Research On Nonlinear Distortion Compensation For LD In Visible Light Communication System

Since the development of traditional mobile communication,the available radio wave fre-quency band has become increasingly crowded.As a new search direction of next-generation mobile communication network,visible light communication technology provides a solution to the problem of the limited spectrum.Similar to the traditional radio frequency communica-tion system,the LD-based visible light communication system also has nonlinear distortion,which will cause rise of bit error rate when the signal is recovered at the receiver side.And laser diode is the main source of nonlinearity.Based on the above background,this thesis introduces the nonlinear modeling method of laser diodes and the commonly used visible light communication system model.Moreover,simulations and analyzations are done to illustrate the influence of laser diode nonlinearity on the signal.In order to realize nonlinear compensation of LD,so as to increase the accu-racy of the recovered signal at the receiver,the nonlinear compensation technology at both transmitter side and receiver side of laser diodes are studied respectively.The main innova-tions are as follows:（1）For the nonlinear compensation of the laser diode at the transmitter in the VLC sys-tem,the adaptive predistortion technique based on memoryless polynomial model is stud-ied.To improve the performance of the traditional adaptive algorithm in the estimation of the predistortion algorithm,a LMS algorithm modified by WOA-DE algorithm is proposed.The performance of the proposed LMS-WOA-DE algorithm is compared with the traditional LMS algorithm and modified LMS algorithms based on other two optimization algorithms.It can be seen from the simulation results that the bit error rate of the system has reduced significantly when the LMS-WOA-DE algorithm is applied to calculate the tap coefficients of the pre-distortion module.When compared with the existing LMS-DE algorithm,under the condition that the system’s bit error rate reaches 1×10^-4,the LMS-WOA-DE algorithm can obtain a 2d B SNR gain.Subsequently,in order to improve the results of the predistortion parameters obtained by the RLS algorithm,the RLS-WOA-DE algorithm was proposed as the parameter estimation method of the predistortion module.The simulation results show that the predistortion system using the parameters estimated by the RLS-WOA-DE algorithm can also achieve better bit error rate performance than the systems using the existing RLS algorithm and RLS-DE algorithm.（2）For the nonlinear compensation of the laser diode at the receiver in the VLC system,a post-distortion compensation technique for inverse modeling of laser diodes based on mem-oryless polynomials is studied.To solve the post-distortion module coefficients,according to its characteristics,the DE-ACO algorithm is combined with the LMS adaptive algorithm to optimize the iterative process of estimating the post-distortion parameters.At the same time,the LMS algorithm optimized by the WOA-DE algorithm is also applied to the parame-ter identification process of the post-distortion module based on the memoryless polynomial model.It can be seen from the simulation results that under certain simulation conditions,when using the LMS algorithm optimized by the WOA-DE algorithm and the post-distortion parameters estimated by the LMS algorithm optimized by the DE-ACO algorithm to perform nonlinear compensation,both the bit error rate of the system can be effectively reduced;and the system using the LMS-DE-ACO algorithm and the LMS-WOA-DE algorithm obtains a signal-to-noise ratio gain of about 2d B compared with the system using the LMS-DE algo-rithm.In order to further optimize the results of the distortion parameters solved by the RLS algorithm,the DE-ACO algorithm was combined with the traditional RLS algorithm,and the RLS-DE-ACO algorithm was proposed as the parameter estimation method of the post-distortion module.The RLS algorithm optimized by WOA-DE algorithm was also applied to estimate the parameters of the post-distortion module.The simulation results show that the post-distortion compensation ability of both RLS-WOA-DE algorithm and the RLS-DE-ACO algorithm are better than the existing RLS-DE algorithm.