  Sparse code multiple access (SCMA) is a promising multiuser communication
technique for the enabling of future massive machine-type networks. Unlike
existing codebook design schemes assuming uniform power allocation, we present
a novel class of SCMA codebooks which display power imbalance among different
users for downlink transmission. Based on the Star-QAM mother constellation
structure and with the aid of genetic algorithm, we optimize the minimum
Euclidean distance (MED) and the minimum product distance (MPD) of the proposed
codebooks. Numerical simulation results show that our proposed codebooks lead
to significantly improved error rate performances over Gaussian channels and
Rayleigh fading channels.
