Research On Mitigation Of Pilot Contamination In Massive MIMO System

Massive MIMO(Multi-Input Multi-Output)can greatly increase the spectrum efficiency and transmission rate of wireless communication,and is therefore considered as a key technology in the fifth generation mobile communication(5G)system.It has been the hot research topic in the communication field.Different from the traditional MIMO system,the basic characteristic of the massive MIMO system is that the base station is equipped with dozens of or even hundreds of antennas,and simultaneously serves multiple users in the same time-frequency resource.The massive MIMO system is based on time division duplex and frequency division duplex modes,this paper focuses on the former.When the massive MIMO system works in the time division duplex mode,the orthogonal pilots will be reused in different cells under the constraint of the coherent time length and the constantly increasing number of users,and then pilot contamination phenomenon occurs.As the number of base station antennas increases,massive MIMO system can mitigate the effect of channel noise.Therefore,the performance of the system will be mainly limited by pilot contamination.This paper focuses on pilot allocation,pilot design and channel estimation in massive MIMO system.The main content and work contained in this paper are as follows:1.The basic principle and main problems of massive MIMO system are introduced.It mainly includes: channel estimation methods and precoding methods commonly used in massive MIMO system;we focus on the cause of pilot contamination problem and the impact of pilot contamination on massive MIMO system;Then a pilot allocation method is introduced,according to the users' large-scale fading coefficient,we divide the users inside the cell into the center user group and the edge user group.2.Aiming at serious pilot contamination of center users in the above pilot allocation method,this paper proposes a soft pilot reuse scheme combined with time-shifted pilots to reduce the pilot contamination actually suffered by the center users.Then,a channel estimation method based on discrete Fourier transform is proposed,which utilizes the angle of arrival information of edge users to distinguish the desired signal from the interference signal.Simulation results show that this scheme can effectively reduce the pilot contamination and increase the data transmission rate.3.In order to solve the above-mentioned difficulty of pilot contamination mitigation method,a non-reused pilot design method is proposed.The pilot sequences with good properties are designed,and assigning them to users in the cell.Simulation results show that the proposed non-reused pilot design method can effectively mitigate the pilot contamination and improve the transmission performance of massive MIMO system.