Research Of Modulation And De-Modulation In OFDM Visible Light Communication System

Visible light communication(VLC)has recently become one hot topic in the area of mobile communications owing to its appealing advantages of rich spectrum resources,high security as well as the properties of energy conservation and environmental protection,etc.And orthogonal frequency division multiplexing(OFDM)is the basic modulation method for high-speed transmission of VLC.This article launched research on direct-current-biased opticalOFDM(DCO-OFDM)technique of VLC,including reducing the peak to average ratio(PAPR)of the DCO-OFDMmodulated signal and the comparison analysis of complementary cumulative distribution function(CCDF)?the improved schemes of the DCO-OFDM system transmitter and receiver?adaptive modulation schemes for multiple-input and multiple-output DCO-OFDM(MIMO-DCO-OFDM)?DCO-OFDM with index modulation(DCO-OFDM-IM)scheme and performance analysis.The main contributions of this paper are summarized as follows:Firstly,DCO-OFDM with phase modulation(DCO-OFDM-PM)schemes are proposed,which can reduce the PAPR of the modulated signal in DCO-OFDM VLC system.The cumulative distribution functions(CDFs)of DCO-OFDM-PM and conventional DCO-OFDM modulated signal are derived.Numerical and simulation results show that DCO-OFDM-PM reduces the PAPR of the system effectively and obtain better bit error rate(BER)performance under the condition of high Signal Noise Ratio(SNR).Secondly,improved schemes are proposed,which uses both lower and upper clipping bounds in DCO-OFDM.Propose an optimization mathematical problem of dc bias under given lower and upper clipping bounds and conclude the numerical method to solve the optimization problem.Simulation results show that the BER performance can be improved when employing the optimum dc bias which obtained by the numerical method under different clipping bounds or different QAM order.Thirdly,propose an optimization mathematical problem of adaptive modulation order scheme of MIMO-DCO-OFDM,and conclude the method to solve the optimization problem.Simulation results show that the QAM modulation order of every subcarriers is adaptive under different channel conditions,At the same time,the spectrum efficiency is much higher than fix QAM modulation order,and sufficiently close to the theoretical capacity.At last,propose the DCO-OFDM-IM scheme wich combines DCO-OFDM and OFDM-IM.Analyze the performance comparison between DCO-OFDM-IM and conventional DCO-OFDM.Simulation results verify that DCO-OFDM-IM outperforms the conventional DCO-OFDM scheme interms of BER performance under high SNR and PAPR performance when the appropriate parameters are selected.