Content for TR 23.700-68 Word version: 18.1.0

6.4 Solution #4: RRC Inactive with CM-IDLE for eDRX > 10.24s 6.4.1 Description 6.4.2 Procedures 6.4.2.1 N2 suspend for UE entering RRC inactive with eDRX > 10.24s 6.4.2.2 UE paging and resume 6.4.3 Impacts on services, entities and interfaces
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6.4 Solution #4: RRC Inactive with CM-IDLE for eDRX > 10.24s p. 20

6.4.1 Description p. 20

This solution proposes, for the case of RRC Inactive and eDRX > 10.24s, to combine:

On the RAN/RRC side, maintain RRC-Inactive state for fast resume procedure.
On the CN/NAS side, use CM IDLE which implicitly already supports proper handling of high latency communications (HLcom) when the UE is unreachable for an extended period of time.

Currently, this CM and RRC state pair has not been defined, therefore this solution proposes how to support it with as minimal impact as possible compared to existing Rel-17 solutions.

The UE and AMF negotiate support of CM-IDLE with RRC inactive and idle mode extended DRX during registration procedure. The AMF provides indication of support of CM-IDLE with RRC Inactive along with the existing extended DRX information in the "RRC Inactive Assistance Information" (see clause 5.3.3.2.5 of TS 23.501).

When the UE is moved to CM-IDLE with RRC-Inactive, the NG-RAN initiates N2 suspend procedure towards the 5GC. When the UE moves back to RRC-Connected, the NG-RAN initiates N2 resume procedure.

When the UE is in CM-IDLE with RRC Inactive, all currently defined 5GC features related to UE being unreachable for CM-ILDE with extended DRX apply with the following addition:

The AMF may send N2 paging message only towards the NG-RAN node where N2 suspension occurred with indication of paging in RRC Inactive with CM_IDLE, and the NG-RAN node performs RAN paging in the RAN notification area.
The AMF may e.g. if initial attempt(s) to reach UE using RAN paging fail, try regular CN paging, i.e. send N2 paging message without indication of RRC Inactive with CM-IDLE. In this case the RAN performs regular CN paging.

From UE perspective, when the UE enters RRC-Inactive with CM-IDLE, the behaviour is the same as for RRC-Inactive with CM-CONNECTED, i.e.:

If the UE receives RAN paging, it attempts RRC resume procedure.
If RRC resume fails the UE falls back to CM-IDLE with RRC Idle and initiates RRC connection establishment request.
If the UE receives CN paging, it moves to CM-IDLE with RRC Idle and initiates RRC connection establishment request.
If the UE steps outside RAN notification area, the UE initiates RAN Notification Area Update.
The UE performs periodic RAN notification Area Update.
The UE performs periodic registration update.

6.4.2 Procedures p. 21

6.4.2.1 N2 suspend for UE entering RRC inactive with eDRX > 10.24s p. 21

Reproduction of 3GPP TS 23.700-68, Fig. 6.4.2.1-1: N2 Suspend for RRC Inactive / CM-IDLE

Figure 6.4.2.1-1: N2 Suspend for RRC Inactive / CM-IDLE

Step 0.

Based on the information received from the AMF, the NG-RAN node may decide to move the UE to CM-IDLE with RRC inactive if the NG-RAN intends to configure the UE with extended DRX longer than 10.24 seconds.

Step 1.

The NG-RAN shall initiate N2 suspend procedure as defined in clause 4.8.1 of TS 23.502 with the addition of: NG-RAN provides RRC Inactive with CM-IDLE indication and provides RRC inactive eDRX cycle length and PTW. The AMF shall enter CM-IDLE with RRC inactive state.

Step 2.

The NG-RAN initiates RRC release with suspend indication towards the UE with eDRX > 10.24s and RRC Inactive with CM-IDLE indication. The UE and the NG-RAN enters CM-IDLE with RRC inactive.

6.4.2.2 UE paging and resume p. 21

When the UE is in CM-IDLE with RRC Inactive, all mobile terminated data handling, high latency communication, and network initiated procedures follow the procedures defined in TS 23.502 when the UE is in CM-IDLE except for UE paging and connection resume.

Reproduction of 3GPP TS 23.700-68, Fig. 6.4.2.2-1: UE paging and resume procedure

Figure 6.4.2.2-1: UE paging and resume procedure

Step 1.

There is a trigger to page the UE, e.g. due to mobile terminated data, steps 1-3 of Network Triggered Service Request (clause 4.2.3.3 of TS 23.502,). The AMF determines the start of PTW as provided by NG-RAN, and sends N2 paging message only towards the NG-RAN node where N2 suspension occurred with indication of paging in RRC Inactive with CM_IDLE.

Step 2.

The NG-RAN initiates RAN paging in the RAN notification area.

Step 3.

When the UE receives RAN paging, it initiates RRC resume procedure from RRC inactive.

Step 4.

The NG-RAN proceeds with Steps 2-4 of clause 4.8.2 of TS 23.502.

(Optional) Fallback to CN paging

Step 5.

If e.g. AMF initiated RAN paging fails, or e.g. due to paging retransmission strategy, the AMF may initiate CN paging.

Step 6.

If the UE receives CN paging, the UE moves to RRC idle/CM idle and initiates service request procedure.

6.4.3 Impacts on services, entities and interfaces p. 22

UE:

Support of RRC inactive with CM-IDLE.

RAN:

Extend N2 Suspend and N2 Resume procedures to CM-IDLE with RRC inactive with eDRX cycle and PTW indication.
Extend the RRC Release message to release the UE to RRC-Inactive with CM-Idle.
Support AMF triggered RAN paging.

AMF:

Support of RRC inactive with CM-IDLE.
Support of N2 paging with RRC inactive indication to trigger RAN paging.
Extend the RRC-Inactive assistance information with the support for RRC-Inactive with CM-idle indication.
Extend N2 suspend and N2 resume procedure to enter CM-IDLE with RRC inactive and CM-CONNECTED respectively.