O-RU

The O-RU is the box bolted up on the tower or wall — the part that actually radiates. It handles the RF front end (power amplifiers, filters, the antenna array) plus low-PHY: the FFT/iFFT, cyclic prefix handling, digital beamforming, that lower edge of the physical layer the 7.2x split assigns to it. Keeping low-PHY on the radio is a deliberate compromise. Push too little down and the fronthaul has to carry raw time-domain samples (the old CPRI problem, enormous bitrates); push too much down and the O-RU gets expensive and vendor-specific again.

The whole reason the O-RU matters commercially is the open fronthaul interface beneath it. Standardise that link and an operator can buy radios from one vendor and the O-DU from another — breaking the lock-in that came with classic integrated RAN. In practice this is also where a lot of O-RAN interoperability pain shows up, because getting a third-party O-RU and O-DU to agree on timing, beamforming descriptors, and the M-plane management details is harder than the spec makes it sound.