HARQ process for sidelink transmission

1. An apparatus for wireless communication, the apparatus comprising:

a processor; and

a memory coupled to the processor, wherein the processor is configured to cause the apparatus to:

receive a sidelink (“SL”) grant for a SL transmission using SL resources, the SL grant indicating a first hybrid automatic repeat request (“HARQ”) process identified by a first HARQ process identifier;

select a second SL HARQ process identified by a second SL HARQ process identifier for the SL transmission using the SL resources,

wherein the second SL HARQ process is selected independently from the first HARQ process in response to the SL grant indicating an SL initial transmission, and

wherein the selection of the second SL HARQ process is based on the first HARQ process identifier in response to the SL grant indicating a SL retransmission;

transmit SL control information (“SCI”) containing the second SL HARQ process identifier; and

perform the SL transmission using the SL resources.

2. The apparatus of claim 1 , wherein the processor is further configured to cause the apparatus to transmit HARQ feedback for the first HARQ process, the HARQ feedback corresponding to the SL transmission using the SL resources.

3. The apparatus of claim 2 , wherein the SL grant indicates a physical uplink control channel resource and/or HARQ feedback timing information, and wherein transmitting the HARQ feedback for the first HARQ process is based on the SL grant.

4. The apparatus of claim 1 , wherein the first HARQ process identifier received in the SL grant is different than the second SL HARQ process identifier signaled within the SCI.

5. The apparatus of claim 1 , wherein, to select the second SL HARQ process, the processor is configured to cause the apparatus to autonomously select the second SL HARQ process from a set of SL HARQ processes.

6. The apparatus of claim 5 , wherein the set of SL HARQ processes is shared between unicast SL transmissions, groupcast SL transmission, and broadcast SL transmissions.

7. The apparatus of claim 1 , wherein the selected second SL HARQ process is associated with a second SL HARQ process for which there is no pending SL HARQ retransmission.

8. The apparatus of claim 1 , wherein the processor is further configured to map the first HARQ process to the second SL HARQ process.

9. The apparatus of claim 1 , wherein the processor is further configured to associate the selected second SL HARQ process to the SL grant.

10. The apparatus of claim 1 , wherein the processor is further configured to associate the second SL HARQ Process identifier corresponding to the second SL HARQ process to the SL grant.

11. The apparatus of claim 1 , wherein the processor is further configured to select the second SL HARQ process being associated with the SL grant in response to the SL grant indicating the SL retransmission.

12. The apparatus of claim 1 , wherein, to perform the SL transmission using the SL resources, the processor is configured to cause the apparatus to transmit the SCI containing the second SL HARQ process identifier for the second SL HARQ process, and wherein the processor is further configured to cause the apparatus to receive SL HARQ feedback for the second SL HARQ process from at least one user equipment (“UE”).

13. The apparatus of claim 12 ,

wherein, to perform the SL transmission using the SL resources, the processor is configured to cause the apparatus to perform an initial groupcast transmission,

wherein the received SL HARQ feedback comprises a negative acknowledgement indication, and

wherein the processor is configured to cause the apparatus to retransmit the SL transmission using unicast SL retransmission.

14. The apparatus of claim 13 , wherein the unicast SL retransmission reuses the second HARQ process identifier.

15. A method of a remote unit comprising:

receiving a sidelink (“SL”) grant for a SL transmission using SL resources, the SL grant indicating a first hybrid automatic repeat request (“HARQ”) process identified by a first HARQ process identifier;

selecting a second SL HARQ process identified by a second SL HARQ process identifier for the SL transmission using the SL resources,

wherein the second SL HARQ process is selected independently from the first HARQ process in response to the SL grant indicating an initial SL transmission,

wherein the selection of the second SL HARQ process based on the first HARQ process identifier in response to the SL grant indicating a SL retransmission;

transmitting SL control information (“SCI”) containing the selected second SL HARQ process identifier; and

performing the SL transmission using the SL resources.

16. The method of claim 15 , further comprising associating the selected second SL HARQ process to the SL grant.

17. The method of claim 15 , further comprising associating the second SL HARQ Process identifier corresponding to the second SL HARQ process to the SL grant.

18. The method of claim 15 , wherein selecting the second SL HARQ process being associated with the SL grant occurs in response to the SL grant indicating the SL retransmission.

19. The method of claim 15 , wherein performing the SL transmission using the SL resources comprises transmitting the SCI containing the second SL HARQ process identifier for the second SL HARQ process, the method further comprising receiving SL HARQ feedback for the second SL HARQ process from at least one user equipment (“UE”).

20. The method of claim 19 , wherein the SCI further contains a new data indicator (“NDI”), and wherein a receiving UE performs soft-buffer management for the received SL transmission based on at least the second SL HARQ process identifier, a source Identifier (“ID”), and the NDI.