Signature Design of Sparsely Spread CDMA Based on Superposed   Constellation Distance Analysis
  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. 
Guanghui Song, Xianbin Wang, and Jun Cheng
cs.IT math.IT