Device and method of handling a radio resource control reestablishment

A method for handling a radio resource control (RRC) reestablishment for a communication device, the method comprising:

• when the communication device is in a RRC connected state:
• detecting a failure;
• transmitting a first RRC Reestablishment Request message to a first radio access network (RAN) in response to the failure;
• receiving a first RRC Setup message for responding to the first RRC Reestablishment Request message, from the first RAN;
• and
• transmitting a first RRC Setup Complete message for responding to the first RRC Setup message, to the first RAN, wherein the first RRC Setup Complete message comprises a Fifth Generation (5G) System Architecture Evolution (SAE) Temporary Mobile Station Identifier (TMSI) (5G-S-TMSI));
• when the communication device is in a RRC idle state,
• transmitting a RRC Setup Request message to the first RAN via a signaling radio bearer 0 (SRB0), wherein the RRC Setup Request message comprises a first part of the 5G-S-TMSI;
• receiving a second RRC Setup message for responding to the RRC Setup Request message, from the first RAN;
• and
• entering the RRC connected state from a RRC idle state in response to the second RRC Setup message,
• transmitting a second RRC Setup Complete message to the first RAN in response to the second RRC Setup message, wherein the second RRC Setup Complete message comprises a second part of the 5G-S-TMSI;
• wherein the first part of the 5G-S-TMSI and the second part of the 5G-S-TMSI form the 5G-S-TMSI;
• wherein the failure comprises an integrity check failure indication from at least one lower layer related to a first SRB 1 (SRB1) or a first SRB 2 (SRB2).

A base station for handling a radio resource control (RRC) reestablishment, comprising:

• at least one storage device;
• and
• at least one processing circuit, coupled to the at least one storage device, wherein the at least one storage device stores instructions, and the at least one processing circuit is configured to execute the instructions of:
• receiving a first RRC Reestablishment Request message from a user equipment (UE) operating in a RRC connected state and detecting a failure;
• transmitting a first RRC Setup message for responding to the first RRC Reestablishment Request message, to the UE;
• receiving a first RRC Setup Complete message for responding to the first RRC Setup message, from the UE, wherein the first RRC Setup Complete message comprises a Fifth Generation (5G) System Architecture Evolution (SAE) Temporary Mobile Station Identifier (TMSI) (5G-S-TMSI);
• receiving a RRC Setup Request message from the UE operating in a RRC idle state via a signaling radio bearer 0 (SRB0), wherein the RRC Setup Request message comprises a first part of the 5G-S-TMSI;
• transmitting a second RRC Setup message for responding to the RRC Setup Request message, to the UE;
• and
• receiving a second RRC Setup Complete message from the UE in response to the second RRC Setup message, wherein the second RRC Setup Complete message comprises a second part of the 5G-S-TMSI;
• wherein the first part of the 5G-S-TMSI and the second part of the 5G-S-TMSI form the 5G-S-TMSI;
• wherein the failure comprises an integrity check failure indication from at least one lower layer related to a first SRB 1 (SRB1) or a first SRB 2 (SRB2).