Demand and response for sidelink synchronization signal block (S-SSB) transmission

1. A method of wireless communications by a receiving sidelink user equipment (UE), comprising:

detecting a lite sidelink synchronization signal (SL-SS) transmitted by a transmitting sidelink UE, the lite SL-SS having no physical sidelink broadcast channel (PSBCH); and

transmitting a demand requesting supplementary SL-SSs from the transmitting sidelink UE on indicated resources in response to detecting the lite-SL-SS having no physical sidelink broadcast channel (PSBCH), the supplementary SL-SSs including a PSBCH.

2. The method of claim 1 in which the supplementary SL-SSs comprise a system information block (SIB).

3. The method of claim 1 , in which the indicated resources comprise pre-configured resources.

4. The method of claim 3 , further comprising transmitting a preamble on the pre-configured resources.

5. The method of claim 4 in which the preamble comprises a reply SL-SS with a reserved sidelink service set identifier (SL-SSID).

6. The method of claim 5 , in which the reserved SL-SSID is pre-configured.

7. The method of claim 6 , in which the reserved SL-SSID is an SL-SSID of the transmitting sidelink UE.

8. The method of claim 4 , in which the pre-configured resources occur a period of time after a sidelink secondary synchronization signal (S-SSS).

9. The method of claim 1 , in which the indicated resources are indicated by the transmitting sidelink UE.

10. The method of claim 9 , in which the indicated resources are indicated by the transmitting sidelink UE with a SL-SSID that defines an offset from a signal after which the transmitting sidelink UE expects to receive the demand.

11. The method of claim 9 , in which the indicated resources further comprise pre-configured resources defining a resource block allocation and/or a transmit power for the demand.

12. The method of claim 1 , in which the demand indicates a coverage status of the receiving sidelink UE, a transmit power of the receiving sidelink UE and/or a mobility level of the receiving sidelink UE.

13. The method of claim 12 , in which the transmit power is indicated by the transmitting sidelink UE.

14. The method of claim 12 , in which the transmit power is pre-configured.

15. The method of claim 1 , further comprising transmitting the demand when a transmit power of the receiving sidelink UE is above a threshold.

16. The method of claim 15 , in which the threshold is defined by the transmitting sidelink UE.

17. The method of claim 15 , in which the threshold is pre-configured.

18. The method of claim 1 , further comprising selecting resources for transmitting the demand based on a coverage status of the receiving sidelink UE, a transmit power of the receiving sidelink UE and/or a mobility level of the receiving sidelink UE.

19. A method of wireless communications by a transmitting sidelink user equipment (UE), comprising:

transmitting a lite sidelink synchronization signal (SL-SS), the lite SL-SS having no physical sidelink broadcast channel (PSBCH);

receiving a demand for supplementary sidelink synchronization signals (SL-SSs) from a receiving sidelink UE; and

transmitting the supplementary SL-SSs in response to the demand, the supplementary SL-SSs including a PSBCH.

20. The method of claim 19 , in which the transmitting the supplementary SL-SSs occurs during a next sidelink synchronization signal block (S-SSB) occasion.

21. The method of claim 19 , in which the transmitting the supplementary SL-SSs occurs after a time offset.

22. The method of claim 21 , in which the time offset is indicated by a base station or pre-configured.

23. The method of claim 19 , further comprising:

starting a timer after transmitting one of the supplementary sidelink synchronization signals (SL-SS) within a sidelink synchronization signal block (S-SSB); and

incrementing a transmit power for a next SL-SS transmission when the timer expires before receiving the demand.

24. The method of claim 19 , further comprising:

starting a timer after transmitting one of the supplementary sidelink synchronization signals (SL-SS) within a sidelink synchronization signal block (S-SSB);

stopping a next SL-SS transmission when the timer expires before receiving the demand and a UE transmit power is at a threshold level; and

transmitting a future SL-SS a time period after the stopping.

25. The method of claim 24 , in which the time period is a random time period, is indicated by a base station, or is pre-configured.

26. An apparatus for wireless communication by a user equipment (UE), comprising:

a processor;

memory coupled with the processor; and

instructions stored in the memory and operable, when executed by the processor, to cause the apparatus:

to detect a lite sidelink synchronization signal (SL-SS) transmitted by a transmitting sidelink UE, the lite SL-SS having no physical sidelink broadcast channel (PSBCH); and

to transmit a demand requesting supplementary SL-SSs from the transmitting sidelink UE on indicated resources in response to detecting the lite-SL-SS having no physical sidelink broadcast channel (PSBCH), the supplementary SL-SSs including a PSBCH.

27. The apparatus of claim 26 in which the supplementary SL-SSs comprise a system information block (SIB).

28. The apparatus of claim 26 , in which the indicated resources comprise pre-configured resources.

29. An apparatus for wireless communication by a user equipment (UE), comprising:

a processor;

memory coupled with the processor; and

instructions stored in the memory and operable, when executed by the processor, to cause the apparatus:

to transmit a lite sidelink synchronization signal (SL-SS), the lite SL-SS having no physical sidelink broadcast channel (PSBCH);

to receive a demand for supplementary sidelink synchronization signals (SL-SSs) from a receiving sidelink UE; and

to transmit the supplementary SL-SSs in response to the demand, the supplementary SL-SSs including a PSBCH.