Resource selection and on-demand request for sidelink synchronization signals

1. A method of operating a user equipment (UE), comprising:

identifying a pool of resources where transmission of sidelink synchronization signals is permitted, the pool of resources comprising at least UE-specific resources with a first reference timing that is independent of any non-UE synchronization source, wherein the UE is synchronized with a second reference timing that is based on a non-UE synchronization source; and

transmitting a sidelink synchronization signal to one or more peer sidelink UEs based on a subset of the pool of resources.

2. The method of claim 1 , wherein the pool of resources further comprises system-wide resources reserved for sidelink synchronization signal transmissions.

3. The method of claim 1 ,

wherein the subset of resources comprises at least part of the UE-specific resources, or

wherein the subset of resources comprises at least part of the reserved system-wide resources, or

any combination thereof.

4. The method of claim 1 , wherein the subset of resources comprises at least part of the UE-specific resources.

5. The method of claim 1 , wherein the transmitting transmits the sidelink synchronization signal in response to a request from at least one of the one or more peer sidelink UEs in an on-demand manner.

6. The method of claim 1 , further comprising:

determining that the subset of resources will not interfere with the one or more peer sidelink UEs and/or a base station,

wherein the transmitting is performed in response to the determining in an opportunistic manner.

7. The method of claim 1 , wherein the transmitting repeats periodically.

8. The method of claim 7 , wherein the periodic transmitting comprises:

transmitting a first set of synchronization signals in accordance with a first periodicity, and

transmitting a second set of synchronization signals in accordance with a second periodicity.

9. The method of claim 8 ,

wherein the first periodicity is higher than the second periodicity,

wherein the first set of synchronization signals is transmitted over a first part of the subset of resources corresponding to system-wide resources reserved for sidelink synchronization signal transmissions, and

wherein the second set of synchronization signals is transmitted over a second part of the subset of resources corresponding to at least part of the UE-specific resources.

10. The method of claim 7 , wherein the periodic transmitting comprises:

transmitting a first set of synchronization signals in accordance with a first periodicity, and

transmitting a second set of synchronization signals in an on-demand manner.

11. The method of claim 10 ,

wherein the first set of synchronization signals is transmitted over a first part of the subset of resources corresponding to system-wide resources reserved for sidelink synchronization signal transmissions, and

wherein the second set of synchronization signals is transmitted over a second part of the subset of resources corresponding to at least part of the UE-specific resources.

12. The method of claim 10 , wherein the second set of synchronization signals is transmitted in response to a synchronization request received from another peer sidelink UE or a synchronization quality dropping below a threshold.

13. The method of claim 1 , wherein the pool of resources and the subset of resources each comprise resources defined in terms of a combination of time and frequency.

14. The method of claim 1 , further comprising:

transmitting an indication signal to the one or more peer sidelink UEs to indicate the subset of the pool of resources.

15. A method of operating a user equipment (UE), comprising:

receiving a sidelink synchronization signal from a peer sidelink UE based on a subset of a pool of resources where transmission of sidelink synchronization signals is permitted, the pool of resources comprising at least UE-specific resources with a first reference timing that is independent of any non-UE synchronization source, wherein the peer sidelink UE is synchronized with a second reference timing that is based on a non-UE synchronization source; and

synchronizing a clock at the UE based at least in part upon the sidelink synchronization signal.

16. The method of claim 15 , wherein the pool of resources further comprises system-wide resources reserved for sidelink synchronization signal transmissions.

17. The method of claim 15 ,

wherein the subset of resources comprises at least part of the UE-specific resources, or

wherein the subset of resources comprises at least part of the reserved system-wide resources, or

any combination thereof.

18. The method of claim 15 , wherein the subset of resources comprises at least part of the UE-specific resources.

19. The method of claim 15 , further comprising:

transmitting a request for the sidelink synchronization signal to the peer sidelink UE,

wherein the sidelink synchronization signal is received in response to the request in an on-demand manner.

20. The method of claim 15 , wherein the receiving repeats periodically.

21. The method of claim 20 , wherein the periodic receiving:

receiving a first set of synchronization signals in accordance with a first periodicity, and

receiving a second set of synchronization signals in accordance with a second periodicity.

22. The method of claim 21 ,

wherein the first periodicity is higher than the second periodicity,

wherein the first set of synchronization signals is received over a first part of the subset of resources corresponding to system-wide resources reserved for sidelink synchronization signal transmissions, and

wherein the second set of synchronization signals is received over a second part of the subset of resources corresponding to at least part of the UE-specific resources.

23. The method of claim 20 , wherein the periodic transmitting comprises:

receiving a first set of synchronization signals in accordance with a first periodicity, and

receiving a second set of synchronization signals in an on-demand manner.

