Cutoff-rate in coded synchronous and asynchronous multiple-access

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 analysis is based on the identification 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 interest in the sense that it allows the extension of the analysis for the most relevant case in which the cutoff-rate is computed for frequency selective channels. These results are of interest and applicable for almost stationary or slowly time-varying operating conditions.