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In this article, we investigate two-tier nonorthogonal multiple access (NOMA)-based wireless powered communication networks. The considered network consists of a hybrid access point and two groups of users, namely near and far users. A near user and a far user are selected to form a NOMA pair, which harvest energy during the downlink phase and transmit information during the uplink phase. Specifically, we consider two cases of energy-transfer time and analyze the corresponding performance. We first analyze the performance of the case with fixed energy-transfer time and derive the exact expression for the ergodic rate. Due to the doubly near-far effect, the far user experiences poor performance. To address this issue, an adaptive energy-transfer time scheme is designed by maximizing the far user’s data rate, and the optimal solution is derived. To reduce the system complexity, a suboptimal solution is proposed, and the ergodic rate's exact expression is derived. Simulation results reveal that the suboptimal solution achieves near-optimal performance and has a lower computational complexity. Moreover, a one-tier NOMA network is set to be a benchmark, and numerical results show that the two-tier system outperforms the one-tier system significantly under the appropriate radius setting. [ABSTRACT FROM AUTHOR]

Recently, Gallager's bounding techniques have been used to derive tight performance bounds for coded systems in fading channels. Most works in this field have thus far dealt with coherent decoding. This paper develops Gallager bounds for noncoherent systems in fading channels. Unlike coherent decoding, the exact error probability of a noncoherent decoder/detector conditioned on the fading coefficients does not admit a closed-form expression. This difficulty is overcome in this paper by employing the Chernoff technique. Although it weakens the bounds to some extent, the Chernoff technique enables the derivations of the limit-before-average (LBA) bound and Gallager bounds in closed form for noncoherent fading channels. Numerical examples show that the proposed bounds are convergent and are tighter than the conventional union bound. Index Terms--Characteristic function, Chernoff bound, fading channels, Gallager bounds, noncoherent decoding.

Novel decision-feedback (DF) linear prediction (LP) receivers, which process multiple samples per symbol interval in conjunction with optimal sample combining, are proposed for differential space-time modulation (DSTM) over Rayleigh fast-fading channels. Performance analysis demonstrates that multisampling DF-LP receivers outperform their symbol-rate sampling counterpart in fast fading substantially. In addition, an asymptotically tight upper bound on the pairwise error probability is derived. In view of this bound, the design criterion of DSTM for fast fading is the same as that for block-wise static lading. To avoid the estimation of the second-order statistics of the channel, a polynomial-model-based DF-LP receiver is proposed. It can approach the performance of the optimum DF-LP receiver at high signal-to noise ratios, provided fading is moderate. Index Terms--Differential detection, diversity combining, linear prediction (LP), space-time modulation, time-selective fading.

In this paper, non-orthogonal multiple access (NOMA) networks assisted by multiple intelligent reflecting surfaces (IRSs) with discrete phase shifts are investigated, in which each user device (UD) is served by an IRS to improve the quality of the received signal. Two scenarios are considered according to whether there is a direct link between the base station (BS) and each UD, and the outage performance is analyzed for each of them. Specifically, the asymptotic expressions for the upper and lower bounds of the outage probability in the high signal-to-noise ratio (SNR) regime are derived. Following that, the diversity order is obtained. It is shown that the use of discrete phase shifts does not degrade diversity order. More importantly, simulation results reveal that a 3-bit resolution for discrete phase shifts is sufficient to achieve near-optimal outage performance. Simulation results also imply the superiority of IRSs over full-duplex decode-and-forward relays. [ABSTRACT FROM AUTHOR]

In this letter, we propose a two-step user-paring scheme for an orthogonal frequency-division multiplexing (OFDM)-based cooperative non-orthogonal multiple-access (NOMA) system where far users cannot communicate with the base station directly. The first step is for allocating near users, and the second step is for allocating far users. To evaluate the performance of the proposed scheme, we derive the closed-form expression for the upper bound of the outage probability, and then obtain the achievable diversity gain. Numerical results show that the proposed scheme can achieve the optimal diversity gain confirmed by exhaustive search. [ABSTRACT FROM AUTHOR]

In this paper, we study the mode selection and subchannel allocation for the downlink of a multi-user multi-subchannel cooperative non-orthogonal multiple access (C-NOMA) system. Specifically, each non-orthogonal multiple access (NOMA) pair can work in two different modes, i.e., non-cooperative NOMA (NC-NOMA) and C-NOMA, where the mode selection is based on the users’ channel conditions. We propose an adaptive mode selection, subchannel allocation and user pairing (AMS-SAUP) scheme and derive the achievable diversity order by analyzing the upper bound for the outage probability. Simulation results reveal that the AMS-SAUP scheme achieves the same diversity order as the exhaustive search scheme. [ABSTRACT FROM AUTHOR]

