Research On Pilot Contamination Suppression Schemes Based On Pilot Allocation In Massive MIMO

Massive MIMO technology has become one of the 5G's core technologies because it can effectively improve the capacity of wireless systems,greatly improve spectrum utilization.However,the contradiction between the limited number of orthogonal pilots and the surge of mobile users in a Massive MIMO communication system makes the base station inevitably subject to interference from users multiplexed with the same pilot sequence when performing channel estimation.The phenomenon is pilot contamination,and such pilot contamination cannot be eliminated by increasing the number of antennas,which seriously restricts the performance of the entire massive MIMO communication system.Reasonable pilot allocation technology can reduce this kind of pilot contamination to a certain extent.Therefore,this paper s proposes a scheme to suppress pilot contamination by pilot coordinated assignment.The main research contents of this paper are as follows:Firstly,this paper starts with the background of massive MIMO technology,briefly describes the existing technology of suppressing the pilot contamination,and analyzes the transmission process of the uplink and downlink data of the system under TDD transmission mode,which leads to the cause of pilot contamination and adverse effects of system performance.Secondly,this paper proposes a semi-dynamic pilot assignment algorithm based on graph coloring.The proposed algorithm starts with dividing each cell user into a cell center user class and a cell edge user class based on the mobile user's SINR.The cell center users of each cell adopt a static allocation mode that reuse the same group of pilot sequences,and the cell edge users use a dynamic allocation based on graph coloring.The minimum SINR in the uplink is used as a criterion to prioritize the assignment of available pilot resources to cell edge users with relatively serious pilot pollution.The simulation results show that the semi-dynamic pilot assignment method proposed in this paper has the effect of suppressing pilot pollution and improving the uplink total available rate of the system compared with the traditional random pilot assignment algorithm and smart pilot assignment algorithm.Finally,this paper proposes a pilot power allocation algorithm based on the pilot assignment.It mainly turns the problem of improving the users uplink total available rates into a convex optimization problem,and obtain the optimal solution by Lagrange multiplier method,and use the water fill algorithm to dynamically set the water level,and finally obtain the optimal arrangement of pilot power of each user in the cell.In order to avoid the phenomenon of zero power,a modified water-filling algorithm is proposed to complete the allocation of user pilot power in the cell.On this basis,all the cells in the system are allocated sequentially with reasonable power in the units of cells to obtain the maximum total available rate.Simulation analysis shows that the algorithm proposed in this paper has great advantages in improving the total available rate and optimizes the system performance compared with the pilot allocation algorithm that assigns equal pilot power to users.