Dimming Scheme Research Articles

Modulating LiFi for dual operation in the visible and infrared spectra

Light-Fidelity (LiFi) has emerged in the last few years as a promising technology for alleviating the stringent demand for wireless data services. Prior works have considered LiFi operating either in the visible light or infrared spectrum. Each spectrum band has its own advantages: visible light allows leveraging existing infrastructure for communication, while infrared is not affected by light dimming. In this work, we propose a modulation scheme that retains the benefits of both bands, introducing a simple, low-cost, yet efficient dimming solution for LiFi networks. We compare the performance of the proposed dimming scheme with both the digital and analog dimming techniques traditionally used in LiFi systems. Simulation results show that our dimming solution offers better communication and illumination performance than previous proposals, providing larger signal-to-noise ratio, spectral efficiency, and a full and fine-grained dimming range. Finally, we prototype our solution by designing an extended version of the OpenVLC 1.3 platform, and we experimentally show its robust communication performance under different dimming conditions. We make the implemented system publicly available to the research community.

THO-OFDM scheme for visible light communication with noise suppression and dimming control.

In recent years, visible light communication (VLC) has emerged as a dual-use technique for illumination as well as communication. In VLC system, dimming control is deemed to be an essential functionality for luminous intensity adjusting and energy saving. In this Letter, a noise-suppressed triple-layer hybrid optical orthogonal frequency division multiplexing (NSTHO-OFDM) is proposed and further combined with pulse width modulation (PWM) to achieve the functionality of dimming control. The proposed NSTHO-OFDM can attain a 4.28-dB maximum power gain compared with THO-OFDM with an overall bit error rate (BER) of 1 × 10-4. Moreover, the proposed dimming scheme can achieve a wide dimming range of 89.0% while maintaining a favorable BER below 3.8 × 10-3.

Dimming Control Scheme of Visible Light Communication Based on Joint Multilevel Time-Shifted Coding

Dimming control is an essential objection in the signal designing of visible light communication (VLC), which requires improving the communication performance of the system as much as possible while considering the illumination quality. Here, we studied the problem of high-efficiency transmission in an indoor VLC multi-core light-emitting diode (LED) communication model while considering dimming constraints, and propose a dimming method based on joint multilevel multi-LED time-shifted coding (ML-MTSC). The scheme utilizes the code structure of time-shifted space–time codes to encode and uses pulse amplitude modulation (PAM) to expand it to achieve the dimming control function in the proposed scenario. Simulation results show that the ML-MTSC dimming control scheme proposed in this paper has improved spectral efficiency and error performance compared with the traditional scheme.

Joint LED Selection and Precoding Optimization for Multiple-User Multiple-Cell VLC Systems

This paper proposes a hybrid dimming scheme based on joint LED selection and precoding design (TASP-HD) for multiple-user (MU) multiple-cell (MC) visible light communications (VLC) systems. In TASP-HD, both the LED selection and the precoding of each cell can be dynamically adjusted to reduce the intra- and inter-cell interferences while satisfying illumination constraints. First, a MU-MC-VLC system model is established, and then a sum-rate maximization problem under dimming level and illumination uniformity constraints is formulated. In this studied problem, the indices of activated LEDs and precoding matrices are optimized, which result in a complex non-convex mixed integer problem. To solve this problem, the original problem is separated into two subproblems. The first subproblem, which maximizes the sum-rate of users via optimizing the LED selection with a given precoding matrix, is a mixed integer problem solved by the penalty method. With the optimized LED selection matrix, the second subproblem which focuses on the maximization of the sum-rate via optimizing the precoding matrix is solved by the Lagrangian dual method. Finally, these two subproblems are iteratively solved to obtain a convergent solution. Simulation results verify that in a typical indoor scenario under a dimming level of 70%, the mean bandwidth efficiency of TASP-HD is 4.8 bit/s/Hz and 7.13 bit/s/Hz greater than AD and DD, respectively.

Independently Controllable Dual-Output Half-Bridge Series Resonant Converter for LED Driver Application

This article presents a dual-output series resonant converter (SRC)-based light-emitting diode (LED) driver, which utilizes three switches to form two half-bridge networks. The proposed three-switch two-half-bridge structure provides reduced switch count and independent control. It uses asymmetric duty cycle control to regulate the LED operating currents and utilizes a low-frequency pulsewidth modulation (PWM) dimming scheme for each half-bridge to control the illumination of each LED load independently. All the switches in the proposed converter operate with zero-voltage turn-on to reduce the switching losses. The operating principle and detailed analysis of the proposed converter is presented and verified with experimental studies for a total load of 50 W (i.e., LED load-1 26 W and LED load-2 24 W). Also, a comprehensive comparative study of the proposed LED driver with recent counterparts is presented.

