Research On The Technology Of Pilot Contamination Mitigation In Massive MIMO Systems

Massive MIMO technology is considered to be a key technology of the future 5G because of a significant increase in spectrum efficiency and in user experience rate.However,the inter-cell interference arising from pilot reusing,that is pilot contamination,is more and more serious and finally limits the system performance.On the one hand,pilot contamination causes interference to the uplink channel estimation and reduces the accuracy of obtaining channel state information,which could adversely affect the system performance.On the other hand,when the criterion of inter-cell user pilot assignment is the traditional pilot reuse mode,the edge users of target cell are often subjected to interference from adjacent cells.As a consequence,the performance of the system is restricted.Based on the above two aspects,in this thesis,we studied the technique of reducing pilot contamination from two aspects: channel estimation method and pilot allocation schemes.Firstly,massive MIMO system model in TDD transmission mode was studied.The cause of pilot contamination and its influence on system performance were analyzed in detail.Then the influence of pilot contamination on system performance is verified by simulation.Secondly,to relieve the problem that pilot contamination decrease channel estimation performance,the channel estimation algorithm based on subspace was improved.This thesis proposed a semi-blind channel estimation method combined the Iterative Least Square with Projection(ILSP)algorithm to solve the problem that there is an error in the process of signal covariance matrix formation.The main idea is the channel estimation of joint iterative channel estimation and signal detection.The channel estimation performance is improved by multiple iterations.The simulation results showed that the algorithm proposed in this thesis can effectively reduce the error of the signal covariance matrix,and improve the accuracy of channel estimation,which can weaken the influence of pilot contamination on the performance of massive MIMO system at a certain extent.Finally,this thesis proposed a pilot allocation scheme based on user grouping,considering the problem of pilot contamination caused by traditional pilot reuse.The user grouping threshold was set according to different characteristics of the large scale fading coefficient of the user in the cell.Users in each cell were divided into two groups: the center group with less pilot contamination and the marginal user group with heavier pilot.The central user group of all the cells reused the same pilot while the edge user groups of per three adjacent cells used mutually orthogonal pilots to weaken pilot contamination of the edge user in target cell.At the same time,the optimal pilot sequence length and the optimal pilot reuse factor are obtained by theoretical analysis.Simulation results showed that the target cell uplink achievable rate increased 0.15bps/Hz and 1.5bps/Hz respectively compared with the traditional pilot reuse factor of 1 and 3,when the proposed pilot allocation scheme meet the optimal pilot reuse factor and the base station is deployed with 200 antennas,thereby mitigating pilot contamination.