Femto cell devices

1. A femto cell device comprising a processor including: a receiver for receiving transmissions from a user terminal in an active call with a macro cellular network node; an analyzer for determining a radio frequency (RF) signature characterizing the transmissions received from the user terminal while the user terminal is in the active call with the macro cellular network; an authorizer arranged to use the RF signature to determine if the user terminal is authorized to use the femto cell device; and a communicator for informing the core network that the femto cell device is a valid candidate handover target for the user terminal when the user terminal is determined by the authorizer to be authorized to use the femto cell device;

wherein the analyzer is configured to use the transmissions received from the user terminal in the active call with the macro cellular network node to determine the RF signature based on a distinguishing characteristic imposed by analogue circuitry of the user terminal.

2. The device as claimed in claim 1 wherein the communicator informs the core network that the device is not a candidate handover target when the authorizer determines that the user terminal is not authorized to use the femto cell device.

3. The device as claimed in claim 1 wherein, when the authorizer determines that the user terminal is not authorized to use the femto cell device, the device does not acknowledge a handover request associated with the user terminal.

4. The device as claimed in claim 1 wherein the analyzer performs time averaging on the transmissions received from the user terminal and uses time averaged transmissions in determining the RF signature.

5. The device as claimed in claim 1 wherein the authorizer determines in which class the user terminal is included based on the RF signature and uses the result of the classification to determine if the user terminal is authorized.

6. The device as claimed in claim 5 wherein the authorizer applies a voting algorithm to determine the class in which the user terminal is included.

7. The device as claimed in claim 1 wherein the authorizer uses the RF signature to identify the user terminal and determines if the identified user terminal in included in a set of authorized user terminals.

8. The device as claimed in claim 1 and including a store configured to store reference RF signature data associated with at least one user terminal; and a comparator configured to compare the determined RF signature with stored reference RF signature data.

9. The device as claimed in claim 8 and including an adder configured to allow an end user to add RF signature data associated with at least one user terminal to the store.

10. The device as claimed in claim 1 and including a training processor for providing a signal for receipt at the receiver from a candidate user terminal to be added to a set of user terminals authorized to use the device.

11. The device as claimed in claim 10 wherein the training processor halts communication with other user terminals when providing said signal for receipt from the candidate user terminal.

12. The device as claimed in claim 10 wherein the training processor is arranged to handover other user terminals to another device when providing said signal for receipt from the candidate user terminal.

13. A device as claimed in claim 10 and including an adjustor configured to adjust the transmit power of the candidate user terminal during training.

14. A device as claimed in claim 1 and including an acquirer configured to acquire reference RF signature data from an external source.

15. A device as claimed in claim 14 and including an adjustor configured to adjust acquired data to take into account the receiver response.

16. The device as claimed in claim 1 wherein the user terminal is a mobile handset.

17. A data storage device storing reference RF signature data for use in a device as claimed in claim 1 .