An apparatus, comprising means for: transmitting one or more reference signals at a transmit power level using a transmit antenna of a repeater; measuring a receive power level of the corresponding one or more reference signals received using a receive antenna of the repeater, wherein one or both of the transmit and receive antennas comprise an antenna array for respectively transmitting and receiving respective reference signals using a plurality of beams; determining a self-interference power value based on the difference between the transmit power level of the one or more reference signals and the receive power level of the one or more corresponding reference signals; determining whether the self-interference power value is above or below a predetermined threshold; and characterised in that comprising further means for: accessing one or more sets of reference data indicative of an amount of gain reduction or gain increase to apply to a gain associated with an amplifying means of the repeater, wherein the reference data is indicative of the amount of gain reduction or gain increase to apply for different combinations of transmit and receive antenna beams for a given self-interference power value; modifying, at a given time, the gain for a given beam combination based on the one or more sets of reference data and based on the determination of whether the self-interference power value is above or below the predetermined threshold and based on the one or more sets of reference data, wherein if the self-interference power is below the predetermined threshold, the modifying means is configured to increase the gain, and wherein if the self-interference power value is above the predetermined threshold, the modifying means is configured to decrease the gain. transmitting one or more reference signals at a transmit power level using a transmit antenna of a repeater; measuring a receive power level of the corresponding one or more reference signals received using a receive antenna of the repeater, wherein one or both of the transmit and receive antennas comprise an antenna array for respectively transmitting and receiving respective reference signals using a plurality of beams; determining a self-interference power value based on the difference between the transmit power level of the one or more reference signals and the receive power level of the one or more corresponding reference signals; determining whether the self-interference power value is above or below a predetermined threshold; and characterised in that comprising further means for: accessing one or more sets of reference data indicative of an amount of gain reduction or gain increase to apply to a gain associated with an amplifying means of the repeater, wherein the reference data is indicative of the amount of gain reduction or gain increase to apply for different combinations of transmit and receive antenna beams for a given self-interference power value; modifying, at a given time, the gain for a given beam combination based on the one or more sets of reference data and based on the determination of whether the self-interference power value is above or below the predetermined threshold and based on the one or more sets of reference data, wherein if the self-interference power is below the predetermined threshold, the modifying means is configured to increase the gain, and wherein if the self-interference power value is above the predetermined threshold, the modifying means is configured to decrease the gain. accessing one or more sets of reference data indicative of an amount of gain reduction or gain increase to apply to a gain associated with an amplifying means of the repeater, wherein the reference data is indicative of the amount of gain reduction or gain increase to apply for different combinations of transmit and receive antenna beams for a given self-interference power value; modifying, at a given time, the gain for a given beam combination based on the one or more sets of reference data and based on the determination of whether the self-interference power value is above or below the predetermined threshold and based on the one or more sets of reference data, wherein if the self-interference power is below the predetermined threshold, the modifying means is configured to increase the gain, and wherein if the self-interference power value is above the predetermined threshold,