Integrated Access and Backhaul (IAB): Self-Backhauling 5G at mmWave

The densification of 5G networks at millimeter wave frequencies creates a paradox: you need hundreds or thousands of small cells to cover a metropolitan area, but each of those cells traditionally requires a dedicated fiber backhaul connection. Trenching fiber to every lamppost, utility pole, and building corner is expensive, slow, and often impossible in congested urban corridors where permits take months. Integrated Access and Backhaul, standardized in 3GPP Release 16 and enhanced in Releases 17 and 18, resolves this paradox by allowing mmWave small cells to wirelessly backhaul their traffic through the same spectrum and radio hardware they use to serve subscribers.

The Backhaul Bottleneck at Millimeter Wave

A single mmWave 5G NR small cell operating at 28 GHz with 400 MHz of channel bandwidth can generate 2 to 4 Gbps of aggregate sector throughput. At 39 GHz with 800 MHz bandwidth, that figure doubles. A dense urban deployment with ten small cells per block produces 20 to 80 Gbps of aggregate backhaul demand from a single city block. Fiber handles this capacity effortlessly, but the cost of fiber construction in a major city runs $50,000 to $200,000 per linear mile depending on conduit availability, surface restoration requirements, and permitting complexity.

Wireless backhaul using dedicated point-to-point links at E-band (71 to 86 GHz) or V-band (60 GHz) has been a partial solution, but each link requires its own radio, antenna, and installation, adding $5,000 to $15,000 per node. IAB eliminates this cost by integrating the backhaul function into the same radio hardware that provides subscriber access. The 5G NR radio that serves user equipment (UE) during one set of time slots uses the remaining slots to relay traffic toward the fiber-connected donor node.

IAB Architecture and Node Types

The 3GPP IAB architecture defines two node types. The IAB donor is a gNB (5G base station) with a wired backhaul connection, typically fiber. It serves as the root of the IAB tree topology, anchoring the CU (Central Unit) and DU (Distributed Unit) functions. The IAB node is a relay that has no wired backhaul; instead, it communicates with the donor (or with another IAB node closer to the donor) over a wireless backhaul link while simultaneously serving UEs on its access links.

Each IAB node contains two functional entities: an IAB-MT (mobile termination) that connects upstream toward the donor, and an IAB-DU (distributed unit) that serves downstream UEs and child IAB nodes. The IAB-MT behaves like a UE from the perspective of the parent node, participating in beam management, power control, and HARQ. The IAB-DU behaves like a standard gNB-DU from the perspective of child nodes and UEs. This dual-role architecture allows multi-hop topologies where traffic traverses two, three, or even four wireless hops before reaching the fiber-connected donor.

Parameter	IAB Donor	IAB Node (Relay)	Traditional Small Cell
Backhaul	Fiber (wired)	Wireless (NR)	Fiber (wired)
Functional entities	CU + DU	IAB-MT + IAB-DU	CU + DU
Hop count	0 (root)	1 to 4 hops	0
Backhaul capacity	10+ Gbps (fiber)	1 to 4 Gbps (wireless)	10+ Gbps (fiber)
Installation cost	$25K to $50K	$3K to $8K	$25K to $50K
Deployment speed	Weeks to months	Days	Weeks to months

Half-Duplex Constraints and Resource Multiplexing

The most significant RF engineering constraint in IAB is the half-duplex limitation. An IAB node cannot transmit on its access link and receive on its backhaul link simultaneously at the same frequency, because the transmit signal would overwhelm the receiver. 3GPP defines two multiplexing approaches to handle this constraint.

Time-division multiplexing (TDM) allocates different OFDM symbols or slots to access and backhaul functions. During "access slots," the IAB node's DU serves UEs. During "backhaul slots," the IAB node's MT communicates with the parent node. The ratio between access and backhaul time determines the effective throughput available to subscribers. A 50/50 split halves the access capacity; a 70/30 split preserves more access capacity but constrains the backhaul.

Spatial-division multiplexing (SDM) uses beamforming to isolate the access and backhaul directions. If the parent node is behind the IAB node and the UEs are in front, directional beams with sufficient front-to-back isolation (typically 30 to 40 dB) can enable simultaneous access and backhaul. This requires antenna arrays with high directivity and careful site planning to ensure the access and backhaul beams point in sufficiently different directions.

