Acknowledgement feedback in unlicensed new radio
Methods, systems, and devices for wireless communications are described. A user equipment (UE) and a base station may communicate in an unlicensed spectrum (e.g., a shared radio frequency spectrum band). As such, the UE may determine a codebook size for transmitting hybrid access request (HARQ) acknowledgement (ACK) feedback with respect to the unlicensed spectrum. Accordingly, the UE may base the HARQ ACK codebook size on a number of HARQ processes with which the UE has been configured. Additionally or alternatively, the UE may base the HARQ ACK codebook size on a number and/or duration of downlink channel monitoring occasions indicated by the base station. In some cases, the UE may base the HARQ ACK codebook size on a combination of the techniques described herein.
1. A method for wireless communication at a user equipment (UE) in a shared radio frequency spectrum band, comprising:
identifying at least one downlink channel monitoring occasion;
receiving, at the UE, a feedback trigger for transmission of hybrid automatic repeat request (HARQ) feedback, wherein the HARQ feedback trigger is embedded in a downlink control information, DCI, that is separate from a grant for a downlink channel and wherein the HARQ feedback trigger is jointly coded with a ACK delay indication;
determining a HARQ acknowledgement (ACK) codebook size based at least in part on a number of configured HARQ processes, on the downlink channel monitoring occasions, or a combination thereof; and
transmitting the HARQ feedback in accordance with the HARQ ACK codebook size and the ACK delay indication.
2. The method of claim 1 , further comprising:
populating the HARQ feedback based at least in part on the ACK delay indication and receipt of the downlink channel.
3. The method of claim 2 , wherein populating the HARQ feedback comprises:
populating the HARQ feedback using decoding results when an ACK ready time for the downlink channel before reception of the feedback trigger, wherein the ACK ready time is a function of the ACK delay indication and the receipt of the downlink channel.
4. The method of claim 2 , wherein populating the HARQ feedback comprises:
populating the HARQ feedback using default values when an ACK ready time for the downlink channel is greater than an ACK transmission time.
5. The method of claim 2 , wherein populating the HARQ feedback with default values comprises:
populating the HARQ feedback using an ACK value, a negative ACK (NACK) value, or a previous ACK/NACK value.
6. The method of claim 1 , further comprising:
identifying a duration for the HARQ feedback to be reported;
identifying a subset of downlink channel monitoring occasions within the duration for the HARQ feedback to be reported; and
determining the HARQ ACK codebook size based at least in part on the number of downlink channel monitoring occasions in the subset.
7. The method of claim 6 , wherein a control message received in downlink control information indicates whether the subset of downlink channel monitoring occasions comprises an ACK transmission time, whether the subset comprises different channel monitoring occasions, or a combination thereof.
8. The method of claim 7 , wherein the ACK transmission time comprises a sum of data transmission time and an ACK delay value corresponding to the ACK delay indication.
9. The method of claim 7 , wherein the message comprises an explicit indication received in downlink control information.
10. The method of claim 7 , wherein the message comprises an implicit indication based at least in part on comparing a HARQ feedback parameter conveyed in downlink control information with a parameter conveyed in radio resource control.
11. The method of claim 6 , wherein a duration of the downlink channel monitoring occasion is based at least in part on:
receiving, during a transmission opportunity (TxOp) in which a downlink channel monitoring occasion is to occur but before the downlink channel monitoring occasion occurs, information from which the UE is able to determine the duration of the downlink channel monitoring occasion.
12. The method of claim 6 , wherein determining the HARQ ACK codebook size comprises:
receiving, in downlink control information, an indication of the number of downlink channel monitoring occasions to include in determining the HARQ ACK codebook size.
13. The method of claim 6 , wherein determining the HARQ ACK codebook size comprises:
receiving, in downlink control information, an indication of a set of downlink channel monitoring occasions to include in determining the HARQ ACK codebook size, wherein the set is one of a plurality of predefined sets of downlink channel monitoring occasions.
14. The method of claim 13 , further comprising:
receiving the plurality of predefined sets of downlink channel monitoring occasions via radio resource control (RRC) signaling.
15. The method of claim 6 , wherein the HARQ ACK codebook size encompasses a number of transmission time intervals (TTIs), wherein at least some of the number of TTIs are non-contiguous.
16. The method of claim 15 , wherein the number of TTIs encompassed by the HARQ ACK codebook size spans two different transmission opportunities (TxOps).
17. The method of claim 6 , further comprising:
determining the HARQ ACK codebook size based at least in part on the duration of the downlink channel monitoring occasion for a first set of HARQ feedback instances of the HARQ feedback; and
determining the HARQ ACK codebook size based at least in part on a number of configured HARQ processes for a second set of HARQ feedback instances of the HARQ feedback.
18. The method of claim 17 , further comprising:
receiving a feedback trigger that indicates whether the HARQ ACK codebook size determination is to be based on the duration of the at least one downlink channel monitoring occasion or on the number of configured HARQ processes.
19. The method of claim 6 , wherein the number of downlink channel monitoring occasions are identified based at least in part on a time location of the HARQ feedback.
20. The method of claim 6 , wherein receiving the feedback trigger comprises:
receiving a trigger for HARQ feedback in DCI for all of the configured HARQ processes or a subset of the configured HARQ processes for the UE via radio resource control (RRC) signaling.
21. The method of claim 20 , wherein the subset of the configured HARQ processes comprises a bitmap indication with each bit in the bitmap corresponding to one HARQ process or a group of HARQ processes.