On Fri, Jun 10, 2011 at 5:53 PM, Morgan Redfield <[email protected]>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 <[email protected]> > wrote: > > On Thu, Jun 9, 2011 at 3:01 PM, Marcus D. Leech <[email protected]> > wrote: > >>> > >>> On Wed, Jun 8, 2011 at 11:24 PM, Morgan Redfield<[email protected]> > >>> 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 > >> > >> > >> > >> _______________________________________________ > >> Discuss-gnuradio mailing list > >> [email protected] > >> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > > > > >
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