Parent User Equipment

Description

The Parent UE is a concept introduced in 3GPP Release 19 to support User Equipment-to-Network (U2N) Relay communication. In this architecture, a U2N Remote UE (the device seeking connectivity) communicates with the network via one or more U2N Relay UEs. The Parent UE is defined as the next hop U2N Relay UE in the upstream direction for serving the U2N Remote UE. In other words, from the perspective of a U2N Remote UE or a U2N Relay UE, the Parent UE is the relay node that provides the connection towards the network. This creates a multi-hop relay chain where each relay node (except the last one) has a Parent UE, and the last relay connects directly to the gNB.

The U2N Relay architecture is designed to extend network coverage and improve service continuity for UEs that are out of direct coverage or in challenging radio conditions. A U2N Remote UE can connect to the network through a U2N Relay UE, which acts as a Layer 3 relay. The relay UE forwards traffic between the remote UE and the network. When multiple relays are chained, the Parent UE concept becomes essential to define the topology. For example, if Remote UE A connects to Relay UE B, and Relay UE B connects to Relay UE C, then for Remote UE A, Relay UE B is its Parent UE. For Relay UE B, Relay UE C is its Parent UE. Relay UE C connects directly to the gNB and does not have a Parent UE (or its Parent UE is the gNB, but the term is specifically for UE-to-UE relay hops).

The Parent UE is responsible for relaying control plane and user plane data between its child UEs (the UEs for which it is the Parent) and its own Parent UE or the network. This includes forwarding paging messages, system information, and data packets. The Parent UE must manage radio resources, handle mobility, and ensure quality of service for the relayed connections. The concept is part of the broader 5G Proximity Services (ProSe) and sidelink framework, where UEs can communicate directly with each other using PC5 interface. The U2N Relay uses sidelink for the access link between the Remote UE and the Relay UE, and the Uu interface for the backhaul link between the Relay UE and the network.

The introduction of the Parent UE term in Rel-19 reflects the maturity of relay technologies and the need for clear terminology in multi-hop scenarios. It enables the specification of procedures for relay selection, reselection, and handover in a hierarchical manner. For instance, a Remote UE may need to select a new Parent UE if the current one moves out of range or if better relay options are available. The Parent UE also plays a role in authorization and security, ensuring that only authorized UEs can act as relays and that the relayed traffic is protected.

In terms of protocol architecture, the Parent UE implements the relay functionality at the adaptation layer (e.g., SDAP, PDCP, RLC, MAC) and may also support the PC5-S protocol for sidelink signaling. The Parent UE must maintain context for each child UE, including security keys and bearer configurations. It also participates in the relay discovery process, broadcasting its presence and capabilities. The Parent UE concept is expected to be further refined in future releases as relay technologies evolve towards more dynamic and mesh-like topologies.

Purpose & Motivation

The Parent UE concept was introduced to address the need for clear and standardized terminology in multi-hop UE-to-Network relay communication. As relay technologies advanced from single-hop to multi-hop scenarios, it became necessary to define the relationship between relay nodes in a hierarchical manner. The term 'Parent UE' provides a unambiguous way to refer to the upstream relay node that serves a given UE, facilitating the specification of procedures and protocols.

Prior to Rel-19, relay architectures primarily considered single-hop scenarios where a Remote UE connects directly to a Relay UE that has a connection to the network. However, as the demand for extended coverage and improved resilience grew, multi-hop relaying became a topic of interest. In a multi-hop chain, each relay node (except the last) acts as a Parent UE to the node below it. This hierarchical structure enables the network to manage the relay chain efficiently, including routing, mobility, and QoS.

The Parent UE concept also solves the problem of ambiguity in signaling and configuration. Without a clear term, specifications would have to use lengthy descriptions like 'the relay UE that is one hop closer to the network.' The term 'Parent UE' simplifies this and allows for concise definitions of procedures such as relay selection, where a UE evaluates candidate Parent UEs based on signal strength, load, and other criteria. It also aids in security by establishing a clear trust relationship between a UE and its Parent UE.

Furthermore, the Parent UE concept supports the integration of relay nodes into the broader network architecture, including network slicing and QoS frameworks. It allows the network to treat the relay chain as a logical extension of the access network, with each hop managed appropriately. This is essential for ensuring end-to-end service quality and for supporting advanced features like edge computing and low-latency communications. The introduction in Rel-19 marks a significant step towards making multi-hop relaying a practical reality in 5G-Advanced and future networks.