Method and device for recovering call service in a mobile communication system

1 . A method performed by a user equipment (UE) in a wireless communication system, the method comprising:

identifying a failure of a registration of an IP multimedia subsystem (IMS) in a first cell;

identifying a second cell satisfies a condition for the registration of the IMS before a waiting time for a next registration of the IMS expires, wherein the waiting time starts due to the failure;

initiating an initial registration of the IMS in a second cell before the waiting time for the next registration of the IMS expires; and

recovering a call service of the UE after the initial registration of the IMS is successful.

2 . The method of claim 1 , wherein the condition for the registration of the IMS comprises at least one of:

a network signal strength of the second cell being greater than a preset threshold;

the second cell being a cell that has successfully registered with the IMS before; or

a link connectivity with an IMS packet data network (PDN) being detected in the second cell.

3 . The method of claim 2 , wherein the second cell is the cell that has successfully registered with the IMS before, the method comprises:

pre-storing cell information of a cell that has successfully registered with the IMS before;

comparing cell information of the second cell with the pre-stored cell information; and

determining that the second cell is the cell that has successfully registered with the IMS before when the cell information of the second cell matches the pre-stored cell information.

4 . The method of claim 2 , wherein the link connectivity with the IMS packet data network (PDN) being detected in the second cell comprises:

sending a transmission control protocol (TCP) handshake packet to the PDN; and

determining, upon receiving a response to the TCP handshake packet, that the link connectivity with the IMS PDN is detected in the second cell.

5 . The method of claim 2 , wherein the initiating of the initial registration of the IMS comprises:

restarting a protocol stack of the UE;

re-camping on a network in the second cell;

re-establishing the link connectivity with the IMS PDN; and

initiating the initial registration of the IMS after successfully establishing the link connectivity with the IMS PDN.

6 . The method of claim 5 , wherein the re-camping on the network in the second cell comprises executing, by the UE, detaching and attaching operations of a wireless network in the second cell.

7 . The method of claim 5 , wherein the re-camping on the network in the second cell comprises executing, by the UE, deregistration and registration operations of a wireless network in the second cell.

8 . The method of claim 2 , wherein the initiating of the initial registration of the IMS comprises:

re-establishing the link connectivity with the IMS PDN; and

initiating the initial registration of the IMS after successfully establishing the link connectivity with the IMS PDN.

9 . The method of claim 8 , wherein the re-establishing of the link connectivity with the IMS PDN comprises:

sending an activation request for re-establishing the link connectivity with the IMS PDN; and

establishing the link connectivity with the IMS PDN successfully after receiving an activation response to the activation request.

10 . The method of claim 1 , wherein the second cell is a different cell from the first cell.

11 . A user equipment (UE) comprising:

a transceiver;

a processor; and

memory storing instructions that, when executed by the processor, cause the UE to:

identify a failure of a registration of an IP multimedia subsystem (IMS) in a first cell,

identify a second cell satisfies a condition for the registration of the IMS before a waiting time for a next registration of the IMS expires, wherein the waiting time starts due to the failure,

initiate an initial registration of the IMS in a second cell before the waiting time for the next registration of the IMS expires, and

recover a call service of the UE after the initial registration of the IMS is successful.

12 . The UE of claim 11 , wherein the condition for the registration of the IMS comprises at least one of:

a network signal strength of the second cell being greater than a preset threshold;

the second cell being a cell that has successfully registered with the IMS before; or

a link connectivity with an IMS packet data network (PDN) being detected in the second cell.

13 . The UE of claim 12 , wherein the memory further comprises the instructions that, when executed by the processor, cause the UE to:

pre-store cell information of a cell that has successfully registered with the IMS before,

compare cell information of the second cell with the pre-stored cell information, and

determine that the second cell is the cell that has successfully registered with the IMS before when the cell information of the second cell matches the pre-stored cell information.

14 . The UE of claim 12 , wherein the memory further comprises the instructions that, when executed by the processor, cause the UE to:

send a transmission control protocol (TCP) handshake packet to the PDN; and

determine, upon receiving a response to the TCP handshake packet, that the link connectivity with the IMS PDN is detected in the second cell.

15 . The UE of claim 12 , wherein the memory further comprises the instructions that, when executed by the processor, cause the UE to:

restart a protocol stack of the UE,

re-camp on a network in the second cell,

re-establish the link connectivity with the IMS PDN, and

initiate the initial registration of the IMS after successfully establishing the link connectivity with the IMS PDN.

16 . The UE of claim 15 , wherein the re-camping on the network in the second cell comprises executing, by the UE, detaching and attaching operations of a wireless network in the second cell.