Research On Pilot Contamination Suppression In Massive MIMO Systems

Massive MIMO technology significantly improves the spectral efficiency and energy efficiency of the system through deploying a large number of antennas at the base station.However,the presence of pilot contamination restricts its performance.To this end,two pilot allocation schemes are proposed in this thesis to reduce pilot contamination in massive MIMO systems.This work sheds light on the characteristics of massive MIMO systems,the causes of pilot contamination,and the impact of pilot contamination on system performance.And the numerical experiments under the uncorrelated Rayleigh fading channel demonstrate the influence of pilot contamination on the spectral efficiency.Furtherly,corresponding reasons are analyzed briefly.The main work of this dissertation as follows:1.Aiming to resolve the issue of overly pilot overhead and low spectral efficiency in the existing pilot multiplexing scheme,a cyclic pilot assignment scheme is proposed in this work.Namely,three sets of orthogonal pilot sequences among cells are cyclically employed to reduce pilot overhead and improve users' signal to interference and noise ratio.Subsequently,in the case of the Maximum Ratio Combining(MRC)and Zero Forcing(ZF)detection adopted in the base station,the expressions of cell's signal to interference and noise ratio and spectral efficiency are introduced,and the interference threshold is determined by the user's asymptotic signal to interference and noise ratio.In addition,the pilot overhead and complexity of the proposed scheme are analyzed.At last,in the experiment results,it is shown that the proposed scheme has higher spectral efficiency,and a larger maximum number of service users compared with the original scheme.2.Because of high complexity in the present cooperative pilot assignment scheme,another pilot assignment scheme associated with user location and spatial distance is proposed in this work.Firstly,users are classified according to whether the arrival angles overlap.Specifically,the users whose arrival angles do not overlap with each other are the first n types of user sets,and the remaining users are the n+1 types of user sets.Secondly,when implementing pilot selection,n sets of orthogonal pilot sequences are allocated to be multiplexed for the first n types of user sets.Meanwhile,pilot assignment borrowing the idea of graph coloring is employed for the n+1 types of user sets.In the end,this thesis analyzes the classification criterion,weight function,assignment complexity and pilot overhead of the proposed scheme.The experiment results illustrate that the proposed pilot assignment scheme dramatically improves the spectral efficiency,and effectively reduces the pilot assignment complexity when the first kind of user increases.