Multi-USIM UE

Description

A Multi-USIM (MUSIM) UE is a User Equipment that contains multiple Universal Subscriber Identity Modules (USIMs), each associated with a different subscription and potentially different Public Land Mobile Networks (PLMNs). Introduced in 3GPP Release 19, the MUSIM UE is capable of initiating and maintaining simultaneous separate registration states over 3GPP access with one or more PLMNs using the identities and credentials associated with those USIMs. This means the UE can be registered with multiple networks at the same time, allowing the user to receive services from multiple subscriptions concurrently. The definition specifies that the UE supports one or more of the following capabilities: N1 NAS signalling connection release, paging indication for voice services, reject paging request, paging restriction, and paging timing collision control.

The architecture for MUSIM UEs involves enhancements to the 5G System (5GS) to handle multiple registrations. The UE maintains separate NAS (Non-Access Stratum) contexts for each USIM, each with its own registration state. The N1 NAS signalling connection is the interface between the UE and the Access and Mobility Management Function (AMF) in the core network. For MUSIM, the UE can release the N1 connection for one USIM while keeping another active, to save resources or manage paging. The paging indication for voice services allows the network to indicate to the UE that an incoming voice call is for a specific USIM, so the UE can respond appropriately. The reject paging request allows the UE to reject a paging request for one USIM if it is busy with another, and the paging restriction allows the UE to restrict paging to certain USIMs or times. Paging timing collision control manages the timing of paging occasions to avoid collisions when multiple USIMs are paged simultaneously.

Key components of MUSIM support include the UE, which must have multiple USIMs and the capability to handle multiple registrations. The network side involves the AMF, which manages the registration and paging for each USIM. The Unified Data Management (UDM) stores subscription data for each USIM. The Policy Control Function (PCF) may apply policies for MUSIM. The 3GPP specifications 24.301 (NAS protocol for EPS) and 24.501 (NAS protocol for 5GS) define the NAS procedures for MUSIM, including registration, paging, and connection management. The UE uses the USIM credentials to authenticate with each PLMN separately. The network must be able to page the UE for each USIM, and the UE must be able to distinguish which USIM the paging is for.

In operation, a MUSIM UE can be in different states for each USIM: for example, one USIM may be in CM-CONNECTED state while another is in CM-IDLE. The UE can initiate a service request for one USIM while maintaining the other. The paging mechanisms are enhanced to include a MUSIM-specific paging indication, which may be a new paging cause or a flag in the paging message. The UE can also send a paging reject message to the network if it cannot respond to a paging for a particular USIM. The paging restriction allows the UE to inform the network that it does not want to receive paging for a certain USIM for a period, which can be useful when the user is actively using one subscription. Paging timing collision control ensures that paging occasions for different USIMs do not overlap, which could cause the UE to miss pages.

The role of MUSIM UEs in the network is to enable users to have multiple subscriptions (e.g., personal and work) on a single device without needing multiple devices. This is increasingly important as users demand flexibility and cost savings. MUSIM support enhances user experience by allowing simultaneous services, such as receiving a voice call on one USIM while data session is active on another. It also helps operators manage network resources by providing mechanisms to control paging and connection release. The feature is part of the ongoing evolution of 5G to support diverse device capabilities and user needs.

Purpose & Motivation

The MUSIM UE concept was created to address the growing need for devices that can support multiple subscriptions simultaneously. Prior to Rel-19, while multi-SIM devices existed, they often had limitations such as not being able to maintain simultaneous registrations or having to manually switch between SIMs. This led to missed calls, delayed data, and poor user experience. The 3GPP recognized the need for standardized support to ensure interoperability and efficient network operation. The motivation was to allow a UE with multiple USIMs to be registered with multiple PLMNs at the same time, and to provide mechanisms to manage paging and connections intelligently.

The limitations of previous approaches included the lack of standardized paging indications for multiple USIMs, which meant the UE could not distinguish which USIM a page was for, leading to unnecessary responses or missed pages. Also, without paging restriction, the network would page all USIMs, causing unnecessary signaling and battery drain. The N1 NAS signalling connection release allows the UE to release connections for idle USIMs to save resources. The paging timing collision control addresses the issue of simultaneous paging occasions that could be missed if they overlap. These features solve the problems of resource efficiency, battery life, and user experience.

MUSIM UE support is a significant step towards accommodating real-world usage where users have multiple SIMs for different purposes (e.g., personal and business). It enables new business models for operators, such as offering secondary subscriptions for IoT or wearables. The standardization in Rel-19 ensures that networks and devices from different vendors can interoperate, providing a seamless experience. This feature is expected to be widely adopted in 5G-Advanced and beyond, as multi-SIM devices become more prevalent.