HARQ feedback and retransmission mechanism for sidelink transmission

1. A method, comprising:

receiving, by a first user equipment (UE) from a base station, a configuration indicating resources for a first set of sidelink (SL) transmissions from the first UE to a second UE, the first set of SL transmissions including an initial SL transmission and a plurality of SL retransmissions;

after the receiving the configuration, transmitting, by the first UE to the second UE, at least one SL transmission of the first set of SL transmissions;

before transmitting a last SL retransmission of the plurality of SL retransmissions receiving, by the first UE from the second UE, an acknowledgement (ACK) for the at least one SL transmission; and

terminating, by the first UE, transmission of the first set of SL transmissions in response to receiving the ACK by refraining from transmitting at least the last SL retransmission of the plurality of SL retransmissions.

2. The method of claim 1 , wherein the transmitting the initial SL transmission comprises:

transmitting SL control information; and

transmitting an initial transmission of SL data, wherein the SL control information indicates the resources for the first set of SL transmissions.

3. The method of claim 1 , further comprising:

receiving, by the first UE, a configuration of a resource pool; and

selecting, by the first UE, the resources for the first set of SL transmissions from the resource pool.

4. The method of claim 1 ,

wherein the first set of SL transmissions is an SL configured grant (CG) transmission, and

wherein the configuration indicates a maximum number of transmissions and retransmissions of a transport block (TB) that the first UE can perform using resources configured by the SL CG transmission.

5. The method of claim 1 , wherein a set of periodic physical resources is available for the second UE to transmit the ACK, and wherein the ACK is received using a first available resource from the set of periodic physical resources that occurs later by at least a defined time gap than a time of successful reception of one of the SL retransmissions by the second UE.

6. The method of claim 1 , wherein each of the at least one SL transmission is a groupcast transmission to a group of UEs including the second UE, the method further comprising:

transmitting all of the first set of SL transmissions when no ACK is received from at least one UE of the group of UEs; or

transmitting fewer than all of the first set of SL transmissions when an ACK is received from all of the group of UEs before all of the first set of SL transmissions has been completed.

7. An apparatus, comprising:

at least one processor; and

a non-transitory computer readable storage medium storing programming, the programming including instructions that, when executed by the at least one processor, cause the apparatus perform operations, the operations comprising:

receiving, from a base station, a configuration indicating resources for a first set of sidelink (SL) transmissions from the apparatus to a second apparatus, the first set of SL transmissions including an initial SL transmission and a plurality of SL retransmissions;

after the receiving the configuration, transmitting, to the second apparatus, at least one SL transmission of the first set of SL transmissions from a first apparatus;

before transmitting a last SL retransmission of the plurality of SL retransmissions, receiving, from the second apparatus, an acknowledgement (ACK) for the at least one SL transmission; and

terminating transmission of the first set of SL transmissions in response to receiving the ACK by refraining from transmitting at least the last SL retransmission of the plurality of SL retransmissions.

8. The apparatus of claim 7 , wherein the transmitting the initial SL transmission comprises:

transmitting SL control information; and

transmitting an initial transmission of SL data, wherein the SL control information indicates the resources for the first set of SL transmissions.

9. The apparatus of claim 7 , the operations further comprising:

receiving a configuration of a resource pool; and

selecting the resources for the first set of SL transmissions from the resource pool.

10. The apparatus of claim 7 ,

wherein the first set of SL transmissions is an SL configured grant (CG) transmission, and

wherein the configuration indicates a maximum number of transmissions and retransmissions of a transport block (TB) that the apparatus can perform using resources configured by the SL CG transmission.

11. The apparatus of claim 7 , wherein a set of periodic physical resources is available for the second apparatus to transmit the ACK, and wherein the ACK is received using a first available resource from the set of periodic physical resources that occurs later by at least a defined time gap than a time of successful reception of one of the SL retransmissions by the second apparatus.

12. The apparatus of claim 7 , wherein each of the at least one SL transmission is a groupcast transmission to a group of apparatuses including the second apparatus, the operations further comprising:

transmitting all of the first set of SL transmissions when no ACK is received from at least one apparatus of the group of apparatuses; or

transmitting fewer than all of the first set of SL transmissions when an ACK is received from all of the group of apparatuses before all of the first set of SL transmissions has been completed.

13. The method of claim 4 ,

wherein the resources configured by the SL CG transmission comprise resources in at least a first period of resources and a second period of resources, and

wherein the first period of resources are for the first set of SL transmissions, and the second period of resources are for a second set of SL transmissions from the first UE to the second UE.

14. The method of claim 1 , wherein the configuration is received using a downlink control information (DCI) signaling.

15. The method of claim 1 , wherein the at least one SL transmission is an SL transmission immediately preceding the ACK, and wherein the refraining from transmitting at least the last SL retransmission comprises:

refraining from transmitting one or more SL transmissions of the first set of SL transmissions after the SL transmission immediately preceding the ACK.

16. The method of claim 1 , further comprising:

receiving, by the first UE from the second UE, an ACK or a negative acknowledgement (NACK) for each SL transmission of the at least one SL transmission.

17. The apparatus of claim 10 ,

wherein the resources configured by the SL CG transmission comprise resources in at least a first period of resources and a second period of resources, and

wherein the first period of resources are for the first set of SL transmissions, and the second period of resources are for a second set of SL transmissions from the apparatus to the second apparatus.

18. The apparatus of claim 7 , wherein the configuration is received using a downlink control information (DCI) signaling.

19. The apparatus of claim 7 , wherein the at least one SL transmission is an SL transmission immediately preceding the ACK, and wherein the refraining from transmitting at least the last SL retransmission comprises:

refraining from transmitting one or more SL transmissions of the first set of SL transmissions after the SL transmission immediately preceding the ACK.

20. The apparatus of claim 7 , further comprising:

receiving, from the second apparatus, an ACK or a negative acknowledgement (NACK) for each SL transmission of the at least one SL transmission.