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

[Tom Rondeau]

On Fri, Jun 10, 2011 at 5:53 PM, Morgan Redfield <address@hidden> wrote:

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

Glad you got it working!

As for the SNR setting, I'd have to look back at the code. I thought it was just for one of the sync methods that we aren't using, so I didn't think it mattered. Must be being used somewhere that I can't recall just now.

Tom

 

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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>
>