Method and system for initial synchronization and collision avoidance in device to device communications without network coverage

1. A method at a first device for enabling a device-to-device wireless link, the method comprising:

detecting whether a presence signal of a second device is received over a first time period, the presence signal of the second device having a time-slot boundary;

if the presence signal of the second device is not detected, initiating a time-slot boundary by the first device including:

transmitting a first presence signal of the first device in a selected time-slot;

checking for an acknowledgment to the first presence signal; and

if the acknowledgement to the first presence signal is received, establishing the time slot-boundary on the first device based on the first presence signal.

2. The method of claim 1 , wherein the presence signal comprises a Zadoff-Chu sequence.

3. The method of claim 1 , wherein the first device transmits the first presence signal in the selected time-slot during a subsequent frame, wherein a frame comprises an integer number of time-slots.

4. The method of claim 1 , wherein a time-slot duration is predetermined.

5. The method of claim 1 , wherein the first time period comprises a predetermined frame period and a non-deterministic contention period.

6. The method of claim 5 , wherein the frame period comprises an integer number of time-slot durations.

7. The method of claim 1 , further comprising:

if the presence signal of the second device is detected, aligning a time-slot boundary on the first device to the time-slot boundary established by the second device; and

after aligning to the time-slot boundary established by the second device, determining a time-slot to use for transmission by the first device.

8. The method of claim 7 , wherein the determining the time-slot comprises:

detecting one or more free time-slots;

choosing a free time-slot;

transmitting the first presence signal on the chosen free time-slot.

9. The method of claim 8 , further comprising:

checking for an acknowledgment to the first presence signal.

10. The method of claim 1 , further comprising sending an acknowledgment to a received presence signal of any device other than the first device.

11. The method of claim 1 , wherein the first presence signal comprises one or more sequences.

12. The method of claim 11 , wherein the first presence signal comprises a primary synchronization signal and a secondary synchronization signal.

13. The method of claim 12 , wherein the primary synchronization signal is used to establish the time-slot boundary.

14. The method of claim 11 , wherein one or more of the sequences varies between two consecutive presence signal transmissions.

15. The method of claim 11 , wherein one or more of the sequences stays the same for a plurality of consecutive presence signal transmissions.

16. The method of claim 1 , wherein the selection of the first presence signal is related to an identity of the first device.

17. A method at a first device for enabling a device-to-device wireless link, the method comprising:

listening for a presence signal on a channel, the presence signal comprising at least one sequence; and

upon detection of the presence signal,

transmitting an acknowledgement to the presence signal; and

aligning to a time-slot boundary associated with the presence signal by utilizing the at least one sequence of the presence signal;

wherein the presence signal comprises a primary synchronization signal relating to an identity of a plurality of devices and a secondary synchronization signal relating to an identity of a device that transmits the presence signal.

18. The method of claim 17 , wherein the presence signal comprises a Zadoff-Chu sequence.

19. The method of claim 17 , wherein the first device uses the secondary synchronization signal to identify signal collisions.

20. A method at a device for enabling a device-to-device wireless link, the method comprising:

listening for a presence signal from another device on a channel; and

transmitting a presence signal on the channel to establish a time-slot boundary,

wherein the transmitting of the presence signal enables another device to align to the established time-slot boundary; and

wherein the presence signal comprises a primary synchronization signal relating to an identity of a plurality of devices and a secondary synchronization signal relating to an identity of the device.

21. The method of claim 20 , wherein the listening is done for a first time period which comprises a predetermined frame period and a randomly generated contention period.

22. The method of claim 20 , wherein the frame period comprises an integer number of time-slot durations.

23. A method at a first device for enabling a device-to-device wireless link, the method comprising:

detecting whether a presence signal of a second device is received over a first time period, the presence signal of the second device having a time-slot boundary;

if the presence signal of the second device is not detected, initiating a time-slot boundary by the first device including:

randomly selecting a first presence signal based on root sequence indexes different from the ones used by a base station; and

transmitting the first presence signal of the first device in a selected time-slot of a radio frame number.

24. The method of claim 23 , wherein the root sequence indexes comprise Zadoff-Chu root sequence indexes.

25. The method of claim 24 , wherein the radio frame number starts from 0.