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


From: David Halls
Subject: Re: [Discuss-gnuradio] OFDM Example, 'Noisy' final symbol
Date: Wed, 30 Apr 2014 11:14:16 +0000

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.

It is only the final symbol of a burst, not the final symbol of each packet.

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.

You have never noticed this happen?

DH
________________________________________
From: Martin Braun address@hidden
Sent: 30 April 2014 12:07
To: David Halls; address@hidden
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


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