Dimming controlled multi header pulse position modulation (MH-PPM) for visible light communication system

Abstract Visible light communication (VLC) has recently drawn more attention due to the existing light sources having a bandwidth that can deliver data transmission at higher bit rates. To maintain desired brightness levels along with reliable data transmission, VLC requires to control dimming levels. In this paper, a dimming control scheme based on pulse position modulation (PPM) is proposed, which constructs a dimming encoded frame per symbol of input data. The scheme utilizes multiple headers, which is selected depending on the dimming levels and the number of bits in a symbol of input data. The proposed scheme for the VLC system supports various dimming levels. The multi header PPM frame generation for input data K = 4 $K=4$ and dimming values α = 0.25, 0.375, and 0.5 is simulated using NI Labview. The proposed dimming scheme has an excellent short-run length property which is well under the maximum flickering time period and is thus capable of flicker mitigation. The slot error performance of the proposed dimming control scheme in a VLC system for varying dimming levels is simulated in Matlab. Simulation results show that the proposed scheme has excellent error performance for a low dimming value of α = 0.25. The experimental results are also carried out in the optisystem considering the practical measured parameters of the indoor environment for free space channel and simulating the VLC system for the proposed scheme. The simulation is carried for different data rates over varying link ranges and the experimental results show that the proposed scheme performs best when dimming value α = 0.25 and data rate 2 Gbps.

Dimming Techniques Focusing on the Improvement in Luminous Efficiency for High-Brightness LEDs

The pulse width modulation (PWM) dimming mode features good dimming linearity and has been widely used for driving high-brightness light-emitting diodes (HBLEDs), in which the brightness change is reached by modulating the duty cycle of the dimming signal to regulate the average current flowing through LEDs. However, the current-illuminance characteristic curve of most LEDs is nonlinear in nature. Namely, under the same lighting power fed, the conventional PWM dimming cannot make the LED exert its best luminous efficiency (LE) specified in datasheets. This paper focuses on the study of further improving LED luminous efficacy via dimming manipulation. Thereby, two multilevel current dimming techniques with varied dimming signal voltage and varied current sensing resistance are presented. With limited dimming capability, the proposed dimming strategies can efficiently raise the luminous flux ratio without increasing the power consumption. A prototype constructed for a 115 W HBLED driver is developed and the devised dimming schemes are realized by a digital signal controller (DSC). Experimental results exhibited with illuminance-power curves and CIE1931 and CIE1976 chromaticity diagrams are given to validate the theoretical derivation and effectiveness. Compared with conventional PWM dimming, under the same illuminance, the driver average output power is respectively reduced by 17.08% and 13.17%; the improvement in average illuminance under the same output power is 13.66% and 11.17%, respectively. In addition, the entire average LE boost has respectively increased by 21.36% and 16.37%.

Dimming control scheme based on hybrid LACO-SCFDM for visible light communications

This paper proposes a hybrid layered asymmetrically clipped optical (HLACO) single-carrier frequency-division multiplexing (SCFDM) scheme for dimmable visible light communication. It designs a signal structure that combines layered asymmetrically clipped optical (LACO)-SCFDM and negative LACO-SCFDM in proportion for improving the inherent weaknesses of orthogonal frequency-division multiplexing (OFDM)-based dimmable schemes and further enhancing the system performance. Compared to the HLACO-OFDM-based dimming scheme, it obtains a lower bit error ratio and enables efficient communication over broader dimming range. Its spectral efficiency realizes 2.875 bit·s−1·Hz−1 within the dimming range of 30%–70%, and the attainable average spectral efficiency gains exceed at least 19.21% compared to other traditional dimmable schemes.

Novel distortionless dimming in high power LED lighting using isolated SEPIC converter

A new low-cost energy-saving concept in the modern light-emitting diode (LED) illumination system is developed and implemented using an isolated single-ended primary inductance converter (SEPIC). The dimming scheme presented in this study is a single-step with negligible distortion in the AC mains current as compared to the traditional low-cost triode for alternating current (TRIAC) dimming system, which distorts the AC mains current wave-shape beyond sinusoidal nature due to the phase-cut techniques. This dimming concept is completely retrofitted, no additional arrangement is required to initiate the dimming as compared to a TRIAC dimmer or other controllers. The isolated SEPIC converter is modelled and analysed in detail using the state-space average technique. The stability of the designed converter and overall closed-loop control are verified using Bode-plot analysis. The full load converter efficiency is found around 93%, which is much higher than the conventional flyback topology-based LED driver. The concept presented in this study is tested with the help of a laboratory prototype with an LED load of 60 W.

