Investigation Of Precoding Technology For Indoor Visible Light Communication

With the development of wireless communication,bandwidth has been a key factor in satisfying the explosive demand in wireless communications.Visible light communication(VLC)has been one of the important supplementary technologies for future indoor networks because of advantages like spectrum-free license,no electromagnetic radiation,and high security.VLC networks are always densely deployed which introduces serious interference between users.Therefore,interference management is one of the key technologies for indoor VLC networks.Precoding is a typical interference management method at the transmitter,which supports multi-user access in the dense network.In this dissertation,differences between traditional wireless communication and indoor visible light communication in the precoding technology from the view of the mathematical model are discussed.The symbol-level precoding technique is investigated by exploiting the constructive interferences.The simulation of the investigated scheme under different factors are implemented,including LED(Light Emitting Diode)distribution,position,light source spacing,transmitter power constraints,FOVs of receivers and LED half-power angle.In order to verify the effectiveness of the investigated symbol-level precoding,the typical scenarios(2 LED/2 users and 3 LED/3 users)are selected for experiments.The experimental results show that the performances of the investigated precoding fit well with the simulation results,which verifies its effectiveness in visible light communication.The current research on precoding technology focuses on the ideal situations.While in the real system,some factors cannot be ignored,including arrival delay,CSI imperfections,and spatial correlation.In this dissertation,the impact of the arrival delay of signals from different LEDs to one user is analyzed with delay matrix.The performance of ZF precoding and the proposed scheme is studied under time delay.In order to verify the effect of arrival delay,some experiments have been implemented.Results show that the symbol-level precoding is robust to small delay when there exist arrival delays between data streams.The arrival delay of the real system is measured by orthogonal sequence,and compensated at the transmitter.The performance before and after the delay compensation are compared.When BER is less than 1×10-3,after time compensation,the minimum transmission distance between users using ZF and the investigated scheme can be decreased by 0.1 m.For the CSI imperfection situation,a robust precoding design algorithm is further investigated.The performance of different precoding schemes assuming imperfect CSI and the performance of the original scheme assuming CSI is perfect are compared.Results show that the investigated robust precoding has the best performance.When BER is less than 1×10-3,the proportion of the communicable area is increased by 11.2%,which proves the superiority of the investigated robust algorithm.In terms of the high spatial correlation in the visible light network,a joint algorithm that optimizes the azimuth,tilt and precoding matrix of the tilt receiver is proposed and the optimal azimuth and tilt angles are obtained.The simulation results show that the diversity of receiving angle can effectively reduce the spatial correlation and improve the performance.