ed over a standardized, network-based interface (nFAPI being a key candidate, see next). This removes any physical layer processing from the centralized location, thus making it very software- and virtualization-friendly. This split is also very natural from a software ecosystem point of view: physical layer and MAC/RLC/PDCP stacks are often provided by different software vendors, and the FAPI interface concepts are well established. This interface builds on both factors, thus mak- ing implementation complexity relatively low-it is an easily understood interface. Options 5, 4, and 3 similarly split the upper stacks to offload more and more processing to the radio head. However, given that the IO bandwidths within the Layer-2 stack are rela- tively similar (so little benefit to be had from a fronthaul capacity point of view) and that MAC/RLC stacks are typically developed in a tightly coupled manner, these options have not proven to be very popular in the industry. A more well-adopted functional split is defined as Open 2 which divides the L2 stack between PDCP and RLC processing domains. Compare this to the functional split between the NIC and the Base Station Line Card in the traditional 3G/4G/5G macro base station to understand why this is a very logical partitioning choice to allow different scaling of PDCP and MAC/RLC components. Fronthaul throughput To contrast the functional split options against each other, compare some popular split options in the chart given in Fig. 1-23 that takes two typical use cases and calculated theo- retical fronthaul throughput for a 100 MHz wide 5G/NR RF channel. Takeaways from this chart that are discussed previously include: Massive MIMO benefits significantly from Option 7-2 where layer-level information is transferred rather than antenna-level information. Option 7 reduces bandwidth sufficiently from Option 8 to warrant moving the FFT function away from the centralized unit. Options 7-3, 6, and 2 all cut fronthaul bandwidth by an order of magnitude compared to more low-layer