Device and method of handling mobility

1 . A communication device for handling mobility, 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 mobility configuration comprising information of a proxy cell and at least one candidate cell from a source cell of a source base station (BS);

performing a mobility procedure to change the source cell to a first target cell according to the mobility configuration;

performing a determination on whether the first target cell is one of the at least one candidate cell according to the mobility configuration;

determining that an ID of the proxy cell is an ID of the at least one candidate cell; and

transmitting a first connection establishment message, comprised of the ID of the proxy cell, to the first target cell according to the determination.

2 . The communication device of claim 1 , wherein the first connection establishment message comprises at least one of a physical cell ID (PCI) of the source cell, information of the source cell and an ID of the proxy cell.

3 . The communication device of claim 2 , wherein the information of the source cell comprises a non-terrestrial network (NTN) mobility indication, a public land mobile network (PLMN) ID of the source cell, a global cell ID (GCI) of the source cell, or a frequency ID of the source cell.

4 . The communication device of claim 2 , wherein the first connection establishment message comprises the PCI of the source cell, and the determination comprises that the first target cell is the one of the at least one candidate cell.

5 . The communication device of claim 4 , wherein the first connection establishment message comprises the ID of the proxy cell, and the ID of the proxy cell is one of at least one cell ID comprised in the at least one candidate cell and is indicated by the source cell by an indicator comprised in the mobility configuration.

6 . The communication device of claim 2 , wherein the first connection establishment message comprises the ID of the proxy cell, the PCI of the source cell and the information of the source cell, and the determination comprises that the first target cell is not the one of the at least one candidate cell.

7 . The communication device of claim 6 , wherein the communication device determines that the ID of the proxy cell is an ID of the at least one candidate cell with a first order.

8 . The communication device of claim 1 , wherein the first connection establishment message comprises a cell radio network temporary identifier (C-RNTI) of the communication device in the source cell.

9 . The communication device of claim 1 , wherein the first connection establishment message comprises an authentication information of the communication device which is configured by the source cell in the mobility configuration.

10 . The communication device of claim 9 , wherein the authentication information of the communication device comprises a message authentication code-integrity (MAC-I) or a part of the MAC-I.

11 . The communication device of claim 2 , wherein the instruction further comprising:

determining a second target cell according to the mobility configuration, when determining that transmitting the first connection establishment message is failed; and

transmitting a second connection establishment message to the second target cell.

12 . The communication device of claim 11 , wherein the second connection establishment message comprises the ID of the proxy cell, the PCI of the source cell and the information of the source cell.

13 . The communication device of claim 1 , wherein the mobility configuration is transmitted by a radio resource control (RRC) message.

14 . The communication device of claim 1 , wherein the mobility configuration comprises configuration of the at least one candidate cell.

15 . The communication device of claim 1 , wherein the information of the proxy cell comprises an indication indicating that the proxy cell is supported for the communication device.

16 . The communication device of claim 1 , wherein the source cell comprises a source cell of the communication device.

17 . The communication device of claim 1 , wherein the source cell comprises a serving cell of the communication device.