In this paper, we investigate physical layer security in a two-tier heterogeneous network with sub-6 GHz massive multi-input multi-output (MIMO) macro cells and millimeter wave (mmWave) small cells. By considering pilot attacks from the eavesdroppers, we analyze the coverage and secrecy performance using stochastic geometry. For the sub-6 GHz tier, we show that increasing the number of BS antennas is more effective than increasing BS density in improving the coverage performance, whereas densifying BS is more effective for security enhancement. For the mmWave tier, we first derive the success probability of beam alignment based on a beam sweeping-based channel training model. It is shown that the mmWave tier may outperform the sub-6 GHz counterpart in terms of both coverage and secrecy through densifying the base stations. Our results also reveal that the mmWave small cell can provide better coverage performance in the high transmission rate region, and can achieve higher security in the low redundant rate region, which reveals the advantage of using mmWave for secure communication. Numerical results verify the analysis. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the issues of joint user pairing and subchannel allocation (JUP-SA) in a multisubchannel multiuser nonorthogonal multiple access (NOMA) system. We first investigate the uplink and propose a JUP-SA scheme so that the minimum achievable diversity order of users is maximized. By deriving the upper and lower bounds of the outage probability of the worst-performance user, the achievable diversity order of the scheme is obtained. Following this, the downlink of the system is considered and a JUP-SA scheme that maximizes the minimum diversity order is presented. The closed-form expression of the worst-case outage probability is then derived and validated by simulation results. Numerical results show that both schemes in uplink and downlink NOMA systems can achieve the same diversity order as that of exhaustive search. [ABSTRACT FROM AUTHOR]

In this paper, the impact of adaptive smarter eavesdroppers on the secrecy performance of a multi-antenna cellular network is investigated. The eavesdroppers can act as either passive eavesdroppers or active jammers based on their distance to the active base stations (BSs). To analyze the secrecy performance, the BSs, cellular users, and eavesdroppers are modeled as independent Poisson point processes. The closed-form expressions of the connection outage probability and lower bound of the secrecy outage probability are derived using the stochastic geometry approach. Following that, the conditions under which the eavesdroppers can act as active jammers are obtained. Finally, the optimal power allocation between artificial noise and information signal and the secrecy code rate at each BS, as well as the guard zone of the eavesdroppers are obtained using the Stackelberg game approach, where the eavesdroppers are modeled as the leader and the BSs as the follower. The Stackelberg equilibrium is obtained through the proposed iterative algorithm. Numerical results verify the theoretical analysis and show that the secrecy performance can be degraded severely by the adaptive eavesdroppers. [ABSTRACT FROM PUBLISHER]

In this letter, an adaptive spatial modulation (SM) scheme, in which the spatial signatures of the transmitted signals are adaptively selected according to the channel responses, is proposed for the uplink of a millimeter-wave communication system. We first use an iterative method to search for the spatial signatures that maximize the minimum Euclidean distance of the SM symbols. Following this, a suboptimal scheme is provided to reduce the computational complexity. Numerical results show that the proposed adaptive SM scheme can effectively reduce the system symbol-error rate. [ABSTRACT FROM PUBLISHER]

We consider an energy-harvesting cognitive radio (CR) system coexisting with an unslotted primary user (PU) system. The secondary user (SU) either harvests energy from the PU transmissions or transmits when the PU is sensed to be idle. In this paper, we optimize the sensing intervals to balance between energy harvesting and spectrum access, considering the PU traffic statistics that affects both the energy arrival and consumption in addition to the opportunistic spectrum access. We propose to use different sensing intervals if the SU decides to transmit or harvest energy. The harvesting and transmission durations can be used to optimize the tradeoff between maximizing the utilization of PU spectrum opportunities and harvesting sufficient energy to transmit. We determine a closed-form expression for the probability of SU being active as a function of the PU traffic statistics and SU's access and harvest durations. Optimal transmission and harvesting durations have been found to maximize the CR throughput while protecting the PU. Furthermore, we enhance the system capability by considering that the SU can choose to purchase energy from the PU during the discovered PU idle periods in addition to the energy harvested opportunistically. It can further exploit the difference in the PU busy and idle durations as per its energy needs. The average energy cost spent in a time slot is minimized to achieve a given SU transmission rate while adhering to the PU collision constraint. [ABSTRACT FROM AUTHOR]

