Re: [Discuss-gnuradio] problems with benchmark_ofdm and N210

[Morgan Redfield]

On Wed, Jun 8, 2011 at 11:24 PM, Morgan Redfield <address@hidden> wrote:
> On Wed, Jun 8, 2011 at 7:20 PM, Marcus D. Leech <address@hidden> wrote:
>> On 06/08/2011 09:51 PM, Morgan Redfield wrote:
>>>
>>> Ok. I found another problem in my code. The transmit_path has a
>>> multiplier in it that was set to 200 (it could go up to 32768). This
>>> was for the original USRP, but I think UHD needs a signal with
>>> magnitude<=1. It seems like the signal was probably clipping. I
>>> changed the magnitude of the multiplier and fiddled with the gain
>>> parameters a bit. I'm now able to get the standard boxcar that I
>>> expect for OFDM. There's still a spike at the center frequency though,
>>> I don't see a gap.
>>>
>>> Thanks,
>>> Morgan
>>
>> If you're doing these tests over-the-air, is it possible that there's a
>> signal on TV channel 44 around you?
>>  Channel 44 starts at 650MHz.
>>
>>
>>
>> --
>> Marcus Leech
>> Principal Investigator
>> Shirleys Bay Radio Astronomy Consortium
>> http://www.sbrac.org
>>
>>
>>
>> _______________________________________________
>> Discuss-gnuradio mailing list
>> address@hidden
>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
>>
>
> When I'm not transmitting, I don't really see anything above the noise
> at the center frequency of 650MHz. There is a small spike (a few dB or
> so) right at the center frequency, but I think that's due to the USRP.
> Other messages on this list seem to indicate that it isn't abnormal.
>
> That spike is only very noticeable when I'm transmitting. I'll try
> changing the tx frequency and playing around with the gain tomorrow
> and let you know how that goes.
>
> Thanks,
> Morgan
>

I found that centering my FFT on a frequency that's offset from what
I'm transmitting at will remove that central spike. I was able to
finally see the gap in the center of the OFDM boxcar and adjust that.
It looks like in my setup I have an offset of about 6kHz.

My OFDM signal never seems to be more than about 10 dB above the noise
floor though. When I bump up the gain or tx-amplitude, everything gets
raised by the same amount. I'm still not able to demodulate packets,
and I think this is why. Do you have any advice about this?

Thanks,
Morgan