From the core to the antenna: a radio-side primer

1. The stack, from the antenna to the core

A 5G gNB is one logical node, but it is usually built from three physical pieces. Reading from the phone toward the core, the five boxes below are what a packet passes through. The core-side reader already owns the box on the right; this article is about the two on the left and the link between them.

2. Split 7.2 versus split 8: where the FFT lives

The "split" number says where the PHY chain is cut between RU and DU. Two cuts matter in practice, and they lead to two different kinds of radio and two different kinds of fronthaul.

Why anyone chooses split 8

Split 7.2 is the right answer for an operator buying radios: the standard fixes what crosses the wire, so any O-RU should work with any O-DU. Split 8 is the right answer when the radio is a software-defined platform and the people using it want to change the signal chain itself. Beam handling, satellite delay and Doppler models, or a non-standard waveform can all be edited on the server when every stage of the PHY runs in software. With split 7.2 the FFT and the beamforming are fixed inside the radio, which is exactly what an operator wants and exactly what a research platform does not.

3. USRP, UHD, ZMQ, Aurora, eCPRI: what each one is and where it sits

These names come up in every radio integration, and they are not the same kind of thing. Some are hardware, some are drivers, some are wire formats, and one is a link protocol inside a box. They line up as layers between the DU and the antenna.

The same layers, four ways

4. The four planes, and where Open M-Plane fits

The four planes are an O-RAN split 7.2 concept. They partition everything that crosses the fronthaul between an O-RU and an O-DU by purpose, and each has its own protocol. The useful thing for a split-8 reader is that every plane has an analogue, even when the analogue is a single field in a packet header.

What Open M-Plane is

In a split-7.2 deployment, Open M-Plane is the piece that implements the O-RAN M-Plane: a NETCONF/YANG management path between the O-RU and the O-DU, so the DU can configure the radio and receive its alarms using the standard YANG models rather than a vendor tool. It exists because in split 7.2 the radio is a smart, standardised box that must be managed the standard way.

Open M-Plane, and the four planes as protocols, are 7.2 concepts. Split 8 has no O-RAN specification for the fronthaul at all. A USRP-style radio has no NETCONF agent, no eCPRI and no PTP requirement. What survives into split 8 is the classification: every parameter that crosses to a radio is still either data, real-time control, timing, or configuration. When we write a parameter inventory for a split-8 driver we still bucket every entry as C, U, S or M, because the bucket tells you the periodicity and the mechanism that parameter needs. Open M-Plane itself is not used with a split-8 radio. gRPC or a vendor API plays that role, and the planes are a vocabulary, not a protocol.

5. Architecture side by side

Read each column top to bottom, core to antenna. The tinted bar is the fronthaul, the only tier that differs in kind. The plane tags show where each plane's job is done.

6. A UE attaches: every message from PRACH to a PDU session

Step through it. The stage shows five lanes. Each step draws the messages of that step, and the panel explains it, with a note on what differs between split 7.2 and split 8 and a line for the core-side reader. Switch the toggle to see the same sequence in the other split: the lane chips under RU and DU move, and the fronthaul hops change. The arrow keys work once a button has focus.

7. What a radio driver actually has to do

An open gNB such as OCUDU has a plug-in point for radios. The UHD driver plugs in there for USRPs, and the ZMQ driver plugs in there for software-only tests. When we integrate a custom radio we write a third driver for the same point. The DU does not know or care which one is loaded. This section describes what that third driver has to do, in general terms, and the questions you have to settle with the radio vendor before you can write it.

The driver slot

The DU talks to its radio through a small interface: a radio session with a transmit call that takes one buffer per antenna and the time the first sample must leave the antenna, and a receive call that fills one buffer per antenna and returns the time the first sample arrived. Buffers hold one slot of int16 IQ samples per antenna. The driver's job is to move those buffers into and out of whatever transport sits below it, and to keep the timestamps honest.

UHD is the reference model. Whatever UHD does with CHDR packets over USB or 10 GbE, your driver does with the vendor's packet format over the vendor's transport. Reading the UHD driver in the open gNB before you start is the fastest way to learn the contract you are implementing.