The quality of energy detector is quite challenging when the system parameters are not stable. On the other hand, in a cognitive radio network, primary users are licensed to use the desired radio spectrum band(s) and the throughput of the primary network has the highest priority to be kept satisfied. Therefore, it is quite important to secure the quality of primary service to a certain level, and it is worthless to blindly improve the throughput of the secondary network without taking its possible effects on the primary throughput into account. Hence, the mentioned issues motivate us to enhance the total throughput of the system when it suffers from an unstable primary network, due to a medium-traffic rate, and a random density of the secondary users, due to a stochastic-modeled secondary network, at the same time. By introducing a 1-persistent method in a cooperative spectrum sensing scheme, we show how effectively it can improve both primary and secondary throughputs, resulting in higher total throughput of the system. [ABSTRACT FROM PUBLISHER]

In this paper, the secure transmission strategy for a multi-input-single-output multi-eavesdropper system with coexistence of a secure user (Bob) and a normal user (NU) is investigated. The NU and Bob require normal and secure data transmissions, respectively, and thus, the stream for the NU can be exploited to confuse the eavesdroppers. To guarantee the security of Bob, artificial noise (AN) is also deliberately injected into the null space of Bob and NU. The power allocation among Bob, NU, and AN, as well as the wiretap code rates, is jointly optimized to maximize the effective secrecy throughput (EST), under the average throughput constraint of the NU and statistical channel state information (CSI) of eavesdroppers. Both non-adaptive and adaptive transmission schemes are proposed, based on the statistical and instantaneous CSI of the legitimate channels, respectively. An alternative optimization algorithm is proposed to obtain the optimal parameters. It is proved that the EST is a quasi-concave function of the secrecy rate and the power allocated to Bob, and for fixed wiretap code rates, the optimal power allocation is derived in a closed-form expression. Numerical results show that the EST increases with the increase in transmitting power and the number of transmit antennas, and decreases with the increasing throughput constraint of the NU. Improved EST can be achieved through injecting AN and concurrent transmission of Bob and NU. [ABSTRACT FROM PUBLISHER]

In this paper, resilient transmission in a multicell multiple-input multiple-output (MIMO) interference wiretap channel model is studied. Each base station (BS) transmits confidential messages to its intended legitimate user with multiple antennas in the presence of eavesdroppers that can overhear the transmission. We study the problem of finding the optimal transmit covariance matrices at the BSs to maximize the secrecy sum rate, which is typically nonconvex and intractable to obtain a globally optimal solution. A distributed iterative optimization algorithm based on a novel decomposition framework across all users is proposed. The decomposition framework preserves the convexity of the objective function and linearizes the nonconvex part. At each iteration, the BSs simultaneously solve a sequence of problems that are decoupled convex approximations of the original secrecy sum-rate function. Moreover, the complexity of the algorithm is analyzed. Numerical results are presented to validate the effectiveness of the proposed distributed algorithm. [ABSTRACT FROM PUBLISHER]

We consider energy-harvesting cognitive radio (CR) systems with finite batteries. The energy-constrained secondary users (SUs) can be coordinated to enhance both the primary user (PU) detection and the opportunistic utilization of the PU spectrum. We employ centralized cooperative spectrum sensing, and the optimal sensors are chosen from among the SUs with varied received PU signal power levels and energy-arrival rates. The finite-horizon partially observable Markov decision process (POMDP) is applied to obtain the optimal cooperation among the SUs for sensing and access to maximize throughput with the available energy, while satisfying the PU detection constraint. The sensing–access policy decides whether to sense, the set of sensing SUs, the sensing detection threshold, and the SU that accesses the spectrum opportunities. The optimal policy achieves an optimum tradeoff between SU transmissions, preventing energy shortage and obtaining sensing information for the future gains. [ABSTRACT FROM PUBLISHER]

The massive multiantenna technique, where base stations (BSs) are equipped with a large number of antennas, has been considered as a candidate for the fifth-generation (5G) wireless systems. Three-dimensional (3-D) beamforming strategies are studied in a two-tier clustered user distribution model. We derive the distribution function of the clustered user distribution. Furthermore, we investigate different 3-D beamforming strategies, namely, the user-specific beamforming, which is the optimal strategy, the region-partition beamforming where user terminals (UTs) located in a partition share a particular downtilt, and the statistic-based beamforming when BSs only have the user's distribution knowledge. Numerical results are presented to validate the effectiveness of the proposed approaches. [ABSTRACT FROM PUBLISHER]

In this paper, the security aspect of an amplify-and-forward relaying network with untrusted relay nodes is considered. The untrusted nodes can help to forward the received signal and they may also try to decode such information, which can be regarded as potential eavesdroppers (Eves). To deal with such a challenging issue, a successive relaying scheme is adopted, where the multi-antenna source transmits to two selected nodes alternately, and the conventional detrimental inter-relay interference is used to jam the untrusted nodes without external helpers. Considering different complexity requirements, several relay selection schemes are proposed, and the closed-form expressions of the lower bound of secrecy outage probability are derived accordingly. To obtain some insights for the network design, an asymptotic analysis is also given, which shows that the maximum secrecy diversity order of $N-1$ can be achieved, where $N$ is the number of the untrusted relays. Moreover, the spectral efficiency is improved dramatically with the successive relaying scheme. Simulation results show that the proposed max–min scheme has almost the same performance as that of the optimal one, and the theoretical results match well with the simulation results. [ABSTRACT FROM PUBLISHER]

