User Equipment-to-Network Relay Communication

Description

UE-to-Network (U2N) Relay communication is a feature introduced in 3GPP Release 19 that enables a User Equipment (UE) to communicate with the network through an indirect path involving one or more relay UEs. This mode of communication is defined in specifications such as TS 38.300 (NR overall description), TS 38.304 (UE procedures in idle and inactive mode), and TS 38.331 (RRC protocol). It supports both single-hop and multi-hop L2 (Layer 2) relay communication, where the relay UE operates at Layer 2, forwarding data between the remote UE and the network.

In single-hop L2 U2N Relay communication, a remote UE connects to the network via a single U2N Relay UE. The relay UE acts as an intermediary, receiving data from the remote UE and forwarding it to the gNB, and vice versa. The relay UE may be a normal UE with relay capability, and it maintains a connection to the network on behalf of the remote UE. The communication between the remote UE and the relay UE is typically over a PC5 interface (sidelink), while the relay UE communicates with the network over the Uu interface.

In multi-hop L2 U2N Relay communication, the remote UE connects to the network through multiple relay UEs, forming a chain. Each relay UE forwards data to the next relay UE until the data reaches the network. This extends coverage further and can be useful in scenarios where direct network coverage is unavailable, such as in disaster relief or remote areas.

The L2 relay architecture means that the relay UE operates at Layer 2 of the protocol stack, forwarding packets based on lower-layer identifiers rather than full IP routing. This allows for efficient forwarding and reduces the processing overhead in the relay UE. The relay UE may also perform functions such as radio link control (RLC) and medium access control (MAC) layer forwarding.

Key procedures for U2N Relay communication include relay selection, relay reselection, and path switching. The remote UE discovers available relay UEs and selects one based on criteria such as signal strength and relay capability. The network may assist in relay selection by providing information about candidate relays. Once connected, the remote UE can switch between direct and indirect paths, depending on coverage and link quality.

The feature is particularly beneficial for extending network coverage to UEs that are out of direct coverage, improving reliability through diversity, and enabling new use cases such as public safety, wearable devices, and industrial IoT. It leverages the sidelink capabilities of NR to enable device-to-device communication, which is a key enabler for 5G Advanced and beyond.

Overall, UE-to-Network Relay communication is a significant enhancement in Release 19 that enhances the flexibility and reach of 5G networks, supporting the vision of ubiquitous connectivity.

Purpose & Motivation

UE-to-Network Relay communication was introduced to address the limitations of direct network coverage in challenging environments. Many UEs, such as those in remote areas, inside buildings, or in disaster zones, may not be able to establish a direct link to a base station due to signal blockage or distance. The relay feature allows such UEs to connect to the network indirectly through other UEs that have good coverage, thereby extending the effective coverage area without the need for additional infrastructure.

Prior to Release 19, 3GPP had specified proximity services (ProSe) and sidelink communication for LTE and NR, but these were primarily focused on direct device-to-device communication or basic relay functionality. The Release 19 feature builds on these foundations to provide a more robust and standardized relay solution for NR, including multi-hop support and enhanced procedures for relay selection and path management.

The motivation for multi-hop relay is to further extend coverage in scenarios where even a single relay may not be sufficient, such as in deep indoor environments or tunnels. By chaining multiple relays, the network can reach UEs that are otherwise unreachable. This is particularly important for public safety and mission-critical communications, where reliable connectivity is essential.

Additionally, the relay feature supports network energy efficiency by allowing UEs to communicate over shorter distances, reducing the transmit power required and thus extending battery life. It also enables new business models, such as relay-as-a-service, where UEs can act as relays for others, potentially with incentives.

In summary, UE-to-Network Relay communication solves the problem of coverage holes and enhances network resilience, making it a key enabler for 5G Advanced use cases.