue. Hence, we commence with the information theory of sensing and communications (S&C), followed by the information-theoretic limits of ISAC systems by shedding light on the fundamental performance metrics. Next, we discuss their clock synchronization and phase offset problems, the associated Pareto-optimal signaling strategies, as well as the associated super-resolution physical-layer ISAC system design. Moreover, we envision that ISAC ushers in a paradigm shift for the future cellular networks relying on network sensing, transforming the classic cellular architecture, cross-layer resource management methods, and transmission protocols. In ISAC applications, we further highlight the security and privacy issues of wireless sensing. Finally, we close by studying the recent advances in a representative ISAC use case, namely the multi-object multi-task (MOMT) recognition problem using wireless signals.

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.

Integrated sensing and communication (ISAC) has received increasing attention as a potential technology to alleviate spectrum shortage. This paper studies the power-saving problem based on power assignment and subcarrier allocation in ISAC. Specifically, we propose a joint design method to jointly optimize the subcarrier and transmit power allocated to sensing services and communication services respectively. To ensure the quality of sensing service (SS) and communication service (CS), the minimum total power required by the system is obtained by optimizing the allocation of subcarriers and transmission power. Since the formulated problem is nonconvex, which is generally difficult to solve effectively. Inspired by the classical subcarrier allocation algorithm based on channel gain information, we decompose the formulated problem into two convex optimization subproblems that are easy to solve. Numerical simulation results show that the proposed algorithm can effectively improve the power-saving performance of the system compared with the existing algorithms.

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.

This paper investigates the problem of fairness in Dual-Function radar-communications system (DFRC) which exploits orthogonal frequency division multiplexing (OFDM) waveforms for performing radar and communication operations simultaneously. A novel iterative algorithm are proposed to maximize the fairness among different communication users (CUs) with the constraint on radar user’s (RU) perfermance. In our considered system, the optimization problem falls into a mixed integer nonlinear programming (MINLP) problem, which is generally NP-hard. In order to make this thorny problem easy to deal with, we propose an approximate algorithm based on BSUM theory to obtain the feasible solution of the original problem in polynomial time. It is shown, using simulation results, that the proposed optimization strategy outperform other strategies in term of fairness.

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.

To improve detection performance, this paper proposes a non-cooperative unmanned aerial vehicle (UAV) detection strategy based on multichannel detection of remote signal with energy detector (ED). More specifically, the sampling point of remote signal on each subchannel adaptively varies with environmental signal-to-noise (SNR) within constrained scope. The decision on the presence or absence of remote signal is made at the fusion center (FC) based on the majority voting rule. Both theoretical derivation and simulation experiments validate the effectiveness of the proposed scheme.

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).

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.

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.

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.

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 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 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 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 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.

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 investigates network coding with full diversity and a high rate for improving the reliability of wireless communications against the co-channel interference. We consider a cellular network composed of two user terminals that are intended to communicate with a common base station (BS), where a two-way relay node is used to assist the bidirectional communications between the users and BS. We propose an opportunistic network coding (ONC) scheme with the two-way relay, called two-way ONC, which operates depending on whether or not the relay node successfully decodes data packets of the user terminals and BS. Closed-form outage probability expressions of the uplink transmission (from users to BS) and downlink transmission (from BS to users) are derived for the proposed two-way ONC scheme in the presence of co-channel interference over Rayleigh fading channels. We also develop the diversity-multiplexing tradeoff (DMT) of the two-way ONC scheme and show that it obtains the full diversity and an increased multiplexing gain, as compared with conventional one-way relay methods. Additionally, numerical outage probability results demonstrate that the proposed two-way ONC scheme outperforms the conventional one-way relay approaches in terms of improving the communication's reliability against co-channel interference.

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 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.

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 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.

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.