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Re: [Discuss-gnuradio] OFDM demodulation problem / example video

From: Jens Elsner
Subject: Re: [Discuss-gnuradio] OFDM demodulation problem / example video
Date: Tue, 4 Apr 2006 19:04:17 +0200
User-agent: Mutt/1.3.28i

I guess I was a little bit too quick with my answer...

In more detail:

> 2) You have done frequency correction manually: did you do this by 
> looking at the DC subcarrier (no power transmitted here) in the
> received signal prior to OFDM demod?
> Frequency synchronization in OFDM is very crucial, because the OFDM 
> symbol rate is small! To see why, try to understand the effect of a 
> small frequency error at the output of the FFT demodulator.
The fractional offset (0.35) was found manually by look at the
cross-correlation in the frequency domain and taking 

df = (left_of_max - right_of_max) / (left_of_max+right_of_max).

Should be correct, shouldn't it?

> 3) You have found the beginning of the frame manually. In your matlab 
> code you posