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


From: Martin Braun
Subject: Re: [Discuss-gnuradio] OFDM Example, 'Noisy' final symbol
Date: Wed, 30 Apr 2014 16:56:04 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.4.0

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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