Non-3GPP Connection indirect path

Description

The N3C indirect path is a concept introduced in 3GPP Release 18 for Multi-path communication in 5G networks. It refers to an indirect communication path between a remote User Equipment (UE) and the network, where the connection between the remote UE and one or more relay UEs uses a Non-3GPP Connection (N3C). The term 'N3C' stands for Non-3GPP Connection, which is a connection that is not defined by 3GPP specifications, such as Wi-Fi, Bluetooth, or other proprietary technologies. In the context of Multi-path, the remote UE can communicate with the network through multiple paths: a direct path (using 3GPP radio) and one or more indirect paths (via relay UEs using non-3GPP connections). The N3C indirect path specifically refers to the indirect path that utilizes a non-3GPP link between the remote UE and the relay UE.

The architecture of the N3C indirect path involves several key components. The remote UE is the device that needs connectivity, possibly because it is out of coverage or has poor signal. The relay UE is a device that has good connectivity to the network and can act as a relay. The non-3GPP connection between them could be, for example, a Wi-Fi Direct link or a Bluetooth link. The relay UE then connects to the network via 3GPP access (e.g., 5G NR). The network side includes the Access and Mobility Management Function (AMF) and the User Plane Function (UPF), which manage the connection and data routing. The N3C indirect path is established when the remote UE and relay UE set up a non-3GPP link and the relay UE registers the remote UE with the network, or the remote UE uses the relay UE's connection to access the network. The 3GPP specifications 38.321 (MAC protocol), 38.323 (PDCP protocol), and 38.331 (RRC protocol) define the procedures for Multi-path and relay operations.

How it works: The remote UE discovers a relay UE that offers connectivity. They establish a non-3GPP connection, which could be managed by a non-3GPP interworking function or directly. The remote UE then uses the relay UE to forward its data to the network. The relay UE may operate at Layer 2 or Layer 3. In Layer 2 relay, the relay UE forwards data at the PDCP or RLC layer, and the remote UE's data is encapsulated. In Layer 3 relay, the relay UE acts as an IP router. For N3C indirect path, the non-3GPP connection is used for the link between remote and relay, while the relay's 3GPP link is used to the network. The Multi-path functionality allows the remote UE to simultaneously use the direct 3GPP path and the N3C indirect path, aggregating bandwidth or providing redundancy. The network can split data across paths based on policies. The N3C indirect path is particularly useful when the direct 3GPP link is congested or unavailable.

Key components include the remote UE, relay UE, non-3GPP connection, and the 3GPP network functions. The remote UE must support Multi-path and non-3GPP access. The relay UE must support relay functionality and have a 3GPP connection. The non-3GPP connection must be established and maintained, with security and QoS considerations. The network must manage the multiple paths, including path selection, data splitting, and mobility. The specifications 38.321, 38.323, and 38.331 provide the protocols for MAC, PDCP, and RRC that enable these operations, including the signaling for path setup and data forwarding.

The role of the N3C indirect path in the network is to enhance coverage, reliability, and throughput. It allows UEs that are out of 3GPP coverage to connect via a relay using non-3GPP technology, extending the network's reach. It also enables multi-path aggregation, where the remote UE can use both direct and indirect paths to increase data rates. This is part of the evolution towards ubiquitous connectivity in 5G-Advanced and 6G. The N3C indirect path is a key enabler for use cases like public safety, industrial IoT, and enhanced mobile broadband in challenging environments.

Purpose & Motivation

The N3C indirect path was created to address the limitations of single-path connectivity in 5G networks, particularly for UEs that are in poor coverage or need higher reliability. Prior to Rel-18, relay solutions existed (e.g., ProSe in LTE), but they primarily used 3GPP links for the relay connection. The use of non-3GPP connections for the indirect path offers flexibility and can leverage unlicensed spectrum or short-range technologies like Wi-Fi and Bluetooth, which are widely available. The motivation was to enable Multi-path communication where a UE can simultaneously use multiple paths, including non-3GPP, to improve throughput, reliability, and coverage.

The limitations of previous approaches included the lack of standardized support for non-3GPP connections in relay scenarios, which meant that such implementations were proprietary and not interoperable. Also, Multi-path operation was not fully specified, so UEs could not seamlessly aggregate 3GPP and non-3GPP paths. The N3C indirect path solves these by providing a standardized way to establish and manage an indirect path using non-3GPP connections, integrated with the 5G core network. It allows the network to control the path selection and data splitting, ensuring QoS and security.

The introduction of N3C indirect path in Rel-18 is part of the broader Multi-path and relay enhancements in 5G-Advanced. It addresses the need for robust connectivity in scenarios like indoor coverage, disaster recovery, and IoT. By leveraging non-3GPP connections, it can offload traffic from 3GPP networks and provide cost-effective coverage extension. This feature is expected to be further refined in Rel-19 and beyond, with improvements in mobility, security, and efficiency.