Codebook Research And Design Of Nonorthogonal Sparse Code Division Multiple Access Technology

The 5th Generation Mobile Communication System(5G)technology puts forward requirements of high spectrum efficiency,massive connectivity,and Ultra-Reliable and Low Latency(URLLC).As one of the most popular Non-Orthogonal Multiple Access(NOMA)technology in 5G,Sparse Code Division Multiple Access(SCMA)technology can meet the above challenging requirements.As an enhanced version of Low-Density Signature Multiple Access(LDS)technology,SCMA technology combines mapping and spreading at the transmitter,so that the input bit stream can be directly mapped into multi-dimensional complex domain codewords.Because the multi-dimensional codewords in the SCMA codebook have sparse characteristics,the low complexity Message Passing Algorithm(MPA)can be used for detective technology at the receiver,and the shaping gain brought by the SCMA multidimensional codewords is also one of the factors that improve the Bit Error Rate(BER)performance.An efficient codebook can improve the BER performance of the system without sacrificing limited spectrum resources,and can also speed up the convergence speed of the detection algorithm.Therefore,this paper analyzes the SCMA uplink and downlink systems,and summarizes the key factors for designing codebook under different SCMA system parameters and channels,so as to design efficient codebook in a targeted manner.The main research contents are as follows:1.Aiming at the problem that the design process of multiple-dimensional complex constellation in traditional codebook design is complicated and the properties of resource block constellation is ignored,an improved SCMA codebook design scheme is proposed for SCMA uplink system.This scheme is designed from the perspective of optimizing the resource block mother constellation.Firstly,the minimum Euclidean distance between the constellation points of the user constellation superimposed on the same resource block is increased by adjusting the parameters of the two-dimensional lattice constellation.Secondly,the signature matrix is optimized.Finally,the signature matrix and the user constellation are combined to generate the corresponding user codebook set.Simulation results show that the optimized codebook which is compared with the existing codebook design methods not only shows better BER performance under the small-scale codebook,but also shows better BER performance under the large-scale codebook under the Gaussian channel,and moreover,the codebook can gradually improve the BER performance of the system as the codebook dimension increases.2.A new multi-stage optimization approach for codebook design is proposed for the SCMA uplink system.The proposed scheme includes two parts: i)multi-dimensional mother constellation design ii)mapping matrix design.Firstly,the multi-dimensional complex mother constellation design is implemented by a series of two-dimensional real constellations which can be obtained by the lattice theory and the Symbol Switch Algorithm(SSA).The Secondly,we construct user-specific mapping matrix by optimizing the phase rotation of the user constellation to effectively reduce the interference between the multi-user codewords superimposed on every single resource block.Finally,the multi-user codebooks are generated by combining the multidimensional mother constellation and the user-specific mapping matrix.The simulation results demonstrate that the proposed codebooks can greatly decrease BER value compared to the existing codebooks under different channels,different codebook sizes and different overloading ratios,and the performance is also improved for the large-scale SCMA codebook.More importantly,in order to verify the proposed codebook applicability,two different MPA are used to simulate the SCMA system with different codebook sizes,both of which obtain faster convergence speed and better BER performance than other existing codebooks.