Method and apparatus for sidelink signaling radio bearer (SRB) establishment in a wireless communication system

1. A method for unicast link establishment by a first User Equipment (UE), comprising:

establishing a first sidelink Signaling Radio Bearer (SRB) for PC5-S message transmission, wherein the first sidelink SRB is associated with a default Destination Layer-2 Identity (ID);

transmitting a first PC5-S message on the first sidelink SRB with a Source Layer-2 ID derived from a Layer-2 ID of the first UE and a Destination Layer-2 ID derived from the default Destination Layer-2 ID;

receiving a second PC5-S message from a second UE, wherein the second PC5-S message is transmitted with a Source Layer-2 ID derived from a Layer-2 ID of the second UE and a Destination Layer-2 ID derived from the Layer-2 ID of the first UE; and

establishing a second sidelink SRB for PC5-S message reception and/or transmission, wherein the second sidelink SRB is associated with the Layer-2 ID of the second UE.

2. The method of claim 1 , further comprising establishing a third sidelink SRB for PC5-Radio Resource Control (PC5-RRC) message transmission in response to a unicast link or the PC5-RRC connection establishment being successfully completed, wherein the third sidelink SRB is associated with the Layer-2 ID of the second UE.

3. The method of claim 2 , further comprising:

establishing a fourth sidelink SRB for protected PC5-S message transmission, wherein the fourth sidelink SRB is associated with the Layer-2 ID of the second UE.

4. The method of claim 2 , further comprising:

not establishing any sidelink SRB for PC5-RRC message transmission before the unicast link or the PC5-RRC connection establishment has been successfully completed.

5. The method of claim 2 , wherein a second Sidelink Control Channel (SCCH) configuration specified in a RRC specification is used for establishing the third sidelink SRB.

6. The method of claim 1 , wherein the second sidelink SRB is established in response to reception of the second PC5-S message by a physical layer of the first UE.

7. The method of claim 1 , wherein a first Sidelink Control Channel (SCCH) configuration specified in a Radio Resource Control (RRC) specification is used for establishing the first sidelink SRB and the second sidelink SRB.

8. The method of claim 1 , wherein the default Destination Layer-2 ID is associated with a sidelink service for which a unicast link or a PC5-Radio Resource Control (PC5-RRC) connection is established.

9. The method of claim 1 , wherein the first PC5-S message is a Direct Communication Request.

10. The method of claim 1 , wherein the second PC5-S message is a security related message.

11. The method of claim 1 , wherein each sidelink SRB corresponds to a Sidelink Control Channel (SCCH).

12. A first UE (User Equipment) for establishing unicast link, comprising:

a processor; and

a memory operatively coupled to the processor, wherein the processor is configured to execute a program code to:

establish a first sidelink Signaling Radio Bearer (SRB) for PC5-S message transmission in response to a unicast link or a PCS-Radio Resource Control (PC5-RRC) connection establishment being initiated, wherein the first sidelink SRB is associated with a default Destination Layer-2 Identity (ID);

transmit a first PC5-S message for the unicast link or the PC5-RRC connection establishment on the first sidelink SRB with a Source Layer-2 ID derived from a Layer-2 ID of the first UE and a Destination Layer-2 ID derived from the default Destination Layer-2 ID;

receive a second PC5-S message from a second UE, wherein the second PC5-S message is transmitted with a Source Layer-2 ID derived from a Layer-2 ID of the second UE and a Destination Layer-2 ID derived from the Layer-2 ID of the first UE; and

establish a second sidelink SRB for PC5-S message reception and/or transmission, wherein the second sidelink SRB is associated with the Layer-2 ID of the second UE.

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

establish a third sidelink SRB for PC5-RRC message transmission in response to the unicast link or the PC5-RRC connection establishment being successfully completed, wherein the third sidelink SRB is associated with the Layer-2 ID of the second UE.

14. The first UE of claim 13 , wherein the processor is further configured to execute a program code to:

establish a fourth sidelink SRB for protected PC5-S message transmission, wherein the fourth sidelink SRB is associated with the Layer-2 ID of the second UE.

15. The first UE of claim 13 , wherein a second Sidelink Control Channel (SCCH) configuration specified in a RRC specification is used for establishing the third sidelink SRB.

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

not establish any sidelink SRB for PC5-RRC message transmission before the unicast link or the PC5-RRC connection establishment has been successfully completed.

17. The first UE of claim 12 , wherein the second sidelink SRB is established in response to reception of the second PC5-S message by a physical layer of the first UE.

18. The first UE of claim 12 , wherein a first Sidelink Control Channel (SCCH) configuration specified in a RRC specification is used for establishing the first sidelink SRB and the second sidelink SRB.

19. The first UE of claim 12 , wherein the first PC5-S message is a Direct Communication Request.

20. The first UE of claim 12 , wherein the second PC5-S message is a security related message.