Section: 4.3.21.4 Wake Up Signal Assistance 4.3.21.4.1 General
Content: 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[ Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) procedures in idle mode ]  [34]). To avoid waking up UEs due to an MME 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 eNodeB 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: a) WUS-capable eNodeBs should provide the Recommended Cells for Paging IE in the Information On Recommended Cells And eNodeBs For Paging IE (see TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36]) to the MME in the S1 UE Context Release Complete or UE Context Suspend Request messages; b) if received during the last S1 UE Context Release Complete or UE Context Suspend Request message, the MME provides (without modification) the Recommended Cells for Paging as Assistance Data for Recommended Cells IE in the S1-AP Paging message to the RAN (see also TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36]); and c) the MME shall delete (or mark as invalid) the Information On Recommended Cells And eNodeBs For Paging when a new S1-AP association is established for the UE. In the S1-AP 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 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36]). When receiving an S1-AP Paging message for a WUS-capable UE that also includes the Assistance Data for Recommended Cells IE then a WUS-capable eNodeB shall only broadcast the WUS on the cell that matches the last used cell ID.