24. The method of claim 23 ,

wherein the first set of synchronization signals is received over a first part of the subset of resources corresponding to system-wide resources reserved for sidelink synchronization signal transmissions, and

wherein the second set of synchronization signals is received over a second part of the subset of resources corresponding to at least part of the UE-specific resources.

25. The method of claim 15 , wherein the pool of resources and the subset of resources each comprise resources defined in terms of a combination of time and frequency.

26. The method of claim 15 , further comprising:

receiving an indication signal from the peer sidelink UE that indicates the subset of the pool of resources.

27. A method of operating a user equipment (UE), comprising:

transmitting a synchronization request message to at least one peer sidelink UE; and

receiving, on one or more UE-specific resources with a first reference timing that is independent of any non-UE synchronization source in advance of a next periodic sidelink synchronization signal that is received at the UE subsequent to the transmission of the synchronization request message, at least one sidelink synchronization signal from the at least one peer sidelink UE in response to the transmitting, wherein the at least one peer sidelink UE is synchronized with a second reference timing that is based on a non-UE synchronization source and is associated with the next periodic sidelink synchronization signal.

28. The method of claim 27 , wherein the transmitting transmits the synchronization request message over a sidelink communications channel as a Medium Access Control (MAC) Command Element (CE).

29. The method of claim 27 , wherein the transmitting transmits the synchronization request message as via unicast, multicast or broadcast.

30. The method of claim 27 ,

determining that the UE has transitioned from being synchronized with respect to a network clock to being unsynchronized with respect to the network clock,

wherein the transmitting is performed in response to the determining.

31. The method of claim 27 ,

determining that a quality associated with a synchronization source of the UE has dropped below a threshold,

wherein the transmitting is performed in response to the determining.

32. A method of operating a user equipment (UE), comprising:

receiving a synchronization request message from a peer sidelink UE; and

transmitting, on one or more UE-specific resources with a first reference timing that is independent of any non-UE synchronization source in advance of a next periodic sidelink synchronization signal transmitted by the UE or a network component subsequent to the reception of the synchronization request message, at least one sidelink synchronization signal to the peer sidelink UE in response to the synchronization request message, wherein the UE is synchronized with a second reference timing that is based on a non-UE synchronization source and is associated with the next periodic sidelink synchronization signal.

33. The method of claim 32 , wherein the receiving receives the synchronization request message over a sidelink communications channel as a Medium Access Control (MAC) Command Element (CE).

34. The method of claim 32 , wherein the receiving receives the synchronization request message as via unicast, multicast or broadcast.

35. A user equipment (UE), comprising:

means for identifying a pool of resources where transmission of sidelink synchronization signals is permitted, the pool of resources comprising at least UE-specific resources with a first reference timing that is independent of any non-UE synchronization source, wherein the UE is synchronized with a second reference timing that is based on a non-UE synchronization source; and

means for transmitting a sidelink synchronization signal to one or more peer sidelink UEs based on a subset of the pool of resources.

36. A user equipment (UE), comprising:

means for receiving a sidelink synchronization signal from a peer sidelink UE based on a subset of a pool of resources where transmission of sidelink synchronization signals is permitted, the pool of resources comprising at least UE-specific resources with a first reference timing that is independent of any non-UE synchronization source, wherein the peer sidelink UE is synchronized with a second reference timing that is based on a non-UE synchronization source; and

means for synchronizing a clock at the UE based at least in part upon the sidelink synchronization signal.

37. A user equipment (UE), comprising:

means for transmitting a synchronization request message to at least one peer sidelink UE; and

means for receiving, on one or more UE-specific resources with a first reference timing that is independent of any non-UE synchronization source in advance of a next periodic sidelink synchronization signal that is received at the UE subsequent to the transmission of the synchronization request message, at least one sidelink synchronization signal from the at least one peer sidelink UE in response to the transmitting, wherein the at least one peer sidelink UE is synchronized with a second reference timing that is based on a non-UE synchronization source and is associated with the next periodic sidelink synchronization signal.

38. A user equipment (UE), comprising:

means for receiving a synchronization request message from a peer sidelink UE; and

means for transmitting, on one or more UE-specific resources with a first reference timing that is independent of any non-UE synchronization source in advance of a next periodic sidelink synchronization signal transmitted by the UE or a network component subsequent to the reception of the synchronization request message, at least one sidelink synchronization signal to the peer sidelink UE in response to the synchronization request message, wherein the UE is synchronized with a second reference timing that is based on a non-UE synchronization source and is associated with the next periodic sidelink synchronization signal.

39. A user equipment (UE), comprising:

a memory;

at least one transceiver; and

at least one processor coupled to the memory and the at least one transceiver and the at least one processor configured to:

identify a pool of resources where transmission of sidelink synchronization signals is permitted, the pool of resources comprising at least UE-specific resources with a first reference timing that is independent of any non-UE synchronization source, wherein the UE is synchronized with a second reference timing that is based on a non-UE synchronization source; and

transmit, via the at least one transceiver, a sidelink synchronization signal to one or more peer sidelink UEs based on a subset of the pool of resources.

