1. A signal processing method for obtaining a diversity gain in a communication system equipping an antenna array when a background noise is a spatial white noise vector, comprising the steps of: a) computing a covariance matrix of an input signal vector; b) performing a eigen-division of the covariance matrix and estimating a time invariance vector forming a signal sub-space according to a matrix rank (r) of the covariance matrix by selecting at least two eigenvectors having comparatively larger eigenvalue; and c) estimating a channel vector by receiving the time invariance vector from the step b) and a channel coefficient computed by using a pre-known symbol.