Study On Technologies Of Pilot Contamination Reduction In Massive MIMO Communication Systems Based On Pilot Scheduling

Massive MIMO which is one of the key technologies of the next generation mobile communication system improves the capacity and reliability greatly by the fact that the base station is equipped with a large number of antennas.For short coherence time,MIMO system must be assigned the same pilot sequences in every cell,which leads to severe pilot contamination,so pilot contamination reduction by pilot scheduling schemes becomes a research hotspot in this field.The thesis mainly focuses on designing the effective pilot scheduling schemes to mitigate the impact of pilot contamination.The main contents are as follows:(1)Build the Massive MIMO system model and discuss the reason and the impact of the pilot contamination as well as the typical pilot contamination reduction schemes.Analyze the relationship between the pilot scheduling and the target cell user's uplink asymptotic SINR by introducing the pilot scheduling matrix.Analyze the impact of pilot contamination problem on the system performance through simulation.Analyze the typical pilot contamination reduction schemes especially the pilot scheduling schemes.(2)For the shortcoming of the existing random pilot scheduling scheme and smart pilot assignment scheme(SPA)which may result in great performance degradation to the users in good channel conditions,a pilot scheduling scheme based on the degradation is proposed.The proposed scheme assigns the optimal pilot sequence to the user who suffers greatest performance degeneration greedily by computing the degradation when the users are assigned every available pilot sequence.The simulation results show that the performance of the target cell's sum achievable rate is improved and the system can meet the requirement of quality of service of more users as well as the impact of shadowing fading on the system is reduced effectively under the proposed pilot scheduling scheme based on the degradation.(3)To solve the problem of the pilot scheduling scheme based on the degradation that the users in bad channel conditions have performance bottleneck,a pilot scheduling scheme based on user grouping is proposed.The scheme is a tradeoff between mitigating pilot contamination and pilot overhead.The proposed scheme divides the users into the center user group and the edge user group according to different levels of pilot contamination on the basis of large-scale fading or path loss.The proposed scheme assigns orthogonal pilot sequences to the edge users on the premise of additional pilot sequences,while the center users reuse pilot sequences.To improve flexibility of the proposed pilot scheduling scheme based on user grouping further,a kind of improved scheme is proposed by the grouping parameter which is used to choose the edge users dynamically.The simulation results show that the proposed scheme can solve the pilot contamination problem of edge users and improve the performance of the target cell's sum achievable rate if reasonable number of edge users or grouping parameter is chosen.