## 2 Background and Related Work
### IAB Features and Standardization  
IAB is characterized by 3 GPP as a flexible technology that should reuse the features and interfaces of 5G NR as much as possible to reduce the impact on the core network. To achieve this, the IAB architecture relies on the concept of network virtualization and, particularly, on the base station functional split, which was proposed in Release 15 TR 38.816 and consists of dividing the base station into logical parts: one central unit (CU) and one or more distributed units (DUs).  
The CU/DU split architecture proposed for IAB in TR 38.874 [7] is depicted in Figure 1. The IAB network consists of an IAB-donor and IAB-nodes. The IAB-donor is a logical node that provides the NR-based wireless backhaul to IAB-nodes and holds a CU and one or more wired donor DUs. The CU hosts the Packet Data Convergence Protocol (PDCP), Service Data Adaptation Protocol (SDAP), or Radio Resource Control (RRC) and terminates the control and user plane interfaces to the core network. The IAB-node, which may have multiple radio antennas, is wirelessly backhauled to the IAB-donor and consists of a DU and a mobile-termination (MT) function. The DU includes the Radio Link Control (RLC), Medium Access Control (MAC), and Physical Layer (PHY) protocols. The CU controls the DU nodes over the F1 interface(s), whereas the DU node hosts the lower layers for the NR Uu interface to the user equipment (UE). The MT allows the IAB-node to connect to an uplink node or donor, while via the DU the IAB-node, it connects to the UEs and to the MTs of downlink IAB-nodes.  
The only IAB-specific protocol--Backhaul Adaptation Protocol (BAP)--was introduced in 3 GPP TS 38.401 [8] and is responsible for forwarding IP data across connected IAB-nodes. The BAP data are carried by backhaul RLC channels on each link. For QoS enforcement, multiple channels can be configured for traffic prioritization. The BAP entity in nodes maps data to the appropriate backhaul RLC channel. Hop-by-hop packet forwarding is provided by the BAP routing identity, which is assigned by the IAB-donor.  
Following [7], all IAB-nodes exactly one hop downlink from a given IAB-node (or the IAB-donor) are called its \"child\" nodes and exactly one hop uplink \"parent\" nodes.  
Figure 1: The CU/DU split architecture for IAB (adapted from [7]).  
Each IAB-node and the IAB-donor can have zero or more child nodes. The different link types present in an IAB network are illustrated in Figure 2. Here, the access links are the links between an IAB-node/donor and a UE, whereas the backhaul links are between two IAB-nodes or between an IAB-node and the IAB-donor.  
IAB is supposed to support out-of-band and in-band backhauling (the use of different or the same spectrum for backhaul and access, respectively). In-band backhauling involves half-duplex restrictions, implying that the MT part of an IAB-node cannot receive while its collocated DU is transmitting and vice versa to avoid intra-site interference.  
The first 3 GPP technical report on the IAB technology, TR 38.874, was published in January 2018. Its current version, v16.0.0 [7], addresses single-hop/multi-hop topology management, route selection, dynamic resource allocation between the access and backhaul links, and spectral efficiency. A detailed description of the IAB RAN deployment is provided in 3 GPP technical specification TS 38.401 v17.0.0 [8]. 3 GPP suggests the following IAB features for research and evaluation:  
* Multi-hop backhauling;
* Multiplexing of the access and backhaul links;
* Multi-beaming;
* Multi-connectivity.  
Multi-connectivity in the context of IAB can be interpreted in different ways, but the most interesting is the ability to connect a UE to several IAB-nodes to enable fast switching in case of channel blocking during traffic transmission.