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Reconfigurable intelligent surface (RIS) can be employed in a cell-free system to create favorable propagation conditions from base stations (BSs) to users via configurable elements. However, prior works on RIS-aided cell-free system designs mainly rely on the instantaneous channel state information (CSI), which may incur substantial overhead due to extremely high dimensions of estimated channels. To mitigate this issue, a low-complexity algorithm via the two-timescale transmission protocol is proposed in this paper, where the joint beamforming at BSs and RISs is facilitated via alternating optimization framework to maximize the average weighted sum-rate. Specifically, the passive beamformers at RISs are optimized through the statistical CSI, and the transmit beamformers at BSs are based on the instantaneous CSI of effective channels. In this manner, a closed-form expression for the achievable weighted sum-rate is derived, which enables the evaluation of the impact of key parameters on system performance. To gain more insights, a special case without line-of-sight (LoS) components is further investigated, where a power gain on the order of $\mathcal {O}(M)$ is achieved, with $M$ being the BS antennas number. Numerical results validate the tightness of our derived analytical expression and show the fast convergence of the proposed algorithm. Findings illustrate that the performance of the proposed algorithm with two-timescale CSI is comparable to that with instantaneous CSI in low or moderate SNR regime. The impact of key system parameters such as the number of RIS elements, CSI settings and Rician factor is also evaluated. Moreover, the remarkable advantages from the adoption of the cell-free paradigm and the deployment of RISs are demonstrated intuitively. [ABSTRACT FROM AUTHOR]

Reconfigurable intelligent surface (RIS) is a promising solution to build a programmable wireless environment with reconfigurable passive elements, which can achieve high spectral and energy efficiency. In this paper, we investigate the ergodic capacity of RIS-assisted multi-user multiple-input single-output (MISO) wireless systems in both uplink and downlink scenarios. Unlike most of prior works, where instantaneous channel state information (CSI) is assumed, we consider the realistic scenario with only statistical CSI. For both scenarios, we first present an analytical expressions for the ergodic sum capacity of the system. Based on which, the joint power control (or transmit beamforming) and phase shift design problem maximizing the ergodic sum capacity is formulated. Capitalizing on the alternating direction method of multipliers (ADMM), fractional programming (FP) and alternating optimization (AO) methods, efficient suboptimal solutions are obtained for the non-convex design problems. Simulation results are presented to validate the accuracy of the analytical ergodic sum capacity expressions and evaluate the impact of key system parameters such as CSI, Rician $K$ factor, number of RIS elements, and RIS location on the ergodic capacity performance. The findings suggest that the proposed statistical CSI design achieves decent performance compared with the instantaneous CSI based design. Moreover, a signal hot spot can be created when placing the RIS close to the users. [ABSTRACT FROM AUTHOR]

This paper considers an angle-domain intelligent reflecting surface (IRS) system. We derive maximum likelihood (ML) estimators for the effective angles from the base station (BS) to the user and the effective angles of propagation from the IRS to the user. It is demonstrated that the accuracy of the estimated angles improves with the number of BS antennas. Also, deploying the IRS closer to the BS increases the accuracy of the estimated angle from the IRS to the user. Then, based on the estimated angles, we propose a joint optimization of BS beamforming and IRS beamforming, which achieves similar performance to two benchmark algorithms based on full CSI and the multiple signal classification (MUSIC) method respectively. Simulation results show that the optimized BS beam becomes more focused towards the IRS direction as the number of reflecting elements increases. Furthermore, we derive a closed-form approximation, upper bound and lower bound for the achievable rate. The analytical findings indicate that the achievable rate can be improved by increasing the number of BS antennas or reflecting elements. Specifically, the BS-user link and the BS-IRS-user link can obtain power gains of order N and NM2, respectively, where N is the antenna number and M is the number of reflecting elements. [ABSTRACT FROM AUTHOR]

We consider a wireless powered two-way relaying system consisting of two energy constrained single antenna sources and one multi-antenna relay with constant power supply. The time division protocol is adopted, where the relay first acts as the energy source and employs energy beamforming to charge the two sources, and then switches its role as a relay to help forward the information to the sources. To maintain user fairness, we aim to maximize the minimum rate of two sources, by jointly optimizing the energy beamforming vector, time splitting factor and relay transformation matrix. To further reduce the complexity of the optimal algorithm, we propose an alternating optimization method, where closed-form expressions for the energy beamforming and time splitting factor are obtained. To gain more insights, we propose a simple suboptimal design and analyze the outage probability and the average rate when the relay applies simple energy beamforming and transformation matrix. The analysis shows that the system can achieve a diversity order of $\frac {N}{3}$ for a system with a relay of $N$ antennas. Numerical results show that the performance of the proposed low-complexity alternating optimization method approaches the optimal algorithm over the entire SNR range, but has a significantly low complexity, and the proposed suboptimal design can also achieves a decent performance especially in small $N$ regime. [ABSTRACT FROM AUTHOR]

This paper investigates the symbol detection of the multiple antenna ambient backscatter communication systems. In particular, to overcome the problem of the noise uncertainty, the maximum-eigenvalue detector is derived from the general likelihood ratio test, and the approximate bit error rate (BER) performance is characterized. The analytical results show that the BER of the maximum-eigenvalue detector converges to a fixed number in large signal-to-noise ratio or number of receiving antennas $M$ regime. Finally, numerical results are presented to confirm the correctness of theoretical analysis, and reveal that the maximum-eigenvalue detector outperforms the energy detector regarding the BER performance. [ABSTRACT FROM AUTHOR]

