UE-to-UE Relay User Equipment

Description

The U2U Relay UE (UE-to-UE Relay User Equipment) is a new entity introduced in 3GPP Release 19 as part of the evolution of 5G ProSe and sidelink relay. It is a user equipment that acts as an intermediary to enable two U2U Remote UEs to communicate with each other when direct sidelink communication is not possible or not desired. The U2U Relay UE essentially extends the concept of relaying from UE-to-Network to UE-to-UE, allowing peer-to-peer communication over multiple hops.

Architecturally, the U2U Relay UE connects to two (or more) U2U Remote UEs via PC5 sidelink interfaces. It receives data from one remote UE and forwards it to the other, effectively acting as a layer-2 or layer-3 relay. The relay UE may be in-coverage of the network or out-of-coverage, depending on the scenario. When in-coverage, the relay may be managed by the network; when out-of-coverage, it operates autonomously based on pre-configuration. The U2U Relay UE must support the necessary sidelink protocols, including discovery, connection establishment, and data forwarding.

The protocol stack for the U2U Relay UE involves multiple PC5 interfaces. At the access stratum, it uses NR sidelink procedures for each link. The relay may operate at different layers: it could be a layer-2 relay (forwarding at the AS level, e.g., RLC/MAC) or a layer-3 relay (forwarding at the IP level). The choice depends on the specific use case and configuration. For U2U communication, the relay typically operates at layer-2 to provide efficient forwarding and maintain QoS. The relay must manage multiple sidelink connections, handle resource allocation, and ensure fairness and priority among the remote UEs.

Key procedures for the U2U Relay UE include discovery (allowing remote UEs to find the relay), connection establishment (establishing PC5 links with each remote UE), and data forwarding (routing data between the remote UEs). The relay may also participate in path selection, deciding whether to relay traffic or allow direct communication. Security is a critical aspect: the relay must ensure that only authorized remote UEs can use its services and that data integrity and confidentiality are maintained. This may involve authentication and key management procedures.

The U2U Relay UE is particularly relevant for public safety (e.g., direct communication between first responders when network coverage is unavailable), social networking (e.g., proximity-based services), and industrial IoT (e.g., machine-to-machine communication). It enables direct UE-to-UE communication over extended ranges by using a relay, thus overcoming the limited range of direct sidelink. It also allows communication between UEs that are not within direct sidelink range of each other.

In terms of network architecture, the U2U Relay UE may be transparent to the core network if the communication is purely local (i.e., no network involvement). However, for charging, policy control, or lawful interception, the network may need to be aware of the relayed communication. Release 19 specifications define the necessary procedures for network authorization and control of U2U relay operations. The U2U Relay UE represents a significant step towards flexible, infrastructure-less communication in 5G and beyond.

Purpose & Motivation

The U2U Relay UE was introduced to enable direct communication between user devices when they are not within direct sidelink range of each other, or when direct communication is not feasible due to obstacles or resource constraints. Prior to Release 19, 3GPP had specified UE-to-Network relaying (U2N) in Release 17, which allowed a remote UE to connect to the network via a relay. However, there was no standardized way for two UEs to communicate with each other via a relay without network involvement. The U2U Relay UE fills this gap by providing a standardized mechanism for UE-to-UE relaying.

The motivation for U2U relaying comes from several use cases. In public safety, first responders may need to communicate directly with each other even when out of network coverage. If two responders are too far apart for direct sidelink, a third responder (or a dedicated relay device) can act as a relay. In commercial scenarios, U2U relaying can enable proximity-based services such as social networking, file sharing, or multiplayer gaming without relying on network infrastructure. In industrial IoT, machines may need to communicate with each other directly, and a relay can extend the range or provide redundancy.

The U2U Relay UE also addresses the limitations of direct sidelink communication, which is typically limited to a few hundred meters and requires line-of-sight or good propagation conditions. By introducing a relay, the range can be extended, and communication can be maintained even when direct links are blocked. Furthermore, U2U relaying can improve energy efficiency for remote UEs, as they can communicate with a nearby relay at lower power rather than transmitting over long distances directly to each other.

The standardization of U2U Relay UE in Release 19 ensures interoperability between different vendors' devices and provides a foundation for future enhancements. It leverages the existing NR sidelink framework and 5G ProSe architecture, ensuring smooth integration with 5G networks.