Status changing of the primary user and the number of secondary users (SUs) are crucial issues for designing a reliable cognitive radio network. In this paper, a medium-traffic primary network is assumed when SUs are stochastically distributed over an area around a secondary base station. By using cooperative spectrum sensing over Rayleigh-faded sensing and imperfect reporting channels, we propose an extra sensing part within the transmission phase of the secondary network in which only some SUs (called sensing SUs) participate in sensing. Although the extra sensing may suffer from interference by transmitting SUs, we show that the total transmission throughput of the system can be improved significantly due to better quality of the detection. Moreover, we reveal that involving all SUs in the extra sensing part does not necessarily increase the total throughput of the system. [ABSTRACT FROM PUBLISHER]

We formulate and study a multi-user multi-armed bandit problem that exploits the temporal–spatial opportunistic spectrum access (OSA) of primary user (PU) channels, so that secondary users (SUs) who do not interfere with each other can make use of the same PU channel. We first propose a centralized channel allocation policy that has logarithmic regret, but requires a central processor to solve an NP-complete optimization problem at exponentially increasing time intervals. To overcome the high computation complexity at the central processor, we also propose heuristic distributed policies that, however, have linear regrets. Our first distributed policy utilizes a distributed graph coloring and consensus algorithm to determine SUs’ channel access ranks, while our second distributed policy incorporates channel access rank learning in a local procedure at each SU at the cost of a higher regret. We compare the performance of our proposed policies with other distributed policies recently proposed for temporal (but not spatial) OSA. We show that all these policies have linear regrets in our temporal–spatial OSA framework. Simulations suggest that our proposed policies have significantly smaller regrets than the other policies when spectrum temporal–spatial reuse is allowed. [ABSTRACT FROM AUTHOR]

The pilot spoofing attack is one kind of active eavesdropping activities conducted by a malicious user during the channel training phase. By transmitting the identical pilot (training) signals as those of the legal users, such an attack is able to manipulate the channel estimation outcome, which may result in a larger channel rate for the adversary but a smaller channel rate for the legitimate receiver. With the intention of detecting the pilot spoofing attack and minimizing its damages, we design a two-way training-based scheme. The effective detector exploits the intrusive component created by the adversary, followed by a secure beamforming-assisted data transmission. In addition to the solid detection performance, this scheme is also capable of obtaining the estimations of both legitimate and illegitimate channels, which allows the users to achieve secure communication in the presence of pilot spoofing attack. The detection probability is evaluated based on the derived test threshold at a given requirement on the probability of false alarming. The achievable secrecy rate is utilized to measure the security level of the data transmission. Our analysis shows that even without any pre-assumed knowledge of eavesdropper, the proposed scheme is still able to achieve the maximal secrecy rate in certain cases. Numerical results are provided to show that our scheme could achieve a high detection probability as well as secure transmission. [ABSTRACT FROM PUBLISHER]

A decentralized channel access strategy is proposed for a multichannel low-power cognitive radio (CR) system. CRs are equipped with rechargeable batteries to harvest radio-frequency (RF) energy from the primary user (PU) transmission. At any time, secondary users (SUs) can exploit either the spatiotemporal spectrum opportunities for transmission or the PU active periods for RF energy harvesting. An optimal channel-selection probability vector is obtained to achieve the maximum SU throughput by finding sufficient spectrum-access and energy-harvesting opportunities while accounting for SU contention. We have a nonconvex optimization problem. It is decomposed to obtain a convex optimization subproblem, but an exhaustive search is required for the master problem. Alternatively, an iterative algorithm is proposed wherein we define a combined metric to consider the spectrum-access and energy-harvesting opportunities on different PU channels. A convex optimization problem is solved with a closed-form solution in each iteration. Numerical results show that the proposed channel-selection policy enables an SU to achieve better expected throughput through an enhanced battery-recharging probability. [ABSTRACT FROM PUBLISHER]

