1. A method carried out in a base station of a cellular radio communications network configured to transmit data to a terminal in an unlicensed radio band, comprising the steps of:determining a contention window, within which to transmit data between the base station and a terminal;
receiving a signal from the terminal, comprising an indication of occupancy of a radio channel, as detected by the terminal;
establishing a metric on a number of Not Acknowledged (NACK) feedback values received on the radio channel in respect to a predetermined period of time;
comparing the indication of occupancy to a first threshold value;
in response to determining that the indication of occupancy meets or exceeds the first threshold value, comparing the metric with a second threshold value;
in response to determining that the metric meets or exceeds the second threshold value, increasing the contention window size; and
transmitting data to the terminal within the increased contention window.