ows a few examples that illustrate topology-wide resource coordination in a spanning tree topology. In one scheme, orthogonal resources are assigned in an 5 NG-RAN Evolution Tier 1 Tier 1 Tier 1 a not Tier 2 Tier 2 Tier 2 Tier 3 Tier 3 Tier 3 Time slots Time slots Time slots Figure 5.2.10 Resource allocation across spanning tree: (a) resources configured per DU, (b) configured per link, and (c) configured per DU with local optimization. alternating pattern across tree levels (Figure 5.2.10a). Within each tier, the gNB-DU scheduler can dynamically multiplex the tier's resources among its downstream links (the link selected by the scheduler is indicated by a solid line with an arrow). While this scheme meets the duplexing constraint with efficient resource reuse, it may lead to suboptimal resource utilization: if node 1 assigns its resource (red) to node 2, the other link to node 3 as well as its child links (links controlled by node 3) remain idle. It is possible to define a topology-wide resource allocation scheme, where each backhaul link obtains its own subset of resources (Figure 5.2.10b). While this multi-edge coloring problem allows optimal resource allocation for a given traffic distribution, it confines the local flexibility of the scheduler and cannot respond to traffic load fluctuations. IAB allows semi-static resource allocation per DU and per link. In addition, a local signal- ing mechanism is supported via the physical downlink control channel, where the DU can dynamically lease the resource it owns to a child node in case it is not needed. Figure 5.2.10c shows an example of DU-wide resource allocation where the scheduler in tier 1 leases its "a" resource to child node 2. In this manner, scheduling flexibility can be retained while local resource leasing can optimize inefficiencies. Another factor to be considered is cross-link interference among non-adjacent access and backhaul links. For mmWaves, it is expected that the use of narrow beams highly reduces cross-link interference. This especially appl