Cognitive radio (CR) has been viewed as a promising solution to spectrum scarcity. In order to design a reliable CR system, many improvements have been proposed to enhance spectrum sensing performance of secondary users (SUs) in a CR network (CRN). Sensing reliability and transmission throughput of SUs are two important performance criteria, which should be optimized to enhance signal protection of primary user (PU) as well as spectrum utilization rate. In this paper, we consider Rayleigh-faded sensing channels and SUs use improved energy detector (IED) to make their local decisions. The final decision is made in a fusion center (FC) through the cooperative spectrum sensing (CSS) scheme with erroneous reporting channels. We show that the improved double-threshold energy detector (IDED) outperforms the conventional energy detector (CED) in terms of the total error rate. Furthermore, we evaluate the transmission throughput of the CRN through various ED schemes with detection constraints over both perfect and imperfect reporting channels. We show that the IDED has the highest achievable throughput among different ED schemes over imperfect reporting channels. [ABSTRACT FROM AUTHOR]

In this paper, the performance of a massive multiple-antenna system under the pilot contamination effect is studied. A 2-D massive antenna array with a uniform circular array (UCA) topology at base stations (BSs) is considered. A 3-D spatial channel model with Von Mises Fisher (VMF) distribution is applied. It is shown that the interfering channels tend to fall into the null space of the desired user's covariance matrix. The pilot contamination can be mitigated if the angle-of-arrival (AoA) pairs of the interfering users have a nonoverlapping region with the AoA of the desired user. The impact of the AoA distribution on the beam pattern is studied as well. Numerical results confirm the analysis and show that the mitigation of pilot contamination depends on the directional dispersion of the multipath AoA distribution. [ABSTRACT FROM AUTHOR]

The pilot spoofing attack is one kind of active eavesdropping conducted by a malicious user during the channel estimation phase of the legitimate transmission. In this attack, an intelligent adversary spoofs the transmitter on the estimation of channel state information (CSI) by sending the identical pilot signal as the legitimate receiver, in order to obtain a larger information rate in the data transmission phase. The pilot spoofing attack could also drastically weaken the strength of the received signal at the legitimate receiver if the adversary utilizes large enough power. Motivated by the serious problems the pilot spoofing attack could cause, we propose an efficient detector, named energy ratio detector (ERD), by exploring the asymmetry of received signal power levels at the transmitter and the legitimate receiver when there exists a pilot spoofing attack. Our analysis shows that by setting the ratio of received signal power levels at the transmitter and the legitimate receiver as the test statistic, the detecting threshold is derived without using the knowledge of the CSI of the legitimate channel as well as the illegitimate channel. Furthermore, we study the performance of the proposed ERD in various special cases in order to obtain useful insights. Numerical results are presented to further demonstrate the performance of our proposed ERD. [ABSTRACT FROM PUBLISHER]

In a cognitive radio network, a primary user (PU) shares its spectrum with secondary users (SUs) temporally and spatially, while allowing for some interference. We consider the problem of estimating the no-talk region of the PU, i.e., the region outside which SUs may utilize the PU's spectrum regardless of whether the PU is transmitting or not. We propose a distributed boundary estimation algorithm that allows SUs to estimate the boundary of the no-talk region collaboratively through message passing between SUs, and analyze the trade-offs between estimation error, communication cost, setup complexity, throughput and robustness. Simulations suggest that our proposed scheme has better estimation performance and communication cost trade-off compared to several other alternative benchmark methods, and is more robust to SU sensing errors, except when compared to the least squares support vector machine approach, which however incurs a much higher communication cost. [ABSTRACT FROM AUTHOR]

In this paper, we consider suboptimal cooperative downlink beamforming strategies to maximize the user sum rate in a clustered multicell system with limited feedback. We decouple the problem into multiple independent subproblems through the uplink–downlink duality theorem. Semidefinite relaxation is applied to transform the original nonconvex problem into a convex semidefinite programming problem, which can be efficiently solved in polynomial time. We propose a novel limited feedback scheme based on a compressive sensing technique that can yield high-quality channel state information (CSI) feedback. We also consider a limited feedback scenario where base stations within one cluster have different estimates of the same CSI knowledge. We investigate the channel quantization criterion by choosing the codeword vector that has the minimum norm of the difference between the codeword and the normalized channel. Numerical results demonstrate the effectiveness of the proposed beamforming algorithm and limited feedback schemes. [ABSTRACT FROM PUBLISHER]

Abstract: In this paper, we present an algorithm to detect unknown interference in slow frequency-hopped -ary frequency-shift-keying (SFH/MFSK) systems over fading channels. Both partial-band noise interference (PBNI) and multitone interference (MTI) are considered. The proposed algorithm performs the detection process after dehopping by making use of square-law detectors. We first analyze the statistical property of the outputs of the square-law detectors over one hop duration, and an appropriate threshold level is derived for detecting the interference based on a binary hypothesis testing. We also formulate the closed-form expressions for the probabilities of detection of both types of interference experienced in any particular frequency hop. The analytical results are validated by the simulation results and they reveal that the proposed algorithm is able to provide good detection performance for both types of interference and outperforms the conventional ratio–threshold test (RTT) method. [Copyright &y& Elsevier]

