Device and method of handling a system redirection procedure

A first base station (BS) of a first radio access technology (RAT) for handling a redirection for a communication device, comprising:

• at least one storage device;
• at least one processing circuit, coupled to the at least one storage device;
• and
• at least one communication interfacing device, coupled to the at least one processing circuit, wherein the at least one storage device stores instructions, and the at least one processing circuit is configured to execute the instructions of:
• receiving an Internet Protocol (IP) Multimedia Subsystem (IMS) signaling message from the communication device;
• determining to comprise, in a redirection message, RAT information indicating a second RAT and first core network (CN) information indicating a first CN;
• and
• transmitting the redirection message to the communication device via a first cell, wherein the redirection message orders the communication device to enter an idle state, comprises the RAT information indicating the second RAT and comprises the first CN information indicating the first CN;
• wherein the communication device selects a second cell of the second RAT according to the RAT information and generates a non-access-stratum (NAS) message for the first CN according to the first CN information, after receiving the redirection message from the first cell of the first RAT;
• wherein a second BS of the second RAT establishes a RRC connection via the second cell with the communication device and receives the NAS message on the RRC connection via the second cell from the communication device;
• wherein the first RAT is a New Radio (NR) and the second RAT is an Evolved Universal Terrestrial Radio Access (E-UTRA);
• wherein the first CN is a fifth generation core (5GC) network or an Evolved Packet Core (EPC) network.

A core network (CN) node for handling a redirection for a first communication device, comprising:

• at least one storage device;
• at least one processing circuit, coupled to the at least one storage device;
• and
• at least one communication interfacing device, coupled to the at least one processing circuit, wherein the at least one storage device stores instructions, and the at least one processing circuit is configured to execute the instructions of:
• connecting to the first communication device via a first BS of a first radio access technology (RAT);
• receiving an Internet Protocol (IP) Multimedia Subsystem (IMS) signaling message from the communication device;
• and
• transmitting a first interface message to the first BS for the first communication device in response to the IMS signaling message, wherein the first interface message indicates the first BS to redirect the first communication device to a second RAT and indicates a first CN;
• wherein the communication device selects a second cell of the second RAT, establishes a radio resource control (RRC) connection via the second cell to a second BS, and generates a non-access-stratum (NAS) message for the first CN;
• wherein the second BS of the second RAT receives the NAS message on the RRC connection via the second cell from the communication device;
• wherein the first RAT is a New Radio (NR) and the second RAT is an Evolved Universal Terrestrial Radio Access (E-UTRA);
• wherein the first CN is a fifth generation core (5GC) network or an Evolved Packet Core (EPC) network.