Device and method of handling tight interworking of two cellular systems

1. A communication device for handling a first radio access technology (RAT) and second RAT tight interworking, comprising:

a storage device, for storing instructions of:

connecting to a first base station (BS) via a first RAT;

receiving a first message from the first BS via the first RAT, wherein the first message comprises a second message and a first transaction identifier (TI), and the second message configures the communication device to connect to a second BS via the second RAT and comprises a second TI;

transmitting a first response message to the first BS via the first RAT in response to the first message, wherein the first response message comprises the first TI and a second response message, and the second response message comprises the second TI;

connecting to the second BS via the second RAT in response to the second message; and

communicating second data via the second RAT with the second BS according to a configuration of the second RAT in the second message, and communicating, simultaneously with the second data, first data via the first RAT with the first BS; and

a processing circuit, coupled to the storage device, configured to execute the instructions stored in the storage device.

2. The communication device of claim 1 , wherein the storage device further stores the instructions of:

receiving a third message of the first RAT from the first BS via the first RAT, wherein the third message comprises a fourth message of the second RAT and a third TI, and the fourth message configures the communication device to connect to a third BS via the second RAT and comprises a fourth TI;

transmitting a third response message to the first BS via the first RAT in response to the third message, wherein the third response message comprises a fourth response message of the second RAT and the third transaction identifier, and the fourth response message comprises the transaction identifier;

connecting to the third BS via the second RAT in response to the fourth message; and

communicating fourth data via the second RAT with the third BS according to a configuration of the second RAT in the fourth message, when communicating third data via the first RAT with the first BS.

3. The communication device of claim 1 , wherein the first message and the first response message are long-term evolution (LTE) radio resource control (RRC) messages and the second message and the second response message are new radio (NR)/fifth generation (5G) RRC messages; or the first message and the first response message are NR/5G RRC messages and the second message and the second response message are LTE RRC messages.

4. The communication device of claim 1 , wherein the first BS deciphers the first response message, performs an integrity protection check on the first response message and transmits the second response message to the second BS via a connection between the first BS and the second BS, when receiving the first response message.

5. The communication device of claim 1 , wherein the storage device further stores the instructions of:

performing at least one of an integrity protection and a ciphering on the second response message according to at least one security key.

6. The communication device of claim 1 , wherein the storage device further stores the instructions of:

receiving a fifth message of the first RAT from the first BS via the first RAT, wherein the fifth message indicates a release of the connection between the communication device and the second BS;

transmitting a fifth response message to the first BS via the first RAT in response to the fifth message; and

stopping communicating with the second BS in response to a release request message.

7. The communication device of claim 1 , wherein the configuration of the second RAT comprises at least one of a Physical configuration, a Media Access Control (MAC) configuration, a Radio Link Control (RLC) configuration and a Packet Data Convergence Protocol (PDCP) configuration.