Research On Retransmission And Multi-Detection Technology In Grant-free NOMA System

With the development of science and technology,the mobile Internet and Internet of Things business has grown explosively.Non-orthogonal multiple access(NOMA) has become one of the key technologies for future 5G wireless communications,which can improve spectrum efficiency and support massive connectivity.NOMA achieve large amount of accesses through grant-free transmission and achieve the separation of superimposed user signals by using advanced multi-user detection technology at the receiver.However,the grant-free design allows users to freely transmit data and that increases the probability of user collisions,which is a severe challenge to system reliability.At present,the research on grant-free multi-user detection is performed on the premise of known user activity information in order to reduce the complexity of the receiver,which is impractical.This thesis focuses on the above issues,studying the retransmission scheme and multi-user detection algorithm in grant-free NOMA system.The main work is as follows:Firstly,the basic principle of the grant-free NOMA system is described,and the problems of retransmission and delay in the grant-free transmission are introduced.Analyzing the key technologies in NOMA by starting from three candidate multiple access technologies and analyzing the multi-user detection algorithm of the receiver by comparing three different types of multi-user detection receivers.Secondly,this thesis studies the retransmission problem of grant-free transmission,an adaptive repeated HARQ scheme is proposed.This scheme reduces the number of initial transmission resources on the basis of traditional HARQ,thereby reducing the probability of multiple user data collisions.Increasing the number of resources of repetitons when subsequent retransmission occurs,and reducing the redundancy brought about by the soft combining information so that making the system performance achieves a better compromise between reliability and resource efficiency.The simulation results verify that compared with the traditional HARQ scheme,the adaptive repeated HARQ scheme not only obtains better BLER performance,but also improves system resource utilization efficiency.Finally,this thesis proposes a multi-user detection algorithm based on compressive sensing and message passing algorithm according to the characteristics of sparseness of active users in grant-free NOMA syetem.The algorithm utilizes Compressive Sampling Matching Pursuit(CoSaMP) to obtain the position information of active users,and then iteratively detects the data of these active users by using a message transfer algorithm to recover the original sparse signal.Through the CoSaMP algorithm,sparse and active user information can be accurately obtained,which greatly reduces the user base for message passing detection,thereby improving multi-user detection performance.The simulation results show that the proposed algorithm can improve the system performance gain under the premise of ensuring better robustness.