In this survey article, we compare the performance of single transmit antenna selection (TAS) systems with different kinds of receiver schemes. These systems include the TAS/switch-and-examine combining (TAS/SEC), TAS/SEC with postselection (TAS/SECPS), TAS/generalized selection combining (TAS/GSC), and TAS/output-threshold generalized selection combining (TAS/OT-GSC) systems. The performance comparisons are made in terms of the average output signal-to-noise ratio (SNR), symbol-error rate (SER), the number of channel estimates Ne, and the number of branches required for processing Np . It is shown that the TAS/SECPS and TAS/OT-GSC systems can achieve similar performance compared with the TAS/selection combining (SC) and TAS/GSC systems, respectively, with a reduced number of both channel estimates and branches required for processing when the predetermined SNR threshold is optimized. [ABSTRACT FROM PUBLISHER]

A diversity-combining system using transmit antenna selection at the transmitter and switch-and-examine combining with postselection (TAS/SECPS) at the receiver is proposed. This system has lower processing complexity as compared with the existing TAS schemes with maximal-ratio combining and selection combining (TAS/SC) at the receiver. The performance of the TAS/SECPS system with different modulation schemes over independent and nonidentically distributed Rayleigh fading channels is examined. The expressions of average output signal-to-noise ratio (SNR) and symbol error rate (SER) are derived and validated by simulation. It is shown that the proposed TAS/SECPS is able to achieve similar average output SNR and SER performance as compared with TAS/SC when the predetermined SNR threshold is optimized. Moreover, the TAS/SECPS requires less number of channel estimates. [ABSTRACT FROM PUBLISHER]

In this paper, we consider a joint design of energy-efficient sensing and transmission durations for a cognitive radio (CR) system in which the primary user (PU) is protected. The design problem is formulated as a function of two variables, which are sensing and transmission durations, subject to the interference to the PU. The optimal solutions are analyzed and a sub-optimal algorithm that maximizes the energy efficiency for the CR system is presented. The performance of the proposed energy-efficient design for the CR system is also evaluated. [ABSTRACT FROM PUBLISHER]

Two-dimensional (2-D) intersymbol-interference (ISI) channels represent an important class of next-generation data-storage systems. In order to mitigate the 2-D ISI and improve system performance, a joint message-passing decoding algorithm is proposed in this paper. The proposed algorithm conducts an iterative decoding of low-density parity-check code and a 2-D ISI channel, which can be implemented in a fully parallel manner. An extrinsic information transfer chart is also introduced to analyze the performance and convergence behavior of the proposed message-passing decoding system. Simulation results show that the proposed joint message-passing decoding algorithm outperforms some existing joint decoding algorithms. [ABSTRACT FROM AUTHOR]

In this paper, we propose a joint iterative detection/decoding scheme for a generalized discrete two-dimensional (2D) interference channel with low-density parity-check (LDPC) codes. A reduced-state form of the channel detector is also proposed to further reduce the system complexity. To achieve better performance, iterations are introduced between the two constituent channel detectors and among the two constituent detectors with the LDPC decoder. The computational complexity and performance of different iteration schemes are analyzed. Simulation results show that the proposed detection scheme can mitigate the 2D interference effectively and the LDPC codes can be well integrated to the 2D iterative detector to further improve the system performance significantly. [ABSTRACT FROM AUTHOR]

In this paper, we propose a diversity combining system with transmit antenna selection at the transmitter and switch-and-examine combining (TAS/SEC) at the receiver. This system has lower processing complexity as compared with the existing TAS systems such as TAS/maximum-ratio combining (TAS/MRC) and TAS/selection combining (TAS/SC). The performance of the TAS/SEC system with different modulation schemes over an independent and nonidentically distributed (i.n.d.) Rayleigh fading channel is investigated. The expressions of the outage probability, average output signal-to-noise ratio (SNR), and symbol error rate (SER) are derived and validated by simulation results. The performance of TAS/SEC is compared with TAS/SC in terms of the SER and the average number of branches to be estimated. It is shown that, as compared with TAS/SC, the TAS/SEC system is able to achieve a lower SER with fewer branches to be estimated. [ABSTRACT FROM PUBLISHER]

An orthogonal space-time block code (OSTBC) with a minimum-selection generalized selection combining (MS-GSC) receiver is proposed and analyzed. The motivation behind the proposition of this scheme is a reduction in complexity and processing power at the receiver, compared with a conventional OSTBC system utilizing the maximum-ratio-combining receiver. In addition, it was shown that the OSTBC/MS-GSC can achieve similar performance, compared with the OSTBC/GSC with reduced processing when the predetermined threshold is optimized. In the analysis of this OSTBC/MS-GSC system over a Rayleigh-fading channel, the moment-generating function is presented, where it can be conveniently used to numerically compute the symbol error rate (SER). The average number of branches that is being selected for processing at the receiver is computed and compared. The analytical results are validated by the simulation results. [ABSTRACT FROM PUBLISHER]