This paper investigates the achievable performance of cell-free massive multiple-input multiple-output (MIMO) systems with low resolution analog-to-digital converters (ADCs) at both the access points (APs) and users. A closed-form expression for the achievable rate is derived, which enables the study of the effects of AP number, antenna number per AP, user number, and ADC resolution on the achievable rate. In addition, a simple asymptotic approximation for the individual user rate is presented, which shows that the user rate is mainly constrained by the ADC resolution at the user. Moreover, we propose an ADC resolution bits allocation scheme aiming at maximizing the sum rate subject to the total ADC resolution bits constraint, which substantially outperforms the equal ADC resolution bits allocation scheme. Furthermore, a max–min power control scheme is proposed, which not only ensures user fairness, but also improves the achievable rate. Finally, simulation results are provided to corroborate the analytical results. [ABSTRACT FROM AUTHOR]

We consider a two-way half-duplex relaying system where multiple pairs of single-antenna users exchange information assisted by a multiple-antenna relay. Taking into account the practical constraint of imperfect channel knowledge, we study the achievable sum spectral efficiency (SE) of the amplify-and-forward protocol, assuming that the relay employs maximum ratio processing. We derive a closed-form expression for the sum SE for arbitrary system parameters and a large-scale approximation for the sum SE when the number of relay antennas $M$ becomes sufficiently large. In addition, we study how the transmit power reduces with $M$ to maintain a desired SE. Our results show that by using a large number of relay antennas, the transmit powers of the user, relay, and pilot symbol can be scaled down proportionally to $1/M^\alpha$ , $1/M^\beta$ , and $1/M^\gamma$ for certain combinations of $\alpha$ , $\beta$ , and $\gamma$ , respectively. This elegant power scaling law reveals a fundamental tradeoff between the transmit powers of the user/relay and pilot symbol. Finally, capitalizing on the new expressions for the sum SE, novel power allocation schemes are designed to further improve the sum SE. [ABSTRACT FROM AUTHOR]

This paper considers a heavy loaded full-duplex (FD) small cell consisting of one FD base station and multiple half-duplex users where the number of subchannels is less than the number of uplink and downlink users. With the objective of maximizing the total sum-rate, a joint power allocation, and user pairing problem is formulated. To solve this problem, a novel decomposition method is proposed, which decouples the power allocation and user pairing problem into two subproblems. It is proven that the optimal solution of subproblems is the optimal solution of the original problem. Then, optimal algorithms are developed for the two subproblems. Numerical results show that the proposed algorithm achieves better performance compared with the baselines of the greedy algorithm and random pairing algorithm. [ABSTRACT FROM AUTHOR]

This paper considers a wirelessly powered wiretap channel, where an energy constrained multi-antenna information source, powered by a dedicated power beacon, communicates with a legitimate user in the presence of a passive eavesdropper. Based on a simple time-switching protocol, where power transfer and information transmission are separated in time, we investigate two popular multi-antenna transmission schemes at the information source, namely, maximum ratio transmission and transmit antenna selection. Closed-form expressions are derived for the achievable secrecy outage probability and average secrecy rate for both schemes. In addition, simple approximations are obtained at the high signal-to-noise ratio (SNR) regime. Our results demonstrate that by exploiting the full knowledge of channel state information (CSI), we can achieve a better secrecy performance, e.g., with full CSI of the main channel, the system can achieve substantial secrecy diversity gain. On the other hand, without the CSI of the main channel, no diversity gain can be attained. Moreover, we show that the additional level of randomness induced by wireless power transfer does not affect the secrecy performance in the high SNR regime. Finally, our theoretical claims are validated by the numerical results. [ABSTRACT FROM PUBLISHER]

In this errata, we make corrections to some mistakes in our published paper [1]. [ABSTRACT FROM AUTHOR]

This paper addresses the joint spectrum sharing and power allocation problem for device-to-device (D2D) communications underlaying a cellular network (CN). In the context of orthogonal frequency-division multiple-access systems, with the uplink resources shared with D2D links, both centralized and decentralized methods are proposed. Assuming global channel state information (CSI), the resource allocation problem is first formulated as a nonconvex optimization problem, which is solved using convex approximation techniques. We prove that the approximation method converges to a suboptimal solution and is often very close to the global optimal solution. On the other hand, by exploiting the decentralized network structure with only local CSI at each node, the Stackelberg game model is then adopted to devise a distributed resource allocation scheme. In this game-theoretic model, the base station (BS), which is modeled as the leader, coordinates the interference from the D2D transmission to the cellular users (CUs) by pricing the interference. Subsequently, the D2D pairs, as followers, compete for the spectrum in a noncooperative fashion. Sufficient conditions for the existence of the Nash equilibrium (NE) and the uniqueness of the solution are presented, and an iterative algorithm is proposed to solve the problem. In addition, the signaling overhead is compared between the centralized and decentralized schemes. Finally, numerical results are presented to verify the proposed schemes. It is shown that the distributed scheme is effective for the resource allocation and could protect the CUs with limited signaling overhead. [ABSTRACT FROM AUTHOR]

This paper investigates the achievable sum-rate of massive multiple-input multiple-output (MIMO) systems in the presence of channel aging. For the uplink, by assuming that the base station (BS) deploys maximum ratio combining (MRC) or zero-forcing (ZF) receivers, we present tight closed-form lower bounds on the achievable sum-rate for both receivers with aged channel state information (CSI). In addition, the benefit of implementing channel prediction methods on the sum-rate is examined, and closed-form sum-rate lower bounds are derived. Moreover, the impact of channel aging and channel prediction on the power scaling law is characterized. Extension to the downlink scenario and multicell scenario is also considered. It is found that, for a system with/without channel prediction, the transmit power of each user can be scaled down at most by 1/\sqrtM (where $M$ is the number of BS antennas), which indicates that aged CSI does not degrade the power scaling law, and channel prediction does not enhance the power scaling law; instead, these phenomena affect the achievable sum-rate by degrading or enhancing the effective signal to interference and noise ratio, respectively. [ABSTRACT FROM AUTHOR]

