Relay Node

Description

A Relay Node (RN) is a low-power base station that wirelessly connects to the core network via a donor cell (DeNB in LTE, gNB in 5G NR). It creates a new cell, visible to UEs as a normal serving cell, but its backhaul connection to the network is provided over the radio interface (the Un interface in LTE, NR-Uu in 5G) rather than a traditional wired fiber or microwave link. Architecturally, the RN contains a base station functionality (eNB/gNB part) towards the UE (Uu interface) and a UE-like functionality towards the donor base station.

The operation involves two distinct radio links: the backhaul link (between RN and Donor eNB/gNB) and the access link (between RN and UE). The RN demodulates and decodes the signal from the donor on the backhaul link, then re-encodes and re-modulates it for transmission on the access link (decode-and-forward relaying). This process improves signal quality compared to simple analog amplification. In-band relaying, where backhaul and access links share the same frequency, requires careful time-division multiplexing to avoid self-interference, often utilizing Almost Blank Subframes (ABS) in the donor cell.

Key protocols are extended to support RN functionality. S1 and X2 interfaces are proxied through the donor eNB. The donor eNB acts as an S1 and X2 proxy, meaning the RN appears as a normal eNB to the core network (MME/S-GW) and to other eNBs. For the UE, the RN is indistinguishable from a normal macro cell. Security is maintained via a secure connection between the RN and the donor eNB. Relay Nodes are categorized by their transparency to UEs (transparent vs. non-transparent) and their layer of operation (Layer 1 repeater, Layer 2 RN, Layer 3 RN). 3GPP primarily standardized Layer 3 RNs, which are full eNBs with wireless backhaul.

Purpose & Motivation

The Relay Node was introduced to address coverage and capacity challenges in a cost-effective manner. Deploying traditional macro cells with fiber backhaul is expensive and time-consuming, especially in rural areas, temporary event locations, or within buildings where coverage holes exist. RNs solve this by leveraging the existing macro network's radio resources to create new cells without the need for costly new backhaul infrastructure.

They specifically improve performance at cell edges and in shadowed regions where the signal from the macro cell is weak, leading to low data rates and poor service quality. By placing an RN closer to such UEs, the access link distance is shortened, enabling higher-order modulation and better throughput. Furthermore, RNs can increase overall network capacity by offloading traffic from the macro cell and enabling more efficient spatial reuse of spectrum. The motivation for their standardization in LTE-Advanced (Rel-10) was to meet IMT-Advanced requirements for enhanced coverage and spectral efficiency, providing a flexible deployment tool for network operators.