5G Proximity Services User Equipment-to-User Equipment Relay

Description

The 5G ProSe UE-to-UE Relay is a key enabler in the 3GPP Release 19 ProSe architecture for direct communication between User Equipment (UEs) without going through the network. It is a function implemented in a 5G ProSe-enabled UE that relays traffic between two other 5G ProSe end UEs (or Remote UEs) over the PC5 sidelink interface. Unlike the UE-to-Network Relay, which connects a Remote UE to the network, the UE-to-UE Relay facilitates communication between two UEs that may be out of direct PC5 range of each other. This effectively extends the range of sidelink communication and enables multi-hop device-to-device connectivity. The relay function can be integrated into any ProSe-enabled UE, allowing it to act as a relay for other UEs while also serving its own applications.

The UE-to-UE Relay operates by establishing PC5 links with both end UEs. It receives data from one UE and forwards it to the other, performing functions such as data forwarding, routing, and potentially QoS mapping. The relay UE must be authorized by the network to perform this function, and it may need to maintain a ProSe context. Discovery procedures allow end UEs to find available relays, either through direct discovery models or network-assisted discovery. The relay can support various types of traffic, including IP, Ethernet, and unstructured data, and it can handle multiple pairs of end UEs simultaneously.

Architecturally, the UE-to-UE Relay is transparent to the network in many cases, but it may interact with the 5G core for authorization, policy control, and charging. The relayed traffic does not traverse the network unless the relay UE chooses to forward it via its Uu interface (e.g., for lawful interception or when the destination is in the network). The UE-to-UE Relay is defined in TS 23.304, with protocol aspects in TS 24.554. It is particularly useful for public safety scenarios where direct communication between first responders is critical, and for commercial applications like social networking or gaming in proximity.

The relay function includes relay selection and reselection procedures to ensure robust connectivity. It also supports security mechanisms to protect the relayed communication, including authentication and encryption of the PC5 links. The relay UE may also provide IP address management for the end UEs if required. Mobility is supported, meaning the relay can move and the end UEs can switch to another relay if needed.

In summary, the 5G ProSe UE-to-UE Relay is a flexible function that enhances direct device-to-device communication by extending range and enabling multi-hop connectivity, forming an essential part of the 5G ProSe feature set for both mission-critical and commercial services.

Purpose & Motivation

The 5G ProSe UE-to-UE Relay was developed to enable direct communication between UEs that are not in direct PC5 range, thereby extending the reach of sidelink communication. Traditional D2D communication has limited range, and in scenarios like public safety operations or large venues, UEs may be spread out. The UE-to-UE Relay solves this by allowing a UE in the middle to relay traffic, effectively creating a mesh network. This is vital for mission-critical push-to-talk (MCPTT) and other group communications where all users must be connected regardless of location. It also supports commercial use cases like proximity-based social media and multiplayer gaming. By standardizing this relay in Release 19, 3GPP ensures interoperability and leverages 5G's advanced capabilities for low-latency, high-reliability communication.