Backhaul Link Budget at 28 and 39 GHz

The wireless backhaul link between IAB nodes must close with enough margin to maintain high modulation orders (64-QAM or 256-QAM) under rain and obstruction conditions. Unlike the access link to mobile UEs, the backhaul link is between fixed, elevated antennas with clear line of sight, which improves the link budget considerably.

Parameter	28 GHz IAB Backhaul	39 GHz IAB Backhaul
Donor EIRP	+55 dBm (per beam)	+55 dBm (per beam)
FSPL at 200m	113 dB	116 dB
FSPL at 500m	121 dB	124 dB
Rain fade (42 mm/hr, 500m)	4 dB	5.5 dB
IAB node Rx gain	+24 dBi	+26 dBi
Rx noise figure	7 dB	8 dB
Received power (500m, clear)	-42 dBm	-43 dBm
SNR for 256-QAM	Requires ~25 dB	Requires ~25 dB
Clear-sky margin	18 dB	16 dB

The backhaul link budget is significantly more favorable than the access link because both ends have high-gain directional antennas mounted at elevated, fixed positions. This is why IAB backhaul inter-site distances of 200 to 500 meters are practical at mmWave, providing sufficient margin to maintain 256-QAM even during moderate rain events.

Multi-Hop Topology Planning

Real IAB deployments rarely use single-hop topologies. A fiber-connected donor station on a rooftop serves several first-hop IAB nodes on nearby poles, and those nodes in turn serve second-hop nodes around corners or down side streets. The topology resembles a tree, with the donor as the root and successive IAB nodes as branches.

Each hop introduces latency (approximately 1 to 4 ms per hop for scheduling and processing) and reduces throughput by the backhaul time fraction. A three-hop chain with 60/40 TDM split delivers only 21.6% of the donor's total capacity to the leaf node's UEs. For this reason, 3GPP recommends limiting IAB to two or three hops maximum in most deployments.

Topology redundancy is critical. If a single-path tree loses one intermediate node, all downstream nodes lose connectivity. 3GPP Release 17 introduced topology adaptation, allowing an IAB node to detect parent failure and reconnect to an alternative parent within seconds. This requires that each IAB node can "see" at least two potential parent nodes with acceptable link quality, which influences site planning and antenna system design at each location.

RF Hardware at the IAB Donor

The IAB donor station carries the heaviest RF hardware burden. It must simultaneously serve multiple UEs on access beams while maintaining high-capacity backhaul beams to several IAB nodes in different directions. This requires a phased array antenna with sufficient elements to form multiple simultaneous beams with high isolation between them.

At RF Essentials, our engineering team supplies the waveguide components used in IAB donor station test and calibration. WR-28 waveguide bends route calibration paths within the antenna assembly, while precision low-power terminations provide matched loads for unused antenna ports during over-the-air pattern verification. The dimensional tolerances of these components directly affect the calibration accuracy that determines beam-pointing precision and sidelobe levels in the deployed system.

Deployment Economics and the Fiber Avoidance Equation

The economic case for IAB is strongest where fiber construction costs are highest and small cell density requirements are most demanding. In Manhattan, where a single fiber pull to a street-level small cell can cost $100,000 or more, IAB's ability to serve that cell wirelessly from a rooftop donor 300 meters away saves the operator six figures per site. Across a deployment of 500 small cells in a dense urban core, IAB can reduce the total network cost by $30 to $50 million compared to a fully fiber-fed architecture.

The trade-off is capacity. Each IAB node delivers less throughput than a fiber-fed small cell, and multi-hop chains compound the reduction. Operators must balance the capital savings from avoiding fiber against the revenue implications of reduced per-cell capacity. In practice, this means deploying fiber to high-traffic locations (stadiums, transit hubs, enterprise buildings) while using IAB for coverage-oriented deployments on residential streets and lower-traffic commercial corridors.

For operators and system integrators evaluating IAB, the RF hardware chain from donor to relay to UE determines whether the deployment meets its performance targets. Every component, from the phased array antenna elements to the calibration standards used during factory acceptance testing, contributes to the link margins that define coverage, capacity, and service availability. That is the engineering reality behind the economic promise of self-backhauled 5G at millimeter wave.