A method for self-interference suppression in a multi-antenna device that is part of a communication channel between a transmitter (Tx) and a receiver (Rx) of a wireless communication system, the communication system using the multi-antenna device, the multi-antenna device being located between the transmitter and the receiver and using antennas (310, 311) and a relay unit for forwarding signals from the transmitter to the receiver, the communication channel having a transmit channel, defined by a transmit channel matrix ( H 1) between the transmitter and the multi-antenna device, a receive channel defined by a receive channel matrix ( H 2), between the multi-antenna device and the receiver and a self-interference channel between an output and an input side of the multi-antenna device, the self-interference channel being defined by a self-interference channel matrix ( H 0), the method comprises the steps: • equipping the multi-antenna device with at least four antennas (310, 311) of which at least two antennas are input antennas located at the input side facing the transmit channel and at least two antennas (310, 311) are output antennas located at the output side facing the receive channel and; characterized in that • suppressing the self-interference channel by using two filtering matrix arrangements, a receive filtering matrix arrangement being inserted between the input antennas and the relay unit and a transmit filtering matrix arrangement being inserted between the output antennas and the relay unit,, the receive filtering matrix arrangement being defined by a receive filtering matrix W r and the transmit filtering matrix arrangement being defined by a transmit filtering matrix W t, said filtering matrix arrangements cancelling at least one eigenmode of the self-interference channel, by choosing the columns of the filtering matrices to be orthogonal to said at least one eigenmode of the self-interference channel that is/are to be cancelled, the self-interference suppression by cancelling a certain eigenmode is made with the filtering matrix arrangements meeting a selection criterion for minimizing risk of loss of one or several dominating eigenmodes of the transmit channel and/or the receive channel, said selection criterion selecting which eigenmode to cancel using which filtering matrix. • equipping the multi-antenna device with at least four antennas (310, 311) of which at least two antennas are input antennas located at the input side facing the transmit channel and at least two antennas (310, 311) are output antennas located at the output side facing the receive channel and; characterized in that • suppressing the self-interference channel by using two filtering matrix arrangements, a receive filtering matrix arrangement being inserted between the input antennas and the relay unit and a transmit filtering matrix arrangement being inserted between the output antennas and the relay unit,, the receive filtering matrix arrangement being defined by a receive filtering matrix W r and the transmit filtering matrix arrangement being defined by a transmit filtering matrix W t, said filtering matrix arrangements cancelling at least one eigenmode of the self-interference channel, by choosing the columns of the filtering matrices to be orthogonal to said at least one eigenmode of the self-interference channel that is/are to be cancelled, the self-interference suppression by cancelling a certain eigenmode is made with the filtering matrix arrangements meeting a selection criterion for minimizing risk of loss of one or several dominating eigenmodes of the transmit channel and/or the receive channel, said selection criterion selecting which eigenmode to cancel using which filtering matrix.