Child User Equipment

Description

The term Child UE is introduced in 3GPP Release 19 to describe a specific role within User Equipment to Network (U2N) relay operations. According to the definition in specifications such as 38.300, 38.304, 38.331, 38.470, and 38.473, a Child UE is the next hop in the downstream direction from a U2N Relay UE that is serving a U2N Remote UE. In simpler terms, when a remote UE connects to the network via a relay UE, the relay UE's downstream neighbor (the next UE in the path towards the remote UE) is called the Child UE. The Child UE can be either the U2N Remote UE itself (in a single-hop relay) or another U2N Relay UE (in a multi-hop relay). The definition further clarifies that U2N Remote UEs beyond the next downstream hop can be referred to as 'indirectly connected Child UEs' in multi-hop scenarios.

The introduction of the Child UE concept is part of the ongoing evolution of relay technology in 3GPP, specifically for 5G-Advanced and beyond. U2N relaying allows a UE to connect to the network through another UE that acts as a relay, extending coverage and improving capacity. This is particularly useful for scenarios like public safety, wearable devices, and IoT, where direct network coverage may be unreliable. In multi-hop relay, the signal can traverse multiple relay UEs before reaching the remote UE, forming a chain. The Child UE term provides a clear nomenclature for the entities in this chain.

The specifications that define Child UE cover various aspects: 38.300 provides overall NR and NG-RAN architecture; 38.304 covers UE procedures in idle and inactive mode; 38.331 specifies Radio Resource Control (RRC) protocol; 38.470 and 38.473 define the F1 interface and related procedures for relay nodes. These specs detail how the relay UE and child UE interact, including connection establishment, bearer configuration, and mobility. For example, when a remote UE connects via a relay UE, the relay UE may need to establish a PC5 connection with the child UE (if the child is another relay) or with the remote UE directly. The Child UE terminology helps to unambiguously describe the direction of data flow and the hierarchy of connections.

In a multi-hop relay operation, the path from the network to the remote UE may consist of multiple segments: network → relay UE 1 → relay UE 2 → remote UE. Here, relay UE 2 is a Child UE of relay UE 1, and the remote UE is a Child UE of relay UE 2. The remote UE is also an indirectly connected Child UE of relay UE 1. This hierarchical relationship is important for routing, resource allocation, and mobility management. The network needs to know the topology to manage the relay path, and the Child UE concept aids in that. The definition also implies that a Child UE could be a relay UE itself, meaning the term is not limited to end devices.

The introduction of Child UE in Release 19 reflects the growing interest in multi-hop relaying as a means to enhance coverage and reliability in 5G-Advanced networks. It builds on earlier relay work in Releases 17 and 18, where single-hop relaying was standardized. The Child UE concept is expected to facilitate more sophisticated relay operations, including dynamic path switching and multi-hop route optimization. As networks move towards 6G, such relay techniques will likely become even more important for ubiquitous connectivity.

Purpose & Motivation

The Child UE concept was created to provide a standardized term for the downstream entities in U2N relay operations, particularly as multi-hop relaying gained prominence in 3GPP Release 19. Prior to this, relay terminology often only distinguished between relay UEs and remote UEs, which was insufficient for describing complex multi-hop topologies. The term solves the problem of ambiguity in referring to the next hop in a relay chain, enabling clearer specification of procedures and interfaces.

Multi-hop relaying itself addresses the need for extended coverage and improved resilience in scenarios where direct network access is challenged. For example, in disaster recovery, a chain of UEs can relay signals to reach a remote area. The Child UE term helps network protocols to identify and manage each link in the chain, ensuring proper routing and QoS. It also aids in mobility scenarios where a remote UE might switch from one relay to another.

The introduction in Release 19 is timely, as 5G-Advanced networks are expected to support a diverse set of devices and applications requiring flexible connectivity. The Child UE concept is likely to be a building block for future relay enhancements, such as dynamic multi-hop routing and network-controlled relay selection. It represents a step towards more autonomous and resilient network architectures.