Research On Key Issues Of Physical Layer Security In Massive MIMO System

With the rapid increasing of mobile wireless communication business volume and the ever-growing of user demands for wireless applications,mobile wireless communication system has experienced an unprecedented data explosion.As a result,5G network research and development have been promoted all around the world.As one of the key technologies of 5G network,large scale multi-input multi-output(Massive MIMO)has tremendous spectrum utilization and energy efficiency,and it can greatly improve the system capacity,so it has wide perspective of development.Compared to the traditional MIMO system,there are many new features and challenges for Massive MIMO system,the biggest of which is that with the increasing of the number of base station antennas,the Massive MIMO performance is limited by inter-cell interference,and the reason is the pilot contamination.Inter-cell interference will make other users receive messages that should only be received by the target user,resulting in information leakage,causing physical layer security problems.In this paper,based on the TDD multi-cell multi-user Massive MIMO system model,we focus on two key issues related to physical layer security:pilot contamination elimination and downlink precoding designing.These two research points eliminate inter-user interference from uplink and downlink perspectives,respectively.Thereby,both of them can ensure the physical layer security.In this paper,a pilot contamination elimination scheme is designed,which is a combination of cells grouping and pilot allocation.In view of the channel reciprocity in TDD Massive MIMO system,we can make channel estimation via an uplink transmission pilot.First we explain the cause of pilot contamination by deriving the formulas related to the uplink channel estimation process.Then several commonly used pilot contamination elimination schemes are briefly introduced.Finally we design a scheme combining of cells grouping and pilot allocation.This scheme can reduce 2/3 pilot contamination and simplify calculation at the cost of doubling the orthogonal pilots.And then through the pilot allocation,we further achieve the purpose of reducing the pilot contamination.Specifically,for a certain pilot,a group of users that make the highest uplink transmission rate are selected using a greedy algorithm.Using Matlab,we simulate the average capacity of the downlink channel of the system by changing users' distribution radius,the parameter ? of the slow fading coefficient,the number of users in each cell,respectively.The simulation results show that this scheme can effectively eliminate the pilot contamination,increasing the average capacity of the channel and improving the performance of the system.A joint precoding scheme is also designed in this paper.First we deduce the formulas of downlink signal transmitting,explaining the concept of downlink precoding.And confirm that it plays an important role in the elimination of inter-cell interference.And then we introduce several commonly used linear precoding schemes.Finally we design a precoding scheme based on the cooperation of base stations,eliminating the pilot contamination and the interference between the downlink signals all at once.Specifically,the base stations of all cells cooperate with each other and exchange some information.We use the information that is got by the interaction of all base stations to design a joint precoding matrix,and then use this matrix to preprocess the signals emitted from the base stations to counteract the influence of pilot contamination.We assume that the number of base station antennas is infinite and do some theoretical derivation.And then we extend it to the situation of limited antennas number,and the theoretical analysis and simulation are carried out.Using Matlab,we simulate the average capacity of the downlink channel of the system by changing the number of base station antennas,the distribution radius of the users,the parameter ? of the slow fading coefficient,respectively.The simulation results show that this scheme can effectively eliminate the pilot contamination,increasing the average capacity of the channel and improving the performance of the system.