Sorry, my fault - I altered the FDE so it *does* give soft outputs, because the
constellation decoder makes hard decisions anyway - so I didn't see the need
for it to be done twice.
I forgot i'd made changes :s But now maybe I understand why people haven't seen
this issue - because they have never looked at the soft outputs of the FDE!!
I am not quite clear how the pulse shaping works, it seems to extend each
*packet* by rolloff_len/2 samples, it works out the flanks by
for (int i = 1; i < d_rolloff_len; i++) {
d_up_flank[i-1] = 0.5 * (1 + cos(M_PI * i/rolloff_len - M_PI));
d_down_flank[i-1] = 0.5 * (1 + cos(M_PI * (rolloff_len-i)/rolloff_len
- M_PI));
}
Which makes sense...
then applies using
if (d_rolloff_len) {
for (int i = 0; i < d_rolloff_len-1; i++) {
out[i] = out[i] * d_up_flank[i] + d_delay_line[i];
d_delay_line[i] = in[i] * d_down_flank[i];
}
}
Is it extending the symbol and then shaping? Not sure I understand why each
*packet* is extended by rolloff_len/2, not each symbol.
Is pulse shaping applied in the USRP, and can this be controlled from USRP_sink?
Phew!
DH
________________________________________
From: Martin Braun [martin.br...@ettus.com]
Sent: 30 April 2014 15:56
To: David Halls; discuss-gnuradio@gnu.org
Subject: Re: OFDM Example, 'Noisy' final symbol
On 30.04.2014 16:03, David Halls wrote:
> It's just the built in basic FDE. Its not awesome, but works well enough
> at high SNR.
Hm, the FDE doesn't output soft info, so I'm stumped why you'd see
anything other than clean constellation points in the first place.
> 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?!
From your other email, it looks like it does... but you can't set the
rolloff to the CP length, you need some space for multipath.
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
---------------------------------------------------------------------------------------
This email has been scanned for email related threats and delivered safely by
Mimecast.
For more information please visit http://www.mimecast.com
---------------------------------------------------------------------------------------
_______________________________________________
Discuss-gnuradio mailing list
Discuss-gnuradio@gnu.org
https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
