Power management in a cellular system

1. A method of controlling respective transmit powers allocated by a base station of a cellular communications network to each of a plurality of sub-bands, the method comprising:

receiving information from at least one other base station of said network, said information comprising information about a sensitivity of a utility function in a cell served by said other base station to changes in downlink transmission powers allocated to respective sub-bands by said base station,

identifying a sub-band in which it would be relatively advantageous to increase a downlink transmit power;

determining whether a factor relating to a service expectation of users in the cell exceeds a threshold value, wherein the factor is defined by each of a respective user's average bit rate divided by the respective user's required bit rate; and

increasing the downlink transmit power in the identified sub-band only if the factor is less than the threshold value.

2. A method as claimed in claim 1 , wherein the factor is defined based on an average service expectation of users in the cell served by the base station.

3. A method as claimed in claim 2 , wherein the factor is defined based on a weighted average service expectation of users in the cell served by the base station.

4. A method as claimed in claim 2 , wherein the factor is defined based on a standard deviation of the service expectation of users in the cell served by the base station.

5. A method as claimed in claim 2 , wherein the factor is defined based on a percentile of the service expectation of users in the cell served by the base station.

6. A method as claimed in claim 5 , wherein the factor is defined based on a scaling factor that introduces a degree of conservativeness.

7. A method as claimed in claim 6 , comprising adjusting the scaling factor based on a current transmit power in the cell.

8. A method as claimed in claim 7 , comprising adjusting the scaling factor based on a measured interference in the cell.

9. A method of determining an effect of interference in a cell served by a base station of a cellular communications network, said interference being caused by transmissions from a base station in at least one neighbouring cell of said network, the method comprising:

obtaining measurements from mobile devices connected to the base station; and

using the measurements to derive a measure of sensitivity of a utility function in said cell served by said base station to changes in downlink transmission powers allocated to respective sub-bands by said base station in said at least one neighbouring cell of said network.

10. A method as claimed in claim 9 , comprising:

obtaining measurements from said mobile devices in a plurality of sub-bands.

11. A method as claimed in claim 10 , comprising:

repeatedly obtaining each of said measurements, and forming a time averaged value for each of said measurements.

12. A method as claimed in claim 11 , comprising forming average measurements from measurements taken by a plurality of mobile devices, for each of said sub-bands.

13. A method as claimed in claim 12 , comprising, for each average measurement that is below a threshold value, setting the sensitivity of the utility function for each of a corresponding sub-band to a negative value.

14. A method as claimed in claim 13 , wherein said measurements are Channel quality indicator measurements.