Method and apparatus for remote UE supporting link identifier update procedure in UE-to-UE relay communication in a wireless communication system

1. A method for a first User Equipment (UE), comprising:

the first UE establishes a PC5 connection with a relay UE for supporting a relay communication between the first UE and a second UE via the relay UE, wherein a direct link context associated with the relay communication includes a first layer-2 Identity (ID) of the second UE;

the first UE initiates a link identifier update (LIU) procedure with the second UE via the relay UE, wherein the first UE receives a LIU response message from the second UE in the LIU procedure, and the LIU response message includes a second layer-2 ID of the second UE; and

the first UE does not update the direct link context with the second layer-2 ID of the second UE.

2. The method of claim 1 , further comprising:

the first UE sends or receives a sidelink relay adaptation protocol (SRAP) packet to or from the relay UE for the relay communication after the LIU procedure is completed, wherein the SRAP packet includes a first local ID associated with the first layer-2 ID of the second UE.

3. The method of claim 1 , further comprising:

the first UE receives a third layer-2 ID of the second UE and a second local Identity (ID) associated with the third layer-2 ID of the second UE from the relay UE.

4. The method of claim 3 , further comprising:

the first UE updates the direct link context with the third layer-2 ID of the second UE.

5. The method of claim 3 , further comprising:

the first UE sends or receives a sidelink relay adaptation protocol (SRAP) packet to or from the relay UE for the relay communication after receiving the third layer-2 ID of the second UE and the second local ID associated with the third layer-2 ID of the second UE from the relay UE, wherein the SRAP packet includes the second local ID associated with the third layer-2 ID of the second UE.

6. The method of claim 3 , wherein the first UE receives the third layer-2 ID of the second UE and the second local ID associated with the third layer-2 ID of the second UE via a PC5 Radio Resource Control (RRC) message.

7. The method of claim 1 , wherein the first UE keeps the first layer-2 ID of the second UE in the direct link context without updating the direct link context with the second layer-2 ID of the second UE.

8. A first User Equipment (UE), comprising:

a control circuit;

a processor installed in the control circuit; and

a memory installed in the control circuit and operatively coupled to the processor;

wherein the processor is configured to execute a program code stored in the memory to:

establish a PC5 connection with a relay UE for supporting a relay communication between the first UE and a second UE via the relay UE, wherein a direct link context associated with the relay communication includes a first layer-2 Identity (ID) of the second UE;

initiate a link identifier update (LIU) procedure with the second UE via the relay UE, wherein the first UE receives a LIU response message from the second UE in the LIU procedure, and the LIU response message includes a second layer-2 ID of the second UE; and

not update the direct link context with the second layer-2 ID of the second UE.

9. The first UE of claim 8 , wherein the processor is further configured to execute a program code stored in the memory to:

send or receive a sidelink relay adaptation protocol (SRAP) packet to or from the relay UE for the relay communication after the LIU procedure is completed, wherein the SRAP packet includes a first local ID associated with the first layer-2 ID of the second UE.

10. The first UE of claim 8 , wherein the processor is further configured to execute a program code stored in the memory to:

receive a third ID of the second UE and a second local Identity (ID) associated with the third layer-2 ID of the second UE from the relay UE.

11. The first UE of claim 10 , wherein the processor is further configured to execute a program code stored in the memory to:

update the direct link context with the third layer-2 ID of the second UE.

12. The first UE of claim 10 , wherein the processor is further configured to execute a program code stored in the memory to:

send or receive a sidelink relay adaptation protocol (SRAP) packet to or from the relay UE for the relay communication after receiving the third layer-2 ID of the second UE and the second local ID associated with the third layer-2 ID of the second UE from the relay UE, wherein the SRAP packet includes the second local ID associated with the third layer-2 ID of the second UE.

13. The first UE of claim 10 , wherein the first UE receives the third layer-2 ID of the second UE and the second local ID associated with the third layer-2 ID of the second UE via a PC5 Radio Resource Control (RRC) message.

14. A method for a second User Equipment (UE), comprising:

the second UE establishes a PC5 connection with a relay UE for supporting a relay communication between a first UE and the second UE via the relay UE, wherein a direct link context associated with the relay communication includes a first layer-2 Identity (ID) of the first UE;

the second UE performs a link identifier update (LIU) procedure with the first UE via the relay UE, wherein the second UE receives a LIU request message from the first UE in the LIU procedure, and the LIU request message includes a second layer-2 ID of the first UE; and

the second UE does not update the direct link context with the second layer-2 ID of the first UE.

15. The method of claim 14 , further comprising:

the second UE sends or receives a sidelink relay adaptation protocol (SRAP) packet to or from the relay UE for the relay communication after the LIU procedure is completed, wherein the SRAP packet includes a first local ID associated with the first layer-2 ID of the first UE.

16. The method of claim 14 , further comprising:

the second UE receives a third layer-2 ID of the first UE and a second local Identity (ID) associated with the third layer-2 ID of the first UE from the relay UE.

17. The method of claim 16 , further comprising:

the second UE updates the direct link context with the third layer-2 ID of the first UE.

18. The method of claim 16 , further comprising:

the second UE sends or receives a sidelink relay adaptation protocol (SRAP) packet to or from the relay UE for the relay communication after receiving the third layer-2 ID layer-2 ID of the first UE and the second local ID associated with the third layer-2 ID layer-2 ID of the first UE from the relay UE, wherein the SRAP packet includes the second local ID associated with the third layer-2 ID layer-2 ID of the first UE.

19. The method of claim 16 , wherein the second UE receives the third layer-2 ID layer-2 ID of the first UE and the second local ID associated with the third layer-2 ID layer-2 ID of the first UE via a PC5 Radio Resource Control (RRC) message.

20. The method of claim 14 , wherein the first UE keeps the first layer-2 ID of the second UE in the direct link context without updating the direct link context with the second layer-2 ID of the second UE.