5G NR is supposed to support many use cases using millimeter wave frequency range (FR2) due to the availability of high bandwidth, but since the propagation loss at these frequencies is high so cell coverage is limited. To cover a large area that requires high capacity, more small cells are needed, each of which needs high transport bandwidth (e.g. fiber) to transfer the subscribers data, which is not possible to be implemented separately for each of them.
To cope the transport bandwidth issue, 3GPP specified IAB in Rel.16 and the enhanced one in Rel.17.
The IAB supports both NR accessing and wireless backhauling i.e. it provides access to UEs, and using the neighbor nodes wirelessly backhauls the access traffic (subscribers payload) to the 5G Base Station which have fiber connection.
3GPP specified IAB Node and IAB Donor within NG-RAN in Rel.16
–The IAB-node, consists IAB-MT (UE function) and IAB-DU (gNB-DU function).
–The IAB-donor consists of an IAB-donor-CU and one or more IAB-donor-DU(s).
Here I am trying to depict an architecture including IAB-node and IAB-donor to clarify that how a UE supporting Release15 can exchange its payload to the 5G Core using 5G NR wireless backhauling.