Research On Technologies Of Pilot Contamination Reduction Based On Location Information In Massive MIMO Communication Systems

Massive multiple input multiple output(Massive MIMO)technology,as one of the key technologies of 5th Generation(5G)mobile communication system,greatly improves system capacity and ensures system reliability,laying the foundation for the application of 5G.Due to the limited coherence time,massive MIMO system cannot assign orthogonal pilots to all users in all cell under time division duplexing(TDD)mode,resulting in pilot multiplexing for some users.The pilot contamination problem caused by this problem seriously restricts the overall performance of massive MIMO system.Therefore,reducing pilot contamination is the key problem to be solved urgently at present.This thesis will use the user space location information to assign the pilot allocation scheme and effectively reduce pilot contamination.The main contents are as follows:(1)Aiming at the problem of imperfect channel state information in traditional pilot allocation schemes,a pilot allocation method based on angle of arrival(AOA)is proposed in the thesis,using the idea of partial frequency reuse and massive MIMO flexible spatial freedom to reduce multiplexing of the same pilot inter-user interference and save pilot overhead.The method divides users into different sectors according to the azimuth angles of arriving for different users at the base station.The intra-sector user allocates orthogonal pilot and the inter-sector user allocates multiplexing pilot.The peripheral primary cell allocates a pilot sequence group orthogonal to the target cell and the secondary cell multiplexes the target cell pilot sequence group or a pilot sequence group that is not adjacent thereto.Compared with the full multiplexing allocation method,the pilot allocation method based on AOA does not increase the pilot overhead and pushes the user interference from the primary cell to the secondary cell,which greatly reduces the impact of pilot contamination.The thesis simulates and analyses the effect of the proposed method on reducing pilot contamination in massive MIMO systems from five aspects.The pilot allocation method based on AOA has obvious performance improvement compared with the other two user grouping methods in ensuring that the user AOA does not overlap or has a slight overlap(within 10°).(2)In order to solve the problem of user AOA overlap caused by random distribution of users,the thesis further perfects the strategy on the basis of "based on AOA pilot allocation method",and makes full use of the user's spatial location information to propose a pilot allocation method based on user location information.The aim is to reduce the impact of pilot contamination on massive MIMO systems when user AOAs overlap and the number of sector users is different.The thesis proposes an interference metric function,analyzes and verifies its rationality,and completes the pilot allocation on this basis.The simulation results show that the proposed pilot allocation method based on position information can significantly reduce the pilot contamination when the number of antennas is low(M ?256),and has a considerable average signal-to-interference-noise ratio;when the number of antennas increases,the algorithm can still maintain good overall performance of the massive MIMO system,and the number of user connections can be increased under the condition that the system performance is not significantly reduced.