Enhancing Signal Space Diversity for SCMA Over Rayleigh Fading Channels

TL;DR

This paper proposes a novel SSD-SCMA system that enhances signal space diversity by introducing quadrature component delay, leading to significant error performance improvements in Rayleigh fading channels.

Contribution

It introduces a new SSD-SCMA system with optimized codebooks based on mutual information and product distance, improving diversity and error performance.

Findings

Significant error performance improvement in uncoded SSD-SCMA.

Enhanced diversity order through quadrature component delay.

Optimized codebooks based on mutual information and product distance.

Abstract

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.