Description
UE-to-Network (U2N) Relay discovery is a fundamental procedure in 5G NR sidelink (PC5) communications that allows a User Equipment (UE) to discover other UEs capable of acting as relays to the network. This capability is essential for extending network coverage to UEs that are out of direct coverage, such as in public safety scenarios, disaster recovery, or remote areas. The discovery process operates over the PC5 interface using dedicated sidelink discovery resources, which can be configured by the network or pre-configured for out-of-coverage operation.
The discovery procedure involves two primary roles: the Relay UE (also known as the UE-to-Network Relay) and the Remote UE. The Relay UE advertises its availability and capabilities by broadcasting discovery messages that include information such as its identity, relay service code, and potentially the Public Land Mobile Network (PLMN) it is registered with. The Remote UE, seeking connectivity, monitors these discovery messages and evaluates potential relays based on criteria such as signal strength, relay capabilities, and service support. Once a suitable relay is discovered, the Remote UE can initiate a connection establishment procedure to route its traffic through the Relay UE to the network.
In 3GPP Release 19, the UE-to-Network Relay discovery was enhanced to support new use cases and improved efficiency. The discovery can be performed in two modes: Model A and Model B. In Model A, the Relay UE broadcasts discovery messages, and the Remote UE monitors for these announcements. In Model B, the Remote UE broadcasts a solicitation message, and the Relay UE responds if it can provide the requested service. Both models are supported over the PC5 interface using either dedicated sidelink discovery resources or shared resources with data communication.
The discovery process is tightly integrated with the 5G core network architecture. The Relay UE must be authorized by the network to act as a relay, and the discovery messages may include security parameters to prevent unauthorized access. The network can control the discovery process by configuring discovery resources, setting discovery parameters, and managing relay authorization through the Access and Mobility Management Function (AMF) and the Policy Control Function (PCF). This ensures that relay discovery is secure, efficient, and aligned with operator policies.
Key technical aspects include the use of sidelink discovery resource pools, which are configured via System Information Block (SIB) messages (e.g., SIB12 for NR sidelink). The discovery messages are transmitted using the PC5-S protocol, which handles the establishment, maintenance, and release of sidelink signaling. The discovery procedure also supports mobility scenarios, where a Remote UE can switch between relays as it moves, ensuring service continuity. Overall, UE-to-Network Relay discovery is a critical enabler for coverage extension, public safety communications, and efficient resource utilization in 5G networks.
Purpose & Motivation
The UE-to-Network Relay discovery mechanism exists to enable UEs that are out of network coverage or experiencing poor signal quality to connect to the network via a relay UE. This is particularly important for public safety (e.g., first responders in remote areas), disaster recovery, and extending coverage in rural or indoor environments. Without relay discovery, such UEs would be unable to access network services, limiting critical communications.
Historically, relay functionality was introduced in earlier 3GPP releases (e.g., LTE Rel-13) for Proximity Services (ProSe). However, these early solutions had limitations in terms of discovery flexibility, resource efficiency, and integration with the 5G core. The NR sidelink in Release 16 provided a more robust foundation for relay communication, and Release 19 further enhanced discovery to support advanced use cases like multi-hop relaying, quality-of-service differentiation, and network slicing awareness.
The UE-to-Network Relay discovery solves the problem of dynamic and efficient relay selection. It allows UEs to autonomously discover relays without manual configuration, while giving the network control over authorization and resource allocation. This balances the need for ubiquitous connectivity with operator control and security.
What changed in its specifications
The latest release-to-release comparison of specifications that define UE-to-Network Relay discovery. Each summary describes the whole specification, not only this term.
Rel-19 adds multi-hop L2 UE-to-Network relay for NR sidelink as the largest item. It also adds low-power wake-up signal and receiver (LP-WUS/WUR) text, on-demand SSB and SIB1 for network energy savings, and sub-band full duplex for TDD carriers.
Full comparison in TS 38.300 →Rel-19 adds TS 38.304 definitions for multi-hop L2 UE-to-Network sidelink relay roles, including child, parent, intermediate, first and last relay UEs. It also adds paging and system information handling for relays, and NTN IoT Phase 3 text on PLMN access technology restrictions and cell categories.
Full comparison in TS 38.304 →Evolution Across Releases
An overview written from the defining specifications, not a verified change history. For the exact text, open the specifications listed below.
Initial introduction of UE-to-Network Relay discovery for NR sidelink, defining the procedures and signaling for UEs to discover relays for U2N communication. This release specifies the discovery models, resource allocation, and interaction with the 5G core network.
Defining Specifications
3GPP specifications that define or reference UE-to-Network Relay discovery, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 38.300 vj40 | NR and NG-RAN Overall Description | Rel-19 read |
| TS 38.304 vj30 | NR UE Idle and Inactive State Procedures | Rel-19 read |
| TS 38.331 vj40 | NR Radio Resource Control (RRC) | Rel-19 read |