Hi, It's just the built in basic FDE. Its not awesome, but works well enough at high SNR.
Yes, when I send multiple packets, its only the last OFDM symbol of the last packet that is affected. I have tried suffixing FFT_len/4 (i.e. 16 = CP) lots of 0's in the TD at the transmitter, and this fixes the problem (low power noise instead of 0's also works). Could it be an analogue issue? It is always the last symbol, i.e. the end of the transmission, and it looks like the power might trail off from the attached picture? But it seems strange that it's exactly the last symbol? Could the pulse shaping performed in the cyclic prefixer have any effect?! Regards, David ________________________________________ From: Martin Braun [martin.br...@ettus.com] Sent: 30 April 2014 14:56 To: David Halls; discuss-gnuradio@gnu.org Subject: Re: OFDM Example, 'Noisy' final symbol On 30.04.2014 13:14, David Halls wrote: > Hi Martin, > > This is OTA or over a cable, 2.48GHz using the N210 with XCVR2450. The > results I sent are over a cable, and the results I sent are post equalizer. Which equalizer are you using? A custom one? Those in GNU Radio aren't fantastic, and they don't output soft info. > It is only the final symbol of a burst, not the final symbol of each packet. So what you're saying is that when you send several packets directly one after another, only the last one gets this? > I agree they should all be equally noisy, and I am certain it's not > 'noise' as such. > > I thought if the timing window is too late, then some TD samples for the > *final* symbol will be included in the FFT that are just noise. I have > stored the TD samples and manually performed FFT and eq'n and shifting > the window of samples to pass to the FFT (either forwards or backwards) > by up 16 samples does not help. If the timing window's late, something's seriously wrong. It should always be early. I currently don't have a good idea, though. M > > You have never noticed this happen? > > DH > ________________________________________ > From: Martin Braun [martin.br...@ettus.com] > Sent: 30 April 2014 12:07 > To: David Halls; discuss-gnuradio@gnu.org <mailto:discuss-gnuradio@gnu.org> > Subject: Re: OFDM Example, 'Noisy' final symbol > > On 30.04.2014 11:51, David Halls wrote: > > Hi Martin, All, > > > > Has anyone noticed that the final symbol of any burst (no matter how > > many packets in a transmission) appears slightly 'noisy' in the OFDM > > example code? Is it something to do with my set-up? > > > > I attach a picture of the eq'd FD constellation when I send two packet: > > a) the first packet, b) the second packet (excluding the final (16th) > > symbol), and c) second packet (with all symbols) > > > > Is it to do with sample alignment/timing error? I have tried sliding the > > samples passed to the FFT at the receiver, moving the window backwards > > and forwards and can't get a better result... > > This was my first thought when I read the subject line... where are you > getting symbols? Before the equalizer? And is this OTA, or simulated? > > Really, all OFDM symbols should be equally noisy. A timing offset will > normally only make the acquisition start somewhere between start and end > of the cyclic prefix. So, I'm unsure what this is without any further > analysis. > > Martin > > > ________________________________ > > NOTE: The information in this email and any attachments may be > confidential and/or legally privileged. This message may be read, copied > and used only by the intended recipient. If you are not the intended > recipient, please destroy this message, delete any copies held on your > system and notify the sender immediately. > > Toshiba Research Europe Limited, registered in England and Wales > (2519556). Registered Office 208 Cambridge Science Park, Milton Road, > Cambridge CB4 0GZ, England. Web: www.toshiba.eu/research/trl > <http://www.toshiba.eu/research/trl> > > > ------------------------------------------------------------------------ > This email has been scanned for email related threats and delivered > safely by Mimecast. > For more information please visit http://www.mimecast.com > ------------------------------------------------------------------------ ________________________________ NOTE: The information in this email and any attachments may be confidential and/or legally privileged. This message may be read, copied and used only by the intended recipient. If you are not the intended recipient, please destroy this message, delete any copies held on your system and notify the sender immediately. Toshiba Research Europe Limited, registered in England and Wales (2519556). Registered Office 208 Cambridge Science Park, Milton Road, Cambridge CB4 0GZ, England. Web: www.toshiba.eu/research/trl --------------------------------------------------------------------------------------- This email has been scanned for email related threats and delivered safely by Mimecast. For more information please visit http://www.mimecast.com ---------------------------------------------------------------------------------------
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