5.38 Support for Multi-USIM UE

23.5013GPPRelease 18System architecture for the 5G System (5GS)TS

5.38.1 General

A network and a Multi-USIM UE may support one or more of the following features for Multi-USIM UE operation:

– Connection Release as described in clause 5.38.2;

– Paging Cause Indication for Voice Service, as described in clause 5.38.3;

– Reject Paging Request, as described in clause 5.38.4;

– Paging Restriction, as described in clause 5.38.5;

– Paging Timing Collision Control, as described in clause 5.38.6.

In the Registration procedure (as specified in clause 4.2.2.2.2), when a Multi-USIM UE has more than one USIM active, supports and intends to use one or more Multi-USIM specific features, it indicates to the AMF the corresponding Multi-USIM feature(s) are supported (except for the Paging Timing Collision Control feature). Based on the received indication of the supported Multi-USIM features from the UE, the AMF shall indicate to the UE the support of the Multi-USIM features based on the Multi-USIM features supported by network and any preference policy by the network, if available. When a UE returns to having only one USIM active from a Multi-USIM UE that previously indicated to the network it supported Multi-USIM feature(s), the UE shall indicate all the Multi-USIM features are not supported to the network for that USIM. The AMF shall only indicate the support of Paging Restriction feature together with the support of either Connection Release feature or Reject Paging Request feature.

The Multi-USIM UE includes the support of individual features for Connection Release, Paging Cause Indication for Voice Service, Reject Paging Request and Paging Restriction as specified in clause 5.4.4a.

NOTE: The Paging Timing Collision Control feature being based on the Mobility Registration Update, and it doesn’t require capability exchange between the UE and network.

The network shall not indicate support for any Multi-USIM feature to the UE as part of the Emergency Registration procedure.

A Multi-USIM UE shall use a separate PEI for each USIM when it registers with the network.

5.38.2 Connection Release

A Multi-USIM UE may request the network to release the UE from RRC-CONNECTED state in 3GPP access for a USIM due to activity on another USIM in 3GPP access, if both UE and network indicate the Connection Release feature is supported to each other.

In the case of NAS connection release procedure, the UE indicates that it requests to be released from RRC-CONNECTED state, by initiating either a Service Request procedure over 3GPP access or a Registration procedure over 3GPP access (if case the UE needs to perform Registration Update at the same time with this network, including the case where the Registration Request is sent due to mobility outside the Registration Area, i.e. before detecting whether the network supports the feature in the new Tracking Area, provided that the network has already indicated support for Connection Release feature in the current stored Registration Area), by including a Release Request Indication. If supported by the UE and network, the UE may also provide, only together with the Release Request Indication, Paging Restriction Information, as specified in clause 5.38.5, which requests the network to restrict paging. If the UE is performing an Emergency Registration then it shall not include a Release Request Indication.

For NR/5G access, an AS method for the UE to request the network to release the UE from RRC-CONNECTED state is specified in TS 38.300 [27]. This mechanism does not allow the UE to indicate Paging Restrictions.

NOTE 1: When both the access stratum and NAS based approaches for requesting the connection release are supported by the UE and the network, it depends on the UE implementation which of the two to use (for example: based on the preferred end state (RRC-INACTIVE or IDLE) and whether Paging Restriction Information is to be provided).

NOTE 2: When there is no PLMN-wide support for the Connection Release feature, it can occur that upon Mobility Registration Update with Release Request indication the UE is not released by the network. The UE behaviour, when it detects that the network does not support the feature in a new RA, is outside the scope of this specification.

5.38.3 Paging Cause Indication for Voice Service

A Multi-USIM UE and the network may support Paging Cause Indication for Voice Service feature.

The network that supports Paging Cause Indication for Voice Service feature shall provide a Voice Service Indication for IMS voice service in the Paging message, only if the UE indicates the Paging Cause Indication for Voice Service feature is supported to the network. The network determines the IMS voice service based on the Paging Policy Indicator as specified in clause 5.4.3.2.

Upon reception of the Voice Service Indication in NGAP Paging Message from AMF, the NG-RAN supporting Paging Cause Indication for Voice Service should include the Voice Service Indication in the Uu Paging message to the UE.

When the UE context in the AMF indicates Paging Cause Indication for Voice Service feature is supported, in order to require NG RAN to deliver the Voice Service Indication in RAN paging for the UE in RRC-Inactive state, the AMF provides an indication indicating the Paging Cause Indication for Voice Service feature is supported to the NG-RAN. Upon reception of the indication, the NG-RAN that supports the feature stores a Paging Cause Indication for Voice Service indication in its the UE context. For a UE in RRC-Inactive, the NG-RAN should provide the Voice Service Indication in the RAN Paging message only when there is Paging Cause Indication for Voice Service indication in the UE context and detects the downlink data which triggers the RAN Paging message is related to voice service based on the Paging Policy Indicator, in the header of the received downlink data, as specified in clause 5.4.3.2.

