The multi-access edge computing (MEC)-based wireless-powered backscatter communication networks (WP-BackComNets) allow wireless devices (WDs) to offload computation resources to lightweight and ...widely deployed MEC servers with the assistance of backscatter devices (BDs), which have substantial application prospects for the emerging Internet-of-Things applications. However, the limited battery capacity of WDs is one of the bottlenecks restricting its further development. Reducing the energy consumption and the computation burden of WDs while ensuring the quality-of-service requirements is an urgent issue. To this end, a joint computation offloading and radio resource allocation problem is formulated to minimize the total energy consumption of WDs for an MEC-based WP-BackComNet by jointly optimizing user association, the transmit power and transmission time of WDs, the computational offloading coefficient of each task, and the reflection coefficients of BDs, where the circuit power consumption of BDs, the computational capabilities of WDs, and the task execution delay budgets are considered. To handle this non-convex problem, we propose an efficient algorithm to obtain a suboptimal solution. Simulation results demonstrate that the proposed scheme can effectively decrease the energy consumption compared with the benchmarks.
This article proposes a new cooperative offloading protocol termed "user reciprocity" for backscatter communication (BackCom)-aided mobile edge computing (MEC) systems, whose quintessence is that ...each user can switch alternately between the active or the BackCom mode in different slots, and one user works in the active mode and the other user works in the BackCom mode in each time slot. Specifically, the user in the BackCom mode can always use the signal transmitted by the user in the active mode for more data transmission in a spectrum-sharing manner. To evaluate the proposed protocol, a computation efficiency (CE) maximization-based optimization problem is formulated by jointly power control, time scheduling, power reflection coefficient (RC) adjustment, and computing frequency allocation, while adhering to constraints on energy budgets, computing frequency thresholds, minimum computed bits, and harvested energy. To solve this non-convex problem, Dinkelbach's method and quadratic transform are first employed to transform the complex fractional forms into linear ones. Then, an iterative algorithm is designed by decomposing the resulting problem to obtain the suboptimal solution. For analytical insights, we not only provide some closed-form solutions, but also derive the analytical performance gain with reciprocal mode. Simulation results demonstrate that the proposed scheme outperforms benchmark schemes regarding the CE.
Secure and fair transmission is essential for wireless-powered backscatter communication networks (WPBCNs), which has not been taken seriously enough by the existing works. In this letter, we ...consider a multi-tag WPBCN in the presence of an eavesdropper, where a power station (PS) serves multiple tags which backscatter wireless information to a reader. Furthermore, the artificial noise (AN) is transmitted from the PS to impair the eavesdropping capability for security deliberately. An energy efficiency (EE)-based resource allocation problem with max-min fairness is formulated by jointly optimizing the backscattering time, the power ratio of the AN, and the reflection coefficient of each tag, where imperfect channel state information and non-linear energy harvesting models are considered. To tackle its non-convexity, we propose a Dinkelbach-based iterative algorithm to obtain the sub-optimal solution. Simulation results verify the effectiveness of the proposed algorithm.
•The development of Zn2+ fluorescence probe in recent years (2015–2020) is systematically reviewed in this paper.•Zn2+ fluorescence probes are classified based on different fluorophore groups.•The ...characteristics of fluorescence probes are introduced by the emission wavelength of different groups.
Zinc is of great importance to humans and other biological systems. For example, too high or low levels of zinc(II) ions (Zn2+) can result in various human disease states, and poor handling of Zn2+ leads to environmental pollution. Owing to the important roles played by this metal, many convenient, low-cost and easy-to-operate fluorescence probes for Zn2+ detection have been developed. Chief among these are fluorescence-based sensors that have high sensitivities and selectivities. In this review, we describe the fluorescence probes for Zn2+ which have been prepared and assessed in the time period since 2015. The presentation is organized according to different fluorophore families, including quinoline, naphthalene, coumarin, rhodamine, naphthalimide, pyrene, fluorescein, phenol derivatives, as well as several other fluorophores. The discussion of each probe concentrates mainly on its mode of action.
In this paper, we investigate and analyze full-duplex-based backscatter communications (BackComs) with multiple backscatter devices (BDs). Different from previous works where only the energy from the ...energy source is harvested, BDs are also allowed to harvest energy from previous BDs by recycling the backscattering energy. Our objective is to maximize the total energy efficiency (EE) of the system via joint time scheduling, beamforming design, and reflection coefficient (RC) adjustment while satisfying the constraints on the transmission time, the transmit power, the circuit energy consumption (EC) and the achievable throughput for each BD by taking the causality and the non-linearity of energy harvesting (EH) into account. To deal with this intractable non-convex problem, we reformulate the problem by utilizing the Dinkelbach's method. Subsequently, an alternative iterative algorithm is designed to solve it. Simulation results show that the proposed algorithm achieves a much better EE than the benchmark algorithms.
