Research On Selective Decision Feedback Equalization For Indoor Visible Light OFDM Communications

Visible light communication(VLC)is a new communication technology.At the same time,orthogonal frequency division multiplexing(OFDM)is also an important modulation technology in the field of wireless communication.The application of OFDM into VLC technology can effectively improve the communication performance indexes such as spectrum efficiency and system bandwidth.Therefore,the OFDM-VLC communication system has always been a hot topic in the field of optical communication.However,VLC technology is inevitably affected by the shelter effect in the application of OFDM modulation mode.The problems arising from the shelter effect mainly include severe attenuation of received signals,the inter-symbol interference(ISI),and nonlinear distortion of light emitting diodes(LED).These problems will lead to a serious decline in BER performance of OFDM-VLC systems.According to the literature,the decision feedback equalization(DFE)technique in wireless communication is a general solution to the above problems.In this thesis,a selective Decision-feedback-equalization(sDFE)technique for visible light OFDM communication is proposed to relieve the ISI and LED nonlinear distortion in the OFDM-VLC systems.The specific work is as follows:(1)The model of OFDM-VLC communication system is built.First,the OFDM-VLC line-of-sight and non line-of-sight channel link model is established.Then,the error performances of the indoor OFDM-VLC channel link model are simulated and compared.At the same time,the indoor diffuse channel link model is established by ray tracing.The indoor diffuse reflection and optical power distribution characteristics are discussed,and then the correlation between the indoor delay distribution and ISI is analyzed.(2)The basic principle of decision feedback equalizer is analyzed.In view of the problem of ISI in the transmission characteristics of the indoor OFDM-VLC systems,the linear equilibrium(LE)and the decision feedback equalization scheme are exhibited and compared.The basic principles of two different time domain equalizers are expounded.Then,the two equilibrium models are tested on the hardware experiment platform,and the corresponding equilibrium is carried out.In comparison of time domain waveforms,the experiment results show that the decision feedback equalization scheme has better effect.(3)A selective decision feedback equalization scheme is proposed.Aiming at nonlinearity distortion caused by LED in OFDM-VLC communication systems,an improved equalization scheme named selective Decision-feedback-equalization is designed to solve the problem of nonlinear distortion.In this scheme,the nonlinear distortion of the OFDM-VLC system is solved by a selective feedback equalization mode that uses a linear signal to compensate for the nonlinear signal.The linear compensation signal can be selected as the selective decision feedback based on the limit range boundary range,and the adaptive equalization compensation for the optical signal is performed.The simulation results show that the method can effectively improve the bit error rate(BER)performance and restore spectral magnitude under different light power conditions.