Relay

Description

In 3GPP terminology, a Relay refers to terminal devices capable of Opportunity Driven Multiple Access (ODMA) relay communications. ODMA is a technique that allows User Equipment (UE) to relay signals for other UEs, effectively creating a multi-hop network without fixed infrastructure. This concept was introduced in early 3GPP releases (Rel-4) as part of efforts to enhance coverage and capacity in cellular systems. The Relay term in this context is distinct from Relay Node (RN), which is a network element introduced later in Rel-10 for E-UTRAN. Here, Relay specifically denotes the capability of terminal devices to act as relays in an ODMA-based ad-hoc network.

ODMA relay communications enable a UE to forward data from another UE to a base station (Node B or eNodeB) or to another UE. This is particularly useful in scenarios where direct communication with the base station is not possible due to shadowing, coverage holes, or high path loss. By using other UEs as relays, the network can extend coverage, improve cell-edge throughput, and reduce transmit power, leading to longer battery life. The relay functionality is opportunistic, meaning that UEs may dynamically discover and select relay paths based on channel conditions and available resources. This requires sophisticated protocols for relay discovery, selection, and data forwarding, which are typically handled at the radio link layer.

The Relay concept in 3GPP has evolved over time. While the initial definition focused on ODMA, later releases introduced more advanced relay architectures, such as Relay Nodes in LTE-Advanced (Rel-10) and Integrated Access and Backhaul (IAB) in 5G (Rel-16). However, the term 'Relay' as defined in TS 21.905 remains tied to the ODMA capability of terminal devices. This definition has been carried over unchanged from Rel-4 through Rel-19, indicating that the fundamental concept of terminal-based relaying has not been redefined in the core vocabulary, even as new relay technologies emerged under different names.

In practice, ODMA relay communications were never widely deployed in commercial networks, partly due to complexity and the need for tight synchronization and coordination. However, the concept laid the groundwork for later device-to-device (D2D) and relay technologies, such as Proximity Services (ProSe) in LTE and sidelink in 5G. These technologies enable direct UE-to-UE communication and UE-to-network relaying, which are essential for public safety, vehicle-to-everything (V2X), and coverage extension. The Relay term thus represents an early vision of cooperative communication that has influenced subsequent 3GPP work on advanced relaying and D2D.

Understanding the Relay concept is important for appreciating the evolution of 3GPP's approach to network topology and coverage enhancement. While the specific ODMA-based relay may not be in active use, the idea of using terminal devices as relays persists in modern standards. The term serves as a historical marker and a foundation for more sophisticated relay architectures that are integral to 5G and beyond.

Purpose & Motivation

The Relay concept was introduced to address coverage and capacity limitations in cellular networks by leveraging terminal devices as relays. In early 3GPP releases, coverage holes and cell-edge performance were significant issues, especially in urban environments with high building penetration loss. Traditional solutions like adding more base stations were costly and sometimes impractical. ODMA relay communications offered a decentralized approach where UEs could cooperate to forward data, effectively creating a virtual antenna array or multi-hop network. This promised to extend coverage without additional infrastructure, reduce transmit power, and improve spectral efficiency. The Relay definition in TS 21.905 captures this capability, providing a standardized term for terminal-based relaying. Although ODMA was not commercially deployed, the concept motivated later work on relay nodes, D2D, and sidelink, which have become key features in LTE and 5G for public safety, V2X, and coverage extension. Thus, the Relay term represents an early attempt to harness user devices for network improvement, paving the way for modern cooperative communication paradigms.