ent requirements on the fron- thaul interface in terms of latency and especially throughput. This is the option supported by CPRI and eCPRI. Generally, one can observe that lower splits offer higher gains in terms of performance (RRM, scheduling, interference mitigation, CoMP) and bigger resource utilization gains, as more functionality is centralized in the CU. Furthermore, a CU may be connected to a large number of DUs controlling multiple cells in a relatively large area - this allows imple- mentation of more advanced RRM and scheduling algorithms, which take into account the radio conditions of multiple cells and large numbers of UEs. This, however, comes at a cost of increased complexity (in terms of standardization, implementation, and interoperabil- ity testing) of the fronthaul interface. Furthermore, lower splits can substantially increase fronthaul requirements in terms of latency and throughput, to the point that certain splits (e.g. split option 8) may not be feasible at all. For example, in the mm Wave frequency range, 4.1 Introduction certain functional splits' fronthaul bandwidth requirements may be hard to satisfy even if fiber backhaul is available. In the course of the study on the gNB split, 3GPP came to realize that there is interest and market demand for two somewhat different split options: Low-level split (options 6, 7, and 8), which is appealing to operators that have sufficient fiber fronthaul transport; High-level split (options 1, 2, 3, and 5), which can be deployed by operators that do not have fiber fronthaul transport or consider investment in fiber transport not justifiable. Therefore, one can generalize a gNB functional split architecture as shown in Figure 4.1.11, in which a gNB is split into three logical nodes: CU, providing high-layer protocol stack functionality (e.g. PDCP); DU, providing low-layer protocol stack functionality (e.g. RLC and MAC); RU, providing lower-layer functionality (e.g. parts of PHY). Notes on Figure 4.1.11: As mentioned above, the antenna interface is no