This paper investigates the average throughput of a wireless-powered communications system, where an energy constrained source, powered by a dedicated power beacon (PB), communicates with a destination. It is assumed that the PB is capable of performing channel estimation, digital beamforming, and spectrum sensing as a communication device. Considering a time-splitting approach, the source first harvests energy from the PB equipped with multiple antennas, and then transmits information to the destination. Assuming Nakagami-$m$ fading channels, analytical expressions for the average throughput are derived for two different transmission modes, namely, delay tolerant and delay intolerant. In addition, closed-form solutions for the optimal time split, which maximize the average throughput are obtained in some special cases, i.e., high-transmit power regime and large number of antennas. Finally, the impact of cochannel interference is studied. Numerical and simulation results have shown that increasing the number of transmit antennas at the PB is an effective tool to improve the average throughput and the interference can be potentially exploited to enhance the average throughput, since it can be utilized as an extra source of energy. Also, the impact of fading severity level of the energy transfer link on the average throughput is not significant, especially if the number of PB antennas is large. Finally, it is observed that the source position has a great impact on the average throughput. [ABSTRACT FROM AUTHOR]

With the growing popularity of mobile Internet, providing secure wireless services has become a critical issue. Physical layer security (PHY-security) has been recognized as an effective means to enhance wireless security by exploiting wireless medium characteristics, for example, fading, noise, and interference. A particularly interesting PHY-security technology is cooperative relay due to the fact that it helps to provide distributed diversity and shorten access distance. This article offers a tutorial on various multi-antenna relaying technologies to improve security at physical layer. The state-of-the-art research results on multi-antenna relay aided PHY-security as well as some secrecy performance optimization schemes are presented. In particular, we focus on large-scale MIMO relaying technology, which is effective in tackling various challenging issues for implementing wireless PHY-security, such as short-distance interception without eavesdropper CSI and with imperfect legitimate CSI. Moreover, the future directions are identified for further enhancement of secrecy performance. [ABSTRACT FROM AUTHOR]

In this paper, we present a comprehensive investigation on the secrecy performance of opportunistic relay selection systems employing the decode-and-forward protocol over Rayleigh fading channels. Considering a practical setting where direct link between the source node (Alice) and the destination node (Bob) is available, we study the secrecy performance of three different diversity combining schemes, namely, maximum ratio combining (MRC), distributed selection combining (DSC), and distributed switch-and-stay combining (DSSC). Throughout the analysis, we consider two different scenarios based on the availability of the eavesdropper's channel state information (CSI), i.e., Scenario A, where the eavesdropper's CSI is not available at Alice and the relay, and Scenario B, where Alice and the relay have knowledge about the eavesdropper's CSI. For Scenario A, we derive exact closed-form expressions for secrecy outage probability and simple asymptotic approximations for the secrecy outage probability, which enable the characterization of the achievable secrecy diversity order and coding gains. For Scenario B, we derive closed-form expressions for the achievable secrecy rates. For both scenarios, we investigate the impact of feedback delay (outdated CSI) on the secrecy performance wherein exact and asymptotic secrecy outage probability and closed-form expressions of the secrecy achievable rates are obtained. Our analytical findings suggest that both the MRC and DSC schemes achieve the maximum diversity order of $K+1$ where $K$ is the number of relays. In addition, the feedback delay has a significant impact on the achievable secrecy performance by reducing the achievable diversity order to two. [ABSTRACT FROM PUBLISHER]

In this paper, an energy harvesting dual-hop relaying system without/with the presence of co-channel interference (CCI) is investigated. Specifically, the energy constrained multi-antenna relay node is powered by either the information signal of the source or via the signal receiving from both the source and interferer. In particular, we first study the outage probability and ergodic capacity of an interference free system, and then extend the analysis to an interfering environment. To exploit the benefit of multiple antennas, three different linear processing schemes are investigated, namely, 1) Maximum ratio combining/maximum ratio transmission (MRC/MRT), 2) Zero-forcing/MRT (ZF/MRT) and 3) Minimum mean-square error/MRT (MMSE/MRT). For all schemes, both the systems outage probability and ergodic capacity are studied, and the achievable diversity order is also presented. In addition, the optimal power splitting ratio minimizing the outage probability is characterized. Our results show that the implementation of multiple antennas increases the energy harvesting capability, hence, significantly improves the systems performance. Moreover, it is demonstrated that the CCI could be potentially exploited to substantially boost the performance, while the choice of a linear processing scheme plays a critical role in determining how much gain could be extracted from the CCI. [ABSTRACT FROM PUBLISHER]

Simultaneous wireless information and power transfer (SWIPT) is a promising solution to increase the lifetime of wireless nodes and hence alleviate the energy bottleneck of energy constrained wireless networks. As an alternative to conventional energy harvesting techniques, SWIPT relies on the use of radio frequency signals, and is expected to bring some fundamental changes to the design of wireless communication networks. This article focuses on the application of advanced smart antenna technologies to SWIPT, including multiple-input multiple-output and relaying techniques. These smart antenna technologies have the potential to significantly improve the energy efficiency and also the spectral efficiency of SWIPT. Different network topologies with single and multiple users are investigated, along with some promising solutions to achieve a favorable trade-off between system performance and complexity. A detailed discussion of future research challenges for the design of SWIPT systems is also provided. [ABSTRACT FROM PUBLISHER]

