  Spatial modulation (SM)-sparse code multiple access (SCMA) systems provide
high spectral efficiency (SE) at the expense of using a high number of transmit
antennas. To overcome this drawback, this letter proposes a novel SM-SCM A
system operating in uplink transmission, referred to as rotational generalized
SM-SCMA (RGSM-SCMA). For the proposed system, the following are introduced: a)
transmitter design and its formulation, b) maximum likelihood and maximum a
posteriori probability decoders, and c) practical low-complexity message
passing algorithm and its complexity analysis. Simulation results and
complexity analysis show that the proposed RGSM-SCMA system delivers the same
SE with significant savings in the number of transmit antennas, at the expense
of close bit error rate and a negligible increase in the decoding complexity,
when compared with SM-SCMA.
