Sidelink group coordination for group-specific synchronization reference

1. An apparatus, comprising:

at least one processor; and

at least one memory including computer program instructions,

wherein the at least one memory and the computer program instructions are configured to, with the at least one processor, cause the apparatus at least to perform:

determining whether a first condition is met for coordination on group synchronization reference for a group of user equipment;

determining whether a second condition is met to act as group synchronization reference user equipment for the group;

transmitting a group synchronization reference signal, performing a full sidelink synchronization search and synchronizing with a suitable synchronization reference, when determined to act as the group synchronization reference user equipment; and

performing a reduced synchronization search when determined not to act as the group synchronization reference user equipment.

2. The apparatus of claim 1 , wherein the at least one memory and the computer program instructions are configured to, with the at least one processor, cause the apparatus at least to perform:

transmitting a synchronization request announcement to the group when the first condition is met.

3. The apparatus of claim 2 , wherein the first condition is met when the apparatus is in an out of coverage area of a serving network or the apparatus requires power saving or the apparatus receives the synchronization request announcement from at least one other user equipment of the group.

4. The apparatus of claim 2 , wherein the synchronization request announcement is sent via at least one of a physical sidelink control channel, a physical sidelink shared channel, or a physical sidelink feedback channel.

5. The apparatus of claim 1 , wherein the at least one memory and the computer program instructions are further configured to, with the at least one processor, cause the apparatus at least to perform:

performing synchronization based on the reduced synchronization search; and

transmitting a synchronization termination announcement to at least one other user equipment of the group upon the first condition no longer being met.

6. The apparatus of claim 5 , wherein the synchronization termination announcement is sent via at least one of a physical sidelink control channel, a physical sidelink shared channel, or a physical sidelink feedback channel.

7. The apparatus of claim 1 , wherein the reduced sidelink synchronization search comprises monitoring only a sidelink synchronization signal transmitted by a group synchronization reference user equipment.

8. The apparatus of claim 1 , wherein, when the second condition is met, the at least one memory and the computer program instructions are further configured to, with the at least one processor, cause the apparatus at least to perform:

indicating to at least one other user equipment of the group that the apparatus will act as group synchronization reference.

9. The apparatus of claim 8 , wherein the indicating comprises sending at least one of a sidelink synchronization signal identifier, coverage indicator, or a sidelink synchronization signal identifier retention flag.

10. The apparatus of claim 8 , wherein the indicating comprises indicating whether reselection of group synchronization reference user equipment is needed at a next discontinuous reception cycle.

11. The apparatus of claim 8 , wherein the indicating comprises sending an indication via a first stage sidelink control information in a physical sidelink control channel or a second stage sidelink control information in a physical sidelink shared channel.

12. The apparatus of claim 1 , wherein the at least one memory and the computer program instructions are further configured to, with the at least one processor, cause the apparatus at least to perform:

after a predetermined number of sidelink discontinuous reception cycles from the determining whether the second condition is met, determining whether the second condition is met, as an updated determination; and

performing, based on the updated determination, transmitting the group synchronization reference signal when determined to act as the group synchronization reference user equipment and performing the reduced synchronization search when determined not to act as the group synchronization reference user equipment.

13. The apparatus of claim 12 , wherein the determining whether the second condition is met is based on at least one criterion, the at least one criterion comprising at least one of a number of times the user equipment has been group synchronization reference, a source identifier, a member identifier, power availability, or coverage status.

14. The apparatus of claim 12 , wherein the determining whether the second condition is met is based on the reception of synchronization request announcement from at least one other user equipment of the group.

15. A method, comprising:

determining, by a user equipment, whether a first condition is met for coordination on group synchronization reference for a group of user equipment;

determining, by the user equipment, whether a second condition is met to act as group synchronization reference user equipment for the group;

determining, by the user equipment, to act as the group synchronization reference user equipment;

transmitting, by the user equipment, a group synchronization reference signal, performing a full sidelink synchronization search and synchronizing with a suitable synchronization reference, when determined to act as the group synchronization reference user equipment;

determining, by the user equipment, not to act as the group synchronization reference user equipment; and

performing, by the user equipment, a reduced synchronization search when determined not to act as the group synchronization reference user equipment.

16. The method of claim 15 , further comprising:

transmitting, by the user equipment, a synchronization request announcement from a user equipment to the group when the first condition is met.

17. The method of claim 16 , wherein the first condition is met when the user equipment is in an out of coverage area of a serving network or the user equipment requires power saving or the user equipment receives the synchronization request announcement from at least one other user equipment of the group.

18. A method, comprising:

determining, by a user equipment, a first condition is met for coordination on group synchronization reference for a group of user equipment;

transmitting, by the user equipment, a synchronization request announcement from the user equipment to the group, wherein the synchronization request announcement is sent via at least one of a physical sidelink control channel, a physical sidelink shared channel, or a physical sidelink feedback channel;

determining, by the user equipment, whether a second condition is met to act as group synchronization reference user equipment for the group;

transmitting, by the user equipment, a group synchronization reference signal, performing a full sidelink synchronization search and synchronizing with a suitable synchronization reference, when determined to act as the group synchronization reference user equipment; and

performing, by the user equipment, a reduced synchronization search when determined not to act as the group synchronization reference user equipment.

19. The method of claim 15 , further comprising:

performing, by the user equipment, synchronization based on the reduced synchronization search; and

transmitting, by the user equipment, a synchronization termination announcement to at least one other user equipment of the group upon the first condition no longer being met.