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Downlink resource allocation for dynamic TDMA-based VLC systems
摘要： Visible light communications (VLC) in general and resource allocation for VLC networks particularly have gained lots of attention recently. In this work, we consider the resource allocation problem of a VLC downlink transmission system employing dynamic time division multiple access where time and power variables are tuned to maximize spectral ef?ciency (SE). As for the operational conditions, we impose constraints on the average optical intensity, the energy budget and the quality-of-service. To solve the non-convex problem, we transform the objective function into a difference of concave functions by solving a second order differential inequality. Then, we propose a low-complexity algorithm to solve the resource allocation problem. Finally, we show by simulations the SE performance gains achieved by optimizing time and power allocation over the initial total power minimization solution for the considered system.
关键词： spectral ef?ciency,resource allocation,DC programming,illumination constraint,Visible light communication
更新于2025-09-09 09:28:46
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2D-Bi<sub>2</sub>MoO<sub>6</sub>/2D-g-C<sub>3</sub>N<sub>4</sub> nanosheet heterojunction composite: synthesis and enhanced visible light photocatalytic mechanism
摘要： In this study, a novel 2D Bi2MoO6 nanosheet/2D g-C3N4 nanosheet heterojunction composite was fabricated through hydrothermal strategy. Afterwards, physicochemical properties of the composite were systematically detected by series of measurements. In addition, photocatalytic activity was evaluated by degradation of Rhodamine B, methylene blue, Amido black 10B and malachite green. Results showed that Bi2MoO6/g-C3N4 played higher photocatalytic efficiency than that of pristine Bi2MoO6 and g-C3N4 sample under visible irradiation. This phenomenon could be attributed to the combination of Bi2MoO6 and g-C3N4 as well as unique 2D/2D heterostructure. Also, the as-fabricated Bi2MoO6/g-C3N4 composite possessed higher surface area, narrower band gap energy, larger electron transport capability and longer charge carrier lifetime. Furthermore, the enhanced visible light driven photocatalytic mechanism was proposed and confirmed, which was mainly ascribed to the Z-scheme structure. That was, the photogenerated electrons (e-) in the CB of Bi2MoO6 and photogenerated holes (h+) in the VB of g-C3N4 would combine with each other quickly, so that the e- in the CB of g-C3N4 and h+ in the VB of Bi2MoO6 participated in reduction and oxidation reactions, respectively. Also, this composite exhibited superior physicochemical stability. Hence, it would be applied in advanced wastewater treatment.
关键词： Nanosheet heterojunction,Bi2MoO6/g-C3N4,Visible light photocatalysis,Z-Scheme,2D/2D
更新于2025-09-09 09:28:46
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Experimental Demonstration of Special-Shaped 32-Quadrature Amplitude Modulation Constellations for Visible Light Communications
摘要： With the rapid development of light-emitting diode, visible light communication (VLC) has become a candidate technology for the next generation of high-speed indoor wireless communication. In this paper, we investigate the performance of the 32-quadrature amplitude modulation (32-QAM) constellation shaping schemes for the first time, where two special circular constellations, named Circular (4, 11, 17) and Circular (1, 5, 11, 15), and a triangular constellation are proposed based on the Shannon’s criterion. Theoretical analysis indicates that the triangular constellation scheme has the largest minimum Euclidian distance while the Circular (4, 11, 17) scheme achieves the lowest peak-to-average power ratio (PAPR). Experimental results show that the bit error rate performance is finally decided by the value of PAPR in the VLC system due to the serious nonlinearity of the LED, where the Circular (4, 11, 17) scheme always performs best under the 7% preforward error correction threshold of 3.8 × 10 with 62.5Mb/s transmission data rate and 1-meter transmission distance.
关键词： constellation shaping,BER,visible light communication,32-QAM,PAPR,circular constellation,triangular constellation
更新于2025-09-09 09:28:46
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Decoration of SrTiO3 nanofibers by BiOI for photocatalytic methyl orange degradation under visible light irradiation
摘要： BiOI decorated electrospun SrTiO3 nanofibers (BE) with a spiky club structure were fabricated using a rapid microwave-assisted solvothermal method. The exposed (102) plane of BiOI preferentially coupled with the electrospun SrTiO3 fibers aiming to achieve charge neutrality. XRD analysis demonstrated that a BE composite coupled with an optimal content of BiOI exhibited few crystal imperfections. The resulting BE samples showed a higher degradation efficiency towards methyl orange dye under visible light irradiation (λ > 400 nm) compared to the individual BiOI and SrTiO3. The BE composite possessed a high degradation rate of 0.015 (kapp, min–1) and a normalized rate constant of 2.46 × 10–3 (kapp/surface area), which was 7.7 and 1120 times higher than those of BiOI and SrTiO3 alone, and a degradation level over 94.6% within 3 h of irradiation under visible light. The high photocatalytic activity could be attributed to the heterostructure formation, p-n junction at the BE interface, high contact between water and dye molecules with the BiOI covered surface, and extended visible light absorption. This work provides an effective approach to produce BE nanofibers for dealing with methyl orange dye and proves that BE composites have the potential to serve as visible-light-driven photocatalysts.
