Content for TR 23.700-68 Word version: 18.1.0
6 Solutions
6.1 Solution #1: Power saving enhancement with long eDRX cycle support for UE in CM-CONNECTED with RRC_INACTIVE state
6.1.1 Introduction
6.1.2 Functional Description
6.1.3 Procedures
6.1.3.1 Notification of eDRX information between CN and gNB
6.1.3.2 Network triggered Connection Resume procedure
6.1.4 Impacts on services, entities and interfaces
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6 Solutions p. 8
6.1 Solution #1: Power saving enhancement with long eDRX cycle support for UE in CM-CONNECTED with RRC_INACTIVE state p. 8
6.1.1 Introduction p. 8
The solution applies to Key Issue #1 and covers both sub-aspects of the key issue.
6.1.2 Functional Description p. 8
The solution is based on existing Rel-17 long eDRX support for NR RedCap device in CM-IDLE mode with the following enhancement:
- eDRX cycle value for CM-CONNECTED with RRC_INACTIVE state is extended from 10.24s up to 10485.76s (i.e. same eDRX cycle value range as for CM-IDLE mode).
- AMF is aware of UE RRC state and long eDRX parameters applied for UE in CM_CONNECTTED with RRC_INACTIVE state based on information notified by RAN.
- AMF also notifies the SMF/UPF to start data buffering/delivery since UE is not reachable or reachable due to power saving. The UPF/SMF is responsible for data buffering and triggers the MT data notification even in case of existing of N3 tunnel.
- Based on the eDRX parameters for CM-CONNECTED with RRC_INACTIVE state, AMF can deduce the needed information and handle MT signalling and MT data the same way as UE is in CM-IDLE while UE is not reachable, i.e. the existing HLcom solutions can be applied to UE in power saving state when UE is in either in CM-IDLE or in CM_CONNECTED with RRC_INACTIVE state as long as AMF knows the information related to power saving.
- Based on the CN notification related to MT signalling or data, RAN triggers the paging when UE is reachable.
- UE performs both AS and NAS level mobility and periodic AS update based on existing Rel-17 functions.
- If extra AMF selection logic is needed for applying long eDRX in CM_CONNECTED with RRC_INACTIVE state, existing mechanisms (e.g., S-NSSAI) can be deployed by the operation based on configuration.
6.1.3 Procedures p. 9
6.1.3.1 Notification of eDRX information between CN and gNB p. 9
This is an updated procedure based on clause 4.8.3 of TS 23.502.
Step 1.
The AMF sends a N2 Request to the NG-RAN (i.e. gNB) to request the RRC state transition report as described in TS 38.413 (e.g. when AMF provides RRC Inactive Assistance Information for UE with reduced NR capability and has negotiated extended DRX longer than 10.24s).
Step 2.
The NG-RAN sends the UE Notification message to report the current RRC state for the UE (i.e. RRC Inactive state or RRC Connected state).
When NG-RAN reports RRC Inactive state for a UE using extended DRX longer than 10.24s in RRC Inactive state, the NG-RAN includes also the extended DRX information (i.e. eDRX cycle value and Paging Time Window) which triggers CN based MT signalling and data handling as in step 3 below.
Step 2b.
When the AMF has requested reporting about subsequent state transitions, the NG-RAN sends subsequent UE Notification messages to the AMF at every RRC state transition until the UE transitions to CM-IDLE or NG-RAN receives a Cancel UE State Notification message from the AMF.
Step 3a.
If the AMF receives extended DRX information when UE enters RRC Inactive state, the AMF stores the extended DRX information for RRC Inactive state and sends the Nsmf_PDUSession_UpdateSMContext request to the SMF enabling the data buffering. When UE enters RRC Connected state, the AMF also sends Nsmf_PDUSession_UpdateSMContext request to SMF enabling the data delivery.
Step 3b.
If data buffering is handled in UPF, the SMF updates the UPF with proper rules for MT data handling.
Step 3c.
The SMF sends the Nsmf_PDUSession_UpdateSMContext response.
Step 4.
The AMF can send a Cancel UE State Notification message to inform the NG-RAN that it should terminate notifications for a given UE when such report is not needed.
6.1.3.2 Network triggered Connection Resume procedure p. 10
This is an updated procedure based on clause 4.8.2.2a of TS 23.502 with enhancement for UE in CM-CONNECTED with RRC_INACTIVE state and eDRX is longer than 10.24s.
The Network Triggered Connection Resume in RRC Inactive procedure is used when the network needs to signal (e.g. the N1 NAS PDU and/or downlink user plane PDU) with a UE in the RRC Inactive state. When the procedure is triggered by other NFs (e.g. SMSF, LMF, GMLC), the UPF (or SMF) in the following Figure should be replaced by the respective NF (the corresponding services operations used by other NFs when they communicate with AMF may also be different from the service operation used by SMF/UPF). During the procedure, the NG-RAN (i.e. gNB) sends a RAN Paging to the UE in order to trigger the Connection Resume in RRC Inactive procedure.
Step 1.
When downlink data is received and the SMF/UPF is requested to perform buffering as specified in clause 6.1.3.1 above, UPF/SMF checks with AMF for the possibility of data delivery, similar to step 2 of clause 4.24.2 of TS 23.502 with the following differences:
Step 2.
- If AMF determines that UE is not reachable in RRC INACTIVE state, the AMF provides the Estimated Maximum Wait time in the response message based on the stored extended DRX information for RRC INACTIVE in AMF (see clause 6.1.3.1 above). This handling is similar to CM-IDLE with eDRX. When the AMF provides the Estimated Maximum Wait time, it can consider the time needed for RRC level procedures (e.g. RRC RNA update procedure) when UE wakes up from the eDRX cycle.
When the AMF determines the UE reachable based on the stored extended DRX inform for RRC INACTIVE, the AMF informs SMF/UPF to deliver data. The NG-RAN receives downlink data (e.g. the N1 NAS PDU and/or the user plane PDU) for a UE in RRC INACTIVE State, the NG-RAN buffers the downlink data and triggers RAN Paging message.
As an alternative option, the AMF may notify the NG-RAN of MT service which can trigger the NG-RAN Paging. Then step for downlink data delivery will be executed after UE enters RRC_CONNECTED mode as show in Figure 6.1.3.2-1b above when SMF/UPF receives the notification as specified in step 3 of clause 6.1.3.1 above.
Step 3.
The NG-RAN sends the RAN Paging message to the UE.
Step 4.
If the UE is in RRC Inactive State, based on the RAN paging, the UE initiates the UE Triggered Connection Resume in RRC Inactive procedure (see clause 4.8.2.2 of TS 23.502). In case of RRC Resume failure, a NAS procedure is triggered by UE to enter RRC_CONNECTED state.
6.1.4 Impacts on services, entities and interfaces p. 11
AMF:
- eDRX parameters for RRC_INACTIVE state exchange on NGAP interface.
- Estimated Maximum Wait time calculation based on eDRX parameters from RRC_INACTIVE state.
- Notification of MT service on NGAP interface (only needed if option as described in Figure 6.1.3.2-1b is used).
- eDRX parameters for RRC_INACTIVE state exchange on NGAP interface.
- Notification of MT service on NGAP interface which triggers the RAN Paging (only needed if option as described in Figure 6.1.3.2-1b is used).
- Supports data buffering/delivery Indication from AMF even in case of the N3 tunnel existence.
- none.
