Research Of Dimming Technology For Visible Light Communication Based On OFDM

As a new type of wireless communication technology,Visible Light Communication(VLC)based on LED has drawn more and more attention due to its low power consumption,anti-electromagnetic interference and high security.VLC uses Light Emitting Diode(LED)lighting devices to communicate,which makes it necessary to take into account the dual needs of illumination and communications.For any lighting system,it is usually necessary to have a dimming function to dynamically adjust its brightness for energy conservation or eco-efficiency.In addition,due to the narrow modulation bandwidth of LED,which is generally a few MHz,it can not meet the requirements of high-speed communications.In order to improve the communication speed of the VLC system,Radio Frequency(RF)Orthogonal Frequency Division Multiplexing(OFDM)(RF-OFDM)technology with high spectral efficiency and ISI(Inter Symbol Interference)is introduced into the VLC system.Because VLC is generally based on Intensity Modulation/Direct Detection(IM/DD),the transmitted signal must be a positive real signal,which makes the RF-OFDM modulation can not be directly applied to the VLC system.Therefore,a large number of researchers have begun to study OFDM modulation suitable for VLC system.Meanwhile,the dimming technology of VLC system based on OFDM also needs to be researched and discussed urgently.Based on the above problems,this dissertation is based on the Huawei Innovation Research Program(HIRP)(HIRP020150101)and focuses on the dimming technology of VLC system based on OFDM(called OFDM-VLC system).The main research contents are as follows:1.This paper first reviews the basic principles and research status of OFDM(referred to as Optical OFDM,O-OFDM)modulation technology in VLC,and summarizes and analysis the status quo of the dimming technology and the dimming technology of OFDM-VLC system.The deficiencies and problems of the existing OFDM-VLC system dimming technology are pointed out through analysis.2.For a single-link VLC system adopting two main O-OFDM modulation schemes:DCO-OFDM(Direct Current Biased Optical OFDM)and ACO-OFDM(Asymmetrically Clipped Optical OFDM),when considering analog dimming,the communication performance of the two schemes in different luminance ranges is complementary.A scheme,named AAD-OFDM(Asymmetrical And Direct Current Biased Optical OFDM),which combines the two modulation schemes and is applied to a VLC system,is proposed in the paper.And the system block diagram of the AAD-OFDM is given.Then the emphasis is placed on how to switch the system.By setting up the optimization problem,the optimal brightness switching threshold is determined.The scheme uses the adaptive brightness switching mode to select the best modulation scheme of the current system under different brightness and makes up for the defects of the original single modulation scheme,so that the communication performance of the system is better than that of a single modulation scheme under whole dimming range.3.In the actual scenario,the brightness of a single LED is limited and there will usually be more than one LED in a single lamp.Therefore,we naturally extend from considering single-link VLC systems to multi-link VLC systems.Compared to single-link VLC systems,multi-link VLC systems undoubtedly add space dimension.How to make full use of space dimensionto further enhance the system performance.In order to solve this problem,this paper proposes a ST-OFDM(Separately Transmitted Optical OFDM)modulation scheme.The basic principle of this scheme is partitionthe bipolar DCO-OFDM signal into two unipolar signals and then they separately drive differentLEDs.The two signals are then summed in spacethus reconstructing DCO-OFDM signals.In addition,in order to improve the dimming performance of the system,a new dimming technology using analog dimming and space dimming is proposedin this paper.It adjusts the brightness of the lamp by jointly controlling the brightness of the glared LED and the number of glared LEDs.The simulation results show that both the communication performance and the dimming performance are superior to the system using SD-OFDM(Spatial Dimming Optical OFDM)and the traditional DCO-OFDM modulation scheme.