SCMA Codebook Design Based On Layered Space-Time Structure

Sparse code multiple access(SCMA)is a competitive non-orthogonal multiple access for future communication systems,supporting the development requirements of large connections,low latency and high efficiency.The transmitter adopts multi-dimensional codeword modulation technology to bring the shaping gain of the constellation,and the receiver uses the codeword sparsity to achieve low complexity detection.Space time block coding(STBC)technology can effectively against fading and improve spectral efficiency through time and antenna diversity.In this thesis,SCMA codebook are designed based on layered space-time(LST)structure,which can comprehensively utilize the diversity characteristics of the two technologies to improve the system BER performance over fading channels.Firstly,this thesis studies the SCMA uplink system,and proposes the SCMA-STBC system combined with SCMA and STBC system.Through the analysis of the system Tanner graph,this thesis gives the uplink joint space-time message passing algorithm(MPA).Then,aiming at the high complexity for SCMA-STBC receiver,the ulink precoding SCMA-LST codebook design scheme based on the channel state information(CSI)is described.Then this thesis proposes the corresponding uplink linear combination MPA detection algorithm according to the characteristics of the codebook.In addition,this thesis derives the theoretical BER lower bound of the precoding codebook scheme.Next,this thesis investigates the downlink SCMA-STBC system based on the uplink system,and the downlink linear combination MPA detection algorithm is proposed.In order to further improve the BER performance of the system,this thesis introduces the downlink SCMA-LST codebook design scheme to further improve system performance over the same space-time resource structure.The codebook design scheme takes the coded modulation capacity(CMC)as the objective function for optimization.The CSI based codebook design process is mainly divided into two parts: the selection of space time resource and the design of constellation points.Then this thesis describes the corresponding resource selection algorithm and constellation design scheme in details to complete the codebook design.Finally,in order to optimize the codebook structure and further improve the system BER performance,this thesis proposes a SCMA downlink system codebook design scheme based on the cycle layered space-time(CLST)structure.According to the system model and the signal characteristics of the transmitter and receiver,this thesis introduces two codebook design structures: multi-weight CLST and multi-matrix CLST.And then the thesis describes the corresponding multi-weight selection algorithms and multi-matrix selection algorithms respectively.Moreover,the interference suppression MPA detection algorithm is given for multi-matrix structure.In addition,this thesis derives the theoretical analysis of CMC and the complexity of different structure's algorithms.