reless radio equipment is also difficult if the site space is limited. IAB solves these issues without needing either extra site space or equipment, provided that part of the UE access bandwidth can be used for backhaul. This is true especially in the higher frequency bands where bandwidth is not as scarce resource. IAB relies on the CU-DU split architecture. Each DU is logically connected to the CU, but the F1 interface may be relayed through intermediate IAB nodes as self-backhaul instead of traditional fiber or other media as shown in Figure 9.11 [28]. The IAB node shares the air interface resources and services both the access UEs attached to it and the backhaul link for the tail-end DU(s) for their connectivity to the CU. The IAB connectivity consists of multiple hops (chaining) and possibly a tree topology. Intermediate IAB nodes provide a relay function with an adaptation layer. The standard work is in progress in 3GPP. In further 3GPP releases, IAB should also support moving IAB nodes, for example, as the backhaul for moving vehicles. 9.5.4 NTNs (Non-Terrestrial Networks) 3GPP has a study item on NTNs [29, 30], with the target of integrating satellite-based communication links and services to the users into the 5G system. Architectural aspects are studied in TR23.737, "Study on Architecture Aspects for using Satellite Access in 5G" [31]. The benefit is the extension of 5G communication service to remote areas, or ships in the middle of the ocean, airplanes, and further cases. With LEO (Low Earth Orbit) platforms, communication latency is drastically reduced (e.g. from 500-800 ms RTT to 10-40 ms) compared to GEO (Geostationary Earth Orbit) 9.5 Specific Topics Access UEs are served by each DU's access links CU terminates F1 interface from all DUs and interfaces the core network Donor IAB node UPF etc Backhaul RLC links serving the tail DU and Logical view carrying F1 to the CU F1 over Backhaul RLC IAB node Donor as a relay Figure 9.11 Integrated access and backhaul. services, and at the same time the possib