Sidelink group management for transmit power controlled group communications

1. A method for wireless communication performed by a user equipment (UE) participating in a sidelink communications group, comprising:

communicating with one or more member UEs of the sidelink communications group; and

transmitting group presence announcement messages for the sidelink communications group based on a determination, based on communicating with the one or more member UEs, of at least a transmit power for the group presence announcement messages and that the UE is expected to transmit the group presence announcement messages for the sidelink communications group, wherein the UE is one of a subset of members of the sidelink communications group that transmits the group presence announcement messages for the sidelink communications group.

2. The method of claim 1 , further comprising:

receiving, from a group leader UE for the sidelink communications group, an indication to transmit the group presence announcement messages.

3. The method of claim 2 , wherein the indication includes a periodicity and a time offset for the group presence announcement messages.

4. The method of claim 3 , wherein the time offset is staggered from time offsets of group presence announcement message transmissions from other member UEs of the sidelink communications group.

5. The method of claim 3 , wherein the UE transmits the group presence announcement messages periodically based on the periodicity.

6. The method of claim 1 , wherein:

the UE is a group leader UE for the sidelink communications group, and

the determination that the UE is expected to transmit the group presence announcement messages is further based on the UE being the group leader UE.

7. The method of claim 1 , wherein:

all member UEs of the sidelink communications group transmit the group presence announcement messages, and

the determination that the UE is expected to transmit the group presence announcement messages is further based on the UE being a member of the sidelink communications group.

8. The method of claim 1 , wherein the determination that the UE is expected to transmit the group presence announcement messages is further based on received signal strength measurements of transmissions from the UE to the one or more member UEs being less than a threshold, or received signal strength measurements of transmissions from the one or more member UEs to the UE being less than the threshold, or both.

9. The method of claim 8 , further comprising:

receiving the threshold from a group leader UE of the sidelink communications group or a base station serving the UE.

10. The method of claim 1 , wherein transmissions from the UE to the one or more member UEs are beamformed transmissions.

11. The method of claim 1 , further comprising:

determining a set of transmit beams that have received signal strength measurements at the one or more member UEs that are less than a threshold; and

selecting at least one transmit beam from the set of transmit beams to use to transmit the group presence announcement messages,

wherein transmitting the group presence announcement messages comprises transmitting the group presence announcement messages on the at least one transmit beam.

12. The method of claim 11 , wherein the set of transmit beams having received signal strength measurements that are less than the threshold indicates that the set of transmit beams are not suitable for communication with the one or more member UEs.

13. The method of claim 1 , wherein the transmit power is determined based on:

a transmit power configuration that the UE is using to transmit other sidelink physical channels to the one or more member UEs,

a downlink pathloss measurement to a serving base station,

a sidelink pathloss to a group leader UE for the sidelink communications group,

a transmit power configuration received from the group leader UE,

a maximum transmit power configuration for the UE that does not exceeds a maximum transmit power capability of the UE, or

any combination thereof.

14. The method of claim 13 , wherein the transmit power configuration received from the group leader UE includes:

a maximum transmit power to use for the group presence announcement messages,

an upper bound on an incremental transmit power increase that the UE is expected to use beyond the transmit power configuration that the UE is using to transmit the other sidelink physical channels to the one or more member UEs, or

any combination thereof.

15. The method of claim 1 , wherein the group presence announcement messages include a group identifier of the sidelink communications group.

16. The method of claim 15 , wherein:

the group identifier is an access stratum layer group identifier, or

the group identifier is an application layer group identifier.

17. The method of claim 1 , wherein the group presence announcement messages comprise:

application layer messages,

non-access stratum (NAS) layer messages, or

radio resource control (RRC) messages.

18. The method of claim 1 , wherein the UE transmits the group presence announcement messages over a physical sidelink discovery channel (PSDCH), a physical sidelink control channel (PSCCH), or a physical sidelink shared channel (PSSCH).

