7.2x Split

When you disaggregate a base station you have to choose where to cut the physical layer between the radio and the rest, and 7.2x is the cut O-RAN settled on for open fronthaul. It puts low-PHY — the FFT/iFFT, cyclic prefix, and digital beamforming — down on the O-RU, and leaves high-PHY (along with MAC and RLC) up on the O-DU. The "x" reflects that there are sub-variants depending on how much of the beamforming and resource-element mapping you push down.

The choice is fundamentally a bandwidth-versus-complexity trade. A lower split (option 8, the old CPRI approach) keeps the radio dumb but forces the fronthaul to carry raw time-domain samples — huge, inflexible bitrates. A higher split shrinks the fronthaul load dramatically but makes the radio more complex and harder to standardise. 7.2x is the negotiated middle ground: fronthaul bandwidth that scales with actual traffic rather than raw bandwidth, while keeping the O-RU simple enough that multiple vendors can build one. That balance is the whole reason it became the basis for the open fronthaul interface.