¶ OFDM Transmission Over FR3
Transmit and receive an OFDM signal through Pi-Radio FR3 front-ends, up-converting a 1.5 GHz IF to 10 GHz and back, using USRP-2974 nodes and GNU Radio flowgraphs.
¶ Overview
This tutorial demonstrates an end-to-end OFDM link in the FR3 band (upper mid-band). An OFDM waveform is generated at 1.5 GHz IF by a USRP-2974 (Krypton — embedded X310) running a GNU Radio transmitter flowgraph. A Pi-Radio FR3 front-end up-converts the 1.5 GHz IF signal to approximately 10 GHz for over-the-air transmission. On the receive side, a second Pi-Radio front-end down-converts the 10 GHz signal back to 1.5 GHz IF, and a second USRP-2974 running a GNU Radio receiver flowgraph demodulates the baseband signal.
The Pi-Radio web UI (port 5006 on each rfdev-* node) is used to configure both the low-side LO (1.5 GHz) and high-side LO (10 GHz) on both front-ends. All OFDM modulation and demodulation is performed in GNU Radio.
¶ Learning outcomes
After completing this tutorial you will be able to:
- Configure Pi-Radio FR3 front-ends via the web UI to set IF and RF LO frequencies.
- Open and run GNU Radio OFDM transmitter and receiver flowgraphs on USRP-2974 nodes.
- Verify an end-to-end OFDM link through a FR3 up/down-conversion chain.
- Understand the signal path from GNU Radio baseband through IF and RF stages in a Pi-Radio setup.
¶ At a glance
| Difficulty | Intermediate |
| Estimated time | TODO: verify |
| Domain / sandbox | sb1.cosmos-lab.org (Sandbox 1) |
| Topic group | FR3 / Pi-Radio |
| Last verified | Not re-tested (migrated 2026-06-20) |
¶ Prerequisites
Background knowledge
- A COSMOS account and an active reservation in Sandbox 1.
- Familiarity with the USRP-2974 (embedded X310, 10 MHz–6 GHz, 160 MHz BW).
- FR3 hardware access (2× Pi-Radio FR3 front-ends) and the
fr3-tutorials.ndzdisk image. - GNU Radio familiarity — understanding of flowgraphs and GNU Radio Companion.
Account & access
- A COSMOS account — register here.
- An approved reservation on Sandbox 1 — see Make a reservation.
- SSH access to the domain console — see SSH access.
- Browser access to the Pi-Radio web UI on
rfdev-sdr1-piradioandrfdev-sdr2-piradioat port 5006.
¶ Resources required
Devices / nodes
| Resource | Role | Qty | Notes |
|---|---|---|---|
| sdr1-piradio | OFDM transmitter host | 1 | USRP-2974 (embedded X310); runs GNU Radio TX flowgraph |
| sdr2-piradio | OFDM receiver host | 1 | USRP-2974 (embedded X310); runs GNU Radio RX flowgraph |
| rfdev-sdr1-piradio | Pi-Radio FR3 TX front-end | 1 | Up-converts 1.5 GHz IF → ~10 GHz RF; web UI on port 5006 |
| rfdev-sdr2-piradio | Pi-Radio FR3 RX front-end | 1 | Down-converts ~10 GHz RF → 1.5 GHz IF; web UI on port 5006 |
Disk images
| Image | Load onto | Provides |
|---|---|---|
| fr3-tutorials.ndz | sdr1-piradio, sdr2-piradio | GNU Radio, OFDM-Tutorial flowgraphs, UHD for X310 |
Software components
| Component | Version | Source |
|---|---|---|
| GNU Radio Companion | TODO: verify | preinstalled in fr3-tutorials.ndz |
| UHD (USRP Hardware Driver) | TODO: verify | preinstalled in fr3-tutorials.ndz |
| OFDM-Tutorial flowgraphs (tx_ofdm.grc, rx_ofdm.grc) | TODO: verify | /root/OFDM-Tutorial/ on the image |
| Pi-Radio web UI | TODO: verify | running on rfdev-* nodes at port 5006 |
Spectrum / RF / special
- TX antenna: Vivaldi (connected to rfdev-sdr1-piradio Pi-Radio front-end TX port).
- RX antenna: Vivaldi (connected to rfdev-sdr2-piradio Pi-Radio front-end RX port).
- IF: 1.5 GHz; RF: ~10 GHz (FR3 band). See the FR3 hardware pages for antenna layout.
- Confirm allowed transmit power and any shielding/OTA constraints for FR3 in Sandbox 1.
