To address the secure downlink communication problem of communication-sensing collaboration in low earth orbit beam hopping integrated sensing and communication (LEO-BH-ISAC) satellite systems, we propose the concept of credible communication and sensing airspace (CCSA). Specifically, CCSA is defined as the intersection of credible communication airspace (CCA), which guarantees secure communication for legitimate users, and credible sensing airspace (CSA), in which eavesdroppers are effectively intercepted. To maximize the CCSA, an optimally balanced beam configuration and power allocation model is established to quantify the relationship between satellite resources and the CCSA. Subsequently, a staged multi-dimensional resource allocation algorithm is proposed, which first accomplishes beam pre-configuration and time slot allocation, and then realizes dynamic power allocation between communication and sensing beams via alternating iterative optimization. Simulation results demonstrate that this method can meet the differentiated throughput requirements of all cells. Meanwhile, compared with the average power scheme, the CCSA is improved by 16.5% on average, and both the secrecy rate and the interception probability approach their theoretical values. Furthermore, the CCSA can be expanded by increasing the power of communication and sensing beams and reducing the secrecy rate threshold. This work validates the effectiveness of the CCSA, providing new ideas for communication-sensing collaboration in LEO-BH-ISAC satellite systems.

ABSTRACT Integrated sensing and communication (ISAC) demonstrates significant advantages in spectrum and resource utilization compared to traditional communication and sensing coexistence systems. To further alleviate the scarcity of spectrum resources, non‐orthogonal multiple access (NOMA), as a potential technology for 6G, is introduced into the ISAC system. Additionally, a transmit antenna selection scheme based on sensing priority and communication user fairness is proposed to further save hardware resources. As a further contribution, the multipath components induced by target reflection are captured and recombined to form constructive signal superposition, thereby further enhancing communication performance. The communication performance of the system proposed is investigated by deriving the exact and asymptotic outage probabilities (OPs), diversity gain, and ergodic communication rate (ECR) of users. Meanwhile, the sensing performance is analyzed in terms of probability of detection (PoD) and sensing rate (SR). Simulation results indicate that: (1) The proposed NOMA ISAC system outperforms the traditional OMA scheme in terms of both OP and PoD; (2) The full‐duplex (FD) scheme sacrifices part of the outage performance in exchange for additional gains in ECR and SR.

Rate Splitting (RS) is a versatile and powerful technique for multi-antenna transmission. In this paper, we study the precoding optimization for RS, which is critically important for improving the system performance but often challenging to address. We first investigate the non-convex sum rate maximization problem under perfect Channel State Information at the Transmitter (CSIT). By constructing a separable structure for the sum-of-functions-of-ratios problem and jointly leveraging the Convex Concave Procedure (CCP) and the Alternating Direction Method of Multipliers (ADMM), we obtain a fast algorithm that substantially reduces the overall computational time through the parallel computation and explicit closed-form solutions. We then investigate the average sum rate maximization problem under imperfect CSIT, which is known as a more challenging non-convex stochastic problem. To obtain a fast algorithm for practical use, we carefully approximate the non-convex stochastic problem to a non-convex deterministic one with acceptable performance loss and tailor the fast algorithm derived from perfect CSIT for imperfect CSIT with modest changes. Numerical results show that compared to the state-of-the-art algorithms, the proposed algorithms achieve comparable sum rates or average sum rates but short computation times for large problem sizes, owing to the unique parallel computation structures and few matrix inverse operations.

In this paper, we investigate reconfigurable intelligent surface (RIS)-assisted integrated sensing and communication (ISAC) systems for robust physical layer security (PLS) schemes. Traditionally, eavesdroppers (Eves) typically avoid interacting with the base station, making it challenging to obtain their relevant information, which limits the implementation of PLS. Fortunately, the sensing information obtained by ISAC can contribute to the design of PLS. Therefore, leveraging imperfect sensing estimation and employing dedicated radar signals as artificial noise, we formulate an RIS-assisted joint active and passive beamforming design problem to maximize the sum secrecy rate while satisfying the user’s quality of service constraints, the transmission power constraints, and the sensing signal strength requirements. To make the problem tractable, we first derive the bound for Eve’s channel state information uncertainty region based on security approximations. Subsequently, we employ the$\mathcal {S}$-procedure and the symbolic-deterministic methods to transform the infinite number of inequalities. We then utilize the first-order Taylor expansion, the second-order cone methods, and the successive convex approximation to address the nonconvexity problem, leading to an efficient suboptimal solution obtained by an iterative algorithm. Finally, the simulation results demonstrate the significant potential of the sensing function in enhancing security and the effectiveness of the proposed robust scheme in flexibly balancing communication and sensing quality.

Movable antennas (MAs) enhance flexibility in beamforming gain and interference suppression by adjusting position within certain areas of the transceivers. In this paper, we propose an MA-assisted integrated sensing and communication framework, wherein MAs are deployed for reconfiguring the channel array responses at both the receiver and transmitter of a base station. Then, we develop an optimization framework aimed at maximizing the sensing signal-to-interference-plus-noise-ratio (SINR) by jointly optimizing the receive beamforming vector, the transmit beamforming matrix, and the positions of MAs while meeting the minimum SINR requirement for each user. To address this nonconvex problem involving complex coupled variables, we devise an alternating optimization-based algorithm that incorporates techniques including the Charnes-Cooper transform, second-order Taylor expansion, and successive convex approximation (SCA). Specifically, the closed form of the received vector and the optimal transmit matrix can be first obtained in each iteration. Subsequently, the solutions for the positions of the transmit and receive MAs are obtained using the SCA method based on the second-order Taylor expansion. The simulation results show that the proposed scheme has significant advantages over the other baseline schemes. In particular, the proposed scheme has the ability to match the performance of the fixed position antenna scheme while utilizing fewer resources.

In this paper, we investigate physical layer security (PLS) for terrestrial RIS-assisted multibeam satellite commu-nications with similar channels. Deploying the reconfigurable intelligent surface (RIS) in each beam, and we use inter-beam in-terference and the channel variability from the RIS to the satellite user and eavesdroppers to enhance satellite security. Specifically, considering the constraints of the secrecy rate, we formulate a problem to minimize the total power by jointly optimizing the satellite beamforming and the RIS beamforming. To solve the problem, the original problem is decoupled into two non-convex optimization sub-problems, i.e., the active and the passive beamforming optimization problem. Then, semidefinite relax-ation (SDR) is used to solve the active beamforming optimization problem. The maximum ratio transmission-based beamforming and SDR are employed separately to solve passive beamforming optimization problem in each beam. The alternating optimization (AO) is utilized to solve the original problem and the convergence is proven. We have carried out simulations to evaluate the effectiveness of our proposed approach, the results show that the proposed algorithms have superior system performance.

The integration of nonorthogonal multiple access (NOMA) with integrated sensing and communication (ISAC) heralds a novel and promising paradigm, advancing the frontier of wireless communication technologies. Nevertheless, this synergistic integration may confront security challenges attributable to intrinsic vulnerabilities within the ISAC frameworks, i.e., sensing targets assume the role of potential eavesdroppers. In this article, we exploit the additional degrees of freedom afforded by reconfigurable intelligent surfaces (RISs) to enhance secure communication and achieve target detection within the NOMA-assisted ISAC system. Specifically, the coexisting radar and NOMA signals are concurrently propagated through both the direct and reflected links. Subsequently, we formulate a secure optimization problem by collaboratively designing the beamforming vectors of the base station and the phase shifts of RIS under the constraints of total transmit power, communication quality of service, and sensing quality. Due to nonconvexity, the optimization problem is decomposed into two subproblems and addressed separately, employing the successive convex approximation approach based on the first-order Taylor expansion and the second-order cone. Finally, we obtain the solution to the original problem utilizing alternating optimization. Simulation results demonstrate that our proposed approach exhibits superior capabilities in secure communication and precise target detection.

