Research Of Pilot Contamination Suppression Based On Channel Estimation And Precoding In Dual-polarized Massive Mimo Systems

The dual-polarized Massive MIMO system is one of the key technologies of 5G,which effectively improves system capacity,energy efficiency and robustness by deploying a large number of dual-polarized antenna arrays in the base station.Considering inter-cell interference caused by pilot contamination does not disappear with the increase of antennas,which becomes a bottleneck restricting the performance improvement of the dual-polarized Massive MIMO systems.Polarization can extend the freedom degree of wireless resources and help solve the problem of pilot contamination.Therefore,it is necessary to study the effective pilot contamination suppression algorithms by using polarization domain wireless resources.The research of this thesis is supported by National Natural Science Foundation:"Research on polarimetric self-interference cancellation in full-duplex system based on additive Gaussianized phase noise(61501050)".In the dual-polarized Massive MIMO system,two kinds of pilot contamination suppression algorithms are proposed,using the polarization domain wireless resources.In the uplink transmission,a channel estimation algorithm based on orthogonal polarization is proposed,which uses the orthogonal polarization characteristics of the dual-polarized antenna.In the downlink transmission,a precoding algorithm based on large-scale fading factor and Cross Polarization Discrimination(XPD)estimation is proposed,with considering estimates spatial fading and depolarization effects.The research content of this thesis are as follows:(1)This thesis systematically expounds the related research of dual-polarized Massive MIMO system,including channel model,uplink and downlink transmission process,polarization information processing and depolarization effect.This thesis also summarizes the causes and impact of pilot contamination,and analyzes the current research status of pilot contamination suppression algorithms.The advantages and challenges of polarization in suppressing pilot contamination are discussed.On this basis,it is points out that it is necessary to use polarization domain radio resources to study pilot contamination suppression algorithm,which is the key to improve the performance of dual-polarization Massive MIMO system.(2)Since pilot contamination limits the accuracy of Channel State Information(CSI),in order to solve this problem,an uplink channel estimation algorithm based on orthogonal polarizations is proposed.Firstly,the XPD in the dual-polarized channel is modeled,and the dual-polarized channel is decomposed by the orthogonal polarization characteristics of the dual-polarized antenna.Then,the unbiased estimation for the combination term of interference caused by pilot contamination and the noise is performed.Thereby,a dual-polarization channel estimation matrix based on orthogonal polarizations is obtained.Finally,theoretical analysis and simulation results show that the proposed algorithm reduces the estimation error under pilot contamination,and the complexity is lower than the Minimum Mean Square Error(MMSE)estimation.Compared with the single-polarized antenna system,when the depolarization XPD effect exists,the orthogonal polarization characteristics of the dual-polarized antenna in the proposed algorithm significantly reduce the estimation error,which effectively suppresses the influence of pilot contamination on CSI accuracy.(3)The spatial fading and depolarization effects in the dual-polarized channel limit the performance of precoding.To solve this problem,a downlink precoding algorithm based on large-scale fading factor and XPD estimation is proposed.Firstly,Maximum Ratio Transmission(MRT)precoding is performed,and the power factor of the precoding matrix is obtained by using the power control strategy.Second,the user estimates the large scale fading factor and XPD to reconstruct the downlink channel estimation matrix.Then,the base station calculates the precoding matrix based on the large-scale fading factor and the XPD estimation,and preprocesses the downlink data.Finally,theoretical analysis and simulation results verify that the proposed algorithm effectively suppresses pilot contamination and improves the sum rate.In order to effectively suppress pilot contamination,this thesis proposes an uplink channel estimation algorithm based on orthogonal polarizations,a downlink precoding algorithm based on large-scale fading factor and XPD estimation.The results show that two kinds of algorithms effectively reduce the estimation error and improve the downlink sum rate,which provides a new idea for pilot contamination suppression research.