User equipment and methods for fast handover failure recovery in 3GPP LTE network

1. User Equipment (UE) comprising:

physical layer circuitry and hardware processing circuitry configured to:

initiate handover (HO) failure recovery by transmission of a random-access channel (RACH) 2 message when both a radio-link failure (RLF) timer and a time- to trigger (TTT) timer are concurrently running,

wherein the RACH 2 message is a message transmitted on a random-access channel for radio-resource control (RRC) connection re-establishment.

2. The UE of claim 1 wherein the processing circuitry is further configured to:

activate the RLF timer as part of a radio-link monitoring (RLM) process based on radio-link conditions with a serving cell;

activate the TTT timer as part of a HO process based on a measurement reporting event; and

determine when both the RLF timer and the TTT timer are active to initiate the HO failure recovery by transmission of the RACH 2 message,

wherein the RACH 2 message is transmitted when the RLF timer and the TTT timer are both active and before either the RLF timer or the TTT timer expires.

3. The UE of claim 2 wherein the RACH 2 message is an unscheduled message that is transmitted on the random-access channel.

4. The UE of claim 2 wherein in response to receipt of a RACH response message, the UE is arranged to:

terminate the RLF timer; and either

transmit a RRC connection reconfiguration complete message to a target cell eNB for non-contention based random access, or

transmit a RRC connection request message to another cell that is neither the target nor the serving cell for contention-based random access.

5. The UE of claim 4 wherein when the UE does not receive a RACH response message and the RLF timer has not reset as part of the RLM process, the UE is arranged to continue to perform the RLM process and perform a connection reestablishment procedure upon expiration of the RLF timer.

6. The UE of claim 2 wherein the RACH 2 message, when transmitted to initiate HO failure recovery when both the RLF timer and the TTT timer are active, is transmitted to an eNB of a cell having a greatest received signal strength,

wherein when the eNB is associated with either a target cell or the serving cell, the RACH 2 message is transmitted in accordance with a non-contention random-access based technique, and

wherein when the eNB is associated with neither the target cell nor the serving cell, the RACH 2 message is transmitted in accordance with a contention-based random-access technique.

7. The UE of claim 2 wherein when the TTT timer is running and the RLF timer is not running, the UE is arranged to continue to perform the HO process and send a RACH 2 message to a target cell eNB after receipt of a RRC reconfiguration message from the serving cell eNB.

8. The UE of claim 2 wherein the UE is arranged to initially set the RLF timer based on a channel quality indicator (CQI) associated with a radio link with the serving cell as part of the RLM process, and

wherein the UE is arranged to set the TTT timer upon satisfaction of a measurement reporting event being triggered.

9. A method for fast handover failure recovery performed by User Equipment (UE), the method comprising:

initiating handover (HO) failure recovery by transmission of a random-access channel (RACH) 2 message when both a radio-link failure (RLF) timer and a time-to trigger (TTT) timer are concurrently running; and

terminating the RLF timer in response to a RACH response message,

wherein the RACH 2 message is a message transmitted on a random-access channel for radio-resource control (RRC) connection re-establishment.

10. The method of claim 9 wherein after receipt of the RACH response message, the method further comprises:

transmitting a RRC connection reconfiguration complete message to a target cell eNB for non-contention based random access, or

transmitting a RRC connection request message to another cell that is neither the target nor a serving cell for contention-based random access.

11. The method of claim 10 further comprising:

activating the RLF timer as part of a radio-link monitoring (RLM) process based on radio-link conditions with a serving cell;

activating the TTT timer as part of a HO process based on a measurement reporting event; and

determining when both the RLF timer and the TTT timer are active to initiate the HO failure recovery by transmission of the RACH 2 message,

wherein the RACH 2 message is an unscheduled message that is transmitted when the RLF timer and the TTT timer are both active and before either the RLF timer or the TTT timer expires.

12. The method of claim 11 wherein when the UE does not receive a RACH response message and the RLF timer has not reset as part of the RLM process, the method further comprises continuing to perform the RLM process and performing a connection reestablishment procedure upon expiration of the RLF timer.

13. The method of claim 9 wherein the RACH 2 message, when transmitted to initiate HO failure recovery when both the RLF timer and the TTT timer are active, is transmitted to an eNB of a cell having a greatest received signal strength.

14. The method of claim 13 wherein when the eNB having the greatest received signal strength is associated with either a target cell or serving cell, the RACH 2 message is transmitted in accordance with a non-contention random-access based technique, and

wherein when the eNB having the greatest received signal strength is associated with neither the target cell nor the serving cell, the RACH 2 message is transmitted in accordance with a contention-based random-access technique.

15. The method of claim 11 wherein when the TTT timer is running and the RLF timer is not running, the UE is arranged to continue to perform the HO process and send a RACH 2 message to the target cell eNB after receipt of a RRC reconfiguration message from the serving cell eNB.

16. A non-transitory computer-readable storage medium that stores instructions for execution by one or more processors to perform operations for fast handover failure recovery by User Equipment (UE), the operations to configure the or more processors to:

initiate handover (HO) failure recovery by transmission of a random-access channel (RACH) 2 message when both a radio-link failure (RLF) timer and a time-to trigger (TTT) timer are concurrently running; and

terminate the RLF timer in response to a RACH response message,

wherein the RACH 2 message is a message transmitted on a random-access channel for radio-resource control (RRC) connection re-establishment.

17. The non-transitory computer-readable storage medium of claim 16 wherein the operations further configure the one or more processors to:

activate the RLF timer as part of a radio-link monitoring (RLM) process based on radio-link conditions with a serving cell;

activate the TTT timer as part of a HO process based on a measurement reporting event; and

determine when both the RLF timer and the TTT timer are active to initiate the HO failure recovery by transmission of the RACH 2 message,

wherein the RACH 2 message is an unscheduled message that is transmitted when the RLF timer and the TTT timer are both active and before either the RLF timer or the TTT timer expires.

18. The non-transitory computer-readable storage medium of claim 17 wherein when the UE does not receive a RACH response message and the RLF timer has not reset as part of the RLM process, the operations further configure the one or more processors to perform the RLM process and perform a connection reestablishment procedure upon expiration of the RLF timer.