Root Relay

Description

The term 'Root Relay' is defined in 3GPP TS 21.905 as an 'ODMA relay node where communications originate or terminate.' ODMA stands for Opportunity Driven Multiple Access, which was a relay-based radio access technology studied in early 3GPP releases. In an ODMA network, relay nodes are used to extend coverage and improve capacity by forwarding traffic between user equipment (UE) and the base station. A Root Relay is a specific type of relay node that serves as the entry or exit point for communication. It is where the communication either starts (originates) or ends (terminates), meaning it is connected to the core network or acts as the source/destination of the data.

ODMA was designed to allow devices to communicate through multiple hops, using other devices as relays. This is similar to modern concepts like device-to-device (D2D) communication or mesh networks. The Root Relay in this context would be the node that has a direct connection to the infrastructure, such as a base station, and thus acts as a gateway for other relay nodes. The other relay nodes would forward traffic towards the Root Relay, which then sends it to the core network. Conversely, for downlink traffic, the Root Relay receives data from the core network and forwards it through the relay chain to the destination UE.

The concept of Root Relay is important for understanding early relay architectures in 3GPP. Although ODMA itself was not widely deployed, it laid the groundwork for later relay technologies, such as LTE Relays (introduced in Rel-10) and 5G Integrated Access and Backhaul (IAB). In these later technologies, the concept of a root node or donor node is similar: it is the node that connects the relay network to the core network. The Root Relay in ODMA can be seen as a precursor to the Donor gNB in IAB or the Relay Node in LTE.

The definition of Root Relay in TS 21.905 is concise and has remained unchanged across many releases. This indicates that the term is stable and well-understood within the context of ODMA. However, since ODMA is no longer actively developed, the term is primarily of historical interest. It is retained in the vocabulary to support the interpretation of older specifications and to provide a complete glossary of terms used in 3GPP.

In summary, a Root Relay is a key component of an ODMA relay network, serving as the origin or destination of communications. It enables multi-hop communication by acting as a gateway to the core network. While ODMA itself is obsolete, the concept of a root relay node persists in modern relay technologies. Understanding this term helps to trace the evolution of relay architectures in 3GPP from early studies to current 5G systems.

Purpose & Motivation

The Root Relay concept was created to support the ODMA (Opportunity Driven Multiple Access) relay technology, which aimed to extend coverage and improve capacity in cellular networks by allowing devices to relay traffic for each other. In ODMA, communications could be routed through multiple relay nodes, and the Root Relay served as the node where the communication originated or terminated, typically connecting to the core network. This was motivated by the need to provide coverage in areas with poor signal strength and to increase network capacity without deploying additional base stations. The Root Relay was essential for anchoring the relay chain and ensuring that traffic could reach the core network.

Historically, ODMA was studied in the early days of 3GPP as a potential enhancement for UMTS. However, it was not standardized for deployment, partly due to complexity and the emergence of other technologies like HSDPA and LTE. The limitations of ODMA included the need for tight synchronization, complex routing, and potential interference. Despite this, the concept of a root node in a relay network proved valuable and was revisited in later releases. For example, in LTE Rel-10, Relay Nodes were introduced, and in 5G, Integrated Access and Backhaul (IAB) uses a Donor gNB that acts similarly to a Root Relay. Thus, the Root Relay concept solved the problem of how to anchor a multi-hop relay network to the core network, a challenge that remains relevant today.

The purpose of defining Root Relay in 3GPP specifications was to provide a clear term for this functional entity within the ODMA architecture. It helped to distinguish between different types of relay nodes: those that are endpoints (Root Relays) and those that are intermediate forwarders. This distinction is important for routing and resource management. Although ODMA is no longer used, the term is retained in TS 21.905 for historical completeness. It serves as a reminder of early efforts to integrate relaying into cellular networks and provides a foundation for understanding modern relay architectures.