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

[Morgan Redfield]

I finally got this working. One of the machine's I was using was
running Windows with the gnuradio port from
http://www.joshknows.com/gnuradio_port. When I switched to Ubuntu the
majority of my problems went away. I've got no idea what was going on
with the Windows machine, but I never got any errors from it. The
transmitted signal was just never very clean.

I'm now able to use the benchmark_ofdm_tx and benchmark_ofdm_rx
scripts to send packets between my N210s. While I was playing with the
settings to get better throughput, I noticed that the SNR setting in
benchmark_ofdm_rx.py seems to break things. As long as I don't use the
--snr flag, everything works ok. If I use --snr with any value, I
receive no packets. I can't even use --snr=30 (the default) without
breaking things. Does anyone know why that would be?

Thanks for all your help,
Morgan

On Thu, Jun 9, 2011 at 12:42 PM, Tom Rondeau <address@hidden> wrote:
> On Thu, Jun 9, 2011 at 3:01 PM, Marcus D. Leech <address@hidden> wrote:
>>>
>>> On Wed, Jun 8, 2011 at 11:24 PM, Morgan Redfield<address@hidden>
>>>  wrote:
>>> 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
>
> Try changing the receiver gain instead. If the noise floor is moving with
> changes in the transmitter, then you are seeing non-linear effects in the
> transmit chain, which is bad. This is the chief problem of OFDM in that you
> need a good, linear PA to transmit with higher power for greater distance
> (which is one reason LTE is using SC-FDMA in the handsets).
>
>>
>> If changing the *TX* amplitude doesn't improve things, then perhaps the
>> frequency offset is the problem.
>>  I'm not much of an OFDM guy, but it seems to me if your OFDM "bins"
>> aren't where they're supposed to be,
>>  to less than a fraction of a bin-width, then there could be problems.
>
> The synchronization algorithms in OFDM correct for both fractional (inner
> subcarrier) offset and integer (greater than a subcarrier) offset, but only
> to an extent. So you can be off by a few subcarriers from the desired
> frequency and have those corrected (I think we put in +/- 5 or 10), and the
> fractional offset is also taken care of. The analysis of this shows that you
> get a significant increase in BER if you are even slightly off carrier after
> sync, so it's a very important part of the process (since OFDM depends on
> things being orthogonal, any frequency offset destroys the orthogonality).
>
> Tom
>
>> Also, to confirm that your RX is sensitive enough, if there's a way you
>> could generate a single-tone signal at
>>  about -110dBm, directly connected to the RX, and see if you can see the
>> tone in an FFT display.  If not then
>>  you have RX sensitivity issues.
>>
>>
>>
>>
>> --
>> Marcus Leech
>> Principal Investigator
>> Shirleys Bay Radio Astronomy Consortium
>> http://www.sbrac.org
>>
>>
>>
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>
>