We consider a dual-hop full-duplex relaying system, where the energy constrained relay node is powered by radio frequency signals from the source using the time-switching architecture, both the amplify-and-forward and decode-and-forward relaying protocols are studied. Specifically, we provide an analytical characterization of the achievable throughput of three different communication modes, namely, instantaneous transmission, delay-constrained transmission, and delay tolerant transmission. In addition, the optimal time split is studied for different transmission modes. Our results reveal that, when the time split is optimized, the full-duplex relaying could substantially boost the system throughput compared to the conventional half-duplex relaying architecture for all three transmission modes. In addition, it is shown that the instantaneous transmission mode attains the highest throughput. However, compared to the delay-constrained transmission mode, the throughput gap is rather small. Unlike the instantaneous time split optimization which requires instantaneous channel state information, the optimal time split in the delay-constrained transmission mode depends only on the statistics of the channel, hence, is suitable for practical implementations. [ABSTRACT FROM AUTHOR]

In this paper, we analyze the ergodic capacity of a dual-hop amplify-and-forward relaying system, where the relay is equipped with multiple antennas and subject to co-channel interference and the additive white Gaussian noise. Specifically, we consider three heuristic precoding schemes, where the relay first applies the: 1) maximal-ratio combining (MRC); 2) zero-forcing (ZF); and 3) minimum mean-squared error (MMSE) principle to combine the signal from the source, and then steers the transformed signal toward the destination with the maximum ratio transmission (MRT) technique. For the MRC/MRT and MMSE/MRT schemes, we present new tight analytical upper and lower bounds for the ergodic capacity, while for the ZF/MRT scheme, we derive a new exact analytical ergodic capacity expression. Moreover, we make a comparison among all three schemes, and our results reveal that, in terms of the ergodic capacity performance, the MMSE/MRT scheme always has the best performance and the ZF/MRT scheme is slightly inferior, while the MRC/MRT scheme is always the worst one. Finally, the asymptotic behavior of ergodic capacity for the three proposed schemes are characterized in large $N$ scenario, where $N$ is the number of relay antennas. Our results reveal that, in the large $N$ regime, both the ZF/MRT and MMSE/MRT schemes have perfect interference cancelation capability, which is not possible with the MRC/MRT scheme. [ABSTRACT FROM AUTHOR]

This paper presents an analytical investigation on the outage performance of dual-hop multiple antenna amplify-and-forward relaying systems in the presence of interference. For both the fixed-gain and variable-gain relaying schemes, exact analytical expressions for the outage probability of the systems are derived. Moreover, simple outage probability approximations at the high signal-to-noise-ratio regime are provided, and the diversity order achieved by the systems are characterized. Our results suggest that variable-gain relaying systems always outperform the corresponding fixed-gain relaying systems. In addition, the fixed-gain relaying schemes only achieve diversity order of one, while the achievable diversity order of the variable-gain relaying scheme depends on the location of the multiple antennas. [ABSTRACT FROM PUBLISHER]

This letter presents an analytical performance investigation on the cognitive broadcast channels (BC) with user selection. Exact closed-form expressions for the outage probability and multiuser interference diversity gain of the cognitive BC are derived. In addition, closed-form upper and lower bounds for the ergodic capacity of the system are presented. These analytical results not only provide fast and efficient means to evaluate the performance of the system, they also enable us to gain valuable insights on the impact of key parameters such as peak transmit power, interference temperature constraint, primary transmit power and channel fading parameters on the system performance. [ABSTRACT FROM AUTHOR]

We investigate the ergodic capacity of amplify-and-forward (AF) dual-hop relaying systems in composite Nakagami-m/inverse-Gaussian fading channels. This type of fading, which is known in the literature as \cal G fading, has recently attracted increasing research interest due to its ability to better approximate the Nakagami- m/lognormal model, compared with the Nakagami- m/gamma model. We study both fixed- and variable-gain relaying systems and present analytical upper and lower bounds for the ergodic capacity of dual-hop relaying systems with not necessarily identical hops; these bounds provide an efficient means to evaluate the ergodic capacity of AF dual-hop relaying systems over \cal G fading channels. We also establish sufficient conditions for the existence of the bounds, depending on the values of the fading parameters. In both cases, our simulation results demonstrate that the proposed upper and lower bounds remain relatively tight for different fading conditions. [ABSTRACT FROM PUBLISHER]

This paper investigates the performance of orthogonal space–time block codes over generalized- K fading multiple-input–multiple-output (MIMO) channels. We present exact analytical expressions for three important performance measures, namely, outage probability, average capacity, and average symbol error rate (SER). In addition, we derive simple expressions for key parameters dictating the system performance at the high and low signal-to-noise ratio (SNR) regimes, i.e., diversity order, coding gain, minimum energy per information bit, and wideband slope, which provide insight on how various key system parameters, such as number of antennas and fading severity level, affect the performance of the system. [ABSTRACT FROM PUBLISHER]

This paper elaborates on the ergodic capacity of fixed-gain amplify-and-forward (AF) dual-hop systems, which have recently attracted considerable research and industry interest. In particular, two novel capacity bounds that allow for fast and efficient computation and apply for nonidentically distributed hops are derived. More importantly, they are generic since they apply to a wide range of popular fading channel models. Specifically, the proposed upper bound applies to Nakagami-m, Weibull, and generalized-K fading channels, whereas the proposed lower bound is more general and applies to Rician fading channels. Moreover, it is explicitly demonstrated that the proposed lower and upper bounds become asymptotically exact in the high signal-to-noise ratio (SNR) regime. Based on our analytical expressions and numerical results, we gain valuable insights into the impact of model parameters on the capacity of fixed-gain AF dual-hop relaying systems. [ABSTRACT FROM PUBLISHER]

This paper introduces some novel upper and lower bounds on the achievable sum rate of multiple-input multiple-output (MIMO) systems with zero-forcing (ZF) receivers. The presented bounds are not only tractable but also generic since they apply for different fading models of interest, such as uncorrelated/correlated Rayleigh fading and Ricean fading. We further formulate a new relationship between the sum rate and the first negative moment of the unordered eigenvalue of the instantaneous correlation matrix. The derived expressions are explicitly compared with some existing results on MIMO systems operating with optimal and minimum mean-squared error (MMSE) receivers. Based on our analytical results, we gain valuable insights into the implications of the model parameters, such as the number of antennas, spatial correlation and Ricean-K factor, on the sum rate of MIMO ZF receivers. [ABSTRACT FROM AUTHOR]

