Contention window adjustment in unlicensed cellular radio network

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.

2. The method of claim 1 , wherein the indication of occupancy represents measured signal strength on the radio channel in the terminal within a measurement period.

3. The method of claim 1 , further comprising:

comparing the indication of occupancy to the first threshold value, and, responsive to determining that the indication of occupancy levels or exceeds the first threshold;

establishing a second metric on a degree of operation by the base station on the radio channel;

comparing the second metric with a third threshold value; and

in response to determining that the second metric meets or exceeds the second threshold, further increasing the contention window size.

4. The method of claim 3 , wherein the step of establishing a metric on a degree number of operation by the base station includes measuring a number busy slots or periods on the radio channel within an observation window.

5. 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, wherein 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;

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, increasing the contention window size; and

transmitting data to the terminal within the increased contention window.

6. The base station of claim 5 , wherein the indication of occupancy represents measured signal strength on the channel in the terminal within a measurement period.

7. 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;

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, (i) 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, and (ii) comparing the metric with a second threshold value,

in response to determining that the metric meets or exceeds the second threshold, increasing the contention window size; and

transmitting data to the terminal within the increased contention window.