Remote User Equipment

Description

The Remote UE is a key concept in 3GPP Proximity Services (ProSe) for public safety, defined in specifications such as TS 23.303, TS 23.468, TS 23.752, TS 33.833, and TS 36.300. It refers to a User Equipment (UE) that is enabled for ProSe and is used by public safety personnel, but is not directly served by the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). Instead, it communicates with a Packet Data Network (PDN) through a ProSe UE-to-Network Relay. This architecture allows devices that are out of network coverage to maintain connectivity by relaying their traffic through another UE that has network coverage.

The Remote UE operates in a direct communication mode with the ProSe UE-to-Network Relay, using the PC5 interface (sidelink). The relay UE acts as a Layer-3 router or a Layer-2 bridge, forwarding IP packets between the Remote UE and the PDN. The Remote UE may be a handheld device, a vehicle-mounted terminal, or any other public safety device. The key requirement is that it supports ProSe direct discovery and communication. The Remote UE can discover a suitable relay UE either through direct discovery procedures or by using a pre-configured relay association.

Architecturally, the Remote UE connects to the relay UE via the PC5 interface, and the relay UE connects to the network via the Uu interface (E-UTRAN). The relay UE may be served by the network directly or may itself be out of coverage but connected to another relay. The ProSe function in the core network manages authorization, authentication, and policy for the Remote UE and the relay. The Remote UE's traffic is encapsulated and forwarded by the relay, which may apply security (e.g., IPsec) to protect the communication. The Remote UE can access PDN services, including IP-based applications, as if it were directly connected to the network.

The Remote UE concept is particularly important for mission-critical push-to-talk (MCPTT) and other public safety applications, where reliable communication is essential even in areas with no network coverage. The relay functionality extends network coverage and improves resilience. The definition of Remote UE has evolved slightly: in Rel-13, it was defined as a ProSe-enabled Public Safety UE that is not served by E-UTRAN and communicates with a PDN via a ProSe UE-to-Network Relay. In Rel-14, the wording changed to remove the explicit 'not served by E-UTRAN' condition, reflecting that the Remote UE may be in coverage but still choose to communicate via a relay for other reasons (e.g., power saving, policy). This change broadened the applicability of the term.

The Remote UE is a fundamental element of the ProSe UE-to-Network Relay architecture, which is part of the broader 3GPP efforts to support public safety and mission-critical communications. It enables devices to communicate beyond network coverage, enhancing the capabilities of first responders. The term is closely related to ProSe UE-to-Network Relay, ProSe, PC5 interface, and public safety.

Purpose & Motivation

The Remote UE concept was created to address the need for public safety users to maintain communication even when they are outside network coverage. In disaster scenarios or remote areas, network infrastructure may be damaged or absent, but first responders still need to communicate with each other and with command centers. Traditional cellular communication requires direct connection to a base station, which is not always available. The ProSe UE-to-Network Relay architecture allows a UE that has network coverage (the relay UE) to extend connectivity to other UEs (the Remote UEs) that are out of coverage.

The Remote UE solves the problem of coverage holes by leveraging device-to-device (D2D) communication. It enables a group of public safety users to stay connected as long as at least one device in the group has network coverage. This is particularly useful for search and rescue operations, where teams may be spread out and some members may be in areas without direct cellular signal. The Remote UE can access the same PDN services as a directly connected UE, including IP-based applications, location services, and group communication.

Historically, the Remote UE was introduced in 3GPP Rel-13 as part of the initial ProSe specifications for public safety. The definition was refined in Rel-14 to remove the restriction that the Remote UE must not be served by E-UTRAN, acknowledging that a UE might still use a relay even when in coverage, for example to conserve battery or to use a more robust link. This evolution reflects the growing understanding of relay use cases and the flexibility required in public safety operations. The Remote UE remains a critical component of mission-critical communication systems, and its definition continues to be maintained in subsequent releases.