Abstract: A relative time offset is measured between transmission of a downlink signal from a cellular radio network base station, and a reference signal (e.g., sync word) included in an uplink signal received at the base station from a network subscriber. If the measured time offset is within a determined range, the uplink signal is judged to originate from a subscriber which is validly assigned to the base station, and the base station will continue to track the valid uplink signal. If the relative time offset is outside the determined range, the uplink signal is judged invalid, e.g., to originate from an interfering co-channel subscriber assigned to a remote base station. The receiving base station will then inhibit tracking of the invalid uplink signal.