SCMA Codebook Design

TL;DR

This paper introduces a systematic lattice-based design approach for SCMA codebooks, demonstrating performance improvements over LDS and OFDMA through simulations.

Contribution

A novel systematic method for designing SCMA codebooks based on lattice constellation principles is proposed, enhancing performance and maintaining low complexity.

Findings

SCMA outperforms LDS and OFDMA in simulations

Lattice-based codebook design improves shaping gain

SCMA maintains low complexity due to sparsity

Abstract

Multicarrier CDMA is a multiple access scheme in which modulated QAM symbols are spread over OFDMA tones by using a generally complex spreading sequence. Effectively, a QAM symbol is repeated over multiple tones. Low density signature (LDS) is a version of CDMA with low density spreading sequences allowing us to take advantage of a near optimal message passing algorithm (MPA) receiver with practically feasible complexity. Sparse code multiple access (SCMA) is a multi-dimensional codebook-based non-orthogonal spreading technique. In SCMA, the procedure of bit to QAM symbol mapping and spreading are combined together and incoming bits are directly mapped to multi-dimensional codewords of SCMA codebook sets. Each layer has its dedicated codebook. Shaping gain of a multi-dimensional constellation is one of the main sources of the performance improvement in comparison to the simple repetition of QAM symbols in LDS. Meanwhile, like LDS, SCMA enjoys the low complexity reception techniques due to the sparsity of SCMA codewords. In this paper a systematic approach is proposed to design SCMA codebooks mainly based on the design principles of lattice constellations. Simulation results are presented to show the performance gain of SCMA compared to LDS and OFDMA.