Spectral efficiency of the feedback receiver for two sets of orthogonal sequences

We present the spectral efficiency analysis of the recently proposed feedback receiver for two sets of orthogonal sequences (TSOS) scheme which is capable of supporting higher number of users than the spreading gain of the DS/CDMA system. The added complexity is minimal compared to the conventional matched filter. We show that in the case of coded system, spectral efficiency of this scheme is close to that of a Gaussian multiple access channel if the incomplete group of orthogonal sequences is detected first. We also discuss the choice of orthogonal sequences that maximizes the spectral efficiency. This analysis is extended for the case of flat fading channels.