Cutoff-rate in coded synchronous and asynchronous multiple-access

Abstract This paper deals with the actual impact of the multiple-access interference (MAI) in the multi-user detection problem for coded-modulations, with applications to the multiple-access scheduling and the optimal multi-user detection problem. The paper analyzes the effect of the cross-correlation between the users ' signatures and the MAI when optimal Maximum Likelihood (ML) multi-user detection schemes are used in coded-modulations. The paper shows that the presence of MAI is not critical for most operating conditions, resulting in a relaxation of the conventional (almost-) orthogonal signature designs commonly adopted when single or multi-user linear detectors are considered. The analy-sis is based on the identication of the cutoff-rate associated to the synchronous and the asynchronous multiple access of DS-CDMA schemes. The study of the asynchronous access is of particular in-terest in the sense that it allows the extension of the analysis for the most relevant case in which the cutoff-rate is computed for fre-quency selective channels. These results are of interest and appli-cable for almost stationary or slowly time-varying operating condi-tions. I