This paper addresses the problem of joint space-time parameter estimation for multicarrier code-division multiple-access (MC-CDMA) systems in the presence of multipath propagation. We propose a novel preprocessing technique termed as subcarrier smoothing (SCS) to decorrelate the coherency of the multipath signals. The basic idea behind the SCS is to restore the rank of the signal correlation matrix by averaging the space-time covariance matrices associated with each subcarrier. Unlike the other existing preprocessing techniques, such as spatial smoothing, the SCS preprocessing is not limited to any specific array geometry and does not decrease the array aperture.