Conclusion
This document has made the following observations:
Observation 1: Due to no consensus in SA2 whether the UP of emergency PDU session can be deactivated, it should be assumed that there may be scenarios in which U-plane connection / PDU Session resources for emergency services are deactivated or remains not activated even though PDU Session for emergency services between UE and CN still exists (based on TS 23.501 5.6.8 Selective activation and deactivation of UP connection of existing PDU Session mechanism).
Observation 2: The RAN node can release the UE to the RRC_IDLE or RRC_INACTIVE state upon the expiry of ue-InactiveTime, regardless of whether the UE has an emergency PDU session.
Observation 3: The UE goes to the RRC_IDLE state upon the expiry of DataInactivityTimer, regardless of whether the UE has an emergency PDU session.
Observation 4: There are scenarios where gNB is not aware that UE has a PDU Session for emergency services and gNB may configure the UE with eDRX.
Observation 5: It is not clear whether TS 23.501 Clause 5.31.7.2.1 ”When the UE has PDU Session associated with emergency services, the UE and AMF follow regular discontinuous reception as defined in clause 5.4.5 and shall not use the extended idle mode DRX.” is applicable to RAN and/or CN eDRX paging in RRC_INACTIVE state.
Observation 6: Even without support from SA2, all observed problems can be solved (although not necessarily in an optimal way) if a UE configured with eDRX uses DRX.
And proposed the following:
Proposal 1: RAN2 confirms and replies SA2 that the RAN node may release the UE to RRC_IDLE or RRC_INACTIVE state from RRC_CONNECTED when the UE has an emergency PDU session.
Proposal 2: RAN2 agrees the UE side solution, which does not require any SA2 solution/conclusion: Clarify in TS 38.304 Clause 7.1 that in RRC_IDLE and RRC_INACTIVE states, the UE shall operate in DRX, not in eDRX, when the UE has a PDU session associated with emergency services. We provide a draft TP for Rel-17 TS 38.304 in the Annex.
4 Annex: TP for TS 38.304
7.1 Discontinuous Reception for paging
The UE may use Discontinuous Reception (DRX) in RRC_IDLE and RRC_INACTIVE state in order to reduce power consumption. The UE monitors one paging occasion (PO) per DRX cycle. A PO is a set of PDCCH monitoring occasions and can consist of multiple time slots (e.g. subframe or OFDM symbol) where paging DCI can be sent (TS 38.213 [4]). One Paging Frame (PF) is one Radio Frame and may contain one or multiple PO(s) or starting point of a PO. A L2 U2N Relay UE monitors the paging occasions of its PC5-RRC connected L2 U2N Remote UEs. In this case, the DRX cycle and UE ID mentioned in this clause refer to those of the L2 U2N Remote UE.
In multi-beam operations, the UE assumes that the same paging message and the same Short Message are repeated in all transmitted beams and thus the selection of the beam(s) for the reception of the paging message and Short Message is up to UE implementation. The paging message is same for both RAN initiated paging and CN initiated paging.
The UE initiates RRC Connection Resume procedure upon receiving RAN initiated paging. If the UE receives a CN initiated paging in RRC_INACTIVE state, the UE moves to RRC_IDLE and informs NAS. However, if a L2 U2N Relay UE in RRC_INACTIVE state receives a CN initiated paging for a L2 U2N Remote UE, the L2 U2N Relay UE does not move to RRC_IDLE state.
NOTE 0a: The L2 U2N Remote UE does not need to monitor the PO in order to receive the paging message.
NOTE 0b: While the SDT procedure is ongoing in RRC_INACTIVE state, the UE monitors the PO in order to receive only the Short Message as specified in TS 38.331 [3].
The PF and PO for paging are determined by the following formulae:
SFN for the PF is determined by:
(SFN + PF_offset) mod T = (T div N)*(UE_ID mod N)
Index (i_s), indicating the index of the PO is determined by:
i_s = floor (UE_ID/N) mod Ns
The PDCCH monitoring occasions for paging are determined according to pagingSearchSpace as specified in TS 38.213 [4] and firstPDCCH-MonitoringOccasionOfPO and nrofPDCCH-MonitoringOccasionPerSSB-InPO if configured as specified in TS 38.331 [3]. When SearchSpaceId = 0 is configured for pagingSearchSpace, the PDCCH monitoring occasions for paging are same as for RMSI as defined in clause 13 in TS 38.213 [4].
When SearchSpaceId = 0 is configured for pagingSearchSpace, Ns is either 1 or 2. For Ns = 1, there is only one PO which starts from the first PDCCH monitoring occasion for paging in the PF. For Ns = 2, PO is either in the first half frame (i_s = 0) or the second half frame (i_s = 1) of the PF.