UE that supports the Paging Cause Indication for Voice Service feature is capable of differentiation between Paging from a network that does not support the Paging Cause Indication for Voice Service feature and Paging without the Voice Service Indication. How the UE distinguishes the Paging from a RAN that does not support the Paging Cause Indication for Voice Service feature and Paging without the Voice Service Indication is defined in TS 38.331 [28]. The UE determines whether the Paging Cause Indication for Voice Service feature is supported in the current Registration Area by 5GC based on the MUSIM capability exchange with the AMF, see clause 5.38.1. The UE determines that the Paging Cause Indication for Voice Service feature is supported if it is supported by both the RAN, as indicated in the received Uu Paging message, and by 5GC, as indicated in the MUSIM capability exchange with the AMF.

The UE uses the Paging Cause Indication for Voice Service as described in TS 24.501 [47] and TS 38.331 [28].

5.38.4 Reject Paging Request

A Multi-USIM UE may set up a connection to respond to a page with a Reject Paging Indication to the network indicating that the UE does not accept the paging and requests to return to CM-IDLE state after sending this response, if both UE and network indicate the Reject Paging Request feature is supported to each other.

Upon being paged by the network, the Multi-USIM UE in CM-IDLE state attempts to send a Service Request message to the paging network including the Reject Paging Indication as the response to the paging, unless it is unable to do so, e.g. due to UE implementation constraints. In addition to the Reject Paging Indication, the UE may include Paging Restriction Information as specified in clause 5.38.5 in the Service Request message, if supported by UE and network.

NOTE: A Multi-USIM UE in RRC_INACTIVE state can decide to not initiate the RRC connection resumption procedure, e.g. when it decides not to respond to the paging message due to UE implementation constraints as specified in TS 24.501 [47] and TS 38.331 [28].

5.38.5 Paging Restriction

A Multi-USIM UE and the network may support Paging Restriction. A Multi-USIM UE, if the AMF indicates that the network supports Paging Restriction feature, may indicate Paging Restriction Information in the Service Request or Registration Request message (including the case where the Registration Request is sent due to mobility outside the Registration Area, i.e. before detecting whether the network supports the feature in the new Tracking Area, provided that the network has already indicated support for Paging Restriction feature in the current stored Registration Area) as specified in clauses 5.38.2 and 5.38.4.

Based on operator policy the AMF may accept or reject the Paging Restriction Information requested by the UE. If the AMF accepts the Paging Restriction Information from the UE, the AMF stores the Paging Restriction Information from the UE in the UE context. If the AMF rejects the Paging Restriction Information, the AMF removes any stored Paging Restriction Information from the UE context and discards the UEs requested Paging Restriction Information. The AMF informs the UE about the acceptance/rejection of the requested Paging Restriction Information in the Registration Accept or Service Accept message.

If the UE does not provide any Paging Restriction Information in the Service Request over 3GPP access or the Registration Request over 3GPP access, the AMF removes any stored Paging Restriction Information from the UE context.

The Paging Restriction Information may indicate any of the following:

a) all paging is restricted; or

b) all paging is restricted, except paging for voice service (IMS voice); or

c) all paging is restricted, except for certain PDU Session(s); or

d) all paging is restricted, except paging for voice service (IMS voice) and certain PDU session(s).

NOTE 1: The UE expects not to be paged for any purpose in case a). The UE expects to be paged only for voice service in case b). The UE expects to be paged only for certain PDU Session(s) in case c). The UE expects to be paged for voice service and certain PDU session(s) in case d). The AMF can page the UE for mobile terminated signalling based on local policy considering the stored Paging Restriction Information, except for case a). In this case, to comply with the UE provided Paging Restriction Information, the AMF can trigger AN release procedure as soon as possible after the mobile terminated signalling procedure is executed.

NOTE 2: In the case of roaming, the Paging Restrictions for voice service implied by bullet b) and d) depends on the existence of an agreement with the HPLMN to support voice service via IMS. Hence the support of Paging Restrictions in bullets b) and d) takes the IMS voice service agreement into consideration.

NOTE 3: When there is no PLMN-wide support for the Paging Restriction feature, it can occur that upon Mobility Registration Update with Paging Restriction Information the UE detects the network does not support the feature. If so, the UE assumes that no Paging Restriction Information is applied.

5.38.6 Paging Timing Collision Control

To avoid possible paging occasion collision and to enhance the likelihood that paging is received successfully for different USIMs, a Multi-USIM UE may need a new 5G-GUTI to modify the timing of the Paging Occasions (POs) for a USIM when the USIM’s registration is not emergency registration. When a Multi-USIM UE needs a 5G-GUTI assignment, it performs a Mobility Registration Update without any specific indication (i.e. it is using a normal Registration procedure). This triggers the AMF to allocate a new 5G-GUTI and provide it to the Multi-USIM UE in the Registration Accept message.

NOTE: It is recommended to avoid excessive signalling load from UE due to this procedure.