¶ Topology
Two USRP-2974 nodes (sdr1-piradio TX, sdr2-piradio RX) are each connected to a dedicated Pi-Radio FR3 front-end (rfdev-sdr1-piradio, rfdev-sdr2-piradio). The USRP-2974 produces and consumes a 1.5 GHz IF signal; the Pi-Radio front-end up-converts (TX path) or down-converts (RX path) between the 1.5 GHz IF and ~10 GHz RF using a low-side LO at 1.5 GHz and a high-side LO at 10 GHz. Vivaldi antennas are used for over-the-air transmission and reception. Both USRP-2974 nodes are accessed via SSH from the console; the Pi-Radio web UIs are accessed via browser from the console or a jump host.
¶ Setup
-
Reserve Sandbox 1 resources and log into the console:
ssh <username>@console.sb1.cosmos-lab.org -
Make sure the nodes are off before imaging:
omf tell -a offh -t system:topo:allres -
Load the FR3 tutorial image onto both USRP-2974 nodes:
omf load -i fr3-tutorials.ndz -t sdr1-piradio,sdr2-piradio -
Turn the nodes on and confirm they are up:
omf tell -a on -t sdr1-piradio,sdr2-piradio omf stat -t sdr1-piradio,sdr2-piradio -
Verify browser access to the Pi-Radio web UI on both rfdev nodes at port 5006 (e.g.,
http://rfdev-sdr1-piradio:5006andhttp://rfdev-sdr2-piradio:5006).
¶ Instructions
¶ Step 1 — Configure the Pi-Radio front-ends
Open the Pi-Radio web UI Frequency panel on both rfdev-sdr1-piradio (TX) and rfdev-sdr2-piradio (RX) and set the following LO values on each:
- Low LO Frequency (GHz):
1.500000 - High LO Frequency (GHz):
10.000000
This pairs a 1.5 GHz IF with a 10 GHz high-side LO (and a 1.5 GHz low-side LO) to place the RF tone at approximately 10 GHz on transmit, and down-convert it back to 1.5 GHz on receive. (Exact placement depends on mixer sign/paths; the screenshots below show the working settings used in this demo.)
Leave Filters, Gain (except where noted), and LO Suppression at defaults for the first run.
¶ Step 2 — Run the GNU Radio OFDM transmitter
Open a new terminal on sdr1-piradio and execute:
sudo -s
cd /root/OFDM-Tutorial/
gnuradio-companion tx_ofdm.grc
This opens GNU Radio Companion with the OFDM transmitter block diagram. Make sure to configure it to use the internal X310 (RIO device).
Run the transmitter flowgraph.
¶ Step 3 — Run the GNU Radio OFDM receiver
Open a new terminal on sdr2-piradio and execute:
sudo -s
cd /root/OFDM-Tutorial/
gnuradio-companion rx_ofdm.grc
This opens GNU Radio Companion with the OFDM receiver block diagram. Make sure to configure it to use the internal X310 (RIO device).
Run the receiver flowgraph.
¶ Verification
- The GNU Radio transmitter flowgraph runs without errors on
sdr1-piradioand the USRP-2974 begins streaming at 1.5 GHz IF. - The GNU Radio receiver flowgraph on
sdr2-piradioshows a received constellation or spectrum consistent with the transmitted OFDM signal. - Confirm what specific output indicators (e.g., EVM, constellation plot, sink output) the flowgraphs display to indicate a successful link — the source tutorial does not describe expected output in detail.
¶ Cleanup
omf tell -a offh -t sdr1-piradio,sdr2-piradio
No omf save needed unless you modified the image.
¶ Troubleshooting
| Symptom | Likely cause | Fix |
|---|---|---|
| GNU Radio cannot find the X310 (RIO device) | RIO device not configured in flowgraph | In GNU Radio Companion, open the flowgraph properties and set the USRP source/sink device to the internal X310 RIO address |
| Pi-Radio web UI not reachable on port 5006 | rfdev node not up or network issue | Confirm the rfdev node is powered and reachable via ping from the console; check that port 5006 is open |
| No signal received on sdr2-piradio | LO frequencies mismatched or TX not running | Verify both front-ends have identical LO settings (Low 1.5 GHz, High 10.000 GHz); confirm the TX flowgraph is actively streaming |
| Transmitter flowgraph errors on startup | Wrong device string for X310 | Confirm the RIO device address in the flowgraph matches the hardware; see USRP-2974 hardware page |
¶ References
- Pi-Radio website
- GNU Radio project
- FR3 hardware pages
- Available SDR / USRP hardware
- Make a reservation
- SSH access
Author(s): COSMOS team · Last verified: Not re-tested (migrated 2026-06-20) · Tested image/release: fr3-tutorials.ndz · Tags: fr3, ofdm, gnuradio, piradio, usrp-2974, x310, mid-band