Air transportation communication jamming recognition model based on deep learning (DL) can quickly and accurately identify and classify communication jamming, to improve the safety and reliability of air traffic. However, due to the vulnerability of deep learning, the jamming recognition model can be easily attacked by the attacker’s carefully designed adversarial examples. Although some defense methods have been proposed, they have strong pertinence to attacks. Thus, new attack methods are needed to improve the defense performance of the model. In this work, we improve the existing attack methods and propose a double level attack method. By constructing the dynamic iterative step size and analyzing the class characteristics of the signals, this method can use the adversarial losses of feature layer and decision layer to generate adversarial examples with stronger attack performance. In order to improve the robustness of the recognition model, we use adversarial examples to train the model, and transfer the knowledge learned from the model to the jamming recognition models in other wireless communication environments by transfer learning. Simulation results show that the proposed attack and defense methods have good performance.

To overcome the inherent defect of centralized data processing in cloud computing, the mobile edge computing (MEC) brings data storage and computing capacities, to the edge closer to end users. However, the uneven distribution of access vehicles, as well the volume of computing data, cause the workload diversity among various mobile edge computing servers (MECSs). In this paper, we propose a hierarchical model with quality of service (QoS)-aware and power-aware resource management for the cooperative edge-computing-based intelligent vehicular network (CEC-IoV), and the system latency and energy efficiency at MECSs are respectively optimized. Specifically, considering the changing response times versus MECSs’ workloads, the Minimum Latency with Migration Loads (MLML) scheme is developed for workload balance among multiple MECSs. By selecting the appropriate response time threshold and migration loads from overloading MECSs to idle MECSs simultaneously, the load-balancing problem can be efficiently solved for multiple MECSs with unbalanced workloads. On the other hand, through performing workload redistribution and dynamic reconfiguration of virtual machines (VMs) instantiated onto the parallel computing platform at one MECS, the energy-efficiency can be also optimized while guaranteeing the QoS requirement on the processing delay. With the latency constraint, the power minimization problem is formulated to be a convex one, and the semi-closed forms for optimal solutions of VMs’ workloads and processing rates are provided using KKT conditions. Compared with the performance obtained by benchmark schemes, numerical results exhibit that our resource management schemes gain lower system latency and higher energy efficiency.

Filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) has been studied as an alternative waveform for post-orthogonal frequency division multiplexing (OFDM) wireless communications. However, due to the intrinsic imaginary inter-carrier/inter-symbol interference of FBMC/OQAM signals, accurate channel estimation and equalization remain open issues for FBMC/OQAM systems in dispersive channels. In this article, two new signal models of the analysis filter bank (AFB) output are first derived in the absence of any channel assumption and in the presence of a relaxed channel assumption, respectively. Based on the new-built models, initial channel estimation with least squares (LS) (IniCE-LS) is presented and its output is used as the initial condition of recursive weighted LS (RWLS) estimation; moreover, non-iterative and iterative channel equalization combined with RWLS channel estimation (i.e., N-ICE-RWLS and ICE-RWLS) are proposed by using the real-time reconstructed FBMC/OQAM data symbols as pilots. The proposed N-ICE-RWLS and ICE-RWLS schemes not only allow to improve channel estimation and equalization performance, but also achieve high transmission efficiency. Considering the requirements of practical application, we further develop the low-complexity implementation methods of N-ICE-RWLS and ICE-RWLS. All these schemes are validated by analyzing the channel estimation accuracy and bit error rate (BER) over various frequency selective channels.

Multiple antenna technology and cooperative relaying are two efficient methods to improve the capacity and reliability of wireless networks. By marrying cooperative relaying with a Rate-Splitting (RS) approach, Cooperative Rate-Splitting (CRS) shows certain benefits compared to several non-cooperative and cooperative schemes in multi-antenna Broadcast Channel (BC). In this paper, we further investigate the CRS with partial Channel State Information at the Transmitter (CSIT), as partial CSIT is an inevitable issue in practical downlink Multi-user Multiple Input Single Output (MU-MISO) systems. The proposed CRS structure includes multiple users, and one user could be selected as a relay to help the other users. With the flexible design, CRS leverages the benefits of user cooperation and relay (or user) selection for cooperative network, and also retains the superiority of RS for multi-antenna BC. Considering the Weighted Average Sum Rate (WASR) performance, we aim to solve the stochastic optimization that jointly involves the precoder design, the time resource allocation, and the relay selection issue. To address the above non-convex problem, a dedicated algorithm for CRS that exploits the Weighted Minimum Mean Square Error (WMMSE) approach and the Convex-Concave Procedure (CCP) is proposed, such that the WASR is maximized by jointly updating the precoding matrix and time allocation factor according to the incoming user channel state estimate. The proposed algorithm then enables us to solve the relay selection through an exhaustive search. Since the optimal relay selection algorithm has a high computational complexity when the number of users is large, we additionally provide a suboptimal heuristic algorithm to strike a balance between complexity and performance. Numerical results demonstrate that the proposed CRS scheme is capable of achieving a larger WASR than its non-cooperative counterpart and several existing beamforming schemes in a wide range of propagation conditions, and the heuristic algorithm achieves a performance close to the optimal one but with lower complexity.

While the prevalent cloud storage platforms are offering convenient services in support of diverse data-driven applications for clients, various security concerns raise in terms of data confidentiality, availability, and retrievability. Among them, servers' dishonesty on the location-specific data backup becomes a serious concern when the data stands out clients' control, considering the strict regulations imposed by many governments and organizations on data storage location. This article studies location-aware data backup verification for the data stored in clouds and aims to design a secure framework, named as ReliableBox, enabling the clients to verify if their data have been backed up on the remote servers with specific geolocation. In the design of ReliableBox, we leverage the prominent proof-of-storage techniques for data possession proof, and take advantage of multilateration geolocation and Intel SGX for the precise communication delay measurement and trust computing delay measurement, respectively. In ReliableBox, a client first computes integrity tags for the files and then outsources both the files and tags to the cloud storage server. In the later attestation, with the precise network delay and distance measurement from location-known verifiers, the client verifies that the outsourced files are intact and backed-up to hosts at the specific geolocation. With the customized design, ReliableBox can support the security needs in terms of both data integrity and backup location verification for clients, even when there exists potential dishonest cloud service providers who may manipulate the network delays or forge verification proofs. We provide security analysis to show the security property of ReliableBox in terms of data access, confidentiality, and verifications. In the end, we implement the system prototype and deploy it into several prevalent and commercial cloud platforms for performance evaluation. The experimental results demonstrate that ReliableBox is secure in support of data integrity checking and location-aware backup auditing, while it is robust to the data possession and location spoofing attacks.

This study presents a full‐duplex (FD) relaying non‐orthogonal multiple access (NOMA) scheme with self‐interference at relay taken into account, where a sender communicates with multiple terminals simultaneously with the help of an FD amplify‐and‐forward relay. The relay is equipped with two antennas, one for transmission and the other for reception. Moreover, with a direct link as a useful information transmission link, selection combining is applied to merge the information from the sender and the relay. General closed‐form expressions of the outage probability are deduced and the upper bound of the ergodic sum rate is derived, including half‐duplex (HD) mode relay as a special case. Compared with the HD relaying scheme, the authors can observe that the FD relaying scheme can achieve higher ergodic sum rate in the moderate‐ and high‐SNR regions and that the outage behaviour can be improved in most cases (if the self‐interference is not extremely severe). More importantly, the use of NOMA in FD relaying system is shown to outperform conventional orthogonal multiple access (OMA) network in terms of terminal fairness. Extensive simulation results are presented to confirm their conclusions and provide a significant reference for the system design.

