Methods for lower layer failure handling when assigning paging subgroup id and network entity

A method of handling lower layer failure when assigning a Paging Early Indication with Paging Subgrouping (PEIPS) subgroup ID is proposed. If a network supports and accepts the use of PEIPS assistance information for a UE, the network can provide to the UE the negotiated PEIPS assistance information containing a new Paging subgroup ID and a new 5G-GUTI in a REGISTRATION ACCEPT message or in a CONFIGURATION UPDATE COMMAND message, and the UE was assigned with an old paging subgroup ID. When lower layer failure occurs, the network may not successfully receive a REGISTRATION COMPLETE message or a CONFIGURATION UPDATE COMPLETE message from the UE. Accordingly, the network needs to use both possible paging subgroup IDs for the UE until one of the paging subgroup IDs is considered invalid.