Research On Pilot Contamination Reduction In Massive MIMO

Massive Multiple-Input Multiple-Output(Massive MIMO)technology is one of the emerging wireless communication technologies,which can increase system’s capacity by using a large number of antennas without additional bandwidth.However,Massive MIMO system in time-division duplex model(TDD)obtains the channel state information(CSI)through the channel reciprocity,which will generate the pilot contamination problem,thereby constraining system capacity.The focus of thesis is how to use effective measures or methods to reduce the influence of pilot contamination and improve the sum rate of users in cell.The major researches are as follows:To begin with,this thesis introduces the Massive MIMO system model and three key technologies of decontamination,including channel estimation,pre-coding and pilot scheduling.Based on this,the theoretical analysis of pilot contamination problem is carried out by using mathematical expressions,which lays a theoretical foundation for the subsequent contamination suppression.Secondly,aiming at the influence of spatial dimension on channel estimation,this thesis gives a 3D MIMO channel model suitable for signal transmission.Then,the two-dimensional compression model in spatial and temporal domain is given.Finally,the signal reconstruction is carried out by using Orthogonal Matching Pursuit(OMP)algorithm.The simulation results show that this algorithm can significantly reduce the Mean Square Error(MMSE)of the channel estimation and the Bit Error Rate(BER)under the same pilot length and in the condition of the same system demand,this method can reduce the number of pilot.Thus it can suppress the pilot contamination effectively at the source.Finally,in order to avoid the matrix inversion and reduce contamination in Massive MIMO,an improved pre-coding algorithm is proposed by using Genetic Algorithm(GA).This thesis improves the traditional genetic algorithm in order to adaptively adjust the mutation probability.After that,the improved genetic algorithm is applied to design the precoding matrix under the Maximize Signal to Interference plus Noise Ratio(Max-SINR)condition.In order to further suppress the contamination,pilot scheduling is combined with the optimized pre-coding algorithm to carry out.The pilot scheduling according to the merits of each cell users’ channel condition.The simulation results show that compared with the exhaustive method of pilot scheduling,this pilot scheduling method can obtain an approximate effect.In the meantime,the algorithm can also reduce the complexity and the impact of pilot contamination on the down-link.