Research On Serial Coded High Order Sparse Code Multiple Access Technology

5G and satellite Internet of things systems connect everything through information processing and transmission technology.Due to the large number of users it supports,the competition for spectrum resources is fierce.Multiple access technology has an important impact on the access capability of the system.Sparse code multiple access technology can achieve overload transmission.However,with the increase of overload,the interference between codewords increases,the system error performance becomes worse,and the decoding complexity is very high.Therefore,reducing the decoding complexity of high-order systems is a problem worthy of further study on the premise of ensuring the BER performance of the system.The main contents of this paper are as follows:Firstly,the theoretical basis of SCMA system is studied,and the key technologies affecting the system performance,such as codebook design,coding process,optimal and suboptimal decoding algorithm,factor graph theory and so on,are analyzed.The simulation environment of SCMA uplink is built to lay the foundation for the later research.Secondly,this thesis makes a detailed study on the transmitter technology of serial coded SCMA system.The coding system is divided into P-Level subsystems,which are composed of SCMA coding subsystem and mode matrix subsystem respectively.Each subsystem processing user information serially to achieve the overall overload factor.Based on the above analysis,an improved codebook design method for a SCMA subsystem is proposed.Firstly,the mother constellation is generated based on the star QAM constellation,and then the factor graph matrix is generated by using the Latin generating matrix which can improve the system performance.For the design of pattern matrix module,Sum Squared Correlation is selected as the measurement criterion to ensure that each pattern vector has as much diversity as possible.Theoretical analysis and simulation results show that the SCMA system based on the above idea introduces more differences between user information,increases the overload factor of the system,and helps to reduce the decoding complexity of the decoder.Finally,in order to solve the problem of high computational complexity at the receiver of SCMA system,a low complexity packet detection algorithm based on dynamic pruning(DPG-MPA)is proposed.In this algorithm,the users are grouped,and the user nodes in the group use serial message passing mode.According to the node convergence degree of user information,the priority of user information update is determined,so that the most useful message for system convergence can be delivered first.After a predetermined number of iterations,the residual valued of the user node is calculated,the node whose residual value is lower than the threshold value is marked as a trusted node,and the update calculation of the information value on the trusted node is stopped in the subsequent iterations.The algorithm can speed up the convergence speed and reduce the redundant calculation on the premise of ensuring the excellent bit error performance of the system.The simulation results show that the decoding complexity of serial coded SCMA system based on DPG-MPA is effectively reduced while maintaining good BER performance.