The 5G base station has been taken apart. The radio unit, the distributed unit and the central unit each own a slice of the stack, connected by fronthaul, midhaul and backhaul that now have to be specified, synchronised and tested as rigorously as the boxes they join. Knowing which function lives where — and which interface carries what — is the foundation of every O-RAN programme.
The base station is no longer one box. Knowing which layer sits in the RU, the DU and the CU — and which interface carries what — is the whole game.
It is the fronthaul split O-RAN standardised between the radio unit (O-RU) and the distributed unit (O-DU). The O-RU keeps the RF and the low-PHY (roughly the FFT/iFFT, cyclic prefix and digital beamforming), and sends frequency-domain IQ to the O-DU over eCPRI. It is a compromise: lower fronthaul bandwidth than a fully centralised split, while keeping the radio simple enough to be cheap and multi-vendor.
The O-RU handles RF and low-PHY; the O-DU handles high-PHY (FEC, modulation, HARQ), MAC and RLC in real time; the O-CU handles PDCP, RRC and SDAP, and is non-real-time and centralisable. The RU-DU link is fronthaul (7.2x over eCPRI), the DU-CU link is the F1 midhaul, and the CU-core link is NG backhaul.
To break the vendor lock of the integrated base station and to place functions where they make economic and latency sense: cheap multi-vendor radios at the cell site, pooled DUs nearby, and centralised CUs that can serve many sites. The cost is that the interfaces between the pieces now have to be rigorously specified and tested — which is where most of the real engineering goes.
The near-real-time RAN Intelligent Controller hosts xApps that optimise the RAN on a 10ms-to-1s timescale, talking to the CU and DU over the E2 interface. It is where O-RAN puts the programmable, data-driven control that a classic base station kept locked inside one vendor's software.
Tell us the specification, the constraint and the deadline.