5.31.7.2.2 Paging for extended idle mode DRX in E-UTRA and NR connected to 5GC

5.31.7.2.2.0 General

For WB-E-UTRA and LTE-M connected to 5GC, the extended idle mode DRX value range will consist of values starting from 5.12s (i.e. 5.12s, 10.24s, 20.48s, etc.) up to a maximum of 2621.44s (almost 44 min). For NB-IoT, the extended idle mode DRX value range will start from 20.48s (i.e. 20.48s, 40.96s, 81.92, etc.) up to a maximum of 10485.76s (almost 3 hours) (see TS 36.304 [52]). For NR, the extended idle mode DRX value range will consist of values starting from 2.56s (i.e. 2.56s, 5.12s, 10.24s, 20.48s, etc.) up to a maximum of 10485.76s (almost 3 hours) (see TS 38.304 [50]). The extended idle mode DRX cycle length is negotiated via NAS signalling. The AMF includes the extended idle mode DRX cycle length for NR, WB-E-UTRA, LTE-M or NB-IoT in paging message to assist the NG-RAN node in paging the UE. For NR, Paging Time Window applies for extended DRX lengths longer than 10.24s as defined in TS 38.304 [50]. For WB-E-UTRA, LTE-M and NB-IoT, Paging Time Window applies for extended DRX lengths of 10.24s and longer as defined in TS 36.304 [52].

The network follows the regular paging strategy as defined in clause 5.4.5 when the extended idle mode DRX cycle length is 5.12s or less for WB-E-UTRA, LTE-M and NB-IoT, or 10.24s or less for NR.

Clauses 5.31.7.2.2.1, 5.31.7.2.2.2 and 5.31.7.2.2.3 apply when the extended idle mode DRX cycle length is 10.24s or longer for WB-E-UTRA, LTE-M and NB-IoT, or longer than 10.24s for NR.

5.31.7.2.2.1 Hyper SFN, Paging Hyperframe and Paging Time Window length

A Hyper-SFN (H-SFN) frame structure is defined on top of the SFN used for regular idle mode DRX. Each H-SFN value corresponds to a cycle of the legacy SFN of 1024 radio frames, i.e. 10.24s. When extended idle mode DRX is enabled for a UE, the UE is reachable for paging in specific Paging Hyperframes (PH), which is a specific set of H-SFN values. The PH computation is a formula that is function of the extended idle mode DRX cycle and a UE specific identifier, as described in TS 36.304 [52] and TS 38.304 [50]. This value can be computed at all UEs and AMFs without need for signalling. The AMF includes the extended idle mode DRX cycle length and the PTW length in paging message to assist the NG-RAN nodes in paging the UE.

The AMF also assigns a Paging Time Window length and provides this value to the UE during Registration Update procedures together with the extended idle mode DRX cycle length. The UE first paging occasion is within the Paging Hyperframe as described in TS 36.304 [52] and TS 38.304 [50]. The UE is assumed reachable for paging within the Paging Time Window. The start and end of the Paging Time Window is described in TS 36.304 [52] and TS 38.304 [50]. After the Paging Time Window length, the AMF considers the UE unreachable for paging until the next Paging Hyperfame.

5.31.7.2.2.2 Loose Hyper SFN synchronization

NOTE: This clause applies when the extended DRX cycle length is 10.24s or longer for WB-E-UTRA, LTE-M and NB-IoT and longer than 10.24s for NR.

In order for the UE to be paged at roughly similar time, the H-SFN of all NG-RAN nodes and AMFs should be loosely synchronized.

Each NG-RAN node and AMF synchronizes internally the H-SFN counter so that the start of H-SFN=0 coincides with the same a preconfigured time epoch. If NG-RAN nodes and AMFs use different epochs, e.g. due to the use of different time references, the GPS time should be set as the baseline and the NG-RAN nodes and AMFs synchronize the H-SFN counter based on the GPS epoch considering the time offset between GPS epoch and other time-reference epoch a preconfigured time. It is assumed that NG-RAN nodes and AMFs are able to use the same H-SFN value with accuracy in the order of legacy DRX cycle lengths, e.g. 1 to 2 seconds. There is no need for synchronization at SFN level.

There is no signalling between network nodes required to achieve this level of loose H-SFN synchronization.

5.31.7.2.2.3 AMF paging and paging retransmission strategy

NOTE: This clause applies when the extended DRX cycle length is 10.24s or longer for WB-E-UTRA, LTE-M and NB-IoT and longer than 10.24s for NR.

When the AMF receives trigger for paging and the UE is reachable for paging, the AMF sends the paging request. If the UE is not reachable for paging, then the AMF pages the UE just before the next paging occasion.

The AMF determines the Paging Time Window length and a paging retransmission strategy and executes the retransmission scheme.

For extended DRX length of 10.24s, in the paging request message the AMF sends the Paging Time Window to the ng-eNB but does not send the Paging Time Window to the gNB.