Abstract: A base transceiver station comprising a first part and a second part, said first part comprising a plurality of antennas and said second part being arranged to provide signals for transmission by said plurality of antennas, said first and second parts being connected by feeder means, said second part being arranged to provide a plurality of calibration signals to said first part, said first part comprising summing means for summing the calibration signals received at the first part and means for providing the summed calibration signals to said second part, said second part being arranged to process said summed calibration signals to determine correction values for application by said second part to signals for said plurality of antennas.