Access link

Description

In 3GPP relay architectures, the Access link is the radio interface between a Relay node (RN) and a User Equipment (UE). It is distinguished from the Backhaul link (also known as the Un interface), which connects the Relay node to the donor eNodeB (DeNB) or donor gNodeB. The Access link allows the UE to communicate with the network via the Relay node, effectively treating the Relay as a regular base station from the UE's perspective. This concept was introduced in Release 10 as part of the LTE-Advanced relay study and specification work, documented in TS 36.116 (Relay radio transmission and reception) and TS 36.117 (Relay lower layers).

The Access link operates using the same air interface technology as the macro cell, typically LTE (E-UTRA) for Rel-10 and beyond. The Relay node appears as a normal eNodeB to the UE, broadcasting its own cell identity (PCI), transmitting synchronization signals, and handling all the lower-layer procedures such as random access, scheduling, and HARQ. This transparency is a key design goal: the UE does not need to be aware that it is communicating through a relay; it simply sees a standard cell. The Access link therefore supports all the physical layer channels and procedures defined for the LTE Uu interface, including PDSCH, PUSCH, PDCCH, PRACH, and others.

From a protocol stack perspective, the Access link carries the full user plane and control plane protocols between the UE and the Relay node. The Relay node terminates the RLC, MAC, and PHY layers for the Access link, and it may also terminate the PDCP layer depending on the relay architecture (e.g., Layer 2 relay vs. Layer 3 relay). In the control plane, the Relay node handles RRC signaling with the UE, including connection establishment, reconfiguration, and mobility procedures. The Relay node then forwards the user data and control information to the DeNB over the Backhaul link, which may use a different set of resources or even a different frequency band to avoid interference.

The Access link is a critical component for coverage extension and capacity enhancement. By placing a Relay node at the cell edge or in a coverage hole, the Access link can provide a much stronger signal to the UE than a distant macro eNodeB, improving throughput and reliability. The Relay node can also help offload traffic from the macro cell, increasing overall network capacity. The design of the Access link must consider interference between the Access link and the Backhaul link, especially when they operate on the same frequency. 3GPP has defined mechanisms such as time-division multiplexing (TDM) of the two links to mitigate self-interference in the Relay node.

In later releases, the concept of the Access link has been extended to other relay-like architectures, including Integrated Access and Backhaul (IAB) in 5G NR (Rel-16 and beyond). In IAB, the Access link is the NR Uu interface between the IAB node and the UE, while the Backhaul link is the NR Un interface between IAB nodes or between an IAB node and the IAB donor. The principles remain similar: the Access link provides the final-hop connectivity to the UE, and the IAB node relays traffic to the core network via the Backhaul. The term 'Access link' is thus a generic concept that applies to various relay-based access technologies across 3GPP releases.

Purpose & Motivation

The Access link concept exists to enable relay-based deployments in cellular networks. Before relays, coverage and capacity were limited by the macro base station's transmission power and the propagation environment. UEs at the cell edge often experienced poor signal quality, leading to low data rates and dropped connections. Increasing the density of macro base stations was one solution, but it was costly and sometimes impractical due to site acquisition and backhaul constraints. Relay nodes offer a more flexible and cost-effective alternative by providing a local access point that is connected to the network via a wireless backhaul.

The Access link is the key enabler of this relay architecture. It allows the Relay node to serve UEs as if it were a normal base station, leveraging the existing LTE air interface and procedures. This means that UEs do not require any special modifications to benefit from relay deployment; they simply connect to the Relay node's cell. The Access link also allows the Relay node to manage radio resources locally, performing scheduling and link adaptation based on the channel conditions between the Relay and the UE, which can be more favorable than the conditions between the UE and the macro eNodeB.

Historically, the Access link was introduced in Release 10 as part of the LTE-Advanced work on relaying. The motivation was to enhance coverage and capacity for LTE-Advanced, which had ambitious targets for peak data rates and spectral efficiency. Relays were seen as a tool to achieve these targets without the need for extensive new macro sites. The specifications TS 36.116 and TS 36.117 define the radio transmission and reception requirements for the Access link, ensuring interoperability and performance. In subsequent releases, the concept has been adapted for 5G NR relays and IAB, demonstrating its enduring relevance in cellular network design.