This paper studies the outage probability performance of decode-and-forward (DF) cognitive dual-hop systems for Nakagami-m fading channels, with consideration of an interference temperature limit. An exact outage probability expression is derived, and the impact of various key system parameters, such as interference temperature and fading severity, is investigated. Our results show that the interference temperature constraint causes the outage saturation phenomenon, but the outage performance gets better when the channel quality of the secondary transmission links improves. However, how the channel quality of the interfering links affects the outage performance depends on the channel quality of the secondary transmission links, as well as the interference temperature constraint and maximum transmit power. [ABSTRACT FROM PUBLISHER]

This paper studies the ergodic capacity limits of multiple-input multiple-output (MIMO) antenna systems with arbitrary finite number of antennas operating on general fading environments. Through the use of majorization theory, we first investigate in detail the ergodic capacity of Nakagami-m fading channels, for which we derive several ergodic capacity upper and lower bounds. We then show that a simple expression for the capacity upper bound is possible for high signal-to-noise ratio (SNR), which permits to analyze the impact of the channel fading parameter in on the ergodic capacity. The asymptotic behavior of the capacity in the large-system limit in which the number of antennas at one or both side(s) goes to infinity, is also addressed. Results demonstrate that the capacity scaling laws for Nakagami-m and Rayleigh-fading MIMO channels are identical. Finally, we employ the same technique to distributed MIMO (D-MLMO) systems undergoing composite log-normal and Nakagami fading, where we derive similar ergodic capacity upper and lower bounds. Monte Carlo simulation results are provided to verify the tightness of the proposed bounds. [ABSTRACT FROM AUTHOR]

• The ratiometric fluorescent approach of sensing melamine was successfully established. • The UiO-66-NH 2 @Ru nanocomposite was fabricated by a "one-pot" process. • UiO-66-NH 2 @Ru was applied to detect melamine in milk powder sample with good performance. • UiO-66-NH 2 @Ru is the first example of MOFs as a fluorescent "on–off" probe toward melamine. Melamine has biological toxicity and is forbidden to be used in food processing and food additives. The long-term ingestion of melamine will cause renal malfunction and metabolic problems of the urinary system, so it is of great significance for the rapid and selective detection of melamine. Herein, a zirconium-based metal–organic frameworks (Zr-based MOFs) encapsulated tris(2,2-bipyridyl)ruthenium (II) chloride hexahydrate ([Ru(bpy) 3 ]2+) composite, donated as UiO-66-NH 2 @Ru, was reported and further studied its sensing performance for melamine based on ratiometric fluorescent method. The UiO-66-NH 2 @Ru was prepared from ZrCl 4 , 2-amino-terephthalic acid (TPA-NH 2) and [Ru(bpy) 3 ]2+ by one pot reaction through solvothermal method, which exhibited dual emission signals at 445 and 595 nm from single excitation of 350 nm. The corresponding two peaks showed different responses to melamine, that was, the obvious enhancement at 445 nm and the slow decrease at 595 nm, thus achieving the "on–off" ratiometric fluorescence detection. When the concentration of melamine was in the range of 0.27 to 110 μM, the ratio of fluorescence intensity F 445 /F 595 had a good linear relation with the correlation coefficient R2 = 0.995 and the detection limit of 90 nM. The synthetic UiO-66-NH 2 @Ru probe implemented rapid determination of melamine in real sample of milk powder with high sensitivity and selectivity. Compared with the reported carbon dots, quantum dots, metal nanoclusters, nanocomposites and other fluorescent sensors, MOFs-based sensor have broadened the application of nanomaterials in melamine detection for the first time. This study also demonstrates the Zr-based MOFs as a platform to encapsulate fluorescent dye for constructing ratiometric sensor in the determination of melamine is feasible and exciting. [ABSTRACT FROM AUTHOR]

In this paper, wepropose a novel two-stage transmission framework for an intelligent reflecting surface (IRS) assisted millimeter wave (mmWave) system in the presence of phase error caused by hardware impairments. Specifically, in Stage I, we introduce a pilot-based phase calibration task and reformulate it as an alternating optimization program. Building on this formulation, an efficient iterative algorithm is developed to jointly estimate the phase error, which needs to be calibrated, and the channels corresponding to the first user. Stage II consists of two blocks, channel estimation and data transmission. During the channel estimation block, by leveraging on the sparsity of mmWave channels, we propose a fast Fourier transform based scheme to estimate the IRS-BS channel and user-IRS channels. During the data transmission block, based on the estimated channel state information, base station precoders and IRS phase shifts are jointly optimized to maximize the sum signal-to-leakage-plus-noise ratio of all users. To handle this non-convex problem, we further develop a low-complexity joint beamforming algorithm by exploiting fractional programming techniques. Moreover, to examine the performance of the proposed phase calibration scheme, we derive the Cramer-Rao lower bound for the intended phase error estimation. Simulation results validate the effectiveness of our proposed transmission framework and the necessity of IRS phase calibration. [ABSTRACT FROM AUTHOR]