Due to its promising performance in a wide range of practical scenarios, Rate-Splitting (RS) has recently received significant attention in academia for the downlink of communication systems. In this letter, we propose and analyse a Cooperative Rate-Splitting (CRS) strategy based on the three-node relay channel where the transmitter is equipped with multiple antennas. By splitting user messages and linearly precoding common and private streams at the transmitter, and opportunistically asking the relaying user to forward its decoded common message, CRS can efficiently cope with a wide range of propagation conditions (disparity of user channel strengths and directions) and compensate for the performance degradation due to deep fading. The precoder design and the resource allocation are optimized by solving the Weighted Sum Rate (WSR) maximization problem. Numerical results demonstrate that our proposed CRS scheme can achieve an explicit rate region improvement compared to its non-cooperative counterpart and other cooperative strategies (such as cooperative NOMA).

In this paper, a novel low-complexity sphere decoding (SD) algorithm (QSM-Tx) designed for quadrature spatial modulation (QSM) is proposed. The reduction of the computational complexity for QSM-Tx is achieved by only searching the specific points lying inside the given sphere rather than all possible points for maximum likelihood (ML) detection. And the bit-error-rate (BER) performance for the proposed QSM-Tx remains close to the ML algorithm. Furthermore, a modified sphere decoding for QSM, called QSM-Rx, with low complexity and the optimum BER performance is presented as well. Finally, the validity of the proposed QSM-Tx scheme is verified by simulation results.

Non-orthogonal multiple access (NOMA) and multiple input multiple output (MIMO) have been widely regarded as two key techniques envisioned for 5G network. In this paper, we apply the core principle of antenna selection (AS) to multi-relay cooperative network, and propose a mechanism that has merits of both MIMO and NOMA. Then, in view of performance and complexity tradeoffs, a suboptimal joint selection scheme with reduced complexity is further presented. Furthermore, to characterize the achievable rates of proposed scheme, the lower and upper bounds of ergodic sum rate at high SNR are derived in closed form. The analytical results are valid for the evaluations for various antenna and relay configurations. Finally, through comprehensive simulations and comparisons, we demonstrate that the proposed scheme can outperform the conventional MIMO scheme or other relevant NOMA schemes in terms of ergodic sum rate and outage performance.

In this study, non‐orthogonal multiple access (NOMA)‐based multiple‐antenna and multiple‐relay networks over Nakagami‐ m fading channels with imperfect channel state information (CSI) and successive interference cancellation (SIC) error are investigated. In order to measure the performance of the system, the exact outage probability and the ergodic sum rate of the system are explored. By analysing the impact of the partial relay selection scheme, it is shown that the performance gain can be achieved significantly by increasing the number of relays from one to two, but when the number of relays is equal to or greater than three, the increment of the performance gain becomes smaller and smaller, and finally saturated as the number of relays increases. Furthermore, due to the complexity of the derivation process for the ergodic sum rate, an upper bound of the ergodic sum rate is taken into account. Finally, by carrying out a series of experimental simulations, the authors prove the correctness of the theoretical derivations above, and from the comparison of NOMA and orthogonal multiple access (OMA), it is found that NOMA can provide better spectral efficiency and user fairness than OMA.

Non‐orthogonal multiple access (NOMA) has been adopted as a potential research candidate in the fifth generation wireless networks owing to its better user fairness and high‐frequency spectrum effect. Energy harvesting has also been attracted for its advantage to prolong the lifetime and without sustained energy supply. Considering the system complexity and lower interference, in this study, a hybrid network is analysed. The multiple users are divided into multiple two‐user groups, where NOMA is applied within each group and orthogonal multiple access (OMA) is applied for inter‐group. Furthermore, with the actual scenario consideration, imperfect channel state information (ICSI) is taken into account in this paper. The authors conduct some works in this study. First, the system performance is analysed and closed‐form expressions for the exact outage probability of the two paired users are derived. By further evaluating the outage probability at the high system signal‐to‐noise ratio regimes, it is observed that error floor exists in the outage probability due to the presence of ICSI. Finally, numerical simulations are performed to compare NOMA and the general OMA to verify the superiority of NOMA and highlight the importance of considering ICSI in wireless networks.

Energy efficiency (EE) plays an indispensable role in the battery constrained systems. In this study, the EE optimisation problem for asymmetric analogue network coding (ANC) protocol is analysed in a two‐way relaying network, where the two sources have different transmission rate requirements. A generalised power consumption model is applied consisting of not only the transmit power and fixed circuit power, but also the most general rate‐dependent dynamic circuit power, with which a power allocation problem is formulated to achieve the maximal EE constrained by the rate requirements as well as the total transmit power budget. By exploiting the two‐layer optimisation method and non‐linear fractional programming theorem, the original EE maximisation problem is solved in an efficient way. Specially, when the rate‐dependent circuit power is modelled as the linear function of transmission rate, a closed‐form solution is obtained. Through simulations, the proposed power allocation algorithm is verified and the impacts of some system parameters such as relay location, transmit power and transmission rate asymmetry are also provided to gain useful insights, which facilitates the energy‐efficient design for the practical asymmetric ANC protocol.

In this paper, the interference statistics in a clustered device-to-device network is studied and the cluster centers are modeled as a Poisson point process. For such, each active receiver device chooses its transmitter randomly following the uniform distribution. We first characterize the statistics of the number of intra/inter-cluster interferers and develop the simplified expressions for several special cases. With the aid of these derived probability distributions of the number of interferers, the Laplace transforms of intra/inter-cluster interference are formulated and simplified approximations for the special cases are also attained. Finally, several applications of the derived interference statistics are shown to illustrate the performance difference between the proposed interference model and the assumed one in the literature.

In this paper, we present a time-domain turbo equalizer for oversampled single carrier (OSC) system under doubly selective fading channels, which is in a new scenario. The detected information symbols are stacked into a state space vector, and its relationship with the received signals is developed, which contributes to realising a Kalman filter. In the receiver, the adaptive equalizer using Kalman filters cooperates with the soft-input soft-output (SISO) decoder to iteratively generate posteriori estimates for each data symbol. Numerical simulation results demonstrate the proposed scheme outperforms both the extended Kalman filter (EKF) method and the linear Kalman filter (LKF) method significantly. Furthermore, complexity analysis is also presented to illustrate the increasing complexity.

Traditional vector orthogonal frequency division multiplexing (V‐OFDM) can obtain both multi‐path diversity and outage gain by transforming an inter‐symbol interference (ISI) channel into multiple ‘ISI‐free’ channel vectors. To achieve full multi‐path diversity and acquire better outage performance, a low‐complexity precoding scheme combined with simple minimum mean square error (MMSE) equalisation for V‐OFDM system is proposed in this study. First, the upper mutual information bound of the V‐OFDM system is derived with channel state information at transmitter. Based on the upper bound, an optimal precoding scheme for V‐OFDM is presented. Meanwhile, the closed‐form and asymptotic expressions on outage probability of the proposed scheme are derived. Simulations show that this low‐complexity scheme can achieve full multi‐path diversity and result in lower outage probability than the traditional V‐OFDM scheme. Besides, the proposed scheme in ISI fading channel is capable of approaching the upper bound with <0.1 bits/s/Hz performance gap and improving average channel capacity with 2 bits/s/Hz over the traditional V‐OFDM scheme.

