| At present,the 5th Generation(5G)mobile communication system has been widely deployed,and major countries and regions in the world have also successively launched research on the 6th Generation(6G)mobile communication system.From 1th Generation(1G)to 5G mobile communication systems are based on cellular cells.However,with the increase in the number of user accesses,traditional cellular networks inevitably have problems such as inter-cell interference and frequent handovers.Therefore,cell-free massive Multiple-Input Multiple-Output(MIMO),as a potential key technology of 6G,has received extensive attention from researchers.It provides services for multiple users on the same time-frequency resources by deploying a large number of distributed Access Points(APs).Compared with traditional cellular systems,cell-free massive MIMO can provide higher spectral efficiency and energy efficiency.In addition,Non-Orthogonal Multiple Access(NOMA)technology is considered to be a way to increase the system capacity when a large number of devices are accessed,which overcomes the limitation of the number of user accesses due to limited wireless resources in traditional multiple access schemes.The combination of these two technologies is expected to improve the spectrum efficiency and energy efficiency of the system while ensuring large-scale access,and meet the development needs of future communication systems.Based on above,the main research of this paper is as follows.First,the distance between the users and the access points in the cell-free massive MIMO varies,and the access points farther away from the target user cannot provide good service to the user,while the backhaul link capacity is limited,assuming that all access points serve all users in a fully connected mode will increase the backhaul link load and power consumption,reducing system performance,which also exists in the cell-free massive MIMO-NOMA system.In order to increase system throughput and reduce the capacity demand on the backhaul link,Quantum Bacterial Foraging Optimization(QBFO)based access point selection scheme is proposed,which encodes the connection relationship between APs and users in the form of qubits.The adaptive quantum rotation gate is used to simulate the chemotaxis of bacteria.By measuring the quantum bacterial population,the selection solution set of APs and the users is obtained,and the dispersal operation is introduced to avoid the algorithm from falling into local optimum.Numerical results show that the proposed scheme can significantly increase the downlink average rate of users while relieving the backhaul burden.Compared with the schemes based on received power and channel estimation mean square error,the proposed scheme has better performance in reducing inter-user interference and improving the total throughput of the system.Second,user clustering is a key design challenge in applying NOMA to MIMO systems.Powerdomain NOMA is able to serve multiple users in the same time-frequency resources by exploiting the channel conditions of the users and supposing the data signals of those users in the power domain.In order to obtain higher spectral efficiency(SE),it is crucial to combine users with different channel conditions to perform NOMA together.Based on this,this paper proposes a new user clustering algorithm based on Jaccard coefficients,and a sum-SE maximization algorithm using Sequential Convex Approximation(SCA)is designed,taking into account spectral efficiency and user fairness,subject to per-user spectral efficiency constraint,access point power constraint and Successive Interference Cancellation(SIC)constraint,the proposed power optimization algorithm maximizes the system sum rate while ensuring a lower bound on the spectral efficiency of users in the system.The numerical results show that the designed sum-SE maximization algorithm has a fast convergence rate,and the proposed scheme can significantly improve the sum-SE of the system compared with the Full Power Control(FPC)scheme. |
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