Method and apparatus for deriving femto cell timing information

1. A method of deriving reference timing information for a femto cell of a cellular wireless communication system, the cellular wireless communication system comprising a plurality of macro cells and a plurality of wireless communication units, the method comprising, at a network element within the wireless cellular communication network:

receiving observed timing data for a femto cell within a macro cell, and receiving observed timing data for the macro cell, from at least one wireless communication unit;

determining observed timing difference values, based at least partly on the observed timing data;

arranging the observed timing difference values into at least one set of observed timing difference values for the femto cell, in accordance with geographical location information corresponding to the observed timing difference values;

determining a central reference timing difference value for each set of observed timing difference values, and thereby for the femto cell; and

setting reference timing information for the femto cell, in accordance with the determined central reference timing difference value for each set of observed timing difference values.

2. The method of claim 1 , wherein:

an observed timing difference value dTm is the difference between an observed timing offset value for the overlying macro cell Tm_macro and an observed timing offset value for the femto cell Tm_femto.

3. The method of claim 1 , comprising:

arranging the observed timing difference values into the at least one set of observed timing difference values for the femto cell, on the basis of a degree of correspondence of the observed timing difference values to previous observed timing difference values for the femto cell.

4. The method of claim 1 , comprising:

arranging the observed timing difference values into the at least one set of observed timing difference values, based at least partly on proximities of the observed timing difference values to current reference timing information for each of at least one previously mapped femto cell.

5. The method of claim 1 , comprising:

periodically using newly derived reference timing information for the femto cell to replace currently held reference timing information for the femto cell.

6. A network element within a wireless cellular communication network, the cellular wireless communication network comprising a plurality of macro cells and at least one femto cell associated with a macro cell, the network element comprising at least one signal processing module arranged to:

receive observed timing data for a femto cell within a macro cell, and receive observed timing data for the macro cell, from at least one wireless communication unit;

determine observed timing difference values, based at least partly on the observed timing data;

arrange the observed timing difference values into at least one set of observed timing difference values for the femto cell, in accordance with geographical location information corresponding to the observed timing difference values;

determine a central tendency timing difference value for each set of observed timing difference values; and

set reference timing information for the femto cell, in accordance with the determined central tendency timing difference value(s).

7. The signal processing module of claim 6 , further arranged to:

receive geographical location information for one or more femto cells associated with the macro cell;

arrange the observed timing difference values into the at least one set of observed timing difference values for the femto cell, by assigning the observed timing difference values to the femto cell whose currently held reference timing information corresponds most closely to the observed timing difference values.

8. A method of updating currently held reference timing information for a femto cell of a wireless communication system, the wireless communication system comprising a plurality of macro cells and a plurality of wireless communication units, the method comprising, at a network element within the wireless cellular communication network:

receiving observed timing offset data for a femto cell and timing offset data for a macro cell from at least one wireless communication unit;

determining a difference in timing offsets between the timing offset data for the femto cell and the timing offset data for the macro cell;

deriving a reference timing difference value from the difference in timing offsets; and

setting the derived reference timing difference value as new reference timing information for the femto cell.

9. The method of claim 8 , further comprising:

a plurality of wireless communication units observing timing offset data for a plurality of femto cells;

deriving a single reference timing value for each femto cell for which timing offset data for the femto cell and timing offset data of a macrocell have been observed;

updating the currently held reference timing information for each femto cell, by setting the single reference timing value as new reference timing information for the femto cell.

10. The method of claim 9 , further comprising:

deriving the single reference timing value for each femto cell from timing offset data that has been obtained during a pre-defined time interval.

11. The method of claim 8 , further comprising:

receiving a measurement report from the wireless communication unit, the measurement report providing timing values for communications between the wireless communication unit and the femto cell, and between the wireless communication unit and more than one macro cell;

deriving reference timing information for the femto cell from the measurement report, the derived reference timing information comprising a reference timing difference value for the femto cell and each of the more than one macro cell.

12. The method of claim 8 , further comprising:

obtaining updated reference timing information for the femto cell during each of a plurality of time intervals; and

using the reference timing information to calculate a rate of drift over time of the difference in timing offsets for the femto cell and one or more macro cells.

13. The method of claim 12 , further comprising:

using the rate of drift to identify a failing femto cell.

14. The method of claim 12 , further comprising:

using the rate of drift to extrapolate a known value of difference in timing offsets for the femto cell, to thereby derive a predicted value of difference in timing offsets.

15. The method of claim 12 , further comprising:

providing information to the femto cell about the rate of drift.

16. The method of claim 15 , further comprising:

the femto cell using the information about the rate of drift to stabilize timing reference circuitry within the femto cell.

17. A non-transitory tangible computer program product having executable program

code stored therein for programming signal processing logic to perform a method of updating currently held reference timing information for a femto cell of a wireless communication system, the wireless communication system comprising a plurality of macro cells and a plurality of wireless communication units, the method comprising, at a network element within the wireless cellular communication network:

receiving observed timing offset data for a femto cell and timing offset data for a macro cell from at least one wireless communication unit;

determining a difference in timing offsets between the timing offset data observed for the femto cell and the timing offset data observed for the macro cell;

deriving a reference timing difference value from the difference in timing offsets; and

setting the derived reference timing difference value as new reference timing information for the femto cell.

18. The non-transitory tangible computer program product of claim 17 wherein the tangible computer program product comprises at least one of: a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a Read Only Memory (ROM), a Programmable Read Only Memory (PROM), an Erasable Programmable Read Only Memory (EPROM), an Electrically Erasable Programmable Read Only Memory (EEPROM), and a Flash memory.