Compared to the traditional orthogonal multiple access, non-orthogonal multiple access (NOMA) technology can achieve higher spectrum efficiency and support more massive connectivity. In this paper, we conduct comprehensive study and comparison on current NOMA technologies that many mainstream companies have proposed for the fifth generation (5G) wireless communication standard. According to the characteristics of the NOMA schemes, we classify these schemes into four categories: scrambling-based NOMA, spreading-based NOMA, coding-based NOMA, and interleaving-based NOMA. We systematically summarize the transceiver block diagram of each category, and detail basic principles, key features, and transmission-reception algorithms of all NOMA schemes. Furthermore, the theoretical analysis based on average mutual information is given to evaluate the achievable sum-rate performance of the NOMA systems and their potential performance gains as compared with OMA. Comprehensive simulations are carried out for the block-error-rate performance evaluation of these NOMA schemes as well, which coincide with the theoretical analysis. By comparing the performance of these technologies, some promising schemes and directions are suggested for the future 5G NOMA development.

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The explosive growth of mobile multimedia services and applications are increasing the demand of access ability for the recent 5G networks. Non-orthogonal multiple access (NOMA) techniques have been recently proposed for 5G to improve access efficiency through allowing multiple users to share the same spectrum resources in a non-orthogonal way. Low Density Superposition Modulation (LDSM) is one of the NOMA techniques with potential to support high spectral efficiency and massive connectivity. In this paper, we utilize Bare-bone Particle Swarm Optimization (BBPSO) algorithm with extrinsic information transfer (EXIT) chart to design degree distribution of LDSM. An optimized design of LDSM scheme has better convergence performance compared with conventional sparse code multiple access (SCMA) and pattern division multiple access (PDMA) scheme over fading channel. Furthermore, the designed irregular LDSM can significantly improve the overall system performance and edge user performance under the nearfar effect scenario. Simulation results validate that the proposed optimized LDSM can achieve about 1.0 dB gains compared to both SCMA and PDMA schemes.