Research On Pilot Contamination For Massive MIMO System

The evolution of wireless cellular network has strongly depended on the advanced Multiple-Input Multiple-Output (MIMO) technology to improve reliability and capacity of the wireless communication fundamentally. Recently proposed massive MIMO, as one of the important technologies of the fifth generation (5G) of wireless communication networks by providing a significant increment in reliability and data rate, has aroused wide interest of researchers.Massive MIMO refers to a wireless communication system that the base station installs tens to hundreds of antennas, serve multiple users simultaneously and use of Time Division Duplexing (TDD) system. The results show that Massive MIMO system is with high frequency efficiency and energy efficiency. Interference can be suppressed within the cell and inter-cell to a large extent, greatly improve the overall capacity of the system. But Massive MIMO play to their strengths on the premise that the system can obtain accurate channel state information. Because of the existence of pilot contamination in system, it is very difficult to acquire the accurate channel state information. Therefore, pilot contamination has become a bottleneck in Massive MIMO which is urgent to be solved.This thesis researches the pilot contamination problem in Massive MIMO system in this background. Firstly, the concept and characteristics of large scale MIMO system are introduced , and the background knowledge is established for the research of reducing the pilot contamination. Then, this thesis introduces the system model of Massive MIMO and studies the performance of a large scale MIMO system under pilot contamination. Secondly, the channel estimation method for Massive MIMO system is introduced, including the least square estimation, the least mean square error estimation and the Bayes estimation method.Secondly, in order to solve the pilot cintamination of Massive MIMO,this thesis introduce the time-shift pilot scheme, and compares with the conventional method, the results show that the inter-cell pilot shift shceme can effectively alleviate the pilot contamination problem compared with the traditional alignment pilot method.Thirdly, this thesis proposes a new pilot decontamination method based on pilot sequences allocation. The cell is divided into central region and edge region, and the users in edge area of adjacent cells is assigned mutual orthogonal pilot to alleviate the pilot contamination, meanwhile the central users are assigned pilots with an itetation method based on Hungarian Algorithm to achieve maximum system sum rate.Simulation results show that the proposed scheme can effectively alleviate the pilot contamination, improve the accuracy of channel estimation and improve the performance of the Massive MIMO system.