What is 5G Integrated Access and Backhaul (IAB)?

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With the ability to support enhanced mobile broadband (eMBB) and ultra-reliable low latency communications (URLLC), the introduction of 5G New Radio (NR) using gigahertz (GHz) spectrum is creating new opportunities for mobile services providers and private enterprises. These high frequencies, however, come with an inherent problem in that the shorter wavelengths have a dramatically smaller signal range and are far more susceptible to interference and degradation. Given that the effective distance of a 5G signal could be as little as 1,000ft, the current design of 4G radio access networks (where the signal can reach up to 10 miles) must be completely rethought when moving to NR. While the obvious answer is to significantly increase the number of antennas in a given serving area - by as many as 50 per square kilometer, in some cases - this solution is complicated by the need to cheaply and efficiently serve the new base stations with backhaul bandwidth capacity.

Originally introduced in 3GPP release 10 in 2011, LTE relaying was never adopted because the low eNodeB densities deemed it unnecessary. Given the inherent limitations of NR operating in the mid-band and above, standardization work on its successor - Integrated Access and Backhaul (IAB) - began in 2017 and was ratified in 2020 as part of release 16 within technical specification TS 38.401. IAB leverages the spectral efficiencies of new radio and the increased capacity afforded by the higher bands available in 5G to deliver an alternative to optical cell site backhaul. This alleviates one of the primary issues surrounding the deployment of 5G that can be employed as a short-term alternative to fiber or as a permanent option for more isolated antennas or those without right of way access. Any future migration is aided by IAB’s adoption of the Internet Protocol (IP) over a new Backhaul Adaption Protocol (BAP) layer defined within 3GPP TS 38.340.

IAB allows for multi-hop backhauling using the same frequencies employed for user equipment (UE) access or a distinct, dedicated, frequency. The IAB Mobile Termination (MT) antenna is either an independent set of arrays (IAB-MT) or they share the same antennas used for access and are referred to as virtual IAB-MTs (vIAB-MT). Shared frequency and combined radio unit implementations are naturally recognized as being more efficient than the decoupled alternatives. Integrated Access and Backhaul specifications define two antenna system types: An IAB node and an IAB donor. IAB donors terminate the backhaul traffic from distributed IAB nodes. These nodes can be backhaul endpoints or relays between those endpoints and the donor. Both IAB donors and nodes serve mobile UEs in the usual way.

5G Integrated Access and Backhaul IAB Architecture

The basic architecture and protocol stack employed in IAB deployments

IAB requires a decomposed radio access network (RAN) model, like that employed in the Open RAN (O-RAN) architecture, which decouples the distributed unit (DU) from the central unit (CU). The DU is present in only the IAB nodes while the doner system also comprises a CU. Defined within standards as including a CU and DU, a single IAB system of one or more IAB nodes and the IAB doner are, together, deemed a single gNodeB (gNB). This approach also ensures the backhaul is insolated within a topologically constrained environment, so routing changes or problems are not propagated into the 5G core (5GC) or other adjacent gNBs.

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