Backhaul Link

Description

The backhaul link is a key architectural element in 3GPP relay deployments, defined in specifications such as 3GPP TS 36.116 and TS 36.117. In a relay-based radio access network, a relay node (RN) provides coverage extension or capacity enhancement by wirelessly connecting to a donor base station (DeNB) that has a backhaul connection to the core network. The link between the relay node and the donor base station is called the backhaul link. It contrasts with the access link, which is the radio link between the relay node and the user equipment (UE).

The backhaul link carries both user plane data and control plane signaling. In the LTE relay architecture (Rel-10 and later), the backhaul link uses the Un interface, which is a variant of the Uu air interface adapted for relay operation. The Un interface supports the transmission of S1 and X2 messages between the relay node and the core network or neighboring base stations, encapsulated over the radio link. The donor eNodeB acts as a proxy for the relay node towards the core network, providing S1 and X2 termination and forwarding.

In 5G and NR relay scenarios, the backhaul link may use similar principles, with the Integrated Access and Backhaul (IAB) architecture introduced in Rel-16 and later. In IAB, the backhaul link is between an IAB node and its parent IAB node or donor gNB. The IAB node multiplexes access and backhaul traffic, potentially using the same NR air interface. The backhaul link can be multi-hop, where an IAB node connects to another IAB node before reaching the donor gNB.

The backhaul link design must address several challenges: it shares radio resources with the access link, and interference between the two links must be managed. The relay node typically cannot transmit and receive simultaneously on the same frequency, so time-division or frequency-division multiplexing is used. The Un interface supports mechanisms such as scheduling, HARQ, and radio link monitoring adapted for relay operation.

From a protocol stack perspective, the backhaul link carries PDCP, RLC, MAC, and PHY layers. The relay node may terminate some protocols locally and forward others to the donor base station. In LTE relays, the relay node acts as a layer 3 relay, terminating IP and S1/X2 protocols, while the donor eNodeB provides S1/X2 proxy functionality. In IAB, the architecture is more flexible, with central unit (CU) and distributed unit (DU) split options.

The backhaul link is essential for extending network coverage to areas where wired backhaul is not available or cost-prohibitive, such as rural areas, temporary deployments, or dense urban environments. It enables flexible network deployment and can improve capacity and coverage without the need for additional fiber or microwave backhaul.

Purpose & Motivation

The backhaul link concept was created to enable wireless relay deployments in 3GPP networks. Before relays, base stations required wired or microwave backhaul to connect to the core network, which limited deployment flexibility and increased cost, especially in remote or temporary locations. Relays offered a way to extend coverage and capacity without the need for a full backhaul connection at each site.

The backhaul link solves the problem of connecting a relay node to the network by using radio resources for backhaul. It allows the relay to appear as a base station to the UE while communicating with the donor base station over a wireless link. This is particularly useful for coverage extension in areas with poor macro coverage, for capacity enhancement in hotspots, and for temporary events.

The evolution towards integrated access and backhaul (IAB) in 5G NR further enhances this concept by allowing the same radio interface to be used for both access and backhaul, simplifying deployment and improving spectral efficiency. The backhaul link is therefore a foundational element for flexible, cost-effective radio access network deployments.