1. Apparatus comprising a receiver for receiving signals via channels defined by a channel transfer matrix H and comprising a transmitter which receiver comprises: a channel estimator for generating first and second channel estimation information; a correlator estimator for correlating the first and second channel estimation information with each other and for, in response to a correlation, generating a correlation factor; and a transmitting feedback unit for, in dependence of the correlation factor, estimating eigenparameters of a covariance matrix HH H and for, in response to an estimation, generating feedback information; and which transmitter comprises: a receiving feedback unit for receiving the feedback information for adjusting the transmitter.