Reducing rare underflows to zero occurrences

All:

I'm developing a comm system involving three SDRs: transmit, channel effects, and receiver.

The channel SDR is controlled by a flowgraph that has various options for the channel: AWGN (h(\tau) = \delta(\tau)), LTI, and a couple time-variant frequency-selective channel models.

I need to run the full three-SDR system over long periods of time to get stable average values for system-performance parameters.

My problem is that the channel flowgraph reports underflows on the order of every hundred seconds or so (sometimes much less). I've tracked down these rare underflows in the channel to transient problems in the receiver flowgraph.

So I put together a minimum flowgraph that shows the problem, which is attached. When I run it, I get output like this:

[INFO] [UHD] linux; GNU C++ version 11.2.0; Boost_107400; UHD_4.1.0.5-3
[INFO] [X300] X300 initialization sequence...
[INFO] [X300] Maximum frame size: 8000 bytes.
[INFO] [X300] Radio 1x clock: 200 MHz
usrp_source :info: set_min_output_buffer on block 1 to 2097152
usrp_sink :info: set_min_output_buffer on block 2 to 2097152
mmse_resampler_cc :info: set_min_output_buffer on block 4 to 2097152
fir_filter_blk<IN_T,OUT_T,TAP_T> :info: set_min_output_buffer on block 5 to 2097152
multiply_cc :info: set_min_output_buffer on block 9 to 2097152
sig_source :info: set_min_output_buffer on block 8 to 2097152
add_cc :info: set_min_output_buffer on block 6 to 2097152
fastnoise_source :info: set_min_output_buffer on block 7 to 2097152
Uusrp_sink :error: In the last 579300 ms, 1 underflows occurred.
Uusrp_sink :error: In the last 256008 ms, 1 underflows occurred.
usrp_sink :error: In the last 84992 ms, 1 underflows occurred.
UUusrp_sink :error: In the last 128008 ms, 1 underflows occurred.

The PSD of the filtered input signal appears correctly on a spectrum analyzer.

System Information:

24-core Intel i9 system
Ubuntu 22.04.5 LTS, kernel 5.19.0-46-generic #47~22.04.1-Ubuntu
gnuradio 3.10.1.1
UHD 4.1.0.5-3

I've taken some pains to optimize the OS settings for gnuradio/UHD along the lines suggested by the online tips at

https://kb.ettus.com/USRP_Host_Performance_Tuning_Tips_and_Tricks

I've also tinkered with the minoutbuf parameter in the various blocks.

Any advice?

Thanks,

Chad
cyclostationary.blog
cmspooner@nwra.com

--   Chad M. Spooner, PhD  NorthWest Research Associates  149 Bonifacio Place, Suite 200  Monterey, CA 93940  cmspooner@nwra.com  831 521 6743  NWRA Scientist/Engineer, VP, BoD  cyclostationary.blog