User equipment and method for sidelink communication in unlicensed frequency spectrum by same

1 . A user equipment (UE) for sidelink (SL) communication in an unlicensed frequency spectrum, comprising:

a memory;

a transceiver; and

a processor coupled to the memory and the transceiver; wherein:

the transceiver is configured to receive a configuration for a SL transmission;

the processor is configured to monitor a hybrid automatic repeat request-acknowledgement (HARQ-ACK) response in a physical sidelink feedback channel (PSFCH) in a slot where the processor expects the HARQ-ACK response to an earlier transmission, and determine whether a retransmission of the earlier transmission is necessary or not according to the HARQ-ACK response;

the processor is configured to perform a listen-before-talk (LBT) operation to gain access to a channel according to the HARQ-ACK response;

the transceiver is configured to perform the SL transmission after the LBT operation; and

the transceiver is further configured to:

receive a configuration of a report cycle;

periodically report, to a higher layer of the UE, a reserved but unused/released SL resource associated with the HARQ-ACK response once every report cycle;

perform, in response to the retransmission of the earlier transmission being determined to be necessary, the retransmission of the earlier transmission; and

perform, in response to the retransmission of the earlier transmission being determined to be not necessary, transmission of dummy data by using the reserved but unused/released SL resource.

2 . The UE of claim 1 , wherein monitoring the HARQ-ACK response in the PSFCH comprises at least one of the following:

detecting a positive acknowledgement (ACK) response in the PSFCH;

decoding a physical sidelink shared channel (PSSCH); or

a non-detection of a negative acknowledgement (NACK) response in the PSFCH.

3 . The UE of claim 2 , wherein detecting the ACK response in the PSFCH is used for a SL unicast transmission.

4 . The UE of claim 2 , wherein decoding the PSSCH is used for a SL unicast transmission.

5 . The UE of claim 2 , wherein the non-detection of the NACK response in the PSFCH is used for a SL groupcast transmission.

6 . The UE of claim 1 , wherein performing the SL transmission after the LBT operation comprises transmitting a PSCCH and/or a PSSCH using the reserved but unused/released SL resource associated with the HARQ-ACK response.

7 . The UE of claim 1 , wherein the retransmission of the earlier transmission is determined to be necessary according to detecting a NACK response in the PSFCH.

8 . The UE of claim 1 , wherein the retransmission of the earlier transmission is determined to be not necessary according to detecting only an ACK response in the PSFCH or a non-detection of a NACK response in the PSFCH.

9 . The UE of claim 1 , wherein the SL transmission is further used for at least one of the following:

the retransmission of the earlier transmission;

a new PSSCH transmission; or

a keep alive message.

10 . The UE of claim 1 , wherein performing the SL transmission after the LBT operation comprises transmitting the SL transmission in a pre-allocated timing slot.

11 . A method for sidelink (SL) communication in an unlicensed frequency spectrum by a user equipment (UE), comprising:

receiving a configuration for a SL transmission;

monitoring a hybrid automatic repeat request-acknowledgement (HARQ-ACK) response in a physical sidelink feedback channel (PSFCH) in a slot where the UE expects the HARQ-ACK response to an earlier transmission, and determine whether a retransmission of the earlier transmission is necessary or not according to the HARQ-ACK response;

performing a listen-before-talk (LBT) operation to gain access to a channel according to the HARQ-ACK response; and

performing the SL transmission after the LBT operation;

wherein the method further comprises:

receiving a configuration of a report cycle;

periodically reporting, to a higher layer of the UE, a reserved but unused/released SL resource associated with the HARQ-ACK response once every report cycle;

performing, in response to the retransmission of the earlier transmission being determined to be necessary, the retransmission of the earlier transmission; and

performing, in response to the retransmission of the earlier transmission being determined to be not necessary, transmission of dummy data by using the reserved but unused/released SL resource.

12 . A non-transitory machine-readable storage medium having stored thereon instructions that, when executed by a user equipment (UE), cause the UE to perform actions comprising:

receiving a configuration for a SL transmission;

monitoring a hybrid automatic repeat request-acknowledgement (HARQ-ACK) response in a physical sidelink feedback channel (PSFCH) in a slot where the UE expects the HARQ-ACK response to an earlier transmission, and determine whether a retransmission of the earlier transmission is necessary or not according to the HARQ-ACK response;

performing a listen-before-talk (LBT) operation to gain access to a channel according to the HARQ-ACK response; and

performing the SL transmission after the LBT operation;

wherein the actions further comprise:

receiving a configuration of a report cycle;

periodically reporting, to a higher layer of the UE, a reserved but unused/released SL resource associated with the HARQ-ACK response once every report cycle;

perform, in response to the retransmission of the earlier transmission being determined to be necessary, the retransmission of the earlier transmission; and

perform, in response to the retransmission of the earlier transmission being determined to be not necessary, transmission of dummy data by using the reserved but unused/released SL resource.