Search

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 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]

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]

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]

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]

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 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, 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]

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]

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]

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]

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]

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 study the effects of different pulse shapes on the bit-error-rate (BER) performance of a space-time transmit diversity scheme for band-limited direct-sequence code-division multiple-access (DS-CDMA) systems in the presence of multitone jamming (MTJ) over generalized-K fading channels. We consider a space-time system with two transmitter and multiple receiver antennas. Two types of band-limited pulse shapes, namely, spectrum raised cosine (SRC) and Beaulieu-Tan-Damen (BTD) pulses, are considered in the BER performance of the conventional matched-filter receiver, the decorrelator receiver, and the minimum-mean-square-error (MMSE) receiver. Numerical results show that the system with BTD pulse outperforms the system with SRC pulse, and the MMSE receiver gives the best BER performance among the three receivers considered. [ABSTRACT FROM AUTHOR]

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]

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]

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]

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]

We investigate low-complexity ultrawideband (UWB) communication systems such as on-off keying (OOK) transmission with an energy-detection receiver and transmitted- reference (TR) signaling with an autocorrelation receiver (AcR). It is observed that the conventional noncoherent energy detector for OOK signals suffers from a huge, performance loss compared with that of the coherent detector, which suggests space of improvement. It is also found that the OOK signaling can be treated as a special case of TR signaling with pulse-amplitude modulation (PAM). Inspired by this observation, we propose a new pseudocoherent detection scheme for OOK transmission, which can also be applied to pulse-position-modulation (PPM) transmission. We also consider the implementation issues of the proposed receiver, such as the high sampling rate requirement. We propose a new scheme named bandpass downsampling (BPDS) to overcome this difficulty. With BPDS, we are able to reduce the sampling rate to one tenth of the Nyquist rate, and yet, it can provide performance with a small degradation. A theoretical analysis is carried out to evaluate the bit-error-rate (BER) performance of the proposed receiver. The numerical results show that the performance of the proposed receiver can drastically be improved from that of the conventional energy detector (CED). [ABSTRACT FROM AUTHOR]