Research On Pilot Allocation For Massive MIMO Systems With Grouping And Game

As a new type of MIMO technology,massive multiple input multiple output(MIMO)is recognized as one of the key technologies of 5G.However,its many excellent performances depend on the accuracy of channel state information(CSI).Existing research shows that the channel can be estimated with the help of pilots to obtain CSI.Under the trend of interconnection of all things,the limited number of orthogonal pilots conflicts with the exponentially increasing number of users in the access network.It is necessary for users in different cells to reuse the same pilot sequences,so that the base stations are unable to distinguish these received pilot signals,further forming the so-called "pilot contamination".At present,reasonable pilot allocation is an effective scheme to reduce pilot contamination.However,in most existing pilot allocation schemes,whether for one-tier or multi-tier networks,it is assumed that the number of users in each cell is fixed and equal or the cell shape is regular(such as regular hexagons,semicircles,etc.),but the future massive MIMO must be a cell-intensive network(or multi-tier heterogeneous networks,ultra-dense heterogeneous networks,etc.),the cell shape is irregular and the number of users is random,so the existing pilot allocation schemes are not applicable in this scenario,that is,the existing schemes lack some flexibility.Based on the previous studies and combining game theory thoughts,this thesis presents three pilot allocation schemes from different perspectives,as follows:1.To be applied in the random distribution of users and obtain the overall optimal solution in the TDD massive MIMO system,the third chapter of this thesis gives a joint cell grouping and coalition game(JCG-CG)pilot allocation scheme.The scheme is mainly divided into two steps: cell grouping and CG among users in the same cell-group.In the CG among users in the same cell-group,we give the definition of the coalition structure,the adjustment principles,and the final stability conditions,analyze the utility function i.e.,the average mean square error(MSE)of channel estimation,and give the coalition formation algorithm to mitigate pilot contamination.Simulation results show that the proposed JCG-CG scheme is superior to joint cell grouping and fixed coalition(JCG-FC)and joint cell grouping and random coalition(JCG-RC).2.In order to reduce the interference between cells and ensure the communication quality and satisfaction of cell-edge users for the TDD massive MIMO system,the fourth chapter of this thesis gives a joint user grouping and coalition game(JUG-CG)pilot allocation scheme.This scheme is not only suitable for random distribution of users,but also can obtain the overall optimal solution through searching.The specific steps of the scheme are as follows: First,according to the user signal strength,the users are divided into two groups,namely A and B.Users with weak strength of the received base stations signals are recorded as group A,and remaining users are group B.The users of group A use mutually orthogonal pilots,and the users of group B share the remaining pilots.Then,in the coalition game for the users of group B,users are divided into several disjoint user sub-coalitions,users belonging to different sub-coalitions are allocated different orthogonal pilot sequences,and users in the same sub-coalition reuse the same pilot sequence.Finally,the corresponding pilot allocation algorithm of JUG-CG is given to reduce pilot contamination.Simulation results demonstrate that the JUG-CG scheme is superior to random pilot allocation(RPA)and greedy algorithm(GA).3.In order to guarantee the communication quality of the edge users in pico-cell and improve the overall performance of system for the TDD massive MIMO three-tier heterogeneous networks,based on the third and fourth chapters,the fifth chapter of this thesis gives the JUG-CG pilot allocation scheme.This scheme is suitable for the random distribution of users and the irregular shape of cells(i.e.,the random distribution of base stations).Specifically,it contains two steps: user group and coalition game among users in the same user group.First,according to the formula(5.2),users in pico-cell and suffered from strong interference are recorded as group B1,and remaining users of heterogeneous network are group B2.Then,the users of group B1 use mutually orthogonal pilots,and the users of group B2 share the remaining pilots.In the coalition game for the users of group B2,we deduce and prove the utility function expression of each user(the MSE of signal detection and the user's spectral efficiency),and give the pilot allocation algorithm of JUG-CG under three-layer heterogeneous networks.Simulation results demonstrate that the JUG-CG scheme is superior to RPA,GA,and coalition game(CG)under three-layer heterogeneous networks.