Sparse code multiple access (SCMA) is a promising technique for the enabling of massive connectivity in future machine-type communication networks, but it suffers from a limited diversity order which is a bottleneck for significant improvement of error performance. This paper aims for enhancing the signal space diversity of sparse code multiple access (SCMA) by introducing quadrature component delay to the transmitted codeword of a downlink SCMA system in Rayleigh fading channels. Such a system is called SSD-SCMA throughout this work. By looking into the average mutual information (AMI) and the pairwise error probability (PEP) of the proposed SSD-SCMA, we develop novel codebooks by maximizing the derived AMI lower bound and a modified minimum product distance (MMPD), respectively. The intrinsic asymptotic relationship between the AMI lower bound and proposed MMPD based codebook designs is revealed. Numerical results show significant error performance improvement in the both uncoded and coded SSD-SCMA systems.

翻译：稀疏码多址接入（SCMA）是实现未来机器型通信网络中海量连接的一种有前景的技术，但受限于有限的分集阶数，这成为显著提升误差性能的瓶颈。本文旨在通过在瑞利衰落信道中的下行SCMA系统中，向传输码字引入正交分量延迟来增强稀疏码多址接入（SCMA）的信号空间分集。该系统在全文中被称为SSD-SCMA。通过分析所提SSD-SCMA的平均互信息（AMI）和成对差错概率（PEP），我们分别基于最大化导出的AMI下界和修正最小乘积距离（MMPD）开发了新颖的码本设计。揭示了AMI下界与所提基于MMPD的码本设计之间的内在渐近关系。数值结果表明，在未编码和编码的SSD-SCMA系统中，误差性能均得到显著提升。