This paper considers a multipair amplify-and-forward massive multipair multiple-input multiple-output relaying system with one-bit analog-to-digital converters and one-bit digital-to-analog converters at the relay. The channel state information is estimated via pilot training, and then utilized by the relay to perform simple maximum-ratio combining/maximum-ratio transmission processing. Leveraging on the Bussgang decomposition, an exact achievable rate is derived for the system with correlated quantization noise. Based on this, a closed-form asymptotic approximation for the achievable rate is presented, thereby enabling efficient evaluation of the impact of key parameters on the system performance. Furthermore, power scaling laws are characterized to study the potential energy efficiency associated with deploying massive one-bit antenna arrays at the relay. In addition, a power allocation strategy is designed to compensate for the rate degradation caused by the coarse quantization. Our results suggest that the quality of the channel estimates depends on the specific orthogonal pilot sequences that are used, contrary to unquantized systems where any set of orthogonal pilot sequences gives the same result. Moreover, the sum rate gap between the double-quantized relay system and an ideal nonquantized system is a moderate factor of ${\text{4}}/\pi ^2$ in the low power regime. [ABSTRACT FROM AUTHOR]

To achieve spectral-efficient massive access in future wireless networks, this paper proposes a comprehensive fully non-orthogonal communication framework. First, we design a fully non-orthogonal communication scheme which consists of non-orthogonal channel estimation and non-orthogonal multiple access. Then, we analyze the performance of the proposed fully non-orthogonal communication, and derive a tight lower bound on the spectral efficiency in terms of key system parameters and channel conditions. Meanwhile, several novel insights are provided on spectral efficiency via asymptotic analysis in three important cases, i.e., a large number of base station (BS) antennas, a high BS transmit power, and perfect channel state information (CSI) at the BS. Finally, we optimize the performance of the proposed fully non-orthogonal communication and present two simple but efficient optimization algorithms for maximizing the weighted sum of spectral efficiency. Extensive simulation results validate the effectiveness of the proposed schemes. [ABSTRACT FROM AUTHOR]

In this paper, we consider a point-to-point multi-input multi-output wireless-powered communication system, where the source S is powered by a dedicated power beacon (PB) with multiple antennas. Employing the time splitting protocol, the energy constrained source S first harvests energy through the radio-frequency signals sent by the PB and then uses this energy to transmit information to the destination D. Unlike several prior works, we assume that the energy transfer link is subjected to Rician fading, which is a real fading environment, due to relatively short range power transfer distance and the existence of a strong line of sight path. We present a comprehensive analysis of the achievable ergodic capacity in two scenarios, depending on the availability of channel state information (CSI) at PB, namely, the absence of CSI and partial CSI. For the former case, equal power allocation is used, while for the later one, energy beamforming is used to enhance the energy transfer efficiency. For both the cases, closed-form expressions for the upper and lower bounds of the ergodic capacity are derived. Furthermore, the optimal time split is discussed, and the capacity in the low and high signal-to-noise ratio regimes is studied through simple closed-form expressions. Numerical results and simulations are provided to validate the theoretical analysis. The results show that the Rician factor $K$ has a significant impact on the ergodic capacity performance, and this impact strongly depends on the availability of the CSI at the PB. [ABSTRACT FROM AUTHOR]

With the fast development of vehicular technologies, the number of mobile terminals as well as their moving speeds has increased conspicuously, making it meaningful to re-examine the impact of mobility on communication performance of the existing cellular systems. In this paper, the uplink sum rate of a multiple-input multiple-output orthogonal frequency-division multiple access (MIMO-OFDMA) cellular system with nodes moving randomly at different speeds is studied. In such a multi-user environment, mobility not only causes inter sub-carrier interferences (ICI) among different users, but also brings the so-called channel aging which together with the above interferences incur outdated and inaccurate channel estimation and thus entail additional pilot overhead. Here, we first derive the relationship between the users’ mobility and the ICI power, characterize the channel aging, and derive the overall multi-user interference caused by the outdated and inaccurate channel state information as a function of mobility. Then, by capitalizing on the above results, the sum rate of the MIMO-OFDMA uplink is theoretically analyzed. Moreover, an adaptive transmission scheme in which the users adjust the pilot percentage according to mobility statistics is proposed to maximize the sum rate. Finally, numerical results are provided to validate our analyses. [ABSTRACT FROM PUBLISHER]

This paper studies the Demmel condition number of Wishart matrices, a quantity which has numerous applications to wireless communications, such as adaptive switching between beamforming and diversity coding, link adaptation, and spectrum sensing. For complex Wishart matrices, we give an exact analytical expression for the probability density function (p.d.f.) of the Demmel condition number, and also derive simplified expressions for the high tail regime. These results indicate that the condition of complex Wishart matrices is dominantly decided by the difference between the matrix dimension and degree of freedom (DoF), i.e., the probability of drawing a highly ill conditioned matrix decreases considerably when the difference between the matrix dimension and DoF increases. We further investigate real Wishart matrices, and derive new expressions for the p.d.f. of the smallest eigenvalue, when the difference between the matrix dimension and DoF is odd. Based on these results, we succeed to obtain an exact p.d.f. expression for the Demmel condition number, and simplified expressions for the high tail regime. [ABSTRACT FROM AUTHOR]

The roll-out of various emerging wireless services has triggered the need for the sixth-generation (6G) wireless networks to provide functions of target sensing, intelligent computing and information communication over the same radio spectrum. In this paper, we provide a unified framework integrating sensing, computing, and communication to optimize limited system resource for 6G wireless networks. In particular, two typical joint beamforming design algorithms are derived based on multi-objective optimization problems (MOOP) with the goals of the weighted overall performance maximization and the total transmit power minimization, respectively. Extensive simulation results validate the effectiveness of the proposed algorithms. Moreover, the impacts of key system parameters are revealed to provide useful insights for the design of integrated sensing, computing, and communication (ISCC). [ABSTRACT FROM AUTHOR]

