Method and apparatus for selecting a free channel in a mobile radio system

A method of enabling a mobile station (M) to select a free duplex channel for communicating with a fixed station (F) in a cellular mobile radio system having a plurality of duplex channels each of which has a rising communication path and an associated descending communication path, respective duplex channels having respective addresses and providing communication between respective ones of a plurality of fixed stations and a plurality of mobile stations, the fixed stations being located in a pattern of cells constituting the cellular system;

• such method comprising:
• each fixed station identifying the addresses of duplex channels having rising paths which are engaged by detecting whether the signal power level in each received rising path exceeds a first given threshold, and transmitting the addresses of such channels to the mobile stations over a signalling path comprising equidistant time interval channels which are successively and periodically allotted to the respective fixed stations in the cellular system;
• determination by each mobile station of the addresses of duplex channels having descending paths which are engaged by detecting whether the signal power level in each received descending path exceeds a second given threshold;
• and
• selection by the mobile station (M) of a free duplex channel from among duplex channels having addresses differing from those of channels which are engaged in the rising paths and those which are engaged in the descending paths;
• such method being characterized by:
• continuous transmission by each of the fixed stations, in the signalling path time interval channels respectively allotted to them, of the addresses of duplex channels which they are using and of which of such channels having engaged descending paths;
• detection by the mobile station (M) of the signal power level in the signalling path time interval channels received from the respective fixed stations, and identification as interfering stations those for which such received signal power level exceeds a predetermined interference threshold but is below the maximum received signal power level;
• and
• determination by the mobile station (M) of the addresses of duplex channels of fixed station (F) having engaged descending paths by eliminating from the address of all duplex channels having engaged descending paths the addresses of duplex channels of interfering fixed stations.