User equipment and method of searching for a cell

1. A user equipment, comprising:

a wireless interface for communication with a cellular communication network,

wherein the user equipment is operative to

detect a device-to-device signal received from a further user equipment at the wireless interface, and

search for a cell of the cellular communication network to camp on based on the device-to-device signal, a scan for signals from a base station of the cell being performed in resources which are selected as a function of the device-to-device signal received from the further user equipment, the user equipment being operative to prioritize, based on the device-to-device signal received from the further user equipment, frequencies or times at which the user equipment searches for the signals from the base station.

2. The user equipment of claim 1 ,

wherein the user equipment is operative to identify at least one frequency at which to search for signals from the base station of the cell based on the device-to-device signal.

3. The user equipment of claim 2 ,

wherein the user equipment is operative to identify several frequencies at which to search for signals from the base station of the cell based on the device-to-device signal.

4. The user equipment of claim 3 ,

wherein the user equipment is operative to limit the search for the signals from the base station of the cell to the several frequencies identified based on the device-to-device signal.

5. The user equipment of claim 3 ,

wherein the user equipment is operative to determine at which times a scan of the several frequencies is performed to search for the signals from the base station of the cell based on the device-to-device signal.

6. The user equipment of claim 1 ,

wherein the user equipment is operative to retrieve information on the cell from the device-to-device signal.

7. The user equipment of claim 6 ,

wherein the user equipment is operative to listen for a paging signal from the base station of the cell based on the information on the cell.

8. The user equipment of claim 6 ,

wherein the information on the cell comprises at least one of cell timing information, physical cell identity, frequency, E-UTRA Absolute Radio Frequency Channel Number, EARFCN, or transmit power.

9. The user equipment of claim 1 ,

wherein the user equipment is operative to determine whether it has moved into coverage of the cell based on the device-to-device signal.

10. The user equipment of claim 9 ,

wherein the device-to-device signal comprises information on a path loss from the base station of the cell to the further user equipment,

wherein the user equipment is operative

to measure a further path loss for the device-to-device signal from the further user equipment to the user equipment, and

to use the path loss and the further path loss to determine whether the user equipment has moved into partial coverage or into coverage of the cell.

11. The user equipment of claim 1 ,

wherein the user equipment is operative to camp on the further user equipment which transmits the device-to-device signal while searching for the cell of the cellular communication network.

12. The user equipment of claim 1 ,

wherein the further user equipment is in coverage of the cell, and

wherein the device-to-device signal is a device-to-device synchronization signal, D2DSS, which includes information on the cell used in the search for the cell to camp on.

13. The user equipment of claim 1 ,

wherein the user equipment is operative to scan for radio access technologies different from the cellular communication network if the search for the cell based on the device-to-device signal fails.

14. A communication system, comprising:

a user equipment, and

a further user equipment which is in coverage of a cell of a cellular communication network and which is operative to transmit a device-to-device signal,

wherein the user equipment comprises a wireless interface for communication with the cellular communication network, and wherein the user equipment is operative to

detect the device-to-device signal received from the further user equipment at the wireless interface, and

search for the cell of the cellular communication network to camp on based on the device-to-device signal, a scan for signals from a base station of the cell being performed in resources which are selected as a function of the device-to-device signal received from the further user equipment, the user equipment being operative to prioritize, based on the device-to-device signal received from the further user equipment, frequencies or times at which the user equipment searches for the signals from the base station.

15. A method of searching for a cell of a cellular communication network to camp on, the method comprising:

detecting, at a wireless interface of a user equipment, a device-to-device signal from a further user equipment, and

performing a cell search procedure by the user equipment based on the detected device-to-device signal, the cell search procedure comprising scanning for signals from a base station of the cell in resources which are selected as a function of the device-to-device signal received from the further user equipment, wherein the user equipment prioritizes, based on the device-to-device signal received from the further user equipment, frequencies or times at which the user equipment searches for the signals from the base station.

16. The method of claim 15 ,

wherein performing the cell search procedure comprises:

identifying several frequencies at which to search for signals from the base station of the cell based on the device-to-device signal.

17. The method of claim 15 ,

wherein performing the cell search procedure comprises:

identifying times at which to search for signals from the base station of the cell based on the device-to-device signal.

18. The method of claim 15 , further comprising:

retrieving information on the cell from the device-to-device signal,

wherein the cell search procedure is performed based on the retrieved information.

19. The method of claim 15 ,

wherein the device-to-device signal comprises information on a path loss from the base station of the cell to the further user equipment, and

wherein performing the cell selection procedure comprises:

measuring, by the user equipment, a further path loss for the device-to-device signal from the further user equipment to the user equipment, and

determining whether the user equipment has moved into partial coverage or into coverage of the cell based on the path loss and the further path loss.