On the UE Context Retrieval Enhancements for Improved Inter-RAT Mobility

Standardization efforts for 5G New Radio (NR) technology are completed and initial deployments based on both Non Standalone (NSA) mode and Standalone (SA) mode have gained momentum in several countries. The 5G rollouts i.e. NSA and SA mode lead to an interim situation where mobile operators need to maintain both Evolved Packet Core (EPC) and 5G Core (5GC). Mobile operators, in future may prefer to migrate to 5GC, albeit maintaining the LTE and NR Radio Access Technology (RAT) to preserve investments. UE inter-RAT (IRAT) mobility between LTE and NR RAT mobility within 5GC, or between EPC and 5GC will be a crucial Key Performance Indicator (KPI) for mobile operators. IRAT mobility for CONNECTED state is already supported in 3GPP specifications but very less attention is paid to mobility when the UE is in INACTIVE state or when the UE performs recovery from Radio Link Failure (RLF). INACTIVE state is supported in both LTE and NR RAT when connected to 5GC, whereas suspend mode in IDLE state is supported when LTE RAT connects to EPC. In this paper we propose UE context retrieval procedure enhancements to support: a) efficient UE mobility between LTE and NR when the UE is in INACTIVE state or suspend mode in IDLE state in LTE and b) enhancements for RLF recovery by allowing reestablishment procedure to be performed if cell selection results in suitable IRAT cell detection. Further, we studied the control plane latency performance and signaling overhead reduction during the IRAT mobility scenarios by applying proposed enhancements. We believe that the UE context retrieval enhancements during IRAT mobility in above mentioned scenarios will improve robustness and minimize service interruption thereby improving the mobility experience.