# OFDM tunnel.py HELP REQUIRED

Hi,For past one week I have been trying to run tunnel.py in ofdm folder.
I have successfully run tunnel.py in digital folder as well as
benchmark_ofdm_*.py with 0 packet loss.I have tried different set of
parameter settings but to no avail. If any body has successfully ran the
members of this forum to guide me in solving this issue.Thank
you.Regards,Chuck.

To follow up with Chuck’s question, I’m also having problem running
tunnel
using OFDM. The individual transmissions between the 2 USRPs were fine
(albeit that I had to manually adjust the center frequency of 1 USRP (A)
to
cope with the frequency offset). I had B operating at 2412MHz and A at
2411.96MHz. The 40kHz offset was figured out by running usrp_fft on A
while
B is transmitting at 2412MHz.

My reasoning for the need of manually adjust A’s center frequency is
that
the synchronization block implemented right now can’t track the offset
that
large. The current synchronization algorithm is due to Schmidt and Cox.
I
had gone through their paper but didn’t find any limit to when the
algorithm
starts to break.

I did some calculation to find the relationship of the offset to the
subcarrier spacing. The DAC runs at 128MSps, I have interpolation = 128
(lowering this gives me overrun, I have a duo-core 1.66GHz laptop with
2.5GB
of RAM). So my bandwidth is 128/128 = 1MHz.
If the FFT length is 512 then the subcarrier spacing is 1000/512 =
1.95kHz.
40kHz offset would be
20.48 subcarrier spacing.
If the FFT length is 32 then the subcarrier spacing is 1000/32 =
31.25kHz.
40kHz offset would be
1.28 subcarrier spacing.

I don’t know the significances of those numbers yet. Hopefully someone
with
better knowledge of OFDM synchronization can shred some lights.

So USRP A can send to B, and B can send to A. But tunnel wouldn’t work.
This
really puzzles me. The BIG QUESTION that I want to know is that
whether
anyone has had tunnel to work with OFDM. So that I can zero out the
possibility of software bugs in tunnel.py.

Thank you very much,

Tuan

Bumping this one up again…
Any luck on this?

-Apurv