How to Capture Over-the-Air Downlink and Uplink Signals from srsRAN gNB and Save as fc32 File?

[PsArzzz]

Hello everyone,

I have set up a full 5G standalone (SA) simulation network using Open5GS and srsRAN. My primary radio unit is a USRP B210.

My Goal:
I want to capture the wireless signals transmitted over the air between the base station (gNB) and the user equipment (UE), and save the raw IQ data as an fc32 file locally for subsequent offline analysis.

My Current Questions & Considerations:
1.Built-in Command or Configuration: Does the srsRAN project itself provide any built-in command-line parameters or configuration file options that would allow the gNB or UE to directly log baseband IQ samples to a file during operation? I've looked through the documentation but haven't found clear guidance on this.
2.External Tool Approach: If capturing directly from within srsRAN isn't feasible, using an external tool is a great alternative. I have an additional, separate USRP that can act as a pure signal probe/receiver to listen and record the over-the-air signals.
For this approach, what toolchains are recommended? (e.g., Using UHD utilities like uhd_rx_cfile, or building a simple receive flowgraph in GNU Radio?)
Are there any critical parameters I should pay special attention to during recording to ensure I correctly capture the srsRAN signal? (For instance, how should I match the center frequency, sampling rate, etc., with the gNB's settings?)

Environment Info:
Core Network: Open5GS
RAN: srsRAN (both gNB and UE)
SDR Hardware: USRP B210 (for the gNB/UE stack), plus an additional USRP available for probing.

I would be very grateful for any advice, example commands, or pointers to relevant resources. Thank you very much for your time and help!

[dvdgrgrtt]

Hey @PsArzzz,
for the uplink signal you can use the following gNB options

log:
  filename: /tmp/gnb.log
  phy_level: debug
  phy_rx_symbols_filename: /tmp/gnb.iq
  phy_rx_symbols_port: all
  phy_rx_symbols_prach: true

Have a look here for an example. Be careful, the IQ file grows very fast.

I'm not sure the output is a proper fc32 file, though: here we store the RE values, alternating real and imaginary parts (4 bytes each), traversing subcarriers first, OFDM symbols second, finally, and antenna ports.

[PsArzzz]

Thank you for your reply and help.
Does srsRAN only provide settings to capture uplink messages?
Is there any configuration that allows saving both uplink and downlink messages into a file?

[dvdgrgrtt]

Hi @PsArzzz,
unfortunately no, we don't currently provide an option for capturing the DL IQ samples, only UL ones as explained above.

[PsArzzz]

ok, thanks!