Handover method in wireless communication system and apparatus therefor

A Source-Radio Access Network (S-RAN) for supporting a User Equipment (UE) handover in a wireless communication system, the S-RAN comprising:

• a transceiver for transmission and receiving a signal;
• a processor for controlling the transceiver, and
• a memory operably connected to the processor and storing instructions that, when executed by the processor, configure the processor to perform operations comprising:
• determining whether a handover of the UE is required;
• transmitting, to a Source-AMF (S-AMF), a first message representing that the handover of the UE is required;
• receiving, from the S-AMF, a second message representing a handover command related to the handover of the UE;
• and
• transmitting, to the UE, a handover command based on the second message,
• wherein, based on the first message, a create UE context request message is transmitted from the S-AMF to a Target-Access and Mobility Management Function (T-AMF),
• wherein the create UE context request message includes (i) Packet Data Unit (PDU) session Identifiers (IDs) which are related to a target of the handover and (ii) Single-Network Slice Selection Assistance Information (S-NSSAI) corresponding to each of the PDU session IDs,
• wherein, based on the create UE context request message, a PDU session update Session Management (SM) context request message is transmitted from the T-AMF to a Session Management Function (SMF) only for the PDU session ID corresponding to the S-NSSAI determined to be available in the T-AMF among the PDU session IDs,
• wherein, based on the PDU session update SM context request message, a PDU session update SM context response message is transmitted from the SMF to the T-AMF, and the PDU session update SM context response message includes a PDU session ID of which handover is accepted by the SMF,
• wherein, based on the PDU session update SM context response message, a non-accepted PDU session list is generated by the T-AMF, the non-accepted PDU session list includes (i) a PDU session ID for which handover is not accepted by the T-AMF and/or the SMF, and (ii) a cause of non-acceptance for the PDU session ID for which handover is not accepted,
• wherein the cause of non-acceptance of handover includes information representing that the handover is not accepted by the T-AMF because the PDU session is related to an S-NSSAI which is determined to be not available in the T-AMF, and
• wherein the non-accepted PDU session list is transmitted from the T-AMF to the S-RAN.

A method performed by a User Equipment (UE) in a wireless communication system, the method comprising:

• establishing one or more Packet Data Unit (PDU) sessions related to Single-Network Slice Selection Assistance Information (S-NSSAI);
• and
• based on determination of a Source-Radio Access Network (S-RAN) that a handover of the UE is required, receiving a handover command from the S-RAN;
• wherein, based on the determination of the S-RAN, a first message representing that the handover of the UE is required is transmitted from the S-RAN to a Source-AMF (S-AMF) and a second message representing the handover command, based on the first message, is transmitted from the S-AMF to the S-RAN;
• wherein, based on the first message, a create UE context request message is transmitted from the S-AMF to a Target-Access and Mobility Management Function (T-AMF),
• wherein the create UE context request message includes (i) PDU session Identifiers (IDs) which are related to a target of the handover and (ii) S-NSSAI corresponding to each of the PDU session IDs,
• wherein, based on the create UE context request message, a PDU session update Session Management (SM) context request message is transmitted from the T-AMF to a Session Management Function (SMF) only for the PDU session ID corresponding to the S-NSSAI determined to be available in the T-AMF among the PDU session IDs,
• wherein, based on the PDU session update SM context request message, a PDU session update SM context response message is transmitted from the SMF to the T-AMF, and the PDU session update SM context response message includes a PDU session ID of which handover is accepted by the SMF,
• wherein, based on the PDU session update SM context response message, a non-accepted PDU session list is generated by the T-AMF, the non-accepted PDU session list includes (i) a PDU session ID for which handover is not accepted by the T-AMF and/or the SMF, and (ii) a cause of non-acceptance for the PDU session ID for which handover is not accepted,
• wherein the cause of non-acceptance of handover includes information representing that the handover is not accepted by the T-AMF because the PDU session is related to an S-NSSAI which is determined to be not available in the T-AMF, and
• wherein the non-accepted PDU session list is transmitted from the T-AMF to the S-RAN.