Research On SCMA Codebook Design

The increasing demand of mobile Internet and the Internet of Things poses challenging requirements for 5G wireless communications,such as high spectral efficiency and massive connectivity.A promising technology,non-orthogonal multiple access(NOMA),which accommodates more users through non-orthogonal resource allocation,is an efficient solution to address some of these challenges for 5G.As a kind of code domain non-orthogonal multiple access technology,Sparse Code Multiple Access(SCMA)stands out as a research hotspot with good compatibility and higher spectral efficiency.In SCMA system,codebook design is the key technology that affects the system performance.Aiming at the complexity of SCMA codebook design,this paper first proposes a suboptimal low complexity codebook design method,which reduces the bit error rate(BER)of the system.The scheme decomposes the multi-user codebook optimization problem into several sub-problems according to the factor graph matrix,and optimizes each sub-problem by maximizing minimum Euclidean distance between superimposed components of the codewords under the constraint of keeping the average power of each codeword does not exceed a constant.By solving the sub-problems,components of the codewords can be obtained,and finally the complete codebooks of all the users can be obtained by combining these codeword components according to the factor graph matrix.This codebook design method could dramatically reduce computational complexity while incurring little loss in performance compared with the existing codebook design method.Furthermore,considering the successful application of spherical code in multi-dimensional modulation system,this paper uses spherical Muller code to construct the SCMA multi-user's mother constellation.Firstly,based on the required dimension of the mother constellation,the corresponding spherical Mueller codes are constructed,and then the corresponding codewords are selected from the spherical Mueller code by maximizing the difference between different codewords.Then we build the mother constellation by making combinations of the selected codes and performing average power normalized.Finally,different layer operators are designed to get the multi users' codebooks.The SCMA codebook constructed by this method has good bit error rate performance.