What crosses the boundary

// transmit: called by the DU once per slot, with one buffer per antenna
for each antenna a:
  for each fixed-size chunk c of slot_buffer[a]:
    hdr.packet_counter = next_packet_counter()
    hdr.slot_counter   = slot_index
    hdr.start_of_burst = (c is the first chunk of the slot)
    hdr.end_of_burst   = (c is the last chunk of the slot)
    hdr.timestamp      = slot_start_ticks + offset_in_samples(c)
    ring_enqueue(tx_ring[a], hdr + c)

// receive: runs continuously, hands the DU one slot at a time
while running:
  pkt = ring_dequeue(rx_ring[a])
  if pkt.packet_counter != expected: log the gap, zero-fill the hole
  copy pkt.samples into rx_buffer[a] at (pkt.timestamp minus slot_start_ticks)
  when rx_buffer[a] is full for every antenna:
    deliver(rx_buffer, slot_start_ticks) to the DU

// start-up, once: the M-Plane job
radio.set_frequency(centre_hz); radio.set_gain(tx_db, rx_db)
radio.set_sample_rate(122.88e6); radio.set_clock_source("pps")

Four questions to settle before you write a line

Each of these has a habit of arriving half-answered. A vendor document usually describes what the radio sends, because that is what the vendor tested. The rest has to be agreed, and the driver cannot be finished until it is.

Glossary for the core-side reader

PRACH / preamble / Msg1

The UE's first transmission: a known sequence on a reserved time-frequency slot. The gNB does not know the UE exists before this.

RAR / Msg2

Random access response: the gNB answers with a timing advance, a temporary identity and an uplink grant.

Timing advance (TA)

How early the UE must transmit so its signal arrives aligned at the gNB. Microseconds on the ground; tens of milliseconds via satellite, which is why NTN adds a common TA and a K-offset.

SSB / SIB1

What the cell broadcasts so a UE can find it and read its configuration. The over-the-air equivalent of a service discovering the NRF.

IQ samples

The digitised radio signal: pairs of 16-bit integers (in-phase, quadrature) at 122.88 million per second for a 100 MHz cell. This is what crosses the split-8 fronthaul.

FFT / IFFT

The transform between time-domain samples and the OFDM subcarriers the standard is written in. Its location defines the split.

eCPRI

The O-RAN fronthaul protocol for split 7.2. Not used in split 8.

UHD / USRP

NI/Ettus software-defined radios and their driver. OCUDU's reference radio path, and the reference model for any custom split-8 driver.

DPDK, hugepages, rings, mbufs

A user-space networking toolkit. The vendor's software owns a large pinned memory area (hugepages) with lock-free queues (rings) of packet buffers (mbufs); your driver attaches to it as a secondary process and reads and writes those queues.

Shim / plugin / adapter / driver

Four names for the same thing: the radio driver you write for a custom SDR platform.

Doppler, CFO

A moving satellite shifts the carrier frequency; carrier frequency offset is the compensation applied to the samples. Open gNBs with NTN support do this in software, driven by ephemeris data broadcast in SIB19.

CHDR

UHD's packet format between host and USRP: a header with sequence number and timestamp, then sc16 samples. The nearest published analogue to a vendor's RF header.

Aurora

An FPGA-to-FPGA link protocol you may meet inside an SDR platform. Below Ethernet, not visible to the DU.

C/U/S/M planes

The O-RAN 7.2 fronthaul partition: control, user, synchronisation, management. Carried by eCPRI, eCPRI, PTP and NETCONF respectively. Used as a vocabulary in split 8.

Open M-Plane / NETCONF / YANG

The O-RAN management path for an O-RU, implemented as a NETCONF/YANG client on the DU side and an agent in the radio. Not used with a split-8 radio; gRPC or a vendor API plays that role.

PTP / PPS

Two ways to give a radio a clock: PTP over the network (split 7.2, S-plane) or a one-pulse-per-second signal from GPS into the radio (split 8, USRP or a custom SDR).

Split 8 vs 7.2

Where the PHY is cut between RU and DU. In split 8 the RU is a plain radio and raw samples cross; in split 7.2 the RU does the FFT and frequency-domain IQ crosses.

Written by telecogeco, a ugeco brand. The stack, the splits and the attach sequence are as the 3GPP and O-RAN specifications define them; the driver section describes the shape of a typical custom-radio integration and names no specific vendor.