  Sparsely spread code division multiple access (SCDMA) is a non-orthogonal
superposition coding scheme that permits a base station simultaneously
communicates with multiple users over a common channel. The detection
performance of an SCDMA system is mainly determined by its signature matrix,
which should be sparse to guarantee large Euclidean distance for the equivalent
signal constellation after spreading and superposition. Good signature matrices
that perform well under both belief prorogation and the maximum likelihood
detections are designed. The proposed design applies to several similar
well-documented schemes, including trellis code multiple access (TCMA), low
density spreading, and superposition modulation systems.
