An optimized code book design and assignment based on 32-QAM constellation in downlink SCMA systems

Abstract

Sparse Code Multiple Access (SCMA) is a multi-dimensional codebook based on a class of Non-Orthogonal Multiple Access (NOMA) to provide many users through non-orthogonal resource elements without detection complexity in 5G wireless communications. The codebook design is one of the main criteria in SCMA downlink systems. This paper proposes an efficient SCMA joint codebook design and assignment model to reduce the detection complexity and maximize the Minimum Euclidean Distance (MED) in Rayleigh fading channels. Initially, we express codebook assignment, which is allotted to group the highest priority subcarriers and then assign to the specific user using k-medoid clustering algorithm and codebook design (i.e., sparse mapping matrix and factor graph constellation design). The obtained mapping matrix with a factor graph uses four ring 32-Quadrature Amplitude Modulation (QAM) features of SCMA that carefully design the mother constellation points for each codebook. In particular, the 32-QAM mother constellation points are obtained for each codebook by phase rotation. In this manner, the codebook is designed without any detection complexity. Further, the Message Passing Algorithm (MPA) is used to reduce the multi-user interference in the receiver side. The experimental results depicts that the proposed SCMA codebook approach achieves low Bit Error Rate (BER) and High Spectral Efficiency (SE) performance comparing with existing known designs in Rayleigh channels.