A base station of a cellular radio communications network configured to operate in an unlicensed radio band, comprising a radio transceiver connected to an antenna, and a controller including a processor and a memory holding computer program code. The processor is configured to execute the computer program code to realize a machine for determining a size for a contention window, within which the base station may transmit data from the transceiver to a radio terminal operating in the network; receiving a signal from the terminal, comprising an indication of occupancy of a radio channel, as detected by the terminal; adjusting the contention window size dependent on the indication of occupancy of the radio channel; transmitting data to the terminal within the adjusted contention window.