A novel codeword grouped SCMA

Abstract

In the future 5G, the fast-growing requirements of massive connectivity, higher data-rate transmissions and multiple services should be matched with new multiple access technology. Non-orthogonal multiple access, nowadays, is considered as an effective method to solve those problems. Some non-orthogonal multiple access technologies, e.g. low density signature (LDS) and sparse code multiple access (SCMA), are proposed nowadays for substantial connectivity together with spectrum efficiency promotion. Instead of spreading data symbols over orthogonal or near orthogonal code sequences in CDMA, those non-orthogonal multiple access technologies take advantage of non-orthogonal code sequences. The LDS repeatedly spreads quadrature amplitude modulation (QAM) symbol according to the low density sequence, while the SCMA assigns different predefined codebooks to multiple layers. However, the SCMA codebooks made from multi-dimensional constellations are designed under the condition of same modulation order for each layer. The design scheme of SCMA codebook is regardless of the orthogonality among some certain layers. A novel codeword grouped SCMA (CG-SCMA) proposed in this paper allows diverse modulations and enhances the applicability of the non-orthogonal multiple access technology. Instead of mapping incoming bits to multi-dimensional codewords from predefined codebooks in SCMA, the procedure of symbols to complex codewords is divided into two steps: grouped indicator sequence spread and grouped symbol sequence rotation. The grouped symbol sequence rotation decreases the interference among layers and increases the signal space diversity, which is regarded as the main source of performance promotion. Simulation results are presented to show the significant performance gain of CG-SCMA comparing with SCMA and LDS.