Research On Modulation And Coding Technology Of Visible Light Communication System For Dimming Control

With the rapid development of the social economy and scientific technology,the demand for the speed of mobile communication in human production and life becomes higher,and traditional wireless communication is difficult to meet the demand.As a kind of wireless communication with great development prospect,visible light communication(VLC)has the advantages of energy conservation,information security,and rich spectrum resources compared with traditional wireless communication.VLC systems utilize widely installed light-emitting diodes(LEDs)as the transmitters and usually utilize photodiodes(PDs)as the receivers.While providing high-speed and reliable communication,VLC also has an illumination function.Therefore,in order to improve the commercial value and practical significance of VLC systems,it is necessary to take the illumination function into consideration while optimizing the communication performance.Dimming control is an essential part of the illumination function for the VLC systems.This function requires meeting the brightness requirements of users and improving the system's communication performance as much as possible.Therefore,many research teams have studied the dimming control function of VLC systems.Based on the background,this paper studies the modulation and coding technology of VLC for dimming in the following three scenarios: the transmitter is a multi-core LED and the receiver is a single PD,the transmitter is a multi-core LED and the receivers are multiple PDs,and the transmitters are multiple LEDs in different positions.We propose four dimming control schemes for the three scenarios.The first part: dimming control scheme for VLC systems in the scenario where the transmitter is a multi-core LED and the receiver is a single PD.1.Dimming control scheme for VLC system based on multilevel parity check codes(MLPCC).In order to make full use of the amplitude resources of the multi-wick LED transmitter and improve the spectral efficiency of the dimmable VLC system,we extend the code words of the weight threshold check codes to multilevel and utilize parity check codes.The code weight of the code words is even.Thus the minimum Euclidian distance of the code words set is increased.In this way,the system error performance is improved while dimming control is realized.By changing the value of the check bit,we can obtain more dimming levels without complement bits.That can improve the spectral efficiency of the system.Simultaneously,a detection algorithm with low complexity is proposed,which can improve the practicability of the system.Simulation results show that the proposed scheme can provide the same spectral efficiency as other multilevel dimming control schemes and achieve better error performance.2.Dimming control scheme for VLC system based on multi-LED phase-shifted space-time codes(MP-STC).For multi-core LED transmitters,the spectral efficiency can be improved by utilizing the amplitude and space resources simultaneously while realizing the dimming control function.Multiple LED cores are utilized as transmitters to carry out superposition and parallel transmission.While obtaining higher spectral efficiency,the number of LED cores utilized and the number of time slots and can be adjusted to meet the dimming requirements of users.Also,by dividing the received signals into several groups according to the code weight,the search range of the maximum likelihood detection can be reduced.The proposed detection scheme can reduce the implementation complexity and provide the performance close to the maximum likelihood detection.Simulation results show that the spectral efficiency of the proposed scheme is better than that of other multi-core LED dimming control schemes.The second part: dimming control scheme for VLC systems in the scenario where the transmitter is multi-core LED and the receiver are multiple PDs.3.Dimming control scheme based on Constant Weight space-time Codes(CWSTC)for MIMO-VLC systems.The multi-core LED at the transmitter and the multiple PDs at the receiver form a MIMO-VLC system.The binary data are mapped to the space-time code matrix,and the MIMO-VLC system can realize dimming control by utilizing the unique codeword structure of constant weight space-time codes.Compared with the existing MIMO-VLC dimming control technology based on space-time codes,the proposed scheme improves spectral efficiency and error performance.Besides,we propose a fast maximum likelihood detection algorithm for binary constant weight codes according to the character of the constant weight space-time codes.The proposed detection algorithm greatly reduces the detection complexity of constant weight spacetime code.The simulation results show that the proposed scheme can ensure efficient and reliable data transmission while realizing the dimming target.The third part: dimming control scheme for VLC systems in the scenario where the transmitters are multiple LEDs in different positions.4.Joint multi-LED dimming control scheme based on additively uniquely decomposable constellation(AUDC).This scenario is mainly a one-dimensional constellation design for a single PD or multiple PDs located in the same position.Based on the current channel state information and the data rates of each LED,we take the dimming control constraint into consideration and obtain the optimal additively uniquely decomposable constellation of the transmitted signal by maximizing the minimum Euclidean distance of the received signal constellation.Thus,the dimming control scheme can adapt to the optical channel state and improve the error performance of the VLC system to realize the joint communication and dimming control of multiple LEDs in different positions.Simulation results show that the proposed scheme has better error performance than the scheme utilizing multiple LEDs separately for data transmission and dimming control.