This invention presents methods for calibrating the channel state information in a multi-antenna system, especially for estimating channels transmitting in one direction using signaling transmitted in the other direction. The methods comprise transmitting a first and a second reference signal wherein one is a function of the other, and using the estimate of the channel in one direction, and the received signals from the two reference signals to obtain an estimate of the channel in the other direction between a transmitter of the multi-antenna system to an external wireless device.