A low-density parity-check (LDPC)-coded multiple-input–multiple-output (MIMO) system is able to achieve excellent bit-error-rate (BER) performance over fading channels. The two types of MIMO systems examined in this paper are the transmit antenna selection/generalized selection combining (TAS/GSC) and orthogonal space–time block code (STBC). The approximate BER expressions of these two systems with LDPC codes using binary phase-shift keying signals over a Rayleigh fading channel are derived based on the Gaussian approximation (GA) approach. These expressions provide a computationally efficient method of analyzing the system performance, as compared with density evolution (DE) and simulation. The stability conditions for both systems using the DE and GA approaches are also presented. [ABSTRACT FROM PUBLISHER]

In this paper, a two-user cooperative multiple access (CMA) system is considered. Using the superposition modulation scheme, we propose a low-complexity suboptimal detector at the destination to achieve the performance close to that of the optimal maximum-likelihood sequence detector (MLSD). For independent and non-identically distributed (i.n.d.) Rayleigh-faded channels, we derive error decision regions and present the exact bit-error rate (BER) expression in an integral form, which is validated by simulation results. We also approximate the decision regions and obtain the BER bound in closed form. Another efficient transmission scheme, i.e., the time-division broadcast (TDBC) protocol operating in three time slots, is also considered and analyzed. BER results show that the superposition modulation outperforms the others and can achieve both full diversity order and high coding gain. [ABSTRACT FROM PUBLISHER]

In this paper, we derive the upper bound of bit-error rate (BER) performance and diversity gain for a decode-and-forward (DF) cooperative system. Either maximal-ratio combining (MRC) or cooperative MRC (C-MRC) is employed at the receiver in the presence of independent, non-identical Nakagami- m fading. Both analytical and simulation results show that C-MRC takes advantage of spatial diversity more efficiently and thus achieves the same or better performance and diversity order as compared to MRC. Copyright © 2009 John Wiley & Sons, Ltd. In this paper, we derive the upper bound of bit-error rate (BER) performance and diversity gain for a decode-and-forward (DF) cooperative system. Either maximal-ratio combining (MRC) or cooperative MRC (C-MRC) is employed at the receiver in the presence of independent, non-identical Nakagami-m fading. Both analytical and simulation results show that C-MRC takes advantage of spatial diversity more efficiently and thus achieves the same or better performance and diversity order as compared to MRC. [ABSTRACT FROM AUTHOR]

In this paper, we address the issue of optimal transmitter design to achieve physical layer security for a multiple-input single-output (MISO) cognitive radio network (CRN), in which a secondary user transmitter (SU-Tx) sends confidential information to a SU receiver (SU-Rx) on the same frequency band with a primary user (PU) in the presence of an eavesdropper receiver (ED-Rx). It is assumed that all the channel state information (CSI) of the secondary, primary and eavesdropper channels is not perfectly known at the SU-Tx. The optimal transmitter design, under the restriction of Gaussian signaling without preprocessing of information, involves a nonconvex semiinfinite optimization problem which maximizes the rate of the secondary link while avoiding harmful interference to the PU and keeping the eavesdropper totally ignorant of the messages sent regardless of the uncertainties in the CSI. We propose two approaches to solve this challenging optimization problem. The first one relates the original problem to a sequence of semiinfinite capacity-achieving transmitter design problems in an auxiliary CRN without any eavesdropper, which can then be solved through transformations and using convex semidefinite programs (SDPs). The second approach explores the hidden convexity of the problem and hence transforms it into a single SDP, which significantly reduces the computational complexity. Furthermore, a few heuristic beamforming solutions for the ease of implementation are also introduced. Finally, simulation results are presented to evaluate the performance of the proposed optimal and suboptimal solutions. [ABSTRACT FROM PUBLISHER]

Bit-patterned media (BPM) is a promising candidate for future high-capacity magnetic storage. The inter-track interference (ITI) in BPM has been taken into account by the “multiple islands per read head” model. However, the complexity of the symbol-based detection for this model is too high for practical use. To reduce complexity, we propose to use the reduced-state Bahl-Cocke-Jelinek-Raviv (BCJR) detection algorithm for such a two-dimensional model. We compare the computational complexity of the proposed reduced-state detector to that of the full-state BCJR detector. Simulation results show that the proposed reduced-state BCJR detector can approach the optimal performance by 0.5 dB while significantly reducing the detector complexity. [ABSTRACT FROM AUTHOR]