Non‐orthogonal multiple access (NOMA), which uses the power domain for multiple users, has been considered as an emerging candidate for fifth generation downlink transmission. Energy harvesting (EH), a promising technology to prolong the lifetime of energy‐constrained wireless networks, has attracted significant attention. In this study, the authors first combine NOMA with EH (NOMA–EH) to enhance system spectral efficiency and user fairness. An NOMA‐based relaying network is analysed, where the relay node is equipped with EH device, and functions as the bridge between the base station and the multiple users. The outage performance of the NOMA–EH scheme‐based relaying network is studied, where the transmitting antenna selection is applied at the base station and maximal ratio combining is applied at the multiple users. Furthermore, closed‐form expression for the system outage probability is also derived. On the basis of the computer simulations, they show the correctness of the numerical analysis and confirm the superiority of the proposed NOMA–EH scheme.

Antenna selection and relay selection are two potent methods to enrich the system capacity by selecting the perfect one to function. In this letter, a downlink multiple-input multiple-output decode-and-forward relaying network is analyzed by considering Nakagami-m fading as a model for the channel gains. The system performance is analyzed by using a joint scheme (JS), which joints the relay selection and the antenna selection at the base station and the selected relay. A closed-form expression corresponding to the outage probability is derived, and the approximate expression of the outage probability in the high-signal-to-noise-ratio regime is obtained. Finally, simulations show the superiority of the JS.

This paper investigates the interference statistics in a clustered device-to-device network, where the cluster centers form a Poisson point process and each active receiver device selects the transmitter randomly following the uniform distribution. The statistics of the number of intra/inter-cluster interferers is first analyzed for both finite and infinite number of receivers per cluster. In addition, for the special cases where the numbers of transmitters and receivers differ significantly, simplified expressions are given. Then, using these derived probability distributions of the number of interferers, the Laplace transforms of intra/inter-cluster interference are established and simple approximations for the special cases are obtained as well. Our results show the following: 1) with the concurrent uniform transmitter selection, the distribution of the number of intra/inter-cluster interfering devices differs greatly from the truncated Poisson distribution assumed in the literature; 2) the model assumed in the literature underestimates the coverage probability specially for a small number of transmitters/receivers; and 3) unlike the model assumed in the literature, the ASE of the proposed model increases with the number of transmitters/receivers when the average number of active receivers is relatively small, while it does not monotonically vary with the number of transmitters/receivers when the average number of active receivers is large.

In this paper, a 5G energy-constrained network is considered, where a non-orthogonal multiple access (NOMA) scheme is applied at the multiple users. The outage probability and the ergodic rate are studied as two benchmarks to evaluate the system performance. First, a closed-form expression for the exact outage probability is derived. A lower bound of the outage probability is obtained and shown to predict well the system performance. In addition, the outage probability in the high signal-to-interference-plus-noise ratio (SINR) regime is studied to assess its upper limit while achieving the diversity order. Second, a closed-form expression for the upper bound of the ergodic rate is obtained. Then the ergodic rate in the high SINR regime is also analyzed. Finally, numerous computer simulations are performed to corroborate the superior performance of NOMA relative to orthogonal multiple access schemes and the validity of the derived analytical results.

In this paper, considering a mixed Rician and Rayleigh fading environment, we investigate the impact of line-of-sight (LOS) path component on the outage performance of dual-hop energy harvesting (EH) amplify-and-forward (AF) relay systems. To this end, a tight asymptotic outage lower bound expression is derived, which reveals that the system diversity order is limited to one and the outage behavior decays as log(SNR)/SNR, with SNR denoting the transmit signal-to-noise ratio (SNR) at the source. In contrast, when the LOS path component in the first-hop link becomes very strong (i.e., a large Rician K factor), our analytical results show that the easing-off factor log(SNR) can be effectively eliminated, becoming the decaying rate of outage curves steeper in this case and, consequently, improving the end-to-end transmission robustness since the outage curves scale as 1/SNR at high SNR regions.

Non‐orthogonal multiple access (NOMA), which is an effective solution to improve spectral efficiency, has been considered as an emerging candidate for the fifth generation multiple access. In addition, simultaneous wireless information and power transfer, which aims to maximise energy efficiency, has received significant attention. In this study, the authors design NOMA for downlink energy harvesting (EH) multiple‐antenna relaying networks. The base station communicates with multiple mobile users simultaneously via an EH relay node. In the first slot, the relay node harvests energy from the received signals; and in the second slot, the relay node uses the harvested energy to broadcast the received signals to all mobile users. Antenna selection is adopted at the base station while maximal‐ratio combining is applied at the mobile users. The outage performance for the NOMA‐EH relaying networks is studied over Nakagami‐ m fading and closed‐form expressions for the outage probability are obtained. In addition, the presented simulation results demonstrate the correctness of the authors’ analysis and the advantage of NOMA over conventional orthogonal multiple access.

Energy harvesting, which is a promising technology to prolong the lifetime of energy-constrained wireless networks, has attracted lots of attention. In this letter, we consider an energy-harvesting multiple-relay multiple-input multiple-output (MIMO) decode-and-forward (DF) cooperation network. A joint relay-and-antenna selection (JRAS) scheme, which jointly determines the best transmit antenna at the source node, the best relay, and the best transmit antenna at the selected relay, is proposed to minimize the system outage probability. Then, we analyze the outage performance of the JRAS scheme, and the exact expression for the outage probability is derived. Finally, simulation results show the correctness of the theoretical analysis and the superiority of the JRAS scheme.

A general distributed multi-antenna two-way relaying network with multiple relays is considered in this paper. We first obtain a tight lower bound of pairwise error probability (PEP) of a maximum likelihood detector for the general distributed linear dispersion code for a half-duplex amplify-and-forward two-way relaying network (TWRN) consisting of two sources with each having single antenna and N relays with each having two antennas. Furthermore, by jointly considering signal precoding at the sources and signal processing at the relays, a general distributed concatenated recursive Alamouti-circulant space-time block code is proposed for the considered TWRNs. Our design ensures that the equivalent channel matrices at both source nodes are the so-called the recursive Alamouti-circulant matrices, with each block being a product of the two Alamouti channel matrices. Based on a lower-upper bound strategy and an induction method, asymptotic PEP formula is attained to show that given the optimal angle rotation matrix and the precoding matrix, the code can meet the lower bound of the diversity gain, as well as the maximum coding gain. In addition, the proposed rate one code turns to be effectively decodable.

In this paper, a three-node energy-harvesting (EH) cooperative system with ROle SElection (ROSE) mechanism, where the cooperative role of each node is dynamically adjusted based on the surrounding wireless fading environment is considered. We first study a benchmark scenario with the fixed role configuration, from which a tight lower bound for the asymptotic outage behavior is derived. For this case, the achievable diversity is limited to two and the system outage behavior at high signal-to-noise ratio (SNR) is affected by the easing-off factor log c0together with log(log c0) denoting the source transmit SNR. After that, a dynamic role selection policy is incorporated and the asymptotic analysis shows that the ROSE mechanism can enhance the system diversity order to three, and the decaying law of outage behavior is no longer affected by the c0and (log c0) factors. In addition, the power-splitting receiver achieves the same asymptotic outage performance as that of the ideal EH receiver. Our results reveal that the line-of-sight component is crucial to improve the performance of the EH-based ROSE cooperative system, and the role decision with imperfect channel information has a detrimental effect on the system's diversity gain.

