Research On Non-orthogonal Multiple Access Schemes Via Code Domain Multiplexing

Nowadays, as the 4G mobile communication system has been wide-ly used in the worldwide scale, mobile Internet business has been grow-ing rapidly. The demand for spectrum efficiency of the communication system in the future will be greatly improved due to the larger amount of network nodes and intelligent terminals, more various business of mobile Internet, as well as a wider range of mobile services. However, the con-ventional scheme of multiple access, orthogonal multiple access (OMA),has been faced with the performance bottleneck and it is difficult to achieve a significantly higher capacity in the future. Therefore,non-orthogonal multiple access (NOMA) technology has become a promising technology in 5G mobile communication system.The contribution of this paper mainly contains three parts, the non-orthogonal multiple access schemes, an iterative multiuser detection receiver based on the message-passing algorithm (MPA), and the analysis of iterative detection performance using EXIT charts.Firstly, two schemes of NOMA are mainly studied in this paper, the spatially coupling multiple access technique and pattern division multiple access (PDMA) technique in the code domain. In spatially coupling mul-tiple access, the data symbols from multiple users are interleaved and su-perimposed by the spatially coupling structure, which is initially used to construct the spatially coupling LDPC code, to generate the transmitted signal. Similarly, PDMA technique via code domain multiplexing uses smaller matrix structure to superimpose the code streams from different users to the same time-frequency resources, and the structure can be op-timized based on the channel coding theory.Secondly, the serial interference cancellation (SIC) algorithm was once used as one of the conventional multi-user detection algorithm.However, the SIC algorithm cannot achieve its ideal performance since error propagation is inevitable in the real system. The receiver based on the message passing algorithm (MPA) is studied in this paper. Unlike SIC algorithm, it can detect the data from different users at the same time, and carry on the exchange of the soft information during the iterative detec-tion process. Based on this, the channel encoding is introduced, and the better performance of multi-user detection can be achieved by iterations between the detector and decoders through the exchanges of the extrinsic message. In addition, a joint multiuser detection and decoding algorithm is proposed in this paper.Finally, the EXIT chart is introduced, which was first used to analyze the performance of a soft-input soft-output iterative decoder. In this paper,the detector and decoder are considered as independent components while using the EXIT curves to analyze the performance of multi-user detection for NOMA system. Numerical results show that lower node degree of the multiple access structure, more iterations inside the detector and LDPC decoder, and an appropriate encoding/decoding scheme jointly contribute to a better performance of the multiuser transmission system.