Research On Pilot Allocation Schemes In Multi-cell Massive MIMO Systems

With the rapid development of various technologies such as mobile health,smart homes and cloud computing,and the rapid popularization of terminals,traditional MIMO technology cannot meet the needs of higher speeds in future mobile communication systems.As a research hotspot in the next generation communication field,massive MIMO technology can effectively increase the capacity and spectrum efficiency of the system by deploying a large number of antennas in base station.Due to the short coherence time of the channel,the number of orthogonal pilots is limited,and the pilot contamination generated by the reuse of pilots in multiple cells will affect the performance of the massive MIMO system.This paper aims to reduce the impact of pilot contamination and improve system performance in multi-cell massive MIMO system through the research of reasonable and effective pilot allocation schemes.The main contents and results are as follows:1)Considering that the number of base station antennas is too large in a massive MIMO system and the high dimensional covariance matrix is difficult to obtain,a pilot allocation scheme based on location information is proposed by analyzing multipath channels.The covariance matrix is obtained by using the locations of the user and the base station to perform channel estimation on the system.According to the overlapping situation of the angle of arrival of the target user and the interference user,if the angle of arrival is non-overlapping,the interference users are allocated to the same pilots with the target users,and the interference can be reduced by path-loss when the angles of arrival are overlapped.Simulation results show that the proposed scheme based on location information can effectively improve the accuracy of channel estimation and suppress pilot contamination.2)Considering the fairness of all users in the different cells,a dynamic pilot allocation scheme based on graph coloring is proposed by using the characteristics of different severity of pilot contamination.Firstly,an edge-weighted interference graph based on cooperation among cells is constructed to describe the strength of pilot contamination among all users.Then,based on the traditional graph coloring theory,greedy algorithm is used to allocate pilot resources for all users.Simulation results show that the proposed pilot allocation scheme can obviously enhance the uplink achievable sum rate and improve the fairness of all users in massive MIMO systems.