Reporting inter-RAT mobility failures

1 . A method for a user equipment, UE, to report failure of an inter-radio access technology (RAT) mobility procedure from a source cell to a target cell in a wireless network, the method comprising:

receiving, from a source node serving the source cell, a command to perform an inter-RAT mobility procedure towards the target cell, wherein the target cell uses a different RAT than the source cell;

detecting a failure associated with the inter-RAT mobility procedure, wherein the failure cause is one of the following:

that the UE was unable to comply with a configuration included in the command, or

a protocol error associated with inter-RAT information included in the command; and

sending a failure report, including an indication of the failure cause, to a node serving a cell in which the UE connected to the wireless network after detecting the failure,

wherein the indication of the failure cause is one of the following:

an explicit indication of the failure cause, or

an implicit indication of the failure cause based on the failure report indicating handover failure but omitting information about random access.

2 . The method of claim 1 , wherein the inter-RAT mobility procedure is one of the following: handover; conditional handover; dual active protocol stack (DAPS) handover; dual-connectivity (DC) setup; or DC modification.

3 . The method of claim 1 , wherein one of the following applies:

the source cell uses a New Radio (NR) RAT and the target cell uses a UTRA RAT or an evolved UTRA (E-UTRA) RAT; or

the source cell uses an E-UTRA RAT and the target cell uses an NR RAT.

4 . The method of claim 1 wherein one or more of the following applies:

the failure report is a radio link failure (RLF) report; and

the failure report also includes a failure type indication, which indicates that the failure is associated with an inter-RAT handover.

5 . The method of claim 1 , wherein:

the UE is in dual connectivity with a master cell group (MCG) and a secondary cell group (SCG) before receiving the command;

the failure is associated with either the MCG or the SCG; and

one of the following applies:

the failure report is a message specific to the cell group associated with the failure; or

the failure report indicates whether the failure is associated with the MCG or the SCG.

6 . The method of claim 1 , wherein the implicit indication is further based on the failure report including an identity of the target cell.

7 . A method for a network node to handle failed inter-RAT mobility procedures of user equipment, UEs, in a wireless network, the method comprising:

receiving a failure report about a UE's failed inter-RAT mobility procedure from a source cell served by a source node to a target cell served by a target node, wherein the target cell uses a different RAT than the source cell, and wherein the failure report indicates one of the following failure causes:

that the UE was unable to comply with a configuration included in a command to perform the inter-RAT mobility procedure, or

a protocol error associated with inter-RAT information included in the command,

wherein the indication of the failure cause is one of the following: an explicit indication of the failure cause, or an implicit indication of the failure cause based on the failure report indicating handover failure but omitting information about random access; and

based on the failure cause indication, performing one or more operations that prevent failures in subsequent mobility procedures involving the source cell and/or the target cell.

8 . The method of claim 7 , wherein:

the failure report is received by the source node from a node serving a further cell in which the UE connected to the wireless network after the failed inter-RAT mobility procedure; and

the one or more operations include one or more of the following:

refraining from requesting subsequent mobility procedures towards the target cell for UEs served by the source cell; and

sending the failure cause indication to the target node.

9 . The method of claim 7 , wherein:

the failure report is received from the UE by a node serving a cell in which the UE connected to the wireless network after the failed inter-RAT mobility procedure; and

the one or more operations include one or more of the following:

sending the failure report to the source node;

determining the target node based on an identifier of the target cell included in the failure report; and

when the indicated failure cause is the protocol error, sending the failure cause indication the to the determined target node.

10 . The method of claim 7 , wherein the inter-RAT mobility procedure is one of the following: handover; conditional handover; dual active protocol stack (DAPS) handover; dual-connectivity (DC) setup; or DC modification.

11 . The method of claim 7 , wherein one of the following applies:

the source cell uses a New Radio (NR) RAT and the target cell uses a UTRA RAT or an evolved UTRA (E-UTRA) RAT; or

the source cell uses an E-UTRA RAT and the target cell uses an NR RAT.

12 . The method of claim 7 , wherein one or more of the following applies:

the failure report is a radio link failure (RLF) report; and

the failure report also includes a failure type indication, which indicates that the failure is associated with an inter-RAT handover.

13 . The method of claim 7 , wherein:

the UE was in dual connectivity (DC) with a master cell group (MCG) and a secondary cell group (SCG) before the failed inter-RAT mobility procedure, which is associated with either the MCG or the SCG; and

one of the following applies:

the failure report is a message specific to the cell group associated with the inter-RAT mobility failure; or

the failure report indicates whether the failure is associated with the MCG or the SCG.

14 . The method of claim 7 , wherein the implicit indication is further based on the failure report including an identity of the target cell.

15 . A user equipment (UE) configured to report failure of an inter-radio access technology (RAT) mobility procedure from a source cell to a target cell in a wireless network, the UE comprising:

radio transceiver circuitry configured to communicate with one or more network nodes in the wireless network; and

processing circuitry operatively coupled to the radio transceiver circuitry, whereby the processing circuitry and the radio transceiver circuitry are configured to:

receive, from a source node serving the source cell, a command to perform an inter-RAT mobility procedure towards the target cell, wherein the target cell uses a different RAT than the source cell;

detect a failure associated with the inter-RAT mobility procedure, wherein the failure cause is one of the following:

that the UE was unable to comply with a configuration included in the command, or

a protocol error associated with inter-RAT information included in the command; and

send a failure report, including an indication of the failure cause, to a node serving a cell in which the UE connected to the wireless network after detecting the failure, wherein the indication of the failure cause is one of the following:

an explicit indication of the failure cause, or

an implicit indication of the failure cause based on the failure report indicating handover failure but omitting information about random access.

16 . A network node configured to handle failed inter-radio access technology (RAT) mobility procedures of user equipment (UEs) in a wireless network, the network node comprising:

radio network interface circuitry configured to communicate with UEs and with one or more other network nodes in the wireless network; and

processing circuitry operatively coupled to the radio network interface circuitry, whereby the processing circuitry and the radio network interface circuitry are configured to perform operations corresponding to the method of claim 7 .