Research On Codebook Design In Sparse Code Multiple Access Systems

The Massive Machine-Type Communications(mMTC)scenario in future fifth 5th Generation mobile communication system(5G)require a large number of devices to be simultaneously accessed.But the lack of spectrum resources makes the traditional Orthogonal Multiple Access(OMA)technology powerless in this scenario.As a result,the non-orthogonal multiple access(NOMA)is considered to be one of the key technologies in mMTC scenario.Sparse Code Multiple Access(SCMA)proposed by Huawei of China is one of competitive one among many NOMA technologies.In it,the users' input bits are mapped into multidimensional codewords of predefined codebooks directly.At the receiver,the factor graph based message passing algorithm(MPA)is exploited for multiuser detection,which can obtain approximate maximum posterior probability(MAP)detection performance with greatly reduced decoding complexity.In SCMA systems,a well designed SCMA codebook can improve the performance of the system without sacrificing the physical resources.Therefore,the theise focused on the study of the codebook design in SCMA system.The main contributions can be concluded as follow.1.SCMA codebook design based on power diversity among mother costellation points.Firstly,under the constraint of minimum Euclidean distance and initial phase,The amplitude phase shift keying(APSK)constellation of four-layer circumferential structure with minimum energy is designed.Then the mother constellation is designed based on the power divergence among each circle of APSK constellation,which can induce power divergence not only among the points in different dimensions,but also among the points in each dimension.After that,the phases of the constellation in the four linear subsets of the designed APSK constellation are taken as the rotation operators to design operators matrices by combining with the permutation sets.At last,SCMA codebooks are designed by combining mother constellation with operators matrices and mapping matrices.Simulation results show that the performance of the proposed SCMA codebooks outperforms that of the traditional SCMA codebooks.2.SCMA codebook design by combining constellation rotation and Q coordinate-interleaving Firstly,by rotating the base constellation and the mother constellation,the minimum Euclidean distance between the projection points of the mother constellation on each dimension,and between the constellation points on the constellations corresponding to each user in the total constellation on a single resource block is maximized in order to improve the performance of the SCMA codebooks over Gaussian channels.On the basis of it,by rotating the constellation of multiple users superimposed on each resource block,the corresponding minimum product distance and the signal space diversity(SSD)order of the users' constellations are optimized.At last,an additional diversity gain is achieved by using Q-coordinate interleaving technology to further improve the performance over the Rayleigh fading channels.Simulation results show that the performance of the proposed SCMA codebooks outperforms that of the HUAWEI' SCMA codebooks and Low Density Signature(LDS)codebooks.