We propose an efficient successive relaying (SR) transmission protocol for multiple-relay cooperative systems under the half-duplex constraint. In this protocol, the process of relay selection is conducted once the instantaneous channel state information changes. During each channel coherent interval, the source keeps transmitting newly generated messages, and two selected relays successively decode and forward the source's information to the destination. In particular, in one time slot, a selected relay jointly decodes both signals transmitted from the source and the other selected relay. In the next time slot, this relay forwards the superposition of these two decoded signals. The destination jointly decodes messages at the end of each interval. The lower bound of outage probability is derived in a closed-form expression and the diversity-multiplexing trade-off (DMT) performance is characterized as well. It is demonstrated by the numerical results that the proposed SR protocol is capable of achieving both full diversity gain and high multiplexing gain. [ABSTRACT FROM PUBLISHER]

Energy-efficient design has become increasingly important to battery-powered wireless devices. In this paper, we focus on the energy efficiency of a cognitive radio network, in which a secondary user senses the channels licensed to some primary users sequentially before it decides to transmit. Energy is consumed in both the channel sensing and transmission processes. The energy-efficient design calls for a careful design in the sensing-access strategies and the sensing order, with the sensing strategy specifying when to stop sensing and start transmission, the access strategy specifying the power level to be used upon transmission, and the sensing order specifying the sequence of channel sensing. Hence, the objective of this paper is to identify the sensing-access strategies and the sensing order that achieve the maximum energy efficiency. We first investigate the design when the channel sensing order is given and formulate the above design problem as a stochastic sequential decision-making problem. To solve it, we study another parametric formulation of the original problem, which rewards transmission throughput and penalizes energy consumption. Dynamic programming can be applied to identify the optimal strategy for the parametric problem. Then, by exploring the relationship between the two formulations and making use of the monotonicity property of the parametric formulation, we develop an algorithm to find the optimal sensing-access strategies for the original problem. Furthermore, we study the joint design of the channel sensing order and the sensing-access strategies. Lastly, the performance of the proposed designs is evaluated through numerical results. [ABSTRACT FROM PUBLISHER]

Low-density parity-check (LDPC) codes combined with cascade combining are able to achieve excellent bit-error rate (BER) performance over multipath fading channels. Cascade combining, which involves two stages of diversity combining, is very useful for situations where site diversity exists and it can reduce the complexity of the combiner when the number of diversity branches increases. The average output signal-to-noise ratio and the uncoded BER expressions of cascade combining over independent and identically distributed (i.i.d.) Nakagami-m fading channels are derived. The BER expressions of cascade combining with LDPC codes over i.i.d. Nakagami-m fading channels are derived based on the Gaussian approximation (GA) approach. These expressions include selection combining and maximum-ratio combining as special cases. The signal-to-noise ratio (SNR) thresholds for error-free decoding are obtained using density evolution (DE). Comparisons among GA analysis, DE analysis, and simulations show that they are in good agreement. The expressions obtained from the GA analysis provide better theoretical understanding of the system performance and allow us to compute the BER and SNR threshold more efficiently as compared to DE and simulations. Lastly, the stability conditions for GA and DE analyses, which are found to be exactly the same, are also presented. [ABSTRACT FROM PUBLISHER]

A maximum-likelihood (ML) receiver structure is proposed for fast frequency-hopped M-ary frequency-shift keying communication systems over Rayleigh-fading channels with the composite effects of multitone jamming (MTJ) and partial-band noise jamming (PBNJ). The corresponding semi-analytical bit-error rate (BER) expressions of the proposed ML receiver are also presented and validated by simulation results. Numerical results show that the BER performance with both MTJ and PBNJ is bounded by the two extreme cases, where only MTJ or PBNJ is present. It is also found that the proposed ML receiver can better counteract MTJ than PBNJ. [ABSTRACT FROM PUBLISHER]

Hand hygiene has been shown to be effective in significantly reducing hospital acquired infections for many years. However it is difficult to maintain and enhance compliance with hand hygiene guidelines. In Hong Kong, we previously reported a strategy to counter campaign fatigue from 50%-55% in 2009-11 to 83% in 2012. Here we report a creative activity that we used to sustain and enhance hand hygiene compliance. In May 2014 we broke the first Guinness World Record for a Hand Sanitizing Relay. A total of 277 participants performed hand hygiene before two official and approved witnesses. Following this team-directed strategy, an increase in hand hygiene compliance was identified in June 2014 in two clinical areas with previously poor compliance. The longer term impact of this strategy remains to be determined. More broadly, further research is urgently needed on meeting the challenge of campaign fatigue, and maintaining and enhancing compliance with hand hygiene guidelines.