Method to Categorize SCG Handover Failures in Dual Connectivity
With MR-DC, UE is connected to both MCG and SCG. When an MCG handover failure occurs, there is a defined method to categorize the handover failures and take corrective actions, however, for SCG handover failures, there are no defined categorizations and corresponding corrective actions currently available. The current method facilitates identification and categorization of SCG handover failures to be categorized as too early, too late and wrong cell handovers. By fine-tuning the handover parameters (based on the SCG handover failure type) SCG handover failure rates would be reduced significantly.
1 . A method for categorizing 5G Secondary Cell Group (SCG) handover failures in Multi-Radio Dual Connectivity (MR-DC) where User Equipment (UE) accesses a Master evolved Node B (MeNB) connected to a 5G Control Unit-Control Plane (CU-CP) and to a 5G Control Unit-User Plane (CU-UP) of the UE, the method comprising the steps of:
the SgNB initiating intra Distributed Unit (DU) handover from a source cell towards a target cell;
the CU-CP allocating UE context in the target cell;
the CU-CP stopping a data flow from the DU when the UE context is successfully setup in the target cell in CU-UP and DU;
the DU stopping transmission of data for the UE and the CU-UP stopping the transmission of data for the UE toward the source cell;
the CU-CP starting a timer for a duration of reconfiguration of the UE for the target cell;
wherein when a SCG random-access failure from the UE to the DU occurs, the UE triggers a Radio Link Failure (RLF) message to the MeNB, which includes New Radio (NR) measurement results at the time of the failure;
the MeNB transmitting the RLF message to the 5G CU-CP; and
the 5G CU-CP decoding the RLF message and categorizing the SCG failure.
2 . The method of claim 1 , wherein the SCG failure is categorized as a too late handover failure.
3 . The method of claim 1 , wherein the SCG failure is categorized as a too early handover failure.
4 . The method of claim 1 , wherein the SCG failure is categorized as a wrong cell handover failure.
5 . The method of claim 1 , wherein the SCG failure is categorized as a SCG RLF too late handover without any handover trigger from SCG.
6 . The method of claim 1 , wherein the SCG failure is categorized as a SCG RLF/handover failure for all MR-DC variants.
7 . The method of claim 1 , wherein the SCG failure is selected from the group consisting of: Intra-DU, Inter-DU, Inter-CU, and combinations thereof.
8 . The method of claim 1 , wherein the SCG failure is selected from the group consisting of: Intra-frequency handovers, Inter-frequency handovers, and combinations thereof.
9 . The method of claim 1 , wherein prior to the step of the SgNB initiating intra Distributed Unit (DU) handover, the method comprises the following steps:
the MeNB receiving from the UE a Radio Resource Control (RRC) transfer message that includes measurement results containing intra-frequency/inter-frequency neighbor cell Physical Cell Id (PCI) and corresponding Reference Signal Received Power (RSRP); and the MeNB transmitting the transfer message as a container in a UE report to the CU-CP; and
the CU-CP decoding the transfer message.
10 . The method of claim 9 , wherein the setup of the UE context comprises the steps:
the CU-UP allocating a new context modification request for all bearers for the UE;
the CU-UP transmitting the context modification request to the CU-CP; and
the CU-CP transmitting the context modification request to the DU.
11 . The method of claim 10 , further comprising the steps of:
the DU transmitting a context modification response to the CU-CP;
the CU-CP transmitting a bearer context modification request to the CU-UP; and
the CU-UP transmitting a bearer context modification response to the CU-CP.
12 . The method of claim 11 , wherein the step of the CU-CP stopping a data flow from the DU comprises the steps:
when a UE Context Modification Request Transmission Action Indicator indicates stop, the gNB-DU stops the transmission for UE;
a Downlink Data Delivery Status is provided from the DU to the CU-UP; and
a UE context Modification Response is provided from the DU to the CU-CP.
13 . The method of claim 12 , further comprising the steps of:
the SCG transmits a Secondary next generation Node B (SgNB) modification required signal containing a Resource Block (RB) configuration for the target cell to the MeNB;
the MeNB transmits a RRC connection reconfiguration signal to the UE;
the UE transmits a RRC connection reconfiguration complete signal to the MeN; and
the MeNB sends a SgNB modification confirm signal to the SgNB.
14 . The method of claim 13 , wherein the RLF message to the MeNB includes current NR measurement results, the method further comprising:
the MeNB triggers SgNB release and the MeNB converts the SCG split bearer to an MCG bearer;
the MeNB sends a SgNB release request to the CU-CP; and
the CU-CP sends a SgNB release request acknowledge to the MeNB.
15 . The method of claim 14 , the method further comprising:
the MeNB sends a RRC connection reconfiguration signal to the UE; and
the UE sends a RRC connection reconfiguration complete signal to the MeNB;
wherein if the conversion from SCG split bearer to MCG bearer is successful, then the UE context is released in the SgNB.