Proximity Services User Equipment-to-Network Relay

Description

The ProSe UE-to-Network Relay is a key entity in the 3GPP Proximity Services (ProSe) architecture, defined to extend network coverage to User Equipments (UEs) that are out of coverage or experiencing poor signal quality. Introduced in Release 13, this relay functionality allows a UE that has a stable connection to the network (the relay UE) to provide connectivity to one or more remote UEs that are in its proximity but cannot directly communicate with the network. This is particularly important for public safety operations, where first responders may need to communicate from areas with no cellular coverage, such as inside buildings, tunnels, or remote locations. The relay UE essentially acts as a Layer 3 router, forwarding data between the remote UEs and the network.

Architecturally, the ProSe UE-to-Network Relay operates using the PC5 interface for communication with remote UEs and the Uu interface for communication with the network. The relay UE must be authorized by the network to perform relay operations, and it must have a subscription that allows relay services. The remote UE discovers the relay UE either through direct discovery (using ProSe discovery procedures) or by using a pre-configured relay selection policy. Once connected, the remote UE sends its data to the relay UE over the PC5 interface, and the relay UE forwards it to the network over the Uu interface. Similarly, downlink data from the network is forwarded by the relay UE to the remote UE. The relay UE may also perform IP address assignment and other network functions for the remote UE, effectively acting as a gateway.

Key components of the ProSe UE-to-Network Relay architecture include the ProSe Function, which authorizes and provisions the relay UE, and the ProSe Application Server, which may manage relay selection policies. The relay UE itself must support the necessary protocols for relaying, including IP routing and possibly NAT (Network Address Translation) if the remote UE uses a private IP address. The remote UE must support ProSe discovery and communication procedures to connect to the relay. Security is a critical aspect: the relay UE must authenticate the remote UE and ensure that only authorized UEs can use the relay service. Additionally, the communication between the remote UE and the network may be secured end-to-end or hop-by-hop, depending on the deployment scenario.

The operation of the ProSe UE-to-Network Relay involves several procedures: relay discovery, where the relay UE advertises its presence and the remote UE discovers it; connection establishment, where the remote UE establishes a PC5 connection with the relay UE; and data forwarding, where the relay UE forwards user data between the remote UE and the network. The relay UE may also support mobility, allowing the remote UE to switch to another relay UE or to direct network connection as it moves. The 3GPP specifications 23.303, 23.468, and 36.300 define the architecture, procedures, and radio aspects of the ProSe UE-to-Network Relay, respectively.

The ProSe UE-to-Network Relay is a vital enabler for extending network coverage and providing connectivity in challenging environments. It is particularly useful for public safety, where it allows first responders to maintain communication even when they are out of direct network coverage. It also has applications in commercial scenarios, such as extending coverage in rural areas, providing connectivity for IoT devices, and enabling vehicle-to-everything (V2X) communication. As 3GPP evolves towards 5G, the relay functionality continues to be enhanced, with improvements in efficiency, security, and support for new services.

Purpose & Motivation

The ProSe UE-to-Network Relay was created to address the problem of limited network coverage, particularly in scenarios where direct communication with the network is not possible due to physical obstructions, distance, or network outages. In public safety operations, first responders often need to communicate from inside buildings, basements, tunnels, or remote areas where cellular coverage is weak or non-existent. Traditional solutions, such as deploying portable base stations or using satellite communication, are expensive, complex, and may not be readily available. The ProSe UE-to-Network Relay provides a cost-effective and flexible solution by leveraging the proximity of UEs to extend coverage. A UE with good network connectivity can act as a relay for other UEs in its vicinity, effectively creating a local coverage extension.

The motivation for the relay also stems from the need to support group communication and data services for remote UEs. In public safety, it is not enough to have voice communication; first responders need to share video, images, and sensor data. The relay enables remote UEs to access the full range of network services, including IP-based applications, by providing a transparent IP connection. This is achieved by the relay UE acting as a router, forwarding IP packets between the remote UE and the network. The relay also helps in conserving energy for remote UEs, as they can use lower power to communicate with the relay UE over the PC5 interface rather than transmitting directly to a distant base station.

Furthermore, the ProSe UE-to-Network Relay supports the evolution towards 5G and the Internet of Things (IoT), where massive numbers of devices may require connectivity. The relay can serve as a gateway for IoT devices, aggregating their traffic and reducing the load on the network. It also enables new use cases such as wearable devices that may not have cellular modems, allowing them to connect via a smartphone relay. The relay functionality has been enhanced over releases to support more efficient discovery, better security, and integration with network slicing and edge computing. Overall, the ProSe UE-to-Network Relay is a key technology for extending coverage, improving resilience, and enabling new services in both public safety and commercial domains.