dio unit An O-RAN radio unit is the logical entity that converts the frequency domain information that the DU produced into a time-domain signal that is transmitted over the air and vice ver- sa. Major components include the RF component that generates (or receives) the analog sig- nal, low-PHY functionality that includes time-frequency domain conversion and signal conditioning, and Enhanced Common Public Radio Interface (eCPRI) termination which converts IQ samples carried in Ethernet frames to IQ sample buffers that are processed by the low-PHY. The bulk of the RU complexity (and cost) is carried in the low-PHY and ana- log/RF components. Generalized block diagram Fig. 2-31 shows a generalized version of an RU block diagram, components of which we will discuss individually. The top half shows the transmit path and the bottom half the receive path, both for a single antenna instance. The example given next shows a zero-IF implemen- tation, but this is only one of multiple design choices. We discussed IF and direct RF conver- sion previously. Note that, more SO than for many other components in the system, the physical imple- mentation and the representation in the figure mentioned above do not need to match. Some elements of partitioning options are driven by the system requirements. The eCPRI and beamforming components, for example, are shared between all the antenna elements where the remainder [fast Fourier transform (FFT) up to antenna] components are (or better Chapter 2 System components, requirements, and interfaces 77 Antenna Connector VCTCXO PLL+VCO Feedback FIGURE 2-31 Generalized RU block diagram. RU, radio unit. said: can be) individually implemented on a per-antenna basis. A logical functional split would colocate the eCPRI and beamforming components for all antennas into a single device, which can lead to scalability challenges in massive MIMO systems with up to 64 antennas. The remainder elements are independent but come with huge IO bandwidths- the IQ bandwidth associated with a single 100 MHz ca