In this paper, we investigate the effects of line-of-sight (LoS) path and opportunistic scheduling (OS) on dualhop energy harvesting (EH) relay systems, where the LoS path exists between source and EH relay. We first study a single-destination case and show that the system diversity order is one and the outage behavior decays as log(SNR)/SNR, with SNR denoting the transmit signal-to-noise ratio (SNR) at the source. In the presence of a strong LoS path, asymptotic results reveal that the easing-off factor log(SNR) can be eliminated and a faster decay rate, 1/SNR, is achieved. Afterward, the OS of the second hops (N destinations) is considered, and its separate as well as joint effects on the system outage behavior are examined. In addition, the choice of power-splitting factor is discussed and the impacts of EH constraint on the performance limit compared with conventional relay system are investigated, showing that under the same transmit-power consumption from the fixed energy supply, the EH relay system could achieve the same performance with that of conventional relay system if the wireless power transfer in the first hop is lossless and the energy conversion efficiency at EH relay tends to be unity.

This paper investigates joint relay and antenna selection schemes for two-way amplify-and-forward (AF) relaying system, where a multiantenna source exchanges information with a single-antenna source by exploiting a group of single-antenna relays. In view of performance and complexity tradeoffs, two optimal and one suboptimal joint selection schemes are proposed and compared. In contrast to the optimal selection schemes, the suboptimal counterpart is characterized by its low overhead and implementation complexity. The performance of these selection strategies has been investigated by deriving the outage probability approximations in closed form. Moreover, the achievable diversity orders and the corresponding coding gains are derived through high signal-to-noise ratio (SNR) approximations of the outage probability. Our theoretical and simulation results show that the performance gap between the optimal and suboptimal schemes disappears for specific scenarios, which provides useful design guidelines for practical two-way relaying system.

Two opportunistic transmission schemes, i.e., joint user-relay selection with beamforming (JURS-BF) and joint user-relay selection with antenna selection (JURS-AS), are proposed and analyzed for multi-user multi-relay bidirectional relaying networks (BRNs) composed of one N-antenna base station (BS)/ access point (AP), M single-antenna users, and K single-antenna relays. In particular, by considering the rate asymmetry between the BS/AP and the users, joint user-relay selection is tackled with beamforming and antenna selection to combine the benefits of multi-antenna diversity, multi-relay diversity, and multi-user diversity. To characterize the performance of the proposed JURS-BF and JURS-AS schemes, lower and upper bounds on the system outage probability are derived in closed form under Nakagami-m fading environment, followed by the diversity, coding gain analysis, and some discussions on the relay deployment. Our analytical results are verified through simulations, and it is proved that the proposed suboptimal schemes perform closely to the traditional outage-optimal counterparts while with much lower implementation complexity, which provides valuable insights into practical BRNs.

This paper investigates the feasibility of deploying dynamic role selection (ROSE) mechanism in three-node energy harvesting (EH) cooperative relay systems. A benchmark case of fixed role configuration is first studied which shows that full diversity order can be achieved and the system outage probability at high signal-to-noise ratio (SNR) is determined by the scaling laws of log c0/c0^2 together with log(log c0)/c0^2, with c0 denoting the transmit SNR at the source. Then, the ROSE mechanism is merged into the EH cooperative system and it is analytically shown that the ROSE mechanism can enhance the system diversity order to three, and the high-SNR outage behavior is no longer affected by the log c0 and log(log c0) factors. Interestingly different from the conventional EH relay system with fixed role configuration, in the high SNR regime our results show that the energy conversion efficiency and the power-splitting factor at the relay have negligible effect on the outage behavior of ROSE cooperation.

Summary With the rapid growth of cloud computing services, there is an increasing need to expand applications across data centers. In these paradigms, common information exchange between two data centers can be assisted via one or more substrate nodes, which correspond to bidirectional relaying communication. In this paper, we investigate the performance of time division broadcast (TDBC) protocol in bidirectional cloud networks in the presence of channel estimation errors (CEEs). The tight asymptotic expressions for individual and system outage probabilities are first presented in closed‐form. It is shown that CEE causes outage probability to remain fixed even if the channel is noiseless. Then, the symbol error rate performance for TDBC protocol with M‐ary Phase Shift Keying and M‐ary quadrature amplitude modulation signals is analyzed. Simulation results validate the accuracy of our analytical results. Furthermore, comparison of signal‐to‐noise ratio gap ratio shows that TDBC protocol is less sensitive to the effect of CEE than analog network coding protocol. Copyright © 2014 John Wiley & Sons, Ltd.

Non‐orthogonal multiple access (NOMA), an emerging technology to improve system capacity and spectrum efficiency, has attracted significant attention. In this study, the authors propose a NOMA‐based downlink cooperative cellular system, where the base station communicates with two paired mobile users simultaneously through the help of a half‐duplex amplify‐and‐forward relay. The outage performance of the system is investigated and closed‐form expressions for their respective exact and asymptotic outage scheme are derived. Furthermore, they study the ergodic sum rate of the two paired users and the upper bound of the ergodic sum rate is obtained. By comparing the NOMA with conventional multiple access (MA) via numerical simulations, they have shown that NOMA can obtain the same diversity order with conventional MA, and achieve nearly the same sum rate with conventional MA. Furthermore, NOMA can offer better spectral efficiency and user fairness since more users are served at the same time, frequency, and spreading code.

Energy harvesting, which is an emerging technology to prolong the lifetime of energy‐constrained wireless networks, has received significant attention. In this study, the authors consider an amplify‐and‐forward‐based asymmetric two‐way relaying system, where two source nodes exchange information through an energy‐constrained relay node. The relay node scavenges energy from the received signals and uses the harvested energy to forward the received signals to the two terminal nodes. The signal power splitting ratio, depicting the trade‐off between the harvesting energy and the forward signals’ power, is a crucial factor for the system outage probability. Therefore, an optimal power allocation (OPA) scheme, which jointly takes into consideration the optimisation of signal power splitting ratio and the transmission power allocation, is proposed to minimise the outage probability. In addition, a close‐form solution for the OPA scheme is obtained. Finally, a joint OPA and relay selection schemes are presented to further improve the system performance. Simulation results highlight the superiority of the author's proposed scheme.

This paper advocates a distributed role selection strategy to coordinate two-way relaying transmissions among three cooperative nodes. For such, the local channel state information comparison and decision feedback mechanism are merged into classical analog network coding (ANC) and time division broadcast (TDBC) protocols such that the cooperative role of each node can be designated in a distributed fashion. We refer to this distributed role selection rule as d-ROSE. In both ANC-based and TDBC-based two-way relaying scenarios, strict proof for the equivalence of d-ROSE and optimal ROSE is given, which indicates that albeit the different form, their final role decision is essentially the same. Outage analysis for the d-ROSE strategy is carried out and the scaling law of the system outage behavior at high signal-to-noise ratio (SNR) is characterized, which manifests that d-ROSE can enhance the system diversity gain to one-order higher relative to the ANC and TDBC protocols. It is also shown that d-ROSE can reduce the signaling overhead upto 60% to perform the outage-optimal role selection. Finally, the impacts of node placement on the outage performance as well as the average signaling overhead of d-ROSE are numerically evaluated and some useful conclusions are drawn.

In this study, the optimal power allocation (OPA) problem is studied for the two‐way relay channel employing the analogue network coding protocol with the asymmetric traffic requirements in a cognitive radio network. The OPA scheme is proposed by maximising the sum‐rate of the success delivery bit from the secondary user (SU) perspective subject to the sum‐power constraint and the interference power threshold (IPT) constraints to the primary user (PU). In particular, for the purpose of accomplishing the power allocation more realistically, the interference to each other between the PU and the SU and the relay is taken into consideration and by solving the optimisation problem composed for each case, the closed form expressions of each node power are obtained. All the simulation results have demonstrated that the system with the proposed scheme outperforms the equal power allocation (EPA) scheme irrespective of the IPT. When the IPT constraint is introduced, the sum‐rate of the success delivery bit of the SU communication converges to a certain level with the increasing total power. Meanwhile, the proposed strategy is superior to the conventional EPA scheme for any relay location, which is more remarkable when the relay is located on either end node regardless of the traffic symmetry or the asymmetry.