Provisioning reliable wireless services for railway passengers is becoming an increasingly critical problem to be addressed with the fast development of high speed trains (HST). In this paper, exploiting the linear mobility inherent to the HST communication scenario, we discover a new type of spatial–temporal correlation between the base station and moving antenna array on the roof top of the train. Capitalizing on the new spatial–temporal correlation structure and properties, an improved differential space-time modulation (DSTM) scheme is proposed. Analytical expressions are obtained for the pairwise error probability of the system. It is demonstrated that the proposed approach achieves superior error performance compared with the conventional DSTM scheme. In addition, an adaptive method, which dynamically adjusts the transmission block length is proposed to further enhance the system performance. Numerical results are provided to verify the performance of the proposed schemes. [ABSTRACT FROM AUTHOR]

Severe doubly selective channel fading, or the time- and frequency-selective fading, prevailing in high-speed train (HST) communications, has been a critical issue that hinders the improvement of system efficiency and reliability. In this paper, a relay-assisted HST communication system is considered, and a low-complexity but effective scheme is proposed to cooperatively exploit the joint multipath-Doppler diversity of both the direct link and the relay link and thus to mitigate the negative effect of channel fading and improve the system reliability. In particular, using a special precoding structure followed by two-dimensional Fourier transform, the transmit signal is generated which in effect parallelizes the doubly selective fading channel. As a result, in the transform domain, pointwise combining and equalization, which are of linear complexity, become feasible and diversity gain is obtained. The maximal diversity order is then analyzed under the amplify-and-forward relay settings, which reveals that by cooperative relaying the joint multipath-Doppler diversity of both links can be well exploited. Simulation results verified our analysis. [ABSTRACT FROM AUTHOR]

In this paper, we propose transmit antenna selection with maximal ratio combining (TAS/MRC) in dual-hop decode-and-forward spectrum-sharing relaying networks with the primary user's interference and outdated channel state information (CSI). In this network, a single antenna that maximizes the received SNR is selected at the secondary transmitter, and the MRC is adopted at the secondary receiver. To efficiently evaluate the impact of key parameters on the system performance, we derive the exact analytical expression for the outage probability of the secondary network in a Rayleigh fading channel. Moreover, we present simple asymptotic expressions for the outage probability in a high SNR regime, which reveal practical insights on the achievable diversity order and coding gain. The findings suggest that whether the outdated CSI concerning the secondary transmission links has significant impact on the outage probability of the system depends on the interference power constraint at primary receivers. Specifically, under the proportional interference power constraint, the achievable diversity order is affected by imperfect CSI regarding the secondary transmission links, and the diversity–multiplexing tradeoff is independent of the primary network. However, under the fixed interference power constraint, the error floor is displayed, and the achievable diversity order reduces to zero regardless of the CSI concerning the secondary transmission links. [ABSTRACT FROM AUTHOR]

The challenge in massive Machine Type Communication (mMTC) is to support reliable and instant access for an enormous number of machine-type devices (MTDs). In some particular applications of mMTC, the access point (AP) only has to know the messages received, but not where they source from, thus giving rise to the concept of unsourced random access (URA). In this paper, we propose a novel URA scheme exploiting the elegant properties of Reed-Muller (RM) sequences. Specifically, after dividing the message of an active user into several information chunks, RM sequences are used to carry those chunks, for exploiting the vast sequence space to improve the spectral efficiency, and their nested structure to enable reliable and efficient sequence detection. Next, we further explore a novel structural property of RM sequences for designing sparse patterns which carry part of the information and serve as the hints of coupling the information chunks of a single user. The factors affecting the performance of our slot-based RM detection are characterized. Besides, the complexity of the proposed message stitching method is analyzed and compared to the commonly used tree coding approach. Our simulation results verify the enhanced performance of the proposed URA scheme in error probability and computational complexity compared to the existing counterpart. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive performance analysis of multiuser multiple antenna amplify-and-forward relaying networks employing opportunistic scheduling with feedback delay and co-channel interference over Rayleigh fading channels. Specifically, we derive exact as well as approximate closed-form expressions for the outage probability and average symbol error rate (SER) of the system. In addition, simple asymptotic expressions at the high signal-to-noise ratio (SNR) regime are obtained, which facilitate the characterization of the achievable diversity order and coding gain of the system. Moreover, two novel ergodic capacity bounds valid for general systems with arbitrary number of antennas and users are proposed. Finally, the optimum power allocation scheme in terms of minimizing the average SER is studied, and simple analytical solutions are obtained. Simulation results are provided to corroborate the derived analytical expressions, and it is demonstrated that the ergodic capacity bounds remain sufficiently tight across the entire range of SNRs and the proposed power allocation scheme offers significant improvements on the SER performance. The findings of the paper suggest that the full diversity order can only be achieved when there is ideal feedback, i.e., no feedback delay, and the diversity order always reduces to one in the presence of feedback delay. Also, the impact of key parameters such as the number of antennas and users on the system performance is intimately dependent on the level of feedback delay. [ABSTRACT FROM PUBLISHER]

In this paper, considering both the antenna correlation and the channel correlation between the legitimate receiver and the eavesdropper, we comprehensively investigate the secrecy performance for transmit antenna selection and maximal ratio combining (TAS/MRC) scheme in a multi-input multi-output multi-antenna eavesdropper (MIMOME) system. Different from the existing works, the correlated legitimate channels and correlated eavesdropper channels experience Nakagmai- ${m}$ fading with distinct fading parameters, and are modeled as a combination of conditionally independent channel gains and independent channel gains. By this novel correlation representation, the combined effect of antenna correlation and channel correlation between the legitimate receiver and the eavesdropper on secrecy performance is quantified via the secrecy outage probability (SOP) under a more general fading circumstance. In addition, two special cases of correlated channels are addressed in detail. Finally, simulations are performed to verify our analysis. The analysis and simulation results show that at high signal-to-noise ratio (SNR), antenna correlation at the legitimate receiver can improve the SOP performance for the correlated MIMOME wiretap channels, and this finding is contrary to the conventional wisdom that antenna correlation degrades the SOP performance. [ABSTRACT FROM AUTHOR]

