Method and apparatus for enhancing handover procedure for supporting conditional handover in wireless communication system

1. A method performed by a source base station in a conditional handover procedure, the method comprising:

receiving a message informing that a wireless device is handed over from the source base station to a target base station, wherein the target base station is one of candidate target base stations in the conditional handover procedure; and

after receiving the message, transmitting a sequence number (SN) status transfer message to the target base station,

wherein a handover request message is transmitted by the source base station to each of the candidate target base stations,

wherein a handover request acknowledge message is received by the source base station from each of the candidate target base stations, and

wherein the handover request acknowledge message includes a handover command for each of the candidate target base stations.

2. The method of claim 1 , wherein data is forwarded to the target base station after transmitting the SN status transfer message to the target base station.

3. The method of claim 1 , wherein the message is a handover success message.

4. The method of claim 1 , wherein the message requests triggering a transmission of the SN status transfer message.

5. The method of claim 1 , wherein the message requests triggering of a handover cancel procedure.

6. The method of claim 5 , wherein the handover cancel procedure is performed towards the candidate target base stations except the target base station.

7. The method of claim 1 , wherein the wireless device is already detached from the source base station.

8. The method of claim 1 , wherein the target base station is a base station which receives a reconfiguration complete message from the wireless device.

9. The method of claim 1 , wherein a user equipment (UE) handover complete message is received by the source base station from the wireless device, before the wireless device is detached from the source base station.

10. The method of claim 9 , wherein the UE handover complete message includes an identifier (ID) of a cell served by the target base station.

11. The method of claim 1 , wherein the wireless device is in communication with at least one of a user equipment, a network, or autonomous vehicles other than the wireless device.

12. A processing device configured to control a source base station to operate a conditional handover procedure, the processing device comprising:

at least one processor; and

at least one computer memory operably connectable to the at least one processor and storing instructions that, based on being executed by the at least one processor, perform operations comprising:

receiving a message informing that a wireless device is handed over from the source base station to a target base station, wherein the target base station is one of candidate target base stations in the conditional handover procedure; and

after receiving the message, transmitting a sequence number (SN) status transfer message to the target base station,

wherein a handover request message is transmitted by the source base station to each of the candidate target base stations,

wherein a handover request acknowledge message is received by the source base station from each of the candidate target base stations, and

wherein the handover request acknowledge message includes a handover command for each of the candidate target base stations.

13. A source base station configured to operate a conditional handover procedure, the source base station comprising:

a transceiver;

at least one processor; and

at least one computer memory operably connectable to the at least one processor and storing instructions that, based on being executed by the at least one processor, perform operations comprising:

receiving, through the transceiver, a message informing that a wireless device is handed over from the source base station to a target base station, wherein the target base station is one of candidate target base stations in the conditional handover procedure; and

after receiving the message, transmitting, through the transceiver, a sequence number (SN) status transfer message to the target base station,

wherein a handover request message is transmitted by the source base station to each of the candidate target base stations,

wherein a handover request acknowledge message is received by the source base station from each of the candidate target base stations, and

wherein the handover request acknowledge message includes a handover command for each of the candidate target base stations.

14. The source base station of claim 13 , wherein data is forwarded to the target base station after transmitting the SN status transfer message to the target base station.

15. The source base station of claim 13 , wherein the message is a handover success message.

16. The source base station of claim 13 , wherein the message requests triggering a transmission of the SN status transfer message.

17. The source base station of claim 13 , wherein the message requests triggering of a handover cancel procedure.

18. The source base station of claim 17 , wherein the handover cancel procedure is performed towards the candidate target base stations except the target base station.

19. The source base station of claim 13 , wherein the wireless device is already detached from the source base station.

20. The source base station of claim 13 , wherein the target base station is a base station which receives a reconfiguration complete message from the wireless device.