Waiting time occupies an important position in user's quality‐of‐service (QoS) evaluation system. However, in conventional overlay cognitive radio networks (CRN), the secondary users are not permitted to transmit any personal information whenever the spectrum is occupied by primary users. It follows that the waiting time of users depends heavily on the primary spectrum occupancy, which, in turn, cannot guarantee the QoS of the secondary users. To remedy this, a game theoretical cooperative waiting‐time reducing method (CWTRM) is proposed in a self‐generated CRN to meet users’ waiting‐time demands. The proposed scheme divides the users to temporary primary and secondary groups instead of introducing an extraneous secondary network. Specifically, waiting users are served as temporary secondary users. The cooperation between them and the temporary primary users is proceeded in each frame to make reuse of the unused time fractions, thereby reducing the global waiting time of the network. The optimal relay power, pricing and corresponding cooperating strategies are determined with the utilisation of Stackelberg game and Hungarian method. Finally, the superiority of the proposed CWTRM over non‐cooperative schemes is validated through simulations.

In downlink multiuser multiple‐input multiple‐output (MU‐MIMO) systems, block diagonalisation (BD) is a well‐known precoding technique that eliminates inter‐user interference. The number of simultaneously supportable users with BD is limited by the number of base station transmit antennas and the number of user receive antennas. Traditional MU‐MIMO scheduling algorithms focus on sum capacity. However, these user scheduling algorithms might not be optimal with respect to energy efficiency. Here, the authors consider the energy‐efficient MU‐MIMO scheduling that maximises the energy efficiency. The brute‐force search for the optimal user set, however, is computationally prohibitive. Therefore they propose a low‐complexity user scheduling algorithm for energy‐efficient MU‐MIMO systems. They first obtain an approximation expression for the energy efficiency by utilising the upper bound of the MU‐MIMO system capacity. Then they show that the maximum energy efficiency can be achieved if the scheduling user set is selected to obtain the largest matrix volume. The numerical results show that the proposed algorithm achieves a good tradeoff between energy efficiency and computational complexity. They also consider the problem of maintaining fairness among users, and propose a simplified proportional fair (PF) scheduling algorithm.

In this letter, we investigate secure multicasting for simultaneous wireless information and power transfer (SWIPT) in the presence of multiple energy receivers who have potential to eavesdrop on the messages of information receivers. Under both, signal-interference-noise ratio (SINR) and harvested energy constraints, we propose a novel secure multicast design to minimize the total power by using the techniques of rank-two beamformed Alamouti coding and semidefinite relaxation (SDR). The sufficient conditions are derived under which the proposed secure multicast design is optimal. We also present the corresponding rank-two Gaussian randomization procedure to obtain a suboptimal solution when SDR is not tight. Simulation results are then provided to demonstrate the efficacy of the proposed design in power saving.

In this paper, we consider a half-duplex amplify-and-forward two-way relaying network consisting of two sources with each having a single antenna and N relays with each having two antennas. For such a system with a general distributed linear dispersion code, a tight lower bound of pairwise error probability (PEP) of the maximum likelihood (ML) detector is derived, showing that diversity gain function cannot decay faster than lnNSNR/SNR2N, where SNR is signal to noise ratio. Particularly for N = 2b, a new distributed concatenated space-time block code (STBC) is proposed and its asymptotic PEP formula is attained, showing that the code presented in this paper achieves the maximum diversity gain, i.e., meeting the lower bound of the diversity gain function, as well as the maximum coding gain.

Tone injection (TI) mitigates the high peak-to-average power ratio problem without incurring data rate loss or extra side information. However, optimal TI requires an exhaustive search over all possible constellations, which is a hard optimization problem. In this paper, a novel TI scheme that uses the clipping noise to find the optimal equivalent constellations is proposed. By minimizing the mean error of the clipping noise and possible constellation points, the proposed scheme easily determines the size and position of the optimal equivalent constellations. The proposed scheme achieves significant PAPR reduction while maintaining low complexity.

In this paper, an optimal and centralized role selection scheme for two-user amplify-and-forward relaying systems is proposed, where two users compete to transmit their own information to a common destination. Relying on the instantaneous channel conditions, either of the two users can opportunely play the role of an information source and the other user will serve as an amplify-and-forward relay. The proposed framework aims to maximize the received signal-to-noise ratio (SNR) at the destination, which enhances the transmission reliability. In order to reveal the inherent impacts of different link statistics on the system outage behavior, a closed-form lower-bound expression for the outage probability is derived. Moreover, an asymptotic outage analysis is carried out and points out that the high-SNR outage behavior of the proposed scheme is dominated by the user-destination links irrespective of the inter-user link. Our analytical results are corroborated by means of Monte Carlo simulations.

Utilizing the recently-developed unique factorization of signals and distributed Alamouti coding scheme, a distributed collaborative Alamouti space-time block code design is presented for a two-way amplify-and-forward (AF) relaying network, where both two sources and the relay are equipped with a single antenna. For such system, two asymptotic pairwise error probability (PEP) formulae are first derived for both fixed-gain AF and variable-gain AF over Rayleigh channels with the maximum likelihood detector. Then, subject to the constraints on a fixed transmission bit rate and unity average transmission energy, the optimal constellation combinations with the optimal energy-scales as the coefficients are attained by minimizing the dominant term of PEP. It is shown that the PEP and the average block error rate of the newly-designed code are superior to those of the conventional distributed Alamouti code.

In this paper, we consider an uncoded cooperative relaying system using an adaptive decode-and-forward (ADF) protocol. Unlike many existing works that adopt single-symbol transmissions, we assume more reasonable frame transmissions here. The primary focus of this paper is on the performance aspects, aiming to offer important theoretical guidance to system designers. Specifically, we derive an accurate bit error rate (BER) expression as well as its asymptotic approximation for the considered ADF cooperative system. The presented analysis shows that the diversity gain can be fully achieved for arbitrary frame length, while the coding gain depends on the frame length. Moreover, by maximizing the coding gain, we obtain an optimum power allocation (OPA) scheme, which can provide a power gain of up to 3 dB over the equal power allocation (EPA) scheme. Simulation results are presented to verify the theoretical analysis.

This paper investigates channel coded transmission schemes for two-way relay networks (TWRNs) where two terminal nodes exchange information through a set of amplify-and-forward (AF) relays over frequency-selective fading channels. Specifically, we assume that the two terminal nodes employ bit-interleaved coded modulation (BICM) and orthogonal frequency division multiplexing (OFDM) for coded data transmission, while the relays employ distributed space-time coding (DSTC) for forwarding the received signals. Our main contribution lies in analyzing the achievable diversity order of such channel coded AF-TWRNs. We show that the two terminal nodes, by using a maximum-likelihood (ML) BICM-OFDM decoder, can harvest the full cooperative and frequency diversity. Simulation results are presented to verify our analytical results.

Energy efficiency (EE) is among the main considerations in the design of modern wireless networks. In this paper, we investigate the EE enhancement for asymmetric analog network coding (ANC) protocol of a two-way relay system based on statistical channel information. A power allocation problem is formulated as the system EE maximization problem with objective function quantified by Goodbit-per-Energy (GPE). Importantly, the EE optimization problem may not be convex and can be categorized into a nonlinear fractional programming problem. Therefore, to solve the problem, a nonlinear fractional programming based algorithm is proposed and closed-form solution is obtained, providing valuable insights into practical system designs. Simulation results highlight the effect of the proposed power allocation.

