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

[Martin Braun]

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 address@hidden
> Sent: 30 April 2014 12:07
> To: David Halls; address@hidden <mailto: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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