Multi-path (Direct and Indirect Path Operation)

Description

Multi-path is a Rel-19 mode of operation for a remote UE in RRC_CONNECTED state, defined in 3GPP TS 38.331 and TS 38.323. In this mode, the remote UE is configured with two simultaneous paths toward the same gNB: a direct path on which the UE connects to the gNB using the NR Uu interface, and an indirect path on which the UE connects to the same gNB via another UE (the relay UE) using a PC5 unicast link or a Non-3GPP Connection. This dual-path architecture is a natural extension of the sidelink relay work that began in Rel-17 with UE-to-Network (U2N) relaying, where a remote UE could reach the network only through a relay UE. Multi-path adds the direct leg so that the remote UE is no longer dependent on a single link.

Architecturally, Multi-path sits at the intersection of the NR Uu air interface, the PC5 sidelink interface, and the SDAP/PDCP layers that manage bearer traffic. Because both paths terminate at the same gNB, the network can treat them as two legs of a single logical connection. The PDCP layer (TS 38.323) is the key enabler: it can split, duplicate, or steer packets across the two legs, and it performs reordering and duplicate detection at the receiver. This is conceptually similar to dual connectivity (DC) or carrier aggregation, but instead of two radio legs to two different nodes or carriers, one leg traverses a relay UE over PC5 or a Non-3GPP Connection. The RRC layer (TS 38.331) configures the paths, the mapping rules, and the switching/steering policies.

How it works in practice: the remote UE establishes an RRC connection with the gNB, either directly or via the relay. The gNB then configures a Multi-path operation, adding the second path. Traffic can be transmitted on both paths simultaneously (for throughput aggregation), on one path with the other as standby (for reliability), or dynamically steered based on link quality, congestion, or policy. The relay UE forwards traffic between the remote UE and the gNB over the indirect path; from the gNB's perspective, the relay is a UE that has its own Uu connection and acts as a Layer-2 or Layer-3 forwarder depending on configuration. When a Non-3GPP Connection is used, the indirect path may traverse Wi-Fi or another access technology, broadening the diversity options.

Key components include the remote UE, the relay UE (or Non-3GPP access), the serving gNB, and the PC5 unicast link that carries the sidelink traffic. The PC5 link is established using the NR sidelink procedures, including discovery, connection establishment, and sidelink RLC/MAC configuration. The gNB coordinates both paths, and the remote UE reports measurements for each so the network can decide when to add, remove, or switch paths. Mobility is handled by the gNB, which can reconfigure the Multi-path setup as the remote UE moves, potentially handing over the direct leg while keeping the indirect leg, or vice versa.

Multi-path matters because it improves reliability, coverage, and throughput for remote UEs that may be at the cell edge or in coverage holes. It also supports service continuity: if one path degrades or fails, traffic can continue on the other, reducing interruption. For public safety, industrial IoT, and wearable scenarios, this resilience is valuable. The feature is part of the broader Rel-19 work on enhanced sidelink relay and multi-path, and it builds on the SDAP/PDCP split-bearer framework already used in DC and MBS.

Purpose & Motivation

Multi-path exists to overcome the fragility of single-link connectivity for remote UEs. In Rel-17 sidelink relay, a remote UE reached the network only through a relay UE, so if the PC5 link degraded or the relay moved away, the remote UE lost connectivity. Conversely, a direct-only UE at the cell edge could suffer poor Uu quality with no fallback. Multi-path solves both problems by giving the remote UE two independent ways to reach the same gNB, allowing the network to exploit whichever link is better at any moment.

The motivation is threefold. First, reliability: mission-critical and public-safety users need uninterrupted service, and path redundancy provides resilience against link failure. Second, throughput: aggregating the direct and indirect paths can increase peak data rates, similar to how dual connectivity aggregates two legs. Third, coverage: a UE outside direct NR coverage can still be served via a relay, while a UE with good direct coverage can use the relay path opportunistically. This flexibility was not available in earlier relay designs, which forced a choice between direct and indirect connectivity.

Historically, 3GPP studied multi-path in the Rel-18/Rel-19 timeframe as part of the evolution of sidelink relay. Earlier releases (Rel-17) standardized U2N relay with a single path, and Rel-18 enhanced relay with improved QoS and service continuity. Rel-19 Multi-path is the next step, introducing simultaneous direct and indirect paths and the PDCP/RRC machinery to manage them. It addresses the limitation that a remote UE could not maintain a direct Uu connection and a relay connection at the same time, which restricted both performance and robustness.