Each base station in a network has an antenna which is connected in the conventional way to a transceiver for handling normal call traffic on one or more radio channels. In addition there is another transmitter which transmits a test signal on each of several test channels, one for each radio channel available for use by the main transceiver. The transceiver also has a receiver for detecting the test channels. The system is controlled by a timer which ensures that the transmitter and receiver do not operate at the same time. The receive times may be random, or may be co-ordinated between individual base stations. When the receiver is in operation, it will detect transmissions of the test signals from other base stations. The signal strength is measured and the result is used by a control system to determine a preference value for each channel; the stronger the signal detected, the lower the preference value. The preference value is used to control an amplifier system which controls the signal strength of each channel transmitted by the transmitter. It is also used to control the traffic transceiver by selecting which traffic channels are to be used in accordance with the preference values.