1. A method performed in a radio station that comprises a first radio node configured to operate a first cell on a first licensed frequency and a second radio node configured to operate both a second cell on a second licensed frequency and a third cell on an unlicensed frequency, the method comprising:communicating with a radio terminal on the first and second licensed frequencies;
transmitting, to the radio terminal from the second radio node in the second cell, control information associated with a discovery signal measurement to be performed on the unlicensed frequency,
wherein the control information includes:
information indicating periodicity at which a first period, in which a discovery signal can be transmitted, occurs periodically, and
an offset value to indicate a subframe number and a System Frame Number (SFN) at which the first period starts; and
receiving, from the radio terminal by the first radio node in the first cell, a result of the discovery signal measurement which is performed within the first period which occurs periodically at the periodicity and starts at the subframe number and the SFN indicated by at least the offset value.