Utilising the recently‐developed unique factorisation of signals and distributed Alamouti coding, a distributed collaborative space‐time block code design is presented for a two‐way amplify‐and‐forward (AF) relaying network, where all nodes are equipped with a single antenna. Two asymptotic pairwise error probability (PEP) formulae are firstly derived for both fixed‐gain AF and variable‐gain AF over Rayleigh channels with the maximum‐likelihood detector. Then, subject to the constraints on a fixed transmission bit rate and unity average transmission energy, the optimal constellation combinations with the optimal coefficients are attained by minimising the dominant term of PEP. It is shown that the PEP and the average block error rate of the newly‐designed code are superior to those of the conventional distributed Alamouti code while the prior still has linear‐decoding complexity.

In the cooperative relay system, different frames from different relays are not aligned in the time domain, and have different carrier frequency offsets (CFOs). Estimation of these timing offsets and CFOs is a crucial and challenging task. In this study, a new training structure is proposed, and a novel time and frequency synchronisation scheme is presented for orthogonal frequency division multiplexing‐based (OFDM‐based) cooperative systems. The coarse time synchronisation is accomplished by using the symmetric conjugate of the training symbol and the fine time synchronisation is accomplished by segment‐moving correlation. The fractional frequency offset is estimated by using the phase difference of the received signal and the integral frequency offset is estimated by utilising the good autocorrelation of the training symbol in frequency domain. The analysis and the simulation results show that, compared with the traditional scheme, the proposed scheme has better timing and frequency offset estimation performance and lower complexity for OFDM‐based cooperative systems.

Finding the capacity region of a Gaussian X network is an open problem in information theory. In this study, the authors characterise the capacity regions of 2 × 2 user multiple‐input multiple‐output Gaussian X channels for very strong interference and generally strong interference. Moreover, the sum capacity is established for noisy interference. Sufficient conditions for Gaussian X networks to have very strong interference, generally strong interference and noisy interference are derived.

Signal space cooperative (SSC) system is bandwidth and energy efficient, which can achieve full‐rate and full‐diversity simultaneously. In this study, the authors first derive accurate bit error rate (BER) of the SSC system with maximum‐likelihood detection (MLD), low‐complexity linear detection (LD) and combined MLD and LD (MLD&LD). Specifically, closed‐form BER expressions for the three detection schemes are derived. Also, asymptotic approximation analysis is performed to show clearly the diversity and coding gains of the three detection schemes. Through the analysis, the authors find out that MLD is optimal in diversity but has a high complexity, whereas LD is of low complexity but only achieves diversity order one. Interestingly, MLD&LD has almost the same BER as the BER‐optimal MLD over the whole SNR range when equal power allocation (EPA) is adopted, whereas having lower complexity than MLD at low‐to‐medium SNR. Then, optimum power allocation for MLD&LD is developed, however, the largest power gain over EPA is proven to be only 0.37 dB. In view of the BER performance of MLD&LD and implementation simplicity of EPA, they conclude that MLD&LD with EPA is a wise choice for the SSC system. Finally, outage probability of MLD&LD with EPA is analysed to characterise the transmission reliability.

In this study, the authors present a cooperative spectrum sensing scheme based on opportunistic amplify‐and‐forward relaying (denoted by CSS‐OAFR), which provides higher detection performance by selecting the best cognitive relay and is interesting in distributed cognitive radio networks. Moreover, the authors focus on the detection performance using the energy detector over independent and non‐identically distributed Nakagami‐ m fading channels. In particular, a tight closed‐form lower bound of the average missed‐detection probability is derived for the convenience of performance evaluation in practice. Finally, simulations are provided to validate the CSS‐OAFR scheme and the derived closed‐form lower bound.

This letter studies cooperative secure beamforming for amplify-and-forward (AF) relay networks in the presence of multiple eavesdroppers. Under both total and individual relay power constraints, we propose two schemes, namely secrecy rate maximization (SRM) beamforming and null-space beamforming. In the first scheme, our design problem is based on SRM. Using a suboptimal, but convex, technique-semidefinite relaxation (SDR), we show that this problem can be handled by performing a one-dimensional search which involves solving a sequence of semidefinite programs (SDPs). To reduce the complexity, in the second scheme, we instead maximize the information rate at the destination while completely eliminating the information leakage to all eavesdroppers. We prove that this problem can be exactly solved by SDR with one SDP only. Simulation results demonstrate the performance gains of the two proposed designs.

A half-duplex amplify-and-forward two-way relay network consisting of two single-antenna sources and N relays with each having two antennas is considered in this paper. For such a system, a tight lower bound of pairwise error probability (PEP) of a maximum likelihood (ML) detector for any distributed linear dispersion code with a fixed average amplifier is derived, revealing that diversity gain function cannot decay faster than lnNSNR/SNR2N, where rm SNR is signal to noise ratio. Particularly for the network having two relays, a novel distributed quasi-orthogonal space-time block code (QO-STBC) is proposed with two significant advantages. One is that the equivalent channel matrices at both source nodes turn out to be the block-circulant matrices, with each block being a product of the two Alamouti channel matrices. The other is that the equivalent noises at the both source nodes are Gaussian and white. Therefore, like QO-STBC for a multi-antennas system, the proposed code for the relay system still maintains low-decoding complexity for the ML detector. Asymptotic PEP formula is attained to show that the proposed code enables the optimal diversity gain function, i.e., meeting the lower bound of diversity gain, and the optimal coding gain. In addition, a near optimal power allocation that maximizes the received SNR of the worse link is presented and examined through comprehensive computer simulations for various kinds of asymmetric relay channels.

The main drawback of orthogonal frequency division multiplexing (OFDM) systems is the high peak-to-average power ratio (PAPR), which leads to performance degradation and power inefficiency. Tone injection (TI) is a distortionless technique that can reduce PAPR efficiently without incurring data rate loss or extra side information. However, optimal TI requires an exhaustive search over all combinations of possible constellations, which is an NP-hard problem. Suboptimal algorithms, achieving different tradeoffs between the PAPR reduction and complexity, have thus been developed. In this paper, we introduce a novel parallel tabu search algorithm for TI. Simulation results show that the proposed algorithm achieves significant PAPR reduction while maintaining low complexity.

This paper investigates the outage performance of dual-hop amplify-and-forward (AF) two-way relaying systems with multiple co-channel interferers at the AF relay and two noisy end-sources. Assuming a Nakagami-m fading environment, a highly precise, easy-to-compute closed-form approximate expression for the outage probability is derived. Our analysis allows for general operating scenarios with distinct Nakagami-m fading parameters and unequal average fading powers between the hops. Monte Carlo simulation results are presented to corroborate the tightness of the proposed approximations.

This study deals with the performance analysis and power allocation of a two-way amplify-and-forward relay system with channel estimation errors. Exact closed-form expressions for outage probability and average symbol error rate (SER) are first presented. To provide more insights, their closed-form asymptotic expressions are then obtained. It is shown that the presence of channel estimation error causes outage probability and average SER maintain a fixed level even when a noiseless channel is adopted. These results are applied further to study the optimal power allocation problem for each node. From the perspective of service quality, rather than minimise the outage probability as much as possible, the goal is to use the minimum energy consumed to satisfy the traffic requirements and thereby conserve the energy resource. Furthermore, based on the optimal solutions to power allocation, the optimal relay location is investigated, which indicates that the system has higher energy efficiency with the relay located around the middle point of the two end nodes for any asymmetric traffic requirement. The simulation results verify that the derived outage probability and average SER expressions are accurate and highlight the effect of power allocation under various traffic requirements and channel estimation errors.