The current work investigated the potential of submerged macrophyte pondweed (Potamogeton malaianus) in treating cadmium, nickel, and lead‐contaminated water through phytoremediation. The adsorption ...for the three metal ions occurred rapidly within 2 h and attained dynamic equilibrium in no more than 72 h. The removal efficiencies of Cd (II), Ni (II), and Pb (II) were high, passing 94% in both single‐ and multi‐metal systems. The kinetic adsorption curves of Cd (II), Ni (II), and Pb (II) were fitted well by both pseudo‐first‐order and pseudo‐second‐order kinetics (R2 = 0.9875–0.9995). The equilibrium adsorption capacities of Cd (II), Ni (II), and Pb (II) for P. malaianus were 6.29–6.97 mg kg−1. In plants, the higher concentration of each metal ions was accumulated in leaves (12.44–38.15 mg kg−1) than in roots (10.32–26.10 mg kg−1). The chlorophyll contents increased from 0.69 to 0.89–1.00 mg g−1 under the treatment of Cd (II), Ni (II), and Pb (II), whereas the chlorophyll a/b ratio was kept constant. There was no significant difference between single‐ and multi‐metal systems. FT‐IR spectra showed that COH and C═N might be involved in the adsorption of Cd (II), Ni (II), and Pb (II). This study demonstrated that P. malaianus could be a suitable submerged macrophyte for the simultaneous removal of Cd (II), Ni (II), and Pb (II) from water.
P. malaianus can simultaneously remove Cd (II), Ni (II), and Pb (II) from water. The removal efficiency of metal ions was about 99%. The bioaccumulation of metal ions in leaves was higher than that in roots. P. malaianus has good tolerance to Cd (II), Ni (II), and Pb (II).
This letter investigates and analyzes covert communications to safeguard backscatter systems, i.e., how to hide backscattering transmission for privacy and security, which is rarely a concern yet. ...Specifically, we consider a backscatter communication system in the presence of an eavesdropper (Eve), in which a passive tag supported by the power source (Alice) attempts to communicate reliably while simultaneously guaranteeing covertness to Eve. Furthermore, artificial noise (AN) is also injected from Alice to mislead Eve for security purposely. Our objective is to maximize the transmission rate via joint power control, AN injection, and reflection coefficient (RC) adjustment while satisfying the physical constraints on the total power budget, the circuit energy consumption, the minimum transmission rate, and the covertness requirement. To deal with this intractable non-convex problem, we reformulate the problem by utilizing the Kullback-Leibler divergence (KLD) and Lambert <inline-formula> <tex-math notation="LaTeX">{W} </tex-math></inline-formula> function. Subsequently, an alternative iterative algorithm is designed to solve it. Besides, we derive the closed forms for RC and transmit power for more insight. Simulation results show that the proposed algorithm outperforms the benchmark algorithms in terms of both security and effectiveness.
For the first time, successful fabrication of the cotton aerogels and cotton-cellulose aerogels is achieved using recycled fibers from environmental waste for oil absorption. The pure cotton and ...cotton-cellulose aerogels are obtained using a cost-effective mixing-blending method with polyamide-epichlorohydrin as strengthening additives. The obtained aerogels are silanized using methyltrimethoxysilane via a facile chemical vapor deposition to endow aerogels with hydrophobic surface. Effects of fiber concentrations and cotton-to-cellulose mass ratio on oil absorption performance in various solvents are also investigated. The cotton aerogel with an initial concentration of 0.25wt% presents the highest oil absorption capacity over 100gg−1. Besides, the cotton/cellulose aerogels also demonstrate good absorption capacity in different pollutant organics. The absorption kinetics of the aerogels with different cotton concentrations are also investigated using pseudo first-order model. Both equilibrium absorption and absorption kinetics demonstrate cotton/cellulose aerogels as promising materials for oil absorption and environmental pollution treatment.
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•Cotton aerogel with absorption capacity over 100gg−1 was prepared through simple solution routine.•Silanized cotton and cotton/cellulose aerogel present good hydrophobicity.•Cotton/cellulose aerogels show remarkable absorption capacity in various contaminates.•Cellulose helps to improve absorption reversibility for the composite aerogels.
The goal of the present paper is to investigate the aeration process and the enhanced effect of baffles in a full-scale commercial membrane bioreactor (MBR) system configured with a flat sheet (FS) ...membrane module. Through a computational fluid dynamics (CFD) simulation, two aerated FS membrane modules for full-scale applications with 26 membrane sheets were simulated. The numerical results indicate that the presence of baffles and the distances between the baffle and the outmost membrane sheet have a minor influence on the area-weighted shear stress for full-scale MBRs. In addition, bubble size and the bottom distance between the aerator and membrane bottom do not affect the average shear stress of full-scale FS membrane modules much. However, an increase in air flow rate has a significant effect on the area-weighted shear stress. A large FS membrane module is recommended, as it could achieve the same cleaning effect as the small one with a lower specific aeration demand for membranes.
Abstract
The change of operation mode of the HVDC project causes a change of the external characteristic of the converter valve, and the reverse recovery characteristic of the thyristor inside the ...converter valve changes accordingly. Therefore, the reverse recovery charge measurement values of thyristors under different operating conditions cannot be directly compared, and the status of thyristors cannot be evaluated online. Therefore, a method of measuring the reverse recovery charge of series thyristor based on the turn-off voltage is presented in this paper. Then, the operation mode of the HVDC project is analyzed, and the external characteristics of the converter valve under different operation modes are discussed. Secondly, the external factors that affect the reverse recovery characteristics of thyristors are analyzed essentially, and the relationship between the external factors and the external characteristics of the converter valve is discussed. A reverse recovery charge model considering external factors is established, and a method to reduce the error is proposed. Finally, the reverse-recovery charge normalization coefficient is derived for the normalization of the measured values in different working conditions. Based on the charge online measurement method and the normalization method proposed in this paper, the reverse-recovery characteristic evaluation method of the thyristor is proposed, and the evaluation steps are introduced.
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