O-RAN split 7.2 in plain terms

A traditional base station is a closed box from one vendor. Open RAN breaks it into parts with published interfaces between them, so an operator can combine a radio from one supplier with baseband from another. The O-RAN Alliance specifications define those parts and interfaces.

The three units

• O-RU, the radio unit, sits at the antenna.
• O-DU, the distributed unit, does the real-time baseband processing.
• O-CU, the central unit, handles the higher protocol layers.

The link between the O-RU and the O-DU is the fronthaul. Where the line between them is drawn is called the functional split.

What split 7.2 puts where

The physical layer (PHY) is a chain of signal-processing steps. Split 7.2 cuts that chain in the middle:

Runs in the O-RU (low PHY)	Runs in the O-DU (high PHY)
FFT and inverse FFT	Channel coding and decoding
Cyclic prefix addition and removal	Modulation and demodulation
Digital beamforming	Layer mapping and precoding
PRACH filtering	Resource element mapping

So frequency-domain samples, not raw time-domain samples, travel over the fronthaul. That needs far less bandwidth than sending raw antenna samples, while keeping the radio unit simpler than one that runs the whole PHY.

The fronthaul carries four planes

• User plane carries the IQ data.
• Control plane tells the radio how to handle that data, slot by slot.
• Synchronisation plane keeps time and frequency aligned across units.
• Management plane configures and monitors the radio.

User and control traffic run over eCPRI on Ethernet, which is what allows ordinary packet networks to carry fronthaul.

Why FPGAs show up in radio units

Low-PHY functions are fixed-rate, highly parallel signal processing with tight timing. That suits FPGAs well, and it leaves room to change the design as the specifications evolve. This is the approach behind Tarang, our O-RAN work, which is still in progress.

Future posts will look at fronthaul timing and at where energy is spent in a radio unit.