5.4.12a.1 General

The RAN and UE may use Low Power Wake UP Signal (LP-WUS) with Paging Subgrouping (LP-WUSPS) to reduce the power consumption of the UE in RRC_IDLE and RRC_INACTIVE over NR (see TS 38.304 [50]).

The LP-WUSPS can be based on a LP-WUS Subgroup ID allocated by the AMF (see clause 5.4.12a.2 for further details), or is determined by the RAN on its own as defined in TS 38.304 [50] and TS 38.331 [28] if the LP-WUS is not disabled.

The AMF, when determining its paging strategy (see clause 5.4.3), may decide to disable or enable LP-WUS per UE and should take into consideration whether a gNB is using LP-WUSPS determined by the RAN.

If LP-WUS disabling is supported by the UE, the UE shall indicate the support of LP-WUS disabling in the Registration Request message via the UE MM Core Network Capability. The AMF may use local configuration information (e.g. when it wants to avoid LP-WUS overload) to determine whether to disable LP-WUS (including both subgroup ID determined by AMF and by RAN) per UE using NAS signalling. If the UE supports disabling LP-WUS and the AMF has determined to disable LP-WUS for the UE, the AMF stores the indication "LP-WUS Disabled" in the UE context and provides it to the UE in every Registration Accept message.

If the UE receives "LP-WUS Disabled" indication in a Registration Accept or UE Configuration Update message, it shall ignore the "per cell" LP-WUS indication defined in TS 38.304 [50] and TS 38.331 [28] and expect to be paged without LP-WUS.

The AMF may use the Registration procedure (as described in clause 4.2.2 of TS 23.502 [3]) or UE Configuration Update procedure (as described in clause 4.2.4 of TS 23.502 [3]), and N2 UE Context Modification procedure (as described in clause 8.3.4 of TS 38.413 [34]) to update the LP-WUS disabling/enabling and AMF LP-WUSPS Assistance Information in the UE and NG-RAN.

If the UE supports AMF disabling LP-WUS and did not receive the "LP-WUS Disabled" indication in Registration Accept or UE Configuration Update message, then the UE shall assume that LP-WUS is enabled, and it shall determine per cell which LP-WUSPS method to use as defined in TS 38.304 [50] and TS 38.331 [28].

If LP-WUS has been disabled for a UE, the AMF shall indicate in the Paging message to the gNB that LP-WUS is disabled and shall not provide LP-WUSPS Assistance Information in the Paging message to NG-RAN when paging the UE in RRC_IDLE. In addition, in order to support UEs in RRC_INACTIVE mode, the AMF shall indicate that LP-WUS is disabled and not provide the AMF LP-WUSPS Assistance Information to NG-RAN as part of the RRC Inactive Assistance Information. If NG-RAN receives the LP-WUS disabled indication, NG-RAN does not use the LP-WUS during paging, otherwise LP-WUS is enabled at RAN for the UE as defined in TS 38.304 [50] and TS 38.331 [28].

If paging subgroups for LP-WUS are being allocated by the AMF, then all AMFs connected to one gNB (including the AMFs in different PLMNs using 5G MOCN network sharing) shall use a consistent policy in allocating UEs to the paging subgroups. The AMF may configure up to 32 different LP-WUS Subgroup IDs.

NOTE: Because the UE uses the AMF allocated LP-WUSPS across all cells in its TAI-list, and, different, overlapping TAI lists can be allocated to different UEs, then to avoid UE power consumption increasing, it is likely to be necessary that all AMFs with an overlapping coverage area use a consistent policy in allocating UEs to the LP-WUSPS.

The NG-RAN node and the UE determine per cell which LP-WUSPS method to use as defined in TS 38.304 [50] and TS 38.331 [28].