5.4.9.1 General

The RAN and UE may use a Wake Up Signal (WUS) to reduce the UE's idle mode power consumption. The RAN sends the WUS shortly before the UE's paging occasion. The WUS feature enables UEs to determine that in the paging occasions immediately following their WUS occasion they will not be paged if their WUS is not transmitted, or that they might be paged if their WUS is transmitted (see TS 36.304 [52]).

To avoid waking up UEs due to an AMF paging other UEs across multiple cells (e.g. due to frequent UE mobility and/or for low paging latency services such as VoLTE), the use of WUS by the ng-eNB and the UE is restricted to the last used cell, i.e. the cell in which the UE's RRC connection was last released. To support this:

• ng-eNBs should provide the Recommended Cells for Paging IE in the Information on Recommended Cells and RAN Nodes for Paging IE (see TS 38.413 [34]) to the AMF in the NGAP UE Context Release Complete, UE Context Suspend Request and UE Context Resume Request messages;
• if received during the last NGAP UE Context Release Complete or UE Context Suspend Request message, the AMF provides (without modification) the Recommended Cells for Paging as Assistance Data for Recommended Cells IE in the Assistance Data for Paging IE within the NGAP Paging message to the RAN (see also TS 38.413 [34]); and
• the AMF shall delete (or mark as invalid) the Information On Recommended Cells And RAND nodes For Paging when a new NGAP association is established for the UE.

In the NGAP Paging message, the last used cell ID is sent in the Assistance Data for Recommended Cells IE in the Assistance Data for Paging IE (see TS 38.413 [34]). When receiving an NGAP Paging message for a WUS-capable UE that also includes the Assistance Data for Recommended Cells IE then a WUS-capable ng-eNB shall only broadcast the WUS on the cell that matches the last used cell ID.