Research Of Modulation Technologies In Visible Light Communication System DCO-OFDM

With rich spectrum resources,high security as well as the properties of energy conservation and environmental protection,visible light communication(VLC)is becoming one of the research hotspots in the field of wireless communication in recent years.And OFDM is the basic modulation method for high-speed transmission of VLC.This article launched research on DCO-OFDM modulation technique of VLC,including reducing the peak to average ratio(PAPR)of the DCO-OFDM modulated signal,adaptive modulation order,the improved schemes of the DCO-OFDM system transceiver and the hardware implementation of the DCO-OFDM system.The specific work of this article is as follows:Firstly,improved schemes are proposed,which can reduce the PAPR of the modulated signal in DCO-OFDM VLC system.Based on the analysis of the PAPR characteristics of DCO-OFDM VLC system signals,for partial transmission sequence method and DFT spread spectrum method to reduce OFDM PAPR in traditional wireless communication,the improved methods in the DCO-OFDM VLC system are respectively proposed.These methods can effectively reduce the PAPR of the system and obtain better BER performance under the condition of high SNR.Secondly,in DCO-OFDM visible light communication system which has multi-light sources and single receiver,the amplitude frequency response of the channel is derived and the adaptive modulation order scheme is proposed.Based on the analysis of the channel frequency selective fading due to the multiple transmitters in the DCO-OFDM system,the adaptive modulation order algorithm is then proposed.And the results of the adaptive modulation order are analyzed under different signal power conditions and the position of receiver.Simulation results show that the modulation order of different subcarriers is set to be different according to the channel condition,and the higher spectrum efficiency is effectively obtained.Thirdly,based on the analysis of the BER performance of DCO-OFDM system in different optical power level,an improved transmission scheme is proposed.This scheme changes the signal transformation form function of conventional DCO-OFDM transmission scheme.By solving the optimization problem,the optimal values of the slope and the parameters of the DC point in the signal transformation form function are given.The DFT spread spectrum module is also added to reduce PAPR.In addition,a fast algorithm for parameter selection is also given.Simulation results show that the BER performance of EDCO-OFDM system transmission scheme is improved obviously.At last,the design and implementation of DCO-OFDM-based VLC digital baseband system is introduced,which is completed by group members.The author is responsible for the hardware implementation of the transmitter.And the content is given in detail,including the overall system design,the physical frame structure(including short sequence,long sequence and physical header,etc),the transmitter detailed scheme,floating-point simulation,fixed-point simulation and FPGA implementation.