Research On Pilot Decontamination Based On Pilot Allocation And Precoding In Massive MIMO System

Massive MIMO as one of the key technologies in 5G,it has attracted wide attention in recent years.Because allocation of large-scale antenna arrays in the base station,it can provide communication service for users at the same time,which enhance the energy efficiency and system capacity greatly,but the pilot contamination generated by pilot multiplexing in uplink adjacent cell which causes channel estimation error,it will affect the performance of downlink precoding.Therefore,this thesis will study the method of pilot contamination mitigation to improve the system performance in Massive MIMO system.This thesis first introduces the research background of Massive MIMO system,three key technologies and pilot contamination problem.Secondly,from the two aspects of pilot allocation strategy and precoding method,we focus on the pilot contamination mitigation method.We use effective pilot allocation strategy and low complexity precoding method to mitigate the influence of pilot contamination,which will improve the performance of system.The main contents of this thesis are as follows:(1)We analysis the reasons of pilot contamination in Massive MIMO systems,from the uplink and downlink,we analysis the impact of system achievable rate for pilot contamination problem in TDD Massive MIMO,and the present research situation of pilot contamination mitigation methods are conducted in analysis and summary.(2)Aiming at the problem of too much pilot cost in traditional pilot allocation method based on users,we propose a pilot allocation method based on cells and combine users grouping to mitigate the impact of pilot contamination.In the uplink,different cells use orthogonal pilot sequences to eliminate inter-cell interference.The users adopt the same pilot in the same cell,and the intra-cell interference is introduced artificially.Then the users are divided into intermediate user groups and edge user groups according to the pilot contamination level in cell,intermediate user groups with lighter pilot contamination use the same pilot and redistribute the orthogonal pilots to edge user groups with heavier pilot contamination.The uplink signal to interference plus noise ratio(SINR)and the system achievable rate are used to measure the effectiveness of the proposed pilot allocation strategy.The performance of pilot allocation method is simulated and verified under the change of base station antennas and SNR.(3)Aiming at the problem of high complexity regularized zero forcing precoding algorithm,we introduce truncated polynomial expansion precoding method,and propose the pilot contamination precoding method of cell-TPE by analyzing the performance of ZF,RZF,TPE precoding algorithm,which mitigates the impact of pilot contamination.Combining the pilot allocation method based on cells,when the number of base station antennas tend to infinity,we use appropriate TPE precoding in downlink transmission process.The truncated polynomial replace matrix inverse of RZF precoding,which reduce the complexity of RZF precoding and the performance close to RZF precoding,the artificial introduction intra-cell pilot interference is reduced,so the pilot contamination is mitigated on the whole.The downlink signal to interference plus noise ratio(SINR)and the system achievable rate are used to measure the effectiveness of the pilot contamination precoding method.The performance of pilot contamination precoding method is simulated and verified under the change of base station antennas and SNR.