Research On Precoding Technology Of Reconfigurable Intelligent Surface

Reconfigurable intelligent surfaces(RIS)can significantly improve the performance of wireless communication networks by intelligently reconfiguring the wireless propagation environment by integrating many low-cost passive reflective elements on a surface.Specifically,RIS can adjust the electromagnetic characteristics of different elements to the incident signal,such as amplitude,phase,etc.,through the precoding technology,to control the signal propagation according to human wishes.The precoding technology is the key to the role of RIS.This thesis focuses on the precoding technology of RIS.The main work is as follows:Firstly,the research status of RIS in wireless communication and the research progress of RIS precoding technology are summarized.The electromagnetic characteristics,typical functions,common RIS precoding technology and basic knowledge of optimization methods of RIS are introduced.It provides theoretical support for the follow-up research work.Secondly,aiming at the problems of high precoding complexity and poor scalability in RIS-assisted multiple-input multiple-output systems,a precoding scheme based on improved particle swarm optimization(PSO)algorithm is proposed.The joint precoding is decomposed into the active precoding at the base station and the phase shift design at the RIS using the idea of alternating optimization.Then,the two problems are solved alternately by using the power allocation of water injection and the improved PSO algorithm.The simulation results show that the precoding scheme based on the improved PSO algorithm can effectively solve the precoding problem,and can improve the spectral efficiency of the studied system by about 30%.Finally,in order to further reduce the complexity and feedback overhead of precoding,a codebook-based RIS precoding scheme is proposed.In the line of sight channel,the optimal precoding matrix form of RIS is derived.The discrete fourier transform(DFT)matrix is used to approximate the RIS precoding matrix,and the precoding can be completed through the feedback precoding matrix index.In addition,in order to improve the performance gain in multipath scenarios,a precoding scheme based on Line-Buzo-Gray(LBG)codebook is proposed.The codebook is generated by LBG algorithm,and the precoding process using this codebook is given.The simulation results show that the two precoding schemes can reduce the complexity and feedback overhead of precoding while ensuring the spectral efficiency gain.