Integrated Access and Backhaul (IAB) MAC

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1. IAB Topology — Multi-Hop Architecture

IAB replaces fiber with wireless backhaul. An IAB donor connects to core via fiber. IAB nodes relay traffic wirelessly through multi-hop paths, each node containing a DU (serves children) and MT (connects to parent).

IAB Topology Tree — Add/Remove Nodes

Build an IAB topology. See how nodes connect through the donor. Each node serves UEs and can relay to child nodes.

Nodes: 3

IAB Donor

Fiber-connected to core. Hosts CU + DU. Controls all downstream IAB nodes and their configuration.

IAB Node (DU + MT)

DU serves child UEs/nodes. MT connects upstream to parent like a UE. Two MAC entities running simultaneously.

BAP Layer

Layer 2.5 routing protocol. Path identifier + BH RLC channel mapping. Routes packets hop-by-hop through the IAB tree.

2. TDM Patterns — Access vs Backhaul

IAB nodes are half-duplex: they cannot transmit to children and receive from parent simultaneously. TDM patterns divide slots between access and backhaul, incurring ~50% capacity loss per hop.

TDM Pattern Visualization

See how slots are allocated between access and backhaul. Different patterns optimize for different traffic directions.

Pattern: Pattern 7

Half-duplex loss: With TDM, each IAB node loses ~50% capacity per hop. 3 hops = ~12.5% of original link capacity. Full-duplex IAB (Rel-18) uses self-interference cancellation to recover this loss.

3. Multi-Hop Packet Flow

Packets traverse multiple wireless hops from UE to donor. Each hop adds scheduling delay, processing time, and potential buffering. Backpressure prevents buffer overflow across hops.

Packet Flow — Hop-by-Hop Latency

Watch a packet travel from UE through multiple IAB hops to the donor. Adjust hop count and per-hop delay.

Hops: 3 Per-hop delay (ms): 3 ms

Hops	Extra Latency (3ms/hop)	Throughput (% of single link)	Use Case
1	3 ms	~50%	Small cell extension
2	6 ms	~25%	Street-level coverage
3	9 ms	~12.5%	Rural extension
4+	12+ ms	<6%	Emergency only

4. Practical Field Considerations

Factor	Challenge	Mitigation
Hop count	Latency accumulation (+3ms/hop)	Max 3 hops for eMBB, 1-2 for URLLC
Half-duplex	50% capacity loss per hop	Full-duplex (Rel-18), more spectrum
Donor bottleneck	All traffic funnels through donor	Multiple donors, load balancing
Self-interference	TX leaks into RX	Guard periods, beam isolation
mmWave backhaul	LoS required, 100-300m range	Sub-6 for NLoS, careful site planning

Multi-parent (Rel-17): IAB nodes connect to 2+ parents for redundancy. If primary parent fails, traffic switches to secondary. Essential for network resilience.

5. 6G Evolution — Mesh IAB + AI Topology

Mesh Topology

Beyond tree: any-to-any backhaul. Auto-reroute around failures. Multiple paths per flow. Like IP routing at Layer 2.

AI Topology Optimization

AI predicts traffic, reconfigures topology. Optimizes parent selection, hop count, load distribution. Self-healing network.

THz Backhaul

100-300 GHz: 100+ Gbps wireless. Replaces fiber for ultra-high-capacity. Short range but massive bandwidth.

NTN + IAB

Satellite as IAB donor. HAPS as relay. 3D topology: space + air + ground integrated backhaul.

IAB Evolution — Rel-16 to 6G

Compare IAB capabilities across 3GPP releases and 6G vision.

Generation:

Dimension	Rel-16	Rel-17	Rel-18	6G Target
Topology	Tree (single-parent)	Multi-parent	Enhanced mesh	Full mesh
Duplex	Half-duplex TDM	Half-duplex	Full-duplex SIC	Full-duplex
Optimization	Static config	Adaptation	AI-assisted	AI-native
Integration	Terrestrial only	Terrestrial	+ NTN study	Space-air-ground

Integrated Access & Backhaul (IAB) MAC