Methods and user equipment for managing internet protocol multimedia subsystem call over long-term evolution in single radio voice call continuity area

1. A method for managing an IP multimedia subsystem (IMS) call over Long-Term Evolution (LTE) in a Single Radio Voice Call Continuity (SRVCC) area, the method comprising:

first detecting, by a User Equipment (UE), whether the IMS call over LTE is in an alert state and whether a Radio Resource Control (RRC) connection is configured for the IMS call over LTE;

initiating, by the UE, a session alert refresh timer with a first value less than a second value of a data inactivity timer from a network entity;

second detecting, by the UE, whether the session alert refresh timer has expired before establishing the IMS call over LTE;

restarting, by the UE, the session alert refresh timer in response to a result of the second detecting indicating that the session alert refresh timer has expired before establishing the IMS call over LTE;

sending, by the UE, a Session Initiation Protocol (SIP) message to the network entity; and

determining, by the UE, whether a caller tune or media pre-configured prior to the IMS call being answered is configured between the first detecting and the initiating,

wherein the initiating a session alert refresh timer is performed in response to the caller tune or the media being configured,

stopping of the session alert refresh timer is performed in response to the caller tune or the media being not configured, and

the method keeps the UE in an RRC connected state by performing the restarting and the sending.

2. The method of claim 1 , further comprising:

receiving, by the UE, the first value of the session alert refresh timer from the network entity before the initiating.

3. The method of claim 1 , wherein the SIP message is a SIP UPDATE message with no media activity to keep the RRC connection active at the UE.

4. The method of claim 1 , further comprising:

receiving, by the UE, an RRC connection release message from the network entity;

determining, by the UE, whether a strength of an LTE signal meets a strength threshold; and

sending, by the UE, the SIP message to the network entity to keep the RRC connection active in response to the strength of the LTE signal meeting the strength threshold.

5. The method of claim 4 , wherein the SIP message is a SIP UPDATE message with no media activity to keep the RRC connection active at the UE.

6. The method of claim 1 , further comprising:

sending, by the UE, a silent message to the network entity to keep the RRC connection active,

wherein the silent message is sent over a Real-time Transport Protocol (RTP) to the network entity.

7. The method of claim 1 , further comprising:

indicating, by the UE, a status of the IMS call over LTE to a Non-access stratum (NAS) client included in the UE;

receiving, by the UE, an RRC connection release message from the network entity;

determining, by the UE, that the IMS call over LTE is in progress; and

sending, by the UE, a service request message to a Access Stratum (AS) client included in the UE to update that the IMS call over LTE is in progress.

8. The method of claim 1 , wherein

the initiating includes initiating, by the UE, the session alert refresh timer with the first value less than the second value of the data inactivity timer from the network entity once the UE has moved to the alert state; and

the sending sends, by the UE, the SIP message to the network entity to keep the RRC connected.

9. The method of claim 8 , further comprising:

restarting, by the UE, the session alert refresh timer in response to the result of the second detecting indicating that the session alert refresh timer has expired before establishing the IMS call over LTE.

10. The method of claim 9 , wherein the restarting is performed in response to a result of the first detecting indicates that the IMS call over the LTE is connected.

11. The method of claim 10 , further comprising:

the second detecting is performed between the initiating and the first detecting so that the UE re-starts the timer to keep the RRC connection active until the IMS call over the LTE is connected.

12. The method of claim 1 , wherein the sending includes sending the SIP message with reason session alert refreshing, the reason session alert refreshing indicating a reason for restarting the session alert refresh timer to the network entity.

13. A User Equipment (UE) for managing an IP multimedia subsystem (IMS) call over Long-Term Evolution (LTE) in a Single Radio Voice Call Continuity (SRVCC) area, the UE comprising:

an IMS client including at least one first processor, the at least one first processor configured to,

first detect whether the IMS call over LTE is in an alert state and whether a Radio Resource Control (RRC) connection is configured for the IMS call over LTE,

initiate a session alert refresh timer with a first value less than a second value of a data inactivity timer,

second detect whether the session alert refresh timer has expired before establishing the IMS call over LTE,

restart the session alert refresh timer in response to a detected result indicating that the session alert refresh timer has expired before establishing the IMS call over LTE,

send a Session Initiation Protocol (SIP) message to a network entity, and

determine, by the UE, whether a caller tune or media pre-configured prior to the IMS call being answered is configured between the first detecting operation of the at least one first processor and the initiating operation of the at least one first processor,

wherein the at least one first processor is configured to initiate the session alert refresh timer in response to the caller tune or the media being configured,

the at least one first processor is configured to stop the session alert refresh timer in response to the caller tune or the media being not configured, and

the at least one first processor is configured to keep the UE in an RRC connected state by being configured to restart the session alert refresh timer and send the SIP message to the network entity.

14. The UE of claim 13 , wherein the first value of the session alert refresh timer is a value received from the network entity.

15. The UE of claim 13 , wherein the SIP message is a SIP UPDATE message with no media activity to keep the RRC connection active.

16. The UE of claim 13 , further comprising:

an Access Stratum (AS) client including at least one second processor, the at least one second processor configured to receive an RRC connection release message from the network entity,

wherein the at least one first processor included in the IMS client is further configured to,

alert state and the Radio Resource Control (RRC) connection is configured for the IMS call over LTE,

determine whether a strength of an LTE signal meets a strength threshold, and

send the SIP message to the network entity if the strength of the LTE signal meets the strength threshold.

17. The UE of claim 16 , wherein the SIP message is a SIP UPDATE message with no media activity to keep the RRC connection active at the UE.

18. The UE of claim 13 , wherein the at least one first processor included in the IMS client is further configured to send a silent message to the network entity to keep the RRC connection active.

19. The UE of claim 18 , wherein the at least one first processor included in the IMS client is further configured to send the silent message over a Real-time Transport Protocol (RTP) to the network entity.

20. The UE of claim 13 , further comprising:

an Access Stratum (AS) client including at least one second processor, the at least one second processor configured to receive an RRC connection release message from the network entity; and

a Non-Access Stratum (NAS) client including at least one third processor, the at least one third processor configured to,

determine whether the IMS call over LTE is in progress, and

send a service request message to the AS client to update that the IMS call over LTE is in progress,

wherein the at least one first processor included in the IMS client is further configured to indicate a status of the IMS call over LTE to the NAS client.