When SearchSpaceId other than 0 is configured for pagingSearchSpace, the UE monitors the (i_s + 1)th PO. A PO is a set of 'S*X ' consecutive PDCCH monitoring occasions where 'S' is the number of actual transmitted SSBs determined according to ssb-PositionsInBurst in SIB1 and X is the nrofPDCCH-MonitoringOccasionPerSSB-InPO if configured or is equal to 1 otherwise. The [x*S+K]th PDCCH monitoring occasion for paging in the PO corresponds to the Kth transmitted SSB, where x=0,1,…,X-1, K=1,2,…,S. The PDCCH monitoring occasions for paging which do not overlap with UL symbols (determined according to tdd-UL-DL-ConfigurationCommon) are sequentially numbered from zero starting from the first PDCCH monitoring occasion for paging in the PF. When firstPDCCH-MonitoringOccasionOfPO is present, the starting PDCCH monitoring occasion number of (i_s + 1)th PO is the (i_s + 1)th value of the firstPDCCH-MonitoringOccasionOfPO parameter; otherwise, it is equal to i_s * S*X. If X > 1, when the UE detects a PDCCH transmission addressed to P-RNTI within its PO, the UE is not required to monitor the subsequent PDCCH monitoring occasions for this PO.
NOTE 1: A PO associated with a PF may start in the PF or after the PF.
NOTE 2: The PDCCH monitoring occasions for a PO can span multiple radio frames. When SearchSpaceId other than 0 is configured for paging-SearchSpace the PDCCH monitoring occasions for a PO can span multiple periods of the paging search space.
The following parameters are used for the calculation of PF and i_s above:
T: DRX cycle of the UE.
If the UE does not operate in eDRX as defined in clause 7.4:
- T is determined by the shortest of the UE specific DRX value configured by RRC (if any), the UE specific DRX value configured by upper layers (if any), and a default DRX value broadcast in system information. For L2 U2N Relay UE, T for a L2 U2N Remote UE is determined by the shortest of the UE specific DRX value provided in PC5-RRC signalling and a default DRX value broadcast in system information.
In RRC_IDLE state, if the UE operates in eDRX and eDRX is configured by upper layers, i.e., TeDRX, CN, according to clause 7.4:
- If TeDRX, CN is no longer than 1024 radio frames:
- T = TeDRX, CN;
- else:
- During CN configured PTW, T is determined by the shortest of UE specific DRX value, if configured by upper layers, and the default DRX value broadcast in system information.
In RRC_INACTIVE state, if the UE operates in eDRX and eDRX is configured by RRC, i.e., TeDRX, RAN (if any), and upper layers, i.e., TeDRX, CN, as defined in clause 7.4:
- If both TeDRX, CN and used TeDRX, RAN are no longer than 1024 radio frames, T = min{TeDRX, RAN, TeDRX, CN}.
- If TeDRX, CN is no longer than 1024 radio frames and no TeDRX, RAN is configured or used, T is determined by the shortest of UE specific DRX value configured by RRC and TeDRX, CN.
- If TeDRX, CN is longer than 1024 radio frames:
- If TeDRX, RAN is not configured or used:
- During CN configured PTW, T is determined by the shortest of the UE specific DRX value configured by RRC, the UE specific DRX value configured by upper layers (if any), and a default DRX value broadcast in system information. Outside the CN configured PTW, T is determined by the UE specific DRX value configured by RRC;
- else if used TeDRX, RAN is no longer than 1024 radio frames:
- During CN configured PTW, T is determined by the shortest of the UE specific DRX value, if configured by upper layers and TeDRX, RAN, and a default DRX value broadcast in system information. Outside the CN configured PTW, T is determined by TeDRX, RAN.
N: number of total paging frames in T
Ns: number of paging occasions for a PF
PF_offset: offset used for PF determination
UE_ID:
If the UE operates in eDRX as specified in clause 7.4:
- 5G-S-TMSI mod 4096
else:
- 5G-S-TMSI mod 1024
Parameters Ns, nAndPagingFrameOffset, nrofPDCCH-MonitoringOccasionPerSSB-InPO, and the length of default DRX Cycle are signaled in SIB1. The values of N and PF_offset are derived from the parameter nAndPagingFrameOffset as defined in TS 38.331 [3]. The parameter firstPDCCH-MonitoringOccasionOfPO is signalled in SIB1 for paging in the BWP configured by initialDownlinkBWP. For paging in a DL BWP other than the BWP configured by initialDownlinkBWP, the parameter first-PDCCH-MonitoringOccasionOfPO is signaled in the corresponding BWP configuration.
If the UE has no 5G-S-TMSI, for instance when the UE has not yet registered onto the network, the UE shall use as default identity UE_ID = 0 in the PF and i_s formulas above.
5G-S-TMSI is a 48 bit long bit string as defined in TS 23.501 [10]. 5G-S-TMSI shall in the formulae above be interpreted as a binary number where the left most bit represents the most significant bit.
In RRC_INACTIVE state, if the UE supports inactiveStatePO-Determination and the network broadcasts ranPagingInIdlePO with value "true", the UE shall use the same i_s as for RRC_IDLE state. Otherwise, the UE determines the i_s based on the parameters and formula above.
In RRC_INACTIVE state, if used eDRX value configured by upper layers is no longer than 1024 radio frames, the UE shall use the same i_s as for RRC_IDLE state.
In RRC_INACTIVE state, if used eDRX value configured by upper layers is longer than 1024 radio frames, during CN PTW, the UE shall use the same i_s as for RRC_IDLE state. Outside CN PTW, the UE shall use the i_s for RRC_INACTIVE state.
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