40. A user equipment (UE), comprising:

a memory;

at least one transceiver; and

at least one processor coupled to the memory and the at least one transceiver and the at least one processor configured to:

receive a sidelink synchronization signal from a peer sidelink UE based on a subset of a pool of resources where transmission of sidelink synchronization signals is permitted, the pool of resources comprising at least UE-specific resources with a first reference timing that is independent of any non-UE synchronization source, wherein the peer sidelink UE is synchronized with a second reference timing that is based on a non-UE synchronization source; and

synchronize a clock at the UE based at least in part upon the sidelink synchronization signal.

41. A user equipment (UE), comprising:

a memory;

at least one transceiver; and

at least one processor coupled to the memory and the at least one transceiver and the at least one processor configured to:

transmit, via the at least one transceiver, a synchronization request message to at least one peer sidelink UE; and

receive, on one or more UE-specific resources with a first reference timing that is independent of any non-UE synchronization source in advance of a next periodic sidelink synchronization signal that is received at the UE subsequent to the transmission of the synchronization request message, at least one sidelink synchronization signal from the at least one peer sidelink UE in response to the transmitting, wherein the at least one peer sidelink UE is synchronized with a second reference timing that is based on a non-UE synchronization source and is associated with the next periodic sidelink synchronization signal.

42. A user equipment (UE), comprising:

a memory;

at least one transceiver; and

at least one processor coupled to the memory and the at least one transceiver and the at least one processor configured to:

receive a synchronization request message from a peer sidelink UE; and

transmit, on one or more UE-specific resources with a first reference timing that is independent of any non-UE synchronization source in advance of a next periodic sidelink synchronization signal transmitted by the UE or a network component subsequent to the reception of the synchronization request message, at least one sidelink synchronization signal to the peer sidelink UE in response to the synchronization request message, wherein the UE is synchronized with a second reference timing that is based on a non-UE synchronization source and is associated with the next periodic sidelink synchronization signal.

43. A non-transitory computer-readable medium containing instructions stored thereon, which, when executed by a user equipment (UE), cause the UE to perform actions, the instructions comprising:

at least one instruction configured to cause the UE to identify a pool of resources where transmission of sidelink synchronization signals is permitted, the pool of resources comprising at least UE-specific resources with a first reference timing that is independent of any non-UE synchronization source, wherein the UE is synchronized with a second reference timing that is based on a non-UE synchronization source; and

at least one instruction configured to cause the UE to transmit a sidelink synchronization signal to one or more peer sidelink UEs based on a subset of the pool of resources.

44. A non-transitory computer-readable medium containing instructions stored thereon, which, when executed by a user equipment (UE), cause the UE to perform actions, the instructions comprising:

at least one instruction configured to cause the UE to receive a sidelink synchronization signal from a peer sidelink UE based on a subset of a pool of resources where transmission of sidelink synchronization signals is permitted, the pool of resources comprising at least UE-specific resources with a first reference timing that is independent of any non-UE synchronization source, wherein the peer sidelink UE is synchronized with a second reference timing that is based on a non-UE synchronization source; and

at least one instruction configured to cause the UE to synchronize a clock at the UE based at least in part upon the sidelink synchronization signal.

45. A non-transitory computer-readable medium containing instructions stored thereon, which, when executed by a user equipment (UE), cause the UE to perform actions, the instructions comprising:

at least one instruction configured to cause the UE to transmit a synchronization request message to at least one peer sidelink UE; and

at least one instruction configured to cause the UE to receive, on one or more UE-specific resources with a first reference timing that is independent of any non-UE synchronization source in advance of a next periodic sidelink synchronization signal that is received at the UE subsequent to the transmission of the synchronization request message, at least one sidelink synchronization signal from the at least one peer sidelink UE in response to the transmitting, wherein the at least one peer sidelink UE is synchronized with a second reference timing that is based on a non-UE synchronization source and is associated with the next periodic sidelink synchronization signal.

46. A non-transitory computer-readable medium containing instructions stored thereon, which, when executed by a user equipment (UE), cause the UE to perform actions, the instructions comprising:

at least one instruction configured to cause the UE to receive a synchronization request message from a peer sidelink UE; and

at least one instruction configured to cause the UE to transmit, on one or more UE-specific resources with a first reference timing that is independent of any non-UE synchronization source in advance of a next periodic sidelink synchronization signal transmitted by the UE or a network component subsequent to the reception of the synchronization request message, at least one sidelink synchronization signal to the peer sidelink UE in response to the synchronization request message, wherein the UE is synchronized with a second reference timing that is based on a non-UE synchronization source and is associated with the next periodic sidelink synchronization signal.