Compact illumination system with variable beam direction and beam divergence

Countless approaches to optimise lighting conditions for indoor and outdoor applications are described in the literature by adaptive dimming schemes or control algorithms. Those methods rely on adapting the emitted flux of individual luminaires in order to adjust lighting conditions to varying circumstances. Complex illumination requirements are tackled by controlling the output of every independent luminaire. In this case, the innovation is rather in software than in hardware, and such approaches are limited by how the LED fixtures can be manipulated. Lighting fixtures with a tuneable radiation pattern are rare, or these fixtures are rather large. This paper introduces a compact lighting system with a collimated light beam of which the beam direction and beam divergence can be adjusted electronically. To realise this optical functionality, a focus tuneable lens and rotatable mirror are combined with two custom-made aplanatic lenses in front of a compact, high-brightness LED. This paper elaborates the optical design of this novel illumination system and discusses the experimental performance of the realised demonstrator. The system functionality is validated by comparing the experimental lighting performance to optical simulations conducted with ray-tracing software. With the proposed system, a fast and flexible adaptation of the generated illumination distribution is possible.

Dimming Correction Scheme considering Luminous Characteristics of R, G, B LEDs in Visible Light Communication

In visible light communication, it is difficult to control dimming of LED lamp or flicker occurs due to changes in luminous flux according to the data. In this paper, to solve this problem, we proposed a new correction scheme for dimming level in which the dimming level of an LED lamp is corrected in real time by considering the data transmission status. A non-linear correction scheme was applied for red and green LEDs because of the non-linear characteristics of the luminous flux according to the dimming level. Since the blue LED’s luminous flux can be assumed to be linear, a linear correction scheme using two-stage current was applied for blue LED in order to lower the minimum possible dimming level. A downscaled model of the visible light communication system was created to verify the validity of the proposed correction scheme for the dimming level. The luminous flux of red, green, and blue LEDs was also measured according to the dimming level. As a result, the dimming range of 9–100% was obtained regardless of the transmitted data condition. Also, transmitting the data of three sensors through red, green, and blue LEDs and constantly maintaining the entire lighting of the LED lamp are possible by applying the proposed dimming scheme.

Multilevel transmission scheme based on parity check codes for VLC with dimming control

In the past few years, visible light communication (VLC), which uses solid-state lightings to transmit information, has been regarded as an emerging way of wireless communication technology. For the combination of illumination and communication, dimming control is a necessary function for VLC systems. Traditional dimming control binary-based dimming schemes are spectral inefficient, while most of the multilevel transmission schemes worsen the error performance of the system. In this paper, motivated by the better characteristics of parity check codes, we propose a multilevel transmission scheme which provides better error performance maintaining spectral efficiency compared with other multilevel dimming schemes.

Research and Design of LED Light Suppression Flicker Technology in Visible Light Communication Technology

“Lighting communication integration” is a key technology research for the future application of visible light communication(VLC). However, the bandwidth characteristics, dynamic modulation range and linear modulation performance of existing white LED still do not meet the requirements of high-speed communication. After adding the dimming function, the naked eye It is noticeable that flicker is felt. In order to solve the problem of LED flicker caused by high-speed communication, this paper proposes a “lighting and communication integration” technology scheme based on visible light communication(VLC), which studies from the aspects of suppressing flickering VLC scheme, LED communication dimming, and high-efficiency modulation driving circuit design. A PWM dimming control technology based on optical OFDM system is used for VLC and LED dimming scheme. Finally, the technology is applied to the typical application scenario of the power grid—cable tunnel, which realizes the two-way real-time voice communication of cable tunnel inspection personnel. And accurate positioning to ensure the personal safety of the inspectors and the safety of the grid production.

디지털 조광 기법을 사용한 LED 가시광통신 시스템

디지털 조광 기법을 사용한 LED 가시광통신 시스템

Spectral-Efficient Hybrid Dimming Scheme for Indoor Visible Light Communication: A Subcarrier Index Modulation Based Approach

Communication and illumination are two basic functions of the indoor visible light communication (VLC). In this paper, a novel spectral-efficient hybrid dimming scheme is proposed for the indoor VLC system, where the frequency-domain subcarrier selection is utilized to maximize the channel capacity and the intensity-domain dimming strategy is used to further adjust the illumination level. Simulations are carried out in both the indoor line-of-sight channel and the indoor multipath channel. Simulation results substantiate the superior performance of the proposed hybrid dimming scheme over the state-of-the-art schemes.

