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A top-down SCMA codebook design scheme based on lattice theory

Recent work on multiple access has shown sparse code multiple access (SCMA) is a promising scheme dealing with massive connection. In SCMA systems, symbol spreading is combined with symbol mapping. The coded bits are directly mapped to the spread symbols, called codewords, according to SCMA codebook. Several codewords are superposed on the same resource. Since codebook is essential to SCMA system performance, this paper aims to present a novel top-down codebook design scheme for SCMA. Firstly, a geometrically uniform top-layer mother constellation with good energy efficiency is proposed based on lattice theory. After dividing the top layer constellation into several bottom layer constellations, we use matrix operation to optimize their distance spectrums. Then a mapping principle between users and resource is proposed to construct the final codewords. Simulation results shows that this top-down codebook outperforms existing schemes in bit error ratio (BER) while sharply reduce the peak to average power ratio (PAPR).