UE-to-Network Remote User Equipment

Description

The U2N Remote UE (UE-to-Network Remote User Equipment) is a fundamental component of 3GPP's proximity services (ProSe) and sidelink relay architecture, introduced in Release 17. It represents a user device that does not have a direct connection to the 5G core network or NG-RAN, but instead accesses network services indirectly through a U2N Relay UE. This relay UE acts as an intermediary, forwarding traffic between the remote UE and the network over the PC5 sidelink interface and the Uu air interface, respectively.

Architecturally, the U2N Remote UE connects to the U2N Relay UE via the PC5 reference point, which is based on the NR sidelink technology. The relay UE then connects to the gNB via the Uu interface. This creates a two-hop path: Remote UE <-> Relay UE <-> gNB. The remote UE can be either in-coverage or out-of-coverage of the network. When out-of-coverage, it relies entirely on the relay for any network communication. The relay UE must be in-coverage and authorized by the network to provide relay services.

The protocol stack for the U2N Remote UE involves both the PC5 interface (for sidelink communication with the relay) and the Uu interface (for the relay's connection to the network). At the access stratum (AS) level, the remote UE uses NR sidelink procedures for discovery, connection establishment, and data transfer with the relay. At the non-access stratum (NAS) level, the remote UE maintains its own NAS connection with the core network (AMF/SMF) through the relay, meaning the remote UE has its own 5G-GUTI, performs its own registration, and maintains its own PDU sessions. The relay simply forwards NAS messages transparently.

Key procedures for the U2N Remote UE include relay discovery (both Model A and Model B), relay selection and reselection, connection establishment over PC5, and data forwarding. The remote UE must discover suitable relay UEs, which involves either the remote UE broadcasting discovery solicitation messages (Model A) or the relay UE broadcasting announcements (Model B). Once a relay is selected, the remote UE establishes a PC5 connection (unicast link) with the relay. The relay then establishes or reuses a Uu connection with the gNB to forward the remote UE's traffic. The gNB and core network must be aware of the remote UE's presence and manage its context appropriately, including mobility and session management.

In terms of mobility, the U2N Remote UE can perform handovers between different relay UEs and also between indirect (relayed) and direct (Uu) connectivity. This is managed through a combination of sidelink measurements, relay reselection procedures, and network-controlled handover mechanisms. The remote UE's mobility is largely transparent to the core network, which sees the remote UE's NAS signaling as coming through the relay. However, the RAN must manage the radio resources and ensure seamless service continuity.

The U2N Remote UE is particularly important for public safety (e.g., mission-critical push-to-talk), industrial IoT, and coverage extension scenarios. It allows devices that are out of network coverage or have poor link quality to still access 5G services by leveraging nearby relay-capable UEs. This enhances network resilience and extends effective coverage without additional infrastructure.

Purpose & Motivation

The U2N Remote UE concept was introduced to address the need for extended coverage and reliable connectivity in scenarios where direct network access is unavailable or unreliable. Traditional cellular networks require a direct Uu link between the UE and the gNB, which can be problematic in remote areas, inside buildings, or during emergencies when infrastructure is damaged. The U2N Remote UE, via a relay, enables devices to connect to the network indirectly, thus extending coverage and providing service continuity.

Prior to Release 17, 3GPP had specified ProSe (Proximity Services) for LTE in Release 12/13, which included UE-to-Network relaying for public safety. However, these were based on LTE technology and had limitations in terms of data rates, latency, and integration with 5G core. The 5G U2N Remote UE leverages NR sidelink and the 5G core architecture to provide enhanced performance, better QoS support, and seamless integration with network slicing and other 5G features.

The motivation also stems from the need to support mission-critical services where network availability is paramount. For example, in disaster relief operations, a relay UE can be deployed to provide connectivity to a group of remote UEs. Similarly, in industrial environments, sensors and actuators (remote UEs) can connect via a relay to reduce power consumption and extend battery life. The U2N Remote UE concept thus solves the problem of limited coverage and enhances the overall robustness of the network.