ed from the CU, to allow it to run in a different environment, less cloud/virtualization centric. The second main split chosen by the O-RAN Alliance is the "Lower Level Split" (LLS) between DU and the RU along (more or less, see next) the 3GPP Option 7-2 dividing line. The balance between fronthaul throughput, implementation complexity, and other aspects is believed to be best struck with this functional split, allowing for an interface that can be sup- ported by a wide ecosystem of vendors. Physical implementations can (and do) of course combine multiple functions into a single hardware box: there is no obstruction to combine CU and DU functions onto a single hard- ware board or even a single (multicore) chip where the physical interface becomes a logical Option 7-2x As we said earlier, the split between DU and RU is key and involves a trade-off between two competing interests: on the one hand, the desire to keep RU complexity low to save power, cost, and implementation complexity, and, on the other hand, the desire to reduce the fronthaul bandwidth. The xRAN standard (later picked up by the O-RAN alliance) defined a new split point called "7-2x," or also called the Lower Layer Split/CU plane ("Ils-CU"). This split is nearly the same as the 3GPP Option 7-2 but allows precoding/digital beamforming either to be implemented in the DU or in the RU, depending on implementation choice. RUs that require precoding done in the DU are called "Category A" and RUs that support precoding in the RU are called "Category B." From the example of massive MIMO, we showed earlier, we can see that Category B is an obvious choice for systems where the antenna count is higher than the logical layer count. 32 Open Radio Access Network (O-RAN) Systems Architecture and Design There are some specific complexities involved in supporting Category B RUs, such as the transport of the necessary control information between the DU and the RU. Relevant 3GPP standards The O-RAN Alliance key interface description is the LLS one that defines the int