5G Proximity Services Remote User Equipment

Description

The 5G ProSe Remote UE is a fundamental element in the 3GPP Release 19 architecture for proximity-based services (ProSe), which enables direct device-to-device (D2D) and relay-based communication. A Remote UE is a user device (such as a smartphone, IoT module, or wearable) that supports 5G ProSe and can connect to a Data Network (DN) not directly through the cellular network but via a chain of relays. Specifically, it communicates with the DN via zero or more 5G ProSe Intermediate UE-to-Network Relays and at least one 5G ProSe UE-to-Network Relay. The term 'zero or more' indicates that a single-hop relay is possible, or multiple hops can be used to extend range further. This architecture is particularly valuable in scenarios where direct network coverage is unavailable or unreliable, such as in disaster zones, remote rural areas, or within buildings.

The Remote UE operates using direct PC5 sidelink communication with the relay UEs. It discovers available relays either through network-controlled discovery or autonomous discovery, depending on its registration and coverage status. Once connected to a relay, it can exchange user data with the DN, and it may also participate in group communications. The Remote UE must maintain a ProSe context and perform authentication and authorization with the network, even when out of coverage, if it has a valid subscription and can reach the network via the relay. The architecture supports mobility of the Remote UE, including handover between relays and between relay and direct network access, ensuring service continuity.

Key functions of the Remote UE include: relay discovery and selection, establishment and maintenance of PC5 links, IP address management (either IPv4, IPv6, or IPv4v6), and support for various services such as IP data, Ethernet, or unstructured data. It also handles security procedures, including authentication and encryption of the sidelink communication. The Remote UE can be in one of several states: in-coverage, out-of-coverage, or partial coverage. In the latter two cases, it relies on the relay to reach the network for control plane and user plane traffic.

The Remote UE interacts with the 5G core network (5GC) through the relay, which acts as a transparent IP router or a Layer-3 relay. The 5G ProSe UE-to-Network Relay encapsulates the Remote UE's traffic and forwards it to the UPF (User Plane Function) in the 5GC. From the network's perspective, the Remote UE may be identified by a ProSe subscription identifier and can be subject to specific QoS policies and charging. This allows operators to manage and monetize relayed traffic appropriately.

In 3GPP Release 19, the Remote UE definition was introduced as part of the enhanced ProSe architecture for 5G, standardized in TS 23.304, with management aspects in TS 32.277. Release 20 continues to refine the concept, ensuring interoperability and addressing new use cases like public safety and commercial proximity services.

Purpose & Motivation

The 5G ProSe Remote UE exists to extend network coverage and enable communication for devices that cannot directly connect to the 5G core network. This is crucial for public safety, disaster relief, and commercial applications where infrastructure may be damaged or absent. Historically, devices out of coverage could not communicate with the network, limiting critical services. The Remote UE, via relays, solves this by allowing indirect connectivity, thereby ensuring service availability and reliability. It also supports group communication for public safety users, allowing them to stay connected even when network coverage is lost, which is a key requirement in mission-critical scenarios.