Research On Multiuser Detection Algorithm For SCMA Based On Improvement

Sparse Code Multiple Access(SCMA),as a typical representative of air interface technologies for the fifth generation mobile communication(5G),is a non orthogonal multiple access technology and has many advantages.First of all,compared with OFDMA,the overall capacity of the system has been effectively improved because of the ability to serve more users at the same time in the same amount of resources.On top of that,message passing algorithm(MPA)could be used as a decoding method for the SCMA codewords have a sparse characteristic,while the performance of the decoding is approximated by maximum posterior probability(MAP),but the complexity is lower.Last but not least,its shaping gain and spectral efficiency have been improved,owing to the use of multi-dimensional codewords.Therefore,SCMA technology has been widely concerned in the industry since it was put forward,and is a very competitive multiple access technology.However,the complexity of the decoding based on the message passing algorithm for SCMA system is still high.Consequently,the thesis mainly concentrates on the multiuser detection method for reducing the complexity of the decoding.Firstly,the past and present of SCMA technology are introduced,starting with CDMA technology,then to Low Density Signature(LDS)technology,and finally to SCMA technology.The structure of LDS,the coding principle of SCMA,the design of multiplexing and codebook are elaborated in detail.Secondly,the principle of message passing algorithm and its application in SCMA system are studied minutely.In the first place,MAP is proved to be the best decoding method.Then the factor graph theory,the expression tree and the message passing rules used in the message passing algorithm are explained detailedly.The SCMA decoding process based on MPA is given,as well as the simulation results in the end.Lastly,a SCMA decoding algorithm based on sphere decoding improvement is proposed.To begin with,the principle of spherical decoding is presented.And then the proposed multiuser detection scheme for SCMA with partial extrinsic information transmission is lucubrated at the same time its improved algorithm by sphere decoding is proposed,by which the complexity is lower.On the function nodes,for one thing,messages whose probability density functions are greater than one threshold are updated,and for another thing,the believable section in the messages are updated.By this way,numbers of external messages decrease again.Simulation results show that the new improved scheme,when the threshold is smaller,can reach lower complexity,compared with conventional MPA and the former algorithm,meanwhile maintain negligible bit error ration performance and convergence rate.