In a mobile radio system, a method and apparatus for enabling a mobile station (M) to select a free duplex communication channel for communication with a fixed station (F). The fixed stations in the system detect duplex channels which are engaged in their rising paths and transmit a list of such engaged duplex channels over a signalling path. The fixed stations further transmit over such signalling path, in time division channels respectively allocated to them, a list of the duplex channels which they use and which are engaged in the descending paths. In the mobile station (M) the power levels of the signals received in the signalling path are measured and compared with an interference threshold power level to identify duplex channels of interfering fixed stations. A free duplex channel of a fixed station (F) is selected by the mobile station (M) by excluding channels of interfering fixed stations and channels of fixed station (F) which are engaged.