An iterative interference alignment (IA) algorithm for the multi-user multi-input multi-output interference channel is proposed, which optimises on the Grassmann manifold to improve the performance of conventional interference subspace (ISS) alignment algorithm without the assumption of channel reciprocity. The proposed algorithm combines the extreme eigenvalues method and the modified steepest descent method on the Grassmann manifold to minimise the distances not only between the ISS and the subspace spanned by interference, but also between the desired signal subspace and the subspace spanned by useful signal. Utilising the subspace optimisation above, both the interference and useful signal are aligned to their respective subspaces. Numerical results show that the proposed algorithm can significantly improve the sum rate of interference channel. Moreover, by properly choosing the attenuation factor of step size, the proposed algorithm can achieve an effective tradeoff between sum rate performance and convergence speed, which gives the IA scheme more design flexibility.

This Letter proposes a mechanism to combine multiuser diversity (MUD) and incremental relaying in a multiuser two-way relaying system, where one end-source exchanges messages with one out of N available end-sources by using a half-duplex amplify-and-forward (AF) relay. Outage analysis for the proposed scheme is carried out and, compared with the time division broadcast (TDBC) protocol, reveals that it not only realizes full diversity order (N+1) but also achieves improved expected spectral efficiency. Also, it is indicated that the expected spectral efficiency of the proposed scheme approaches that of the analog network coding (ANC) protocol in high signal-to-noise ratio (SNR) regime. Furthermore, the finite-SNR diversity-multiplexing tradeoff (DMT) is examined, from which the superiority of the proposed scheme to TDBC is confirmed as well. Other insightful comparisons in terms of the outage performance are presented that show the practical usefulness of our multiuser two-way relaying framework.

The combination of bit-interleaved coded modulation (BICM), orthogonal space-time block coding (OSTBC) and orthogonal frequency division multiplexing (OFDM) has been shown recently to be able to achieve maximum spatial-frequency diversity in frequency selective multi-path fading channels, provided that perfect channel state information (CSI) is available to the receiver. In view of the fact that perfect CSI can be obtained only if a sufficient amount of resource is allocated for training or pilot data, this paper investigates pilot-efficient noncoherent decoding methods for the BICM-OSTBC-OFDM system. In particular, we propose a noncoherent maximum-likelihood (ML) decoder that uses only one OSTBC-OFDM block. This block-wise decoder is suitable for relatively fast fading channels whose coherence time may be as short as one OSTBC-OFDM block. Our focus is mainly on noncoherent diversity analysis. We study a class of carefully designed transmission schemes, called perfect channel identifiability (PCI) achieving schemes, and show that they can exhibit good diversity performance. Specifically, we present a worst-case diversity analysis framework to show that PCI-achieving schemes can achieve the maximum noncoherent spatial-frequency diversity of BICM-OSTBC-OFDM. The developments are further extended to a distributed BICM-OSTBC-OFDM scenario in cooperative relay networks. Simulation results are presented to confirm our theoretical claims and show that the proposed noncoherent schemes can exhibit near-coherent performance.

Partial transmit sequences (PTS) is a distortionless phase optimization technique that significantly improves peak-to-average power ratio (PAPR) performance with a small amount of redundancy for orthogonal frequency division multiplexing (OFDM) systems. However, the computational complexity in conventional PTS increases exponentially with the number of subblocks. In this paper, a novel global parallel algorithm, combining the immune and versatile evolutionary mechanism, is proposed for PTS technique to reduce computational complexity and improve PAPR performance. The proposed scheme restrains the degenerate phenomenon effectively during the evolutionary process, thus making the fitness of population increase steadily. The simulation results show that the proposed scheme achieves significant PAPR reduction while maintaining low complexity.

Two-way opportunistic relaying (TWOR) systems in amplify-and-forward (AF) strategy have been widely studied because of simplicity and high spectral efficiency. However, there have been few investigations into the performance of these systems. In this study, the authors present a performance analysis of the equal power allocation (EPA) scheme for the TWOR-AF systems over independent and non-identically distributed Rayleigh fading channels. Closed-form lower and upper bounds as well as an approximation of the outage probability are established to show the asymptotic performance of the EPA-TWOR-AF scheme, which are all tight at high signal-to-noise ratio. The diversity gain, coding gain and diversity-multiplexing tradeoff are obtained from the tight approximation. Using the moment generating function approach, a general symbol-error-rate expression in closed-form is derived for the EPA scheme with M-PSK or M-QAM modulation. Moreover, an instantaneous power allocation (IPA) scheme is proposed for the TWOR-AF systems, which significantly outperforms the EPA scheme in outage probability. The analytical outage probability and closed-form lower and upper bounds are also derived for the IPA-TWOR-AF scheme. Extensive simulation results are provided to validate the theoretical analysis.

We are concerned with transmit power optimization for two-way opportunistic relaying (TWOR) systems in amplify-and-forward (AF) strategy over independent and non-identically distributed (i.ni.d.) Nakagami-m fading channels. With instantaneous squared channel strength known to source nodes before transmitting, we derive an optimum power allocation (OPA) scheme by solving an optimization problem of signal-to-noise ratio (SNR) balancing. While with only statistical channel knowledge, we propose a sub-OPA scheme that minimizes the high-SNR approximation of the system outage probability. Simulation results show that the proposed schemes significantly outperform the equal power allocation (EPA) scheme available in outage probability.

In this paper, a novel OFDM symbol timing recovery method, which is robust to doubly selective fading channels and carrier frequency offset, is proposed for the mobile terminals of CMMB system. The time slot timing synchronization is accomplished by using the inverse sliding correlation between the segmenting Sync sequences, which possess the inverse conjugate symmetry (ICS) characteristic. Furthermore, a fast fine OFDM symbol timing algorithm based on the auto-correlation property of Sync sequences in frequency domain is also presented. According to a detection strategy for the valid taps of current channel, the specific multi-path distribution situation can be obtained. The simulation results show that compared with conventional methods, two proposed algorithm can provide a more accurate and fast estimation for symbol timing offset even in very adverse fading channels.

This study addresses the problem of time and frequency offset estimation in the case of the delays, and the frequency offsets of all the transmitter/receiver pairs are different for distributed multiple-input multiple-output orthogonal frequency division multiplexing (MIMO–OFDM) systems. A new training structure is designed and an efficient time and frequency offset estimator is proposed. With the symmetric conjugate property of the preamble, the timing metric proposed has an impulse shape, making the time offset estimation accurate. Using the repetition property of the preamble, iterative frequency offset estimation algorithm proposed can acquire a high precision and a wide estimation range. To distinguish different transmit antennas, the preamble is weighted by pseudo-noise (PN) sequences with low cross-correlations. The analysis and the simulation results show that the proposed estimator offers an accurate estimation on the different time delays and frequency offsets caused by the distributed transmitters of the MIMO–OFDM system. This is a situation that the conventional estimators cannot handle as far.

An ad hoc network is a collection of wireless computers (nodes), communicating among themselves over possibly multi-hop paths, without the help of any infrastructure. Although many ad hoc network routing protocols have been proposed (DSR, AODV, ZRP etc), none of them considers the security problems. We put forward an efficient security mechanism based on the AODV routing protocol. In this security mechanism the double Hash authentications are adopted to protect routing information.

Since the publication of 2-party Diffie-Hellman (DH) key exchange (1976), various solutions have been proposed to extend Diffie-Hellman key exchange to multiparty key distribution. For the special operational requirement of ad hoc networks, none of these protocols are suitable. This is mainly because they demand the network topology follow a predestined structure. In this paper we utilize the thought of CEDAR (Core-Extraction Distributed Ad hoc Routing) algorithm and integrate the two DH-based protocols, the octopus protocol and the BD protocol, then present a group key establishment system suitable for ad hoc networks.