Combining techniques for MC-CDMA

This paper investigates combining techniques for multi-carrier CDMA (MC-CDMA) systems. Specifically, based on the maximum likelihood criteria, optimal combining techniques are derived and discussed for the following scenarios: (1) channel information is not available at the receiver; (2) a statistical characterization of the channel fades is available at the receiver, but the actual, instantaneous channel information is not; and (3) the instantaneous channel fade information is available at the receiver. The MLC proposed in this paper is optimal from the standpoint of minimizing the probability of error performance. It is shown that MLC is very close to MMSEC in a fully loaded system, justifying the selection of MMSEC over other combining schemes. When the load is small (as measured by number of users), MLC is shown to outperform MMSEC (and other combining schemes). Furthermore, maximum-likelihood combining techniques are also derived for MC-CDMA systems employing QAM modulations.