In-band relay

Description

An in-band relay is a type of relay node defined by 3GPP for cellular networks, where the access link and the backhaul link operate in the same operating band. The access link connects the relay node to user equipment (UEs) within its coverage area, while the backhaul link connects the relay node to the donor base station (eNodeB or gNB) that provides connectivity to the core network. By using the same frequency band for both links, the relay node can be deployed without the need for additional spectrum, making it a cost-effective solution for extending coverage and enhancing capacity.

The operation of an in-band relay requires careful management of radio resources to avoid interference between the access and backhaul links. Since both links share the same frequency band, the relay node must coordinate transmissions in time or frequency to prevent self-interference. In 3GPP Long Term Evolution (LTE) Release 10 and later, in-band relays are specified to use time-division multiplexing (TDM) for the backhaul and access links. The relay node communicates with the donor eNodeB during specific subframes (backhaul subframes) and with UEs during other subframes (access subframes). This scheduling is controlled by the donor eNodeB, which allocates resources for the backhaul link and ensures that the relay node does not transmit and receive simultaneously on the same frequency, thus avoiding interference.

The architecture of an in-band relay involves several key components. The relay node itself consists of a UE-like entity (the relay UE) that communicates with the donor eNodeB, and an eNodeB-like entity (the relay eNodeB) that serves UEs. The donor eNodeB is responsible for managing the relay node, including resource allocation, mobility management, and security. The relay node may also support self-backhauling, where the backhaul link uses the same air interface as the access link, but with modifications to support the relay functionality. The core network sees the relay node as a regular eNodeB, and the UEs served by the relay are treated as if they were connected directly to the donor eNodeB, with the relay acting as an intermediary.

In-band relays are particularly useful in scenarios where deploying fiber or other high-capacity backhaul is impractical or expensive, such as in rural areas, indoor environments, or temporary deployments. They can also enhance capacity in dense urban areas by providing additional coverage and offloading traffic from the macro network. However, the shared spectrum between access and backhaul links imposes constraints on the achievable throughput and latency, as the relay node must divide its time between the two links. This is in contrast to out-of-band relays, which use separate frequency bands for access and backhaul, allowing simultaneous operation and potentially higher performance.

3GPP specifications 36.116 and 36.117 provide the detailed requirements and test procedures for in-band relays in LTE. These specifications cover aspects such as radio resource management, synchronization, and interference coordination. With the evolution to 5G New Radio (NR), the concept of in-band relaying has been extended to integrated access and backhaul (IAB) in Release 16 and beyond, where the access and backhaul links can share the same spectrum in a more flexible manner, supporting multi-hop relaying and advanced beamforming. In-band relays remain a fundamental building block for flexible and cost-effective network deployments.

Purpose & Motivation

In-band relays were introduced to address the challenge of extending network coverage and capacity without the need for additional spectrum or costly backhaul infrastructure. In traditional cellular deployments, extending coverage to remote or indoor areas often required either deploying new base stations with dedicated backhaul (e.g., fiber or microwave) or using repeaters that amplify signals but do not provide the same level of performance as a base station. In-band relays offer a middle ground: they act as small base stations that connect to the macro network wirelessly using the same frequency band, eliminating the need for separate backhaul spectrum and reducing deployment costs.

The motivation for in-band relays came from the need to provide high-quality broadband services in areas where wired backhaul is not available or is too expensive. For example, in rural areas, laying fiber to every cell site is not economically viable, and microwave backhaul may be limited by line-of-sight requirements. In-band relays can be deployed on lampposts or building walls, using the same licensed spectrum as the macro network for both access and backhaul. This allows operators to quickly and cost-effectively extend coverage and capacity.

However, the shared spectrum introduces challenges such as self-interference and resource partitioning, which required new 3GPP specifications to define the necessary protocols and procedures. The introduction of in-band relays in LTE Release 10 was a significant step towards more flexible network topologies, and it laid the groundwork for more advanced relay technologies in 5G, such as IAB. The concept continues to evolve, but the fundamental purpose remains: to provide a scalable and efficient means of enhancing network coverage and capacity using existing spectrum resources.