Performance Analysis Of Polar Code In The SCMA System

This paper mainly discusses the performance of Sparse Code Multiple Access(SCMA)system.The two significant factors that influence the performance of SCMA are codebook and multi-user detection algorithm.The thesis is divided into two aspects to discuss.Firstly,a mathematical model of SCMA is proposed.The issue of SCMA codebook design is evolved to design of a multi-dimensional constellation.From the perspective of main constellation and sub constellation,the codebook is designed based on the principle of maximizing the Minimum Euclidean Distance between point and point on the main constellation on the basis of the same mean energy.Then,the problem of codebook design is divided into two sub-problems,design of modulation scheme and construction of generator matrix.To improve the generator matrix,the principle to optimize phase rotation factor is proposed,and the method to construct Latin generator matrix is developed.The effectiveness of the structure of the codebook designed to maximize the Minimum Euclidean Distance between point and point on the main constellation is demonstrated.The codebook is excellent in performance and could be used in the AWGN channel.Then the influence of the multi-user detection algorithm on the performance of SCMA system is studied.The pros and cons of Maxmium Likelihood Algorithm and Message Passing Algorithm are analysed.Maxmium Likelihood Algorithm could achieve the best performance but have high computation complexity.Message Passing Algorithm is sub-optimal but is low in complexity.Also the effect of Message Passing Algorithm and Maxmium Likelihood Algorithm on the performance of SCMA system is analysed.On the premise of excellent codebook,the performance of Message Passing Algorithm in SCMA system is close to the performance of Maxmium Likelihood Algorithm,but still has the advantage of lower complexity.Finally,when it comes to the impact of error control codes on the performance of SCMA system,the thesis researched the performance of Polar Code in SCMA system.In general,employing polar codes as error control codes,the performance of the SCMA system is improved distinctly.The BER performance of the SCMA system based on polar codes as error control codes with different code lengths is analysed simulatively.As the code length increases,the BER performance of the SCMA system is promoted.However,when the code length achieves to some extent,the BER performance of the SCMA syatem is improved to a limited extent.At the same time,the complexity would increase exponentially.Hence,in practice,the code length should be decided by balancing the BER performance and the complexity.