Analysis, design, and experimentation of the active hybrid‐series‐parallel PWM dimming scheme for high‐efficient off‐line LED drivers

This study presents a high power factor (PF) light-emitting diode (LED) driver featuring a pulse width modulation (PWM) dimming technique. The integrated buck-flyback-buck converter (IBFBC) is built by connecting a buck converter to the integrated buck-flyback converter's output. The IBFBC ensures a constant output current regulation as well as high PF at all dimming ratios. The purpose of the output buck converter is to clear up the discontinuous power operation created by the PWM dimming. This occurs when the buck converter absorbs the output power, at LED switched-off, and sends it back to the input. Thus, the IBFBC maintains the operational enhancement shown by the hybrid-series-parallel (HSP)-PWM technique, ensuring greater efficiency. This study explains the operating principles of the IBFBC using circuit diagrams as well as an average model for the entire converter. Moreover, the study presents a mathematical analysis of the IBFBC power flow. It provides guidance on how to select the optimum value for the output buck inductance. A 230V, 50Hz input, 160V output, 100W AC-DC converter operating at 100 kHz has been implemented. The prototype is fully tested to demonstrate the active HSP-PWM technique. An efficiency ranging 50-86% corresponding to the dimming ratio of 5-100% is reached, where 5% dimming refers to an output power of 4W.

A Novel Coding Based Dimming Scheme with Constant Transmission Efficiency in VLC Systems

Visible light communication (VLC) has attracted tremendous attention due to two functions: communication and illumination. Both reliable data transmission and lighting quality need to be considered when the transmitted signal is designed. To achieve the desired levels of illumination, dimming control is an essential technology applied in VLC systems. In this paper, we propose a block coding-based dimming scheme to construct the codeword set, where dimming control can be achieved by changing the ratio of two levels (ON and OFF) based on on-off keying (OOK) modulation. Simulation results show that the proposed scheme can maintain good error performance with constant transmission efficiency under various dimming levels.

Spectral-Efficient L/E-ACO-SCFDM-Based Dimmable Visible Light Communication System

In this paper, we propose a dimmable visible light communication (VLC) system based on spectral-efficient layered/enhanced asymmetrically clipped optical single-carrier frequency-division multiplexing (L/E-ACO-SCFDM). L/E-ACO-SCFDM superimposes multi-layer ACO-SCFDMs occupying different subcarriers for enhancing the spectral efficiency of the system. Both the adjustable layer polarities and the low peak-to-average-power ratio property of ACO-SCFDM make it better suited to the linearity-limited VLC system. Meanwhile, L/E-ACO-SCFDM has the signal-to-noise ratio averaging advantage, which can effectively resist the high-frequency distortion caused by the bandwidth limitation of the VLC system. Based on the maximization of spectral efficiency of the system, the layer amplitudes, layer spectral efficiencies, and bias voltage are jointly optimized for providing the efficient communication under different illumination requirements. Benefiting from the numerous strengths, the proposed L/E-ACO-SCFDM-based dimming scheme can achieve at least 22.0% average spectral efficiency gain compared with several state-of-the-art OFDM-based dimming schemes within the same dimming range, which shows the great superiority for future practical applications of VLC.

An In-depth Survey of Viisiiblle Liight Communiicatiion Modullatiing and Codiing schemes for Diimmiing/Flliicker Avoiidance Technollogiies

Visible light communication (VLC) is considered as a promisingtechnique for the wireless technology of the 21st century. UtilizingLight Emitting Diode (LED) technology adds a wide area ofapplications for VLC. Most important one is that VLC can provideillumination/communication jointly. To achieve efficientcommercial VLC communication/illumination system one needsto profit from dimming schemes/flicker. Availability of suitablesystem designs and enhancing system's power requirements arekey advantages for using dimming schemes. Effective dimmingschemes/flicker are mainly achieved through several VLC basedon modulation techniques. In last few years many kinds ofliterature discussed independently wide range of modulationschemes to address the effectively previous issue. However, fewsurveys for these works are presented in the field. This paperprovides an in-depth survey of VLC based dimming scheme.Therefore, the kinds of modulation and coding techniques arecollected and categorized of more than 60 papers extending frompioneering papers to the state of the art in the field. In addition,research and current issues are discussed especially in whichtrends in VLC based dimming/flicker.

Dimming control scheme for VLC systems based on multilevel data transmission.

As an emerging wireless communications technology, visible light communications (VLC) have attracted much attention for synergistically providing both high-speed data transmission and high-quality illumination. To provide energy savings and ecological benefits, dimming control is an essential function in VLC systems where users can obtain the required illumination levels. However, the conventional binary-based dimming schemes are spectrally inefficient. In this paper, we propose a novel multilevel data transmission scheme for VLC systems that can achieve dimming control and communication functions simultaneously. Additionally, the encoding/decoding structure is provided where the specific symbol set can maintain the specific dimming level. Finally, simulation results show that the proposed multilevel transmission scheme can achieve increased spectral efficiency and better error performance.