In this article, we study the combination of NOMA and full-duplex operation as a promising solution to improve the capacity of next-generation wireless systems. We study the application of full-duplex NOMA transmission in wireless cellular, relay and cognitive radio networks, and demonstrate achievable performance gains. It is shown that the effects of self-interference and inter-user interference due to full-duplex operation can be effectively mitigated by optimizing/enhancing the beamforming, power control, and link scheduling techniques. We also discuss research challenges and future directions so that full-duplex NOMA can be made practical in the near future. [ABSTRACT FROM AUTHOR]

One-bit quantization can significantly reduce the massive multiple-input and multiple-output (MIMO) system hardware complexity, but at the same time it also brings great challenges to the system algorithm design. Specifically, it is difficult to recover information from the highly distorted samples as well as to obtain accurate channel estimation without increasing the number of pilots. In this paper, a novel inference algorithm called variational approximate message passing (VAMP) for one-bit quantized massive MIMO receiver is developed, which attempts to exploit the advantages of both the variational Bayesian inference algorithm and the bilinear generalized approximated message passing algorithm to accomplish joint channel estimation and data detection in a closed form with first-order complexity. Asymptotic state evolution analysis indicates the fast convergence rate of VAMP and also provides a lower bound for the data detection error. Moreover, through extensive simulations, we show that VAMP can achieve excellent detection performance with low pilot overhead in a wide range of scenarios. [ABSTRACT FROM AUTHOR]

Mobility can potentially increase communication opportunities due to the channel variation it causes. This paper studies a fundamental scheduling problem in a three-node mobile relay system in which data packets are sent from a source to a destination via a mobile relay. Our aim is to fully exploit the communication opportunities brought by relay mobility to minimize the energy consumption under a data throughput constraint. In particular, first, we apply a 2-D finite-state Markov chain (2D-FSMC) channel model to reflect both the large- and small-scale channel fading in the mobile environment. Then, the scheduling problem is formulated as a constrained Markov decision process (C-MDP) and is approximately solved through the Lagrange relaxation approach. Based on this, we proposed a probabilistic algorithm, called the opportunistic packet scheduling (OPS) algorithm, to further reduce energy consumption. The proposed OPS algorithm is proved to be optimal with polynomial complexity. Simulation results show that the proposed algorithm generally outperforms the conventional algorithms in terms of energy consumption. Furthermore, some observations are analyzed for the effect of mobility on the energy-efficient transmission. [ABSTRACT FROM AUTHOR]

We consider a full-duplex (FD) decode-and-forward system in which the time-switching protocol is employed by the multiantenna relay to receive energy from the source and transmit information to the destination. The instantaneous throughput is maximized by optimizing receive and transmit beamformers at the relay and the time-split parameter. We study both optimum and suboptimum schemes. The reformulated problem in the optimum scheme achieves closed-form solutions in terms of transmit beamformer for some scenarios. In other scenarios, the optimization problem is formulated as a semidefinite relaxation problem and a rank-one optimum solution is always guaranteed. In the suboptimum schemes, the beamformers are obtained using maximum ratio combining, zero-forcing, and maximum ratio transmission. When beamformers have closed-form solutions, the achievable instantaneous and delay-constrained throughput are analytically characterized. Our results reveal that beamforming increases both the energy harvesting and loop interference suppression capabilities at the FD relay. Moreover, simulation results demonstrate that the choice of the linear processing scheme as well as the time-split plays a critical role in determining the FD gains. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the impact of multiuser diversity on the performance of secondary users (SUs) in dual-hop decode-and-forward (DF) spectrum sharing systems over Nakagami- $m$ fading channels, taking into consideration the interference from primary users (PUs). In particular, we present a detailed performance comparison for two different opportunistic scheduling algorithms, i.e., the signal-to-noise ratio (SNR)-based scheduling algorithm and the signal-to-interference-plus-noise ratio (SINR)-based scheduling algorithm. For both scheduling algorithms, exact and asymptotic analytical expressions for the outage probability are derived. Our findings show that the SINR-based scheduling algorithm always outperforms the SNR-based scheduling algorithm. Nevertheless, when the number of destination is large, both scheduling algorithms attain almost the same outage performance, which suggests that, in such a scenario, the SNR-based scheduling algorithm is preferred because it requires less channel state information. [ABSTRACT FROM PUBLISHER]

Continuous and high silica SSZ-13 zeolite membranes were prepared on porous mullite supports from high SiO2/Al2O3 ratio or aluminum-free precursor synthesis gel. Single gas permeance (CO2 and CH4) of the high silica SSZ-13 zeolite membrane was decreased with the SiO2/Al2O3 ratio in the precursor synthesis gel, while the ideal CO2/CH4 selectivity of the membrane was gradually increased. Moreover, effects of synthesis conditions (such as H2O/SiO2 and RNOH/SiO2 ratios of precursor synthesis gel, crystallization time) on the single gas permeance performance of high silica SSZ-13 zeolite membranes were studied in detail. Medium H2O/SiO2 and RNOH/SiO2 ratios in the initial synthesis gel were crucial to prepare the good CO2 perm-selective SSZ-13 zeolite membrane. When the molar composition of precursor synthesis gel, crystallization temperature and time were 1.0 SiO2: 0.1 Na2O: 0.1 TMAdaOH: 80 H2O, 160 °C and 48 h, CO2 permeance and ideal CO2/CH4 selectivity of the SSZ-13 zeolite membrane were 0.98 × 10−7 mol/(m2·s·Pa) and 47 at 25 °C and 0.4 MPa. In addition, the SiO2/Al2O3 ratio of the corresponding SSZ-13 zeolite was 410 by X-ray fluorescence spectroscopy. [ABSTRACT FROM AUTHOR]