Method and apparatus for reselecting relay based on SL RLF

1. A method for performing wireless communication by a first user equipment (UE), the method comprising:

establishing a first PC5 radio resource control (RRC) connection between the first UE and a second UE, wherein the first UE is a remote UE that communicates with a network via a relay UE, and the second UE is a relay UE that provides connectivity to the network for the remote UE;

detecting a radio link failure (RLF) for the first PC5 RRC connection between the first UE and the second UE; and

performing a reselection of a relay UE based on detecting the RLF for the first PC5 RRC connection between the first UE and the second UE,

wherein the reselection of a relay UE comprises:

selecting a third UE as a relay UE; and

establishing a second PC5 RRC connection between the first UE and the third UE, and

wherein each of the first PC5 RRC connection and the second PC5 RRC connection comprises a sidelink connection.

2. The method of claim 1 , further comprising:

transmitting, to the second UE, first sidelink control information (SCI) through a physical sidelink control channel (PSCCH);

transmitting, to the second UE, second SCI and data through a physical sidelink shared channel (PSSCH) related to the PSCCH; and

determining a physical sidelink feedback channel (PSFCH) resource based on an index of a subchannel and a slot related to the PSSCH,

wherein, based on a number of failures to receive a sidelink (SL) hybrid automatic repeat request (HARQ) feedback for the data from the second UE on the PSFCH resource reaching a first threshold, the RLF for the first PC5 RRC connection is detected by the first UE.

3. The method of claim 1 , wherein, based on a number of out-of-syncs (DDSs) transferred from a physical layer of the first UE to an RRC layer of the first UE reaching a second threshold, the RLF for the first PC5 RRC connection is detected by the first UE.

4. The method of claim 1 , wherein, based on detecting an integrity check failure, the RLF for the first PC5 RRC connection is detected by the first UE.

5. The method of claim 1 , wherein the first UE is allowed to perform the reselection of a relay UE while a timer is running.

6. The method of claim 5 , wherein the timer is configured for the first UE.

7. The method of claim 1 , wherein, based on detecting the RLF for the first PC5 RRC connection, a signaling radio bearer (SRB) and a data radio bearer (DRB) related to the first PC5 RRC connection between the first UE and the second UE is suspended.

8. The method of claim 1 , further comprising:

based on detecting the RLF for the first PC5 RRC connection, transmitting a message including (i) a cause of the RLF, (ii) a list of candidate relay UEs, and (iii) a result of a channel measurement for the candidate relay UEs, to the network.

9. The method of claim 8 , wherein the cause of the RLF includes at least one of information representing that the RLF is related to expiration of a timer, information representing that the RLF is related to a packet data convergence protocol (PDCP) layer, information representing that the RLF is related to a radio link control (RLC) layer, information representing that the RLF is related to a medium access control (MAC) layer, or information representing that the RLF is related to a physical layer.

10. The method of claim 1 ,

wherein the reselection of the relay UE further comprises performing a channel measurement for at least one candidate relay UE, and

wherein selecting the third UE comprises selecting the third UE for which a result of the channel measurement exceeds a third threshold from among the at least one candidate relay UE as a relay UE.

11. The method of claim 10 , further comprising:

transmitting a message including a UE ID of the third UE to the network.

12. The method of claim 10 , wherein the third threshold is configured by the network for the first UE.

13. The method of claim 1 , further comprising:

based on failure of the reselection of a relay UE, transmitting a message including (i) information representing the failure of the reselection and (ii) a cause of the RLF, to the network.

14. The method of claim 13 , wherein the cause of the RLF includes at least one of information representing that the RLF is related to expiration of a timer, information representing that the RLF is related to a packet data convergence protocol (PDCP) layer, information representing that the RLF is related to a radio link control (RLC) layer, information representing that the RLF is related to a medium access control (MAC) layer, or information representing that the RLF is related to a physical layer.

15. A first user equipment (UE) configured to perform wireless communication, the first UE comprising:

one or more memories storing instructions;

one or more transceivers; and

one or more processors connected to the one or more memories and the one or more transceivers, wherein the one or more processors execute the instructions to:

establish a first PC5 radio resource control (RRC) connection between the first UE and a second UE, wherein the first UE is a remote UE that communicates with a network via a relay UE, and the second UE is a relay UE that provides connectivity to the network for the remote UE;

detect a radio link failure (RLF) for the first PC5 RRC connection between the first UE and the second UE; and

perform a reselection of a relay UE based on detecting the RLF for the first PC5 RRC connection between the first UE and the second UE,

wherein the reselection of a relay UE comprises:

selecting a third UE as a relay UE; and

establishing a second PC5 RRC connection between the first UE and the third UE, and

wherein each of the first PC5 RRC connection and the second PC5 RRC connection comprises a sidelink connection.

16. The first UE of claim 15 ,

wherein the one or more processors execute the instructions to:

transmit, to the second UE, first sidelink control information (SCI) through a physical sidelink control channel (PSCCH);

transmit, to the second UE, second SCI and data through a physical sidelink shared channel (PSSCH) related to the PSCCH; and

determine a physical sidelink feedback channel (PSFCH) resource based on an index of a subchannel and a slot related to the PSSCH,

wherein, based on a number of failures to receive a sidelink (SL) hybrid automatic repeat request (HARQ) feedback for the data from the second UE on the PSFCH resource reaching a first threshold, the RLF for the first PC5 RRC connection is detected by the first UE.

17. An apparatus configured to control a first user equipment (UE), the apparatus comprising:

one or more processors; and

one or more memories operably connected to the one or more processors and storing instructions, wherein the one or more processors execute the instructions to:

establish a first PC5 radio resource control (RRC) connection between the first UE and a second UE, wherein the first UE is a remote UE that communicates with a network via a relay UE, and the second UE is a relay UE that provides connectivity to the network for the remote UE;

detect a radio link failure (RLF) for the first PC5 RRC connection between the first UE and the second UE; and

perform a reselection of a relay UE based on detecting the RLF for the first PC5 RRC connection between the first UE and the second UE,

wherein the reselection of a relay UE comprises:

selecting a third UE as a relay UE; and

establishing a second PC5 RRC connection between the first UE and the third UE, and

wherein each of the first PC5 RRC connection and the second PC5 RRC connection comprises a sidelink connection.

18. The apparatus of claim 17 , wherein the one or more processors execute the instructions to:

transmit, to the second UE, first sidelink control information (SCI) through a physical sidelink control channel (PSCCH);

transmit, to the second UE, second SCI and data through a physical sidelink shared channel (PSSCH) related to the PSCCH; and

determine a physical sidelink feedback channel (PSFCH) resource based on an index of a subchannel and a slot related to the PSSCH,

wherein, based on a number of failures to receive a sidelink (SL) hybrid automatic repeat request (HARQ) feedback for the data from the second UE on the PSFCH resource reaching a first threshold, the RLF for the first PC5 RRC connection is detected by the first UE.