According to aspects of the invention, a radio signal that constitutes a secondary use is provided with a predefined identifier (11) that identifies the radio signal as a secondary use. The identifier (11) can be any suitable signal characteristic that somehow can be detected by a suitable radio receiver. The identifier (11) enables identification of an origin of the radio signal. That is, whether it originates from a primary user or a secondary user. It therefore becomes possible to give all secondary users a more equal status, facilitating resource sharing. A transmitter is provided with an identifier inserter (25) for providing the identifier (11) in a transmitted radio signal. In a corresponding manner, a receiver is provided with identifier detector (29) for detecting a presence of the identifier in a received signal.