19. A user equipment (UE), comprising:

a memory;

a communication interface; and

at least one processor communicatively coupled to the memory and the communication interface, the at least one processor configured to:

communicate with one or more member UEs of a sidelink communications group in which the UE is participating; and

cause the communication interface to transmit group presence announcement messages for the sidelink communications group based on a determination, based on communication with the one or more member UEs, of at least a transmit power for the group presence announcement messages and that the UE is expected to transmit the group presence announcement messages for the sidelink communications group, wherein the UE is one of a subset of members of the sidelink communications group that transmits the group presence announcement messages for the sidelink communications group.

20. The UE of claim 19 , wherein the at least one processor is further configured to:

receive, via the communication interface, from a group leader UE for the sidelink communications group, an indication to transmit the group presence announcement messages.

21. The UE of claim 20 , wherein the indication includes a periodicity and a time offset for the group presence announcement messages.

22. The UE of claim 21 , wherein the time offset is staggered from time offsets of group presence announcement message transmissions from other member UEs of the sidelink communications group.

23. The UE of claim 19 , wherein:

the UE is a group leader UE for the sidelink communications group, and

the determination that the UE is expected to transmit the group presence announcement messages is further based on the UE being the group leader UE.

24. The UE of claim 19 , wherein:

all member UEs of the sidelink communications group transmit the group presence announcement messages, and

the determination that the UE is expected to transmit the group presence announcement messages is further based on the UE being a member of the sidelink communications group.

25. The UE of claim 19 , wherein the determination that the UE is expected to transmit the group presence announcement messages is further based on received signal strength measurements of transmissions from the UE to the one or more member UEs being less than a threshold, or received signal strength measurements of transmissions from the one or more member UEs to the UE being less than the threshold, or both.

26. The UE of claim 19 , wherein the at least one processor is further configured to:

determine a set of transmit beams that have received signal strength measurements at the one or more member UEs that are less than a threshold; and

select at least one transmit beam from the set of transmit beams to use to transmit the group presence announcement messages,

wherein the at least one processor being configured to transmit the group presence announcement messages comprises the at least one processor being configured to transmit the group presence announcement messages on the at least one transmit beam.

27. The UE of claim 19 , wherein the transmit power is determined based on:

a transmit power configuration that the UE is using to transmit other sidelink physical channels to the one or more member UEs,

a downlink pathloss measurement to a serving base station,

a sidelink pathloss to a group leader UE for the sidelink communications group,

a transmit power configuration received from the group leader UE,

a maximum transmit power configuration for the UE that does not exceeds a maximum transmit power capability of the UE, or

any combination thereof.

28. The UE of claim 27 , wherein the transmit power configuration received from the group leader UE includes:

a maximum transmit power to use for the group presence announcement messages,

an upper bound on an incremental transmit power increase that the UE is expected to use beyond the transmit power configuration that the UE is using to transmit the other sidelink physical channels to the one or more member UEs, or

any combination thereof.

29. A user equipment (UE), comprising:

means for communicating with one or more member UEs of a sidelink communications group; and

means for transmitting group presence announcement messages for the sidelink communications group based on a determination, based on communicating with the one or more member UEs, of at least a transmit power for the group presence announcement messages and that the UE is expected to transmit the group presence announcement messages for the sidelink communications group, wherein the UE is one of a subset of members of the sidelink communications group that transmits the group presence announcement messages for the sidelink communications group.

30. A non-transitory computer-readable medium storing computer-executable instructions that, when executed by a user equipment (UE), cause the UE to:

communicate with one or more member UEs of a sidelink communications group; and

transmit group presence announcement messages for the sidelink communications group based on a determination, based on communicating with the one or more member UEs, of at least a transmit power for the group presence announcement messages and that the UE is expected to transmit the group presence announcement messages for the sidelink communications group, wherein the UE is one of a subset of members of the sidelink communications group that transmits the group presence announcement messages for the sidelink communications group.