关键词： Visible light irradiation,Photocatalyst,SrTiO3,BiOI,Nanofiber
更新于2025-09-09 09:28:46
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Optimization, kinetics and thermodynamics of photocatalytic degradation of Acid Red 1 by Sm-doped CdS under visible light
摘要： Sm-doped CdS nanoparticles were synthesized through an ultrasound-assisted co-precipitation method and their photocatalytic efficiency was investigated by applying them to photodegradation of Acid Red 1 (AR1) under visible light. The effect of the operational factors on the photocatalytic process was systematically evaluated using response surface methodology. Under the optimal conditions ([AR1]0= 15 mg L-1, [2% Sm-CdS]0= 1 g L-1, pH= 4 and t= 94 min), more than 83% of the AR1 molecules were degraded. The kinetics of the process was well described by the Langmuir-Hinshelwood’s pseudo-first-order model (kapp= 0.0163 min-1). Furthermore, thermodynamics of the process was demonstrated by the activated complex theory of Eyring, which declared that the photocatalytic process is endothermic and nonspontaneous in the temperature range of 25–45°C. In addition, the main products and intermediates of AR1 degradation were determined by the GC-MS technique.
关键词： Sm-doped CdS,Thermodynamics,Visible light photocatalysis,Kinetics,Acid Red 1,RSM
更新于2025-09-09 09:28:46
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Nanocrystalline ZnO doped lanthanide oxide: An efficient photocatalyst for the degradation of diesel pollutant in seawater under visible light irradiation
摘要： In this study, a ZnO doped Er2O3 photocatalyst is employed to degrade diesel pollutant in seawater under visible light irradiation. The photocatalyst was prepared by a precipitation method and was characterized by SEM and XRD analysis. The experimental results proved that the nanocrystalline photocatalysts were highly active in the visible region. The photocatalytic degradation efficiency of diesel was analysed by various experimental parameters namely dosage, doping ratio, initial concentration of diesel, pH value, concentration of H2O2 and illumination time. The degradation of diesel pollution in seawater was optimized by orthogonal experiment. According to the results, the removal rate of diesel is less than 30 % without any catalysts (only evaporation). The best effect exists when the dose of catalysts was 0.6 g/L, doping ratio of catalysts was 10%, initial concentration of diesel was 0.2 g/L, pH value was 8, concentration of H2O2 was 10 mg/L, illumination time was 1 h. The removal rate of diesel can reach 99.38 %. This study would make ZnO utilize sunlight more efficiently and accelerate the practical application of photocatalytic technology in organic pollutants treatment region.
关键词： visible light irradiation,photocatalyst,Er2O3,diesel pollutant,seawater,ZnO
更新于2025-09-09 09:28:46
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A Second-Generation Chameleon N-Heterocyclic Carbene–Borane Coinitiator for the Visible-Light Oxygen-Resistant Photopolymerization of Both Organic and Water-Compatible Resins
摘要： N-Heterocyclic carbene?boranes (NHC-boranes, NHCBs) were recently described as efficient coinitiators for the visible light photopolymerization of hydroxyethyl methacrylate (HEMA) in the presence of water. In this work, a new, more water-soluble, NHC-borane—2,4-dimethyl-1,2,4,5-tetrazol-3-ylidene borane—has been synthesized, and its efficiency in three-component systems (dye (Acridine Orange), disulfide, and NHC-borane coinitiators) for the polymerization of methacrylate resins under visible light under air has been studied. In fluid resins this new photoinitiating system (PIS) gives better results than the one previously studied. More importantly, this system is competitive with the widely used type II system—camphorquinone/4-(dimethylamino)benzonitrile (DMABN)—for the polymerization of methacrylate resins but better for the photopolymerization of poly(2-hydroxyethyl methacrylate) leading to hydrogels, where the former PIS fails. The excellent ability of the NHC-boranes, and especially the new one, to be used as photopolymerization coinitiators has been analyzed by laser flash photolysis (LFP). The rate constants for elementary reactions of the three boranes and their derived NHC-boryl radicals obtained by LFP correlated well with the molecular modeling data and show that the key for the observed reactivity is the ability of the tetrazolydinyl NHC-borane to repair the peroxyl radicals formed by the reactions of the macroradicals with oxygen.
