Research On Pilot Decontamination Based On Pilot Allocation In Multi-Cell Massive MIMO System

In the Massive MIMO(Multiple-Input Multiple-Output)system,since the number of base station antennas is large,the channels tend to be orthogonal,and part of the noise can be suppressed,so that the base station can detect the transmitted signals,and these require the base station to obtain accurate channel state information(Channel State Information,CSI).The CSI can be obtained from channel estimation.The currently used channel estimation methods are mainly based on pilot assistance.In multi-cell Massive MIMO system,since the same set of orthogonal pilots are multiplexed between cells,pilot contamination is generated,which increases channel estimation error.Where there is a large amount of communication traffic,deploying a Massive MIMO cellular cell with a small radius often meets the communication requirements.As a result,the number of cells in the local area is large,and there are many interfering cells in the vicinity of the central cell.As the communication demand increases,the cell radius becomes smaller and smaller.The interference originally ignored by the second-circle cells outside the central cell due to large-scale fading still exists.The traditional pilot power control pilot decontamination method ignores the interference from the second-circle cells outside the central cell.It is not realistic,and there is still a large pilot contamination.In order to solve the pilot contamination problem between cells in multi-cell Massive MIMO system with 19 cells,and to improve the time slot resource utilization of the system as much as possible,this paper adopts two solutions.The main contents are as follows:1.In order to facilitate the processing of the received signal by the receiving end of the base station,a joint method based on time division multiplexing pilot power control and orthogonal identification is adopted.The pilot slot is segmented by pilot power control so that the pilot transmission slots of each cell are staggered to divide all cells into different types of cells,and the same type of cells are further grouped by orthogonal identifiers,and corresponding orthogonal identifiers are added to the head of time division multiplexed pilot sequence.Thereby enabling the base station to distinguish between time division multiplexing pilots transmitted by users of different groups of cells.The simulation results show that the joint method based on time division multiplexing pilot power control and orthogonal identification can effectively eliminate the pilot contamination between multiple cells and facilitate the processing of the received pilot signals by the base station.2.The time division multiplexing pilot in the above system needs to sacrifice valuable time slot resources,and the time slot resource utilization rate is not high.In this paper the method of mitigating pilot contamination by using superimposed pilots is studied.A joint control method of superimposed pilot time slots and power based on iterative channel estimation is adopted.The pilots are divided into different time slots periodically superimposed on the data and the power of the control transmission signal is mainly studied at the superimposed pilot angle.By iterative channel estimation method of superimposed pilots,the interference of the transmitted data to the pilot is reduced,thereby obtaining more accurate channel estimation information.The simulation results show that the proposed superimposed pilot joint conrtrol method improves the channel estimation accuracy compared with the traditional first-order statistical property method,and is close to the time division multiplexing pilot joint method used in this paper.The method effectively reduces pilot contamination between multiple cells.The superimposed pilot joint control method used in this paper improves the time slot resource utilization compared with the time division multiplexing pilot joint method used in this paper.And the method is suitable for shorter coherence time or a larger number of users means that the required pilot is longer.