A node (105, 200) for a communications system (100), arranged to operate as a secondary user of part of a frequency spectrum, comprising a detection unit (210) for detecting other secondary users (110) who use all or part of said frequency spectrum and for detecting changes in such other secondary user's usage of their part of the frequency spectrum. The node (105, 200) also comprises a signaling unit (225) for signaling to detected other secondary users (110) that the node (105, 200) wishes to operate within their frequency spectrum as a secondary user, by means of transmitting interference signals according to a predefined pattern in the frequency spectrum of detected other secondary users. The node also comprises a decision unit (230) for deciding how the node (105, 200) should arrange its secondary operation in the frequency spectrum together with detected other secondary users (110).