Re: [Discuss-gnuradio] OFDM tunnel.py HELP REQUIRED

[Tuan Ta]

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

On Fri, Jul 9, 2010 at 3:34 AM, chuck lorres <address@hidden> wrote:

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 tunnel.py in ofdm folder, please help me out. I would request the senior members of this forum to guide me in solving this issue.

Thank you.

Regards,

Chuck.

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