The Research On Pilot Contamination In Cell-free Massive MIMO Systems

The user's requirement for communication technology are gradually increasing with the continuous progress of wireless mobile communication systems.User's requirement in term of spectral efficiency and network throughput have been unable to be satisfied by traditional massive MIMO(multiple-input multiple-output)systems.The cell-free massive MIMO system has wide spectral and high power efficiency.And it is expected to become one of the key technologies of post5G(the fifth generation mobile communication)and 6G(the sixth generation mobile communication).However,in the real scenarios,the pilot contamination is an important factor that affects the network throughput of a cell-free massive MIMO system.This thesis studies how to reduce pilot contamination in a cell-free massive MIMO system from the aspects of pilot assignment,pilot power control,and the number of user's antennas,and increase the system's spectral efficiency and the network throughput.First,for a cell-free massive MIMO system with single antenna users and single antenna AP(Access Point),the uplink pilot training phase and the uplink and downlink data transmission process are introduced.The closed form expression of the user's achievable rate of the uplink and downlink is studied.The random pilot assignment scheme and the greedy pilot assignment scheme are analyzed and compared.The greedy pilot assignment scheme iteratively updates the pilot sequence of the users with lower downlink achievable rates.The simulation results verify that the greedy pilot assignment algorithm improves the limit to the network throughput of the lower-performance users.At the same time,the purpose of the data power control is to increase the overall network throughput of the cell free massive MIMO system,and to maximize the minimum value of the achievable rate of the uplink and downlink for all users.Then,the transmission process of the uplink and downlink in a cell free massive MIMO system with single-antenna users and multi-antenna AP is analyzed.On the basis of optimizing the pilot assignment method,the pilot power control coefficient is designed.The pilot power control scheme minimizes the maximum value of the normalized mean square error of the all users.The pilot contamination has been greatly alleviated under this scheme.The greedy pilot assignment algorithm combined with the pilot power control proposed in this thesis significantly improves the network throughput on both the uplink and downlink propagation,thereby improving the overall performance of the system.Finally,the pilot power control algorithm is extended to be used in a cell-free massive MIMO system with multi-antennas users and multi-antennas AP.It specifically introduces the information propagation process of its uplink.And the uplink spectral efficiency expression of the user is studied and analyzed.By equipping each user with multiple antennas,the spectral efficiency of the system can be improved.At the same time,the pilot power control is performed,which greatly reduces pilot contamination.The study results of this thesis verify that the overall performance of the system can be greatly improved by both equipping each user with multiple antennas and pilot power control.