UE-to-UE Remote User Equipment

Description

The U2U Remote UE (UE-to-UE Remote User Equipment) is a user equipment that communicates with other UEs indirectly through a U2U Relay UE. It is a key component of the 5G ProSe UE-to-UE relay architecture introduced in 3GPP Release 19. Unlike the U2N Remote UE, which connects to the network via a relay, the U2U Remote UE connects to another UE (the peer) via a relay, enabling direct device-to-device communication even when the two UEs are not within direct sidelink range.

Architecturally, the U2U Remote UE establishes a PC5 sidelink connection with a U2U Relay UE. The relay then forwards traffic to another U2U Remote UE (the peer). Thus, the communication path is: Remote UE A <-> Relay UE <-> Remote UE B. The U2U Remote UE may be in-coverage or out-of-coverage of the network. When in-coverage, it may be managed by the network for relay selection and authorization; when out-of-coverage, it relies on pre-configuration for relay discovery and selection. The U2U Remote UE must support NR sidelink procedures for discovery, connection establishment, and data transfer.

The protocol stack for the U2U Remote UE involves the PC5 interface for communication with the relay. At the access stratum, it uses NR sidelink protocols, including discovery, PC5-S signaling, and data transfer over SL-DRB. The U2U Remote UE may operate at layer-2 or layer-3, depending on the relay's operation. For layer-2 relaying, the remote UE's data is forwarded at the RLC/MAC level; for layer-3, forwarding is at the IP level. The remote UE must handle the fact that its peer is not directly connected but reached via a relay, which may involve addressing and routing considerations.

Key procedures for the U2U Remote UE include relay discovery, relay selection, connection establishment with the relay, and data communication with the peer. Discovery can be Model A (the remote UE broadcasts solicitation) or Model B (the relay broadcasts announcement). Once a suitable relay is found, the remote UE establishes a PC5 unicast link with the relay. The relay then establishes a link with the peer remote UE (or uses an existing link). The remote UE then sends data to the relay, which forwards it to the peer. The remote UE may also receive data from the peer via the relay.

Mobility for the U2U Remote UE involves relay reselection when the current relay becomes unavailable or when a better relay is found. This is similar to U2N remote UE mobility but without network involvement in out-of-coverage scenarios. The remote UE must monitor the link quality to the relay and perform reselection based on criteria such as signal strength, relay load, and service requirements. In-coverage, the network may assist in relay selection and handover.

The U2U Remote UE is essential for public safety direct communication, where two users may be separated by distance or obstacles but need to communicate. It also enables commercial proximity services, such as social networking or file sharing between devices that are not in direct range. By using a relay, the effective communication range is extended, and connectivity is maintained even when direct links are not possible. The U2U Remote UE thus enhances the flexibility and robustness of 5G ProSe.

Purpose & Motivation

The U2U Remote UE was introduced to enable direct communication between user devices when they are not within direct sidelink range, or when direct communication is not feasible. Prior to Release 19, 3GPP had standardized 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 mechanism for a UE to communicate with another UE via a relay without network involvement. The U2U Remote UE fills this gap by defining the remote UE's role in UE-to-UE relay communication.

The motivation for U2U relaying comes from public safety and commercial use cases. In public safety, first responders 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 can act as a relay. The U2U Remote UE is the responder that uses the relay to reach the peer. In commercial scenarios, U2U relaying can enable proximity-based services such as social networking, file sharing, or multiplayer gaming without relying on network infrastructure. The U2U Remote UE allows devices to communicate even when they are not in direct range.

The U2U Remote UE also addresses the limitations of direct sidelink communication, which is typically limited to a few hundred meters and requires good propagation conditions. By using 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 Remote 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.