关键词： laser flash photolysis,methacrylate resins,N-Heterocyclic carbene?boranes,hydrogels,visible light photopolymerization
更新于2025-09-09 09:28:46
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An Overview of OFDM-Based Visible Light Communication Systems From the Perspective of Energy Efficiency Versus Spectral Efficiency
摘要： In this paper, the energy ef?ciency and the spectral ef?ciency for orthogonal frequency division multiplexing (OFDM)-based visible light communication schemes are studied, which is crucial for practical application with limited energy resources. The conventional schemes, including asymmetrically clipped optical OFDM (ACO-OFDM), pulse-amplitude-modulated discrete multitone, and direct current biased optical OFDM, are compared in terms of energy ef?ciency and spectral ef?ciency relationship. The in?uence of power allocation for asymmetrically clipped dc biased optical OFDM and hybrid ACO-OFDM is also investigated. The energy ef?ciency and spectral ef?ciency of layered ACO-OFDM with a variable layer number are calculated and their relationship is also formulated. These conventional and hybrid modulation schemes are analyzed and compared through computer simulations, which should be considered in practice according to the requirements of illumination and transmission.
关键词： visible light communication (VLC),spectral ef?ciency,orthogonal frequency division multiplexing (OFDM),Energy ef?ciency
更新于2025-09-09 09:28:46
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Fast electron transfer and enhanced visible light photocatalytic activity by using poly-o-phenylenediamine modified AgCl/g-C3N4 nanosheets
摘要： Exfoliation of bulk graphitic carbon nitride (g-C3N4) into two-dimensional (2D) nanosheets is one of the effective strategies to improve its photocatalytic properties so that the 2D g-C3N4 nanosheets (CN) have larger specific surface areas and more reaction sites. In addition, poly-o-phenylenediamine (PoPD) can improve the electrical conductivity and photocatalytic activity of semiconductor materials. Here, the novel efficient composite PoPD/AgCl/g-C3N4 nanosheets was first synthesized by a precipitation reaction and the photoinitiated polymerization approach. The obtained photocatalysts have larger specific surface areas and could achieve better visible-light response. However, silver chloride (AgCl) is susceptible to agglomeration and photocorrosion. The PoPD/AgCl/CN composite exhibits an extremely high photocurrent density, which is three times that of CN. Obviously enhanced photocatalytic activities of PoPD/AgCl/g-C3N4 are revealed through the photodegradation of tetracycline. The stability of PoPD/AgCl/CN is demonstrated based on four cycles of experiments that reveal that the degradation rate only decreases slightly. Furthermore, ?O2? and h+ are the main active species, which are confirmed through a trapping experiment and ESR spin-trap technique. Therefore, the prepared PoPD/AgCl/CN can be considered as a stable photocatalyst, in which PoPD is added as a charge carrier and acts a photosensitive protective layer on the surface of the AgCl particles. This provides a new technology for preparing highly stable composite photocatalysts that can effectively deal with environmental issues.
关键词： AgCl,Visible light irradiation,Poly-o-phenylenediamine,g-C3N4 nanosheets,Photocatalytic
更新于2025-09-09 09:28:46
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[IEEE 2018 11th International Symposium on Communication Systems, Networks and Digital Signal Processing (CSNDSP) - Budapest (2018.7.18-2018.7.20)] 2018 11th International Symposium on Communication Systems, Networks & Digital Signal Processing (CSNDSP) - Performance Comparison of MIMO CAP Receivers in Visible Light Communication
摘要： Multiple-input, multiple-output (MIMO) techniques have recently been proposed to improve the spectral ef?ciency of the conventional carrierless amplitude and phase (CAP) implementation modulation scheme while maintaining its simplicity in visible light communication (VLC). The proposed MIMO CAP uses the optimum Maximum Likelihood (ML) detection scheme which has high computational complexity. As a result, four practical, low-complexity detection schemes are investigated for MIMO CAP in this work and their performances are compared with that of ML. It is found that the minimum mean square error detection with optimally-ordered successive interference cancellation (MMSE-OSIC) offers the best choice between complexity and bit-error rate (BER) performance. To achieve a spectral ef?ciency of 8 bits/s/Hz at BER of 10?4 in a 4 × 8 MIMO CAP system, the signal-to-noise ratio (SNR) requirement of the MMSE-OSIC is 5 dB higher than that of the optimum ML receiver. However, the ML receiver’s complexity is an order of magnitude higher than that of the MMSE-OSIC.
关键词： spatial multiplexing (SM),visible light communication (VLC),carrierless amplitude and phase modulation (CAP),optical communication,MIMO
更新于2025-09-09 09:28:46