1. Apparatus comprising a receiver for receiving signals via channels defined by a channel transfer matrix H and a transmitter, wherein: the receiver comprises: a channel estimator adapted to generate first and second channel estimation information; a correlator estimator adapted to correlate the first and second channel estimation information with each other and, in response to the correlation, to generate a correlation factor; and a transmitting feedback unit adapted to estimate eigenparameters of a covariance matrix HH H in dependence on the correlation factor, and to generate feedback information in response to the estimation; and the transmitter comprises: a receiving feedback unit adapted to receive the feedback information for adjusting the transmitter.