Re: [Discuss-gnuradio] OFDM Example, 'Noisy' final symbol

[David Halls]

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 address@hidden
Sent: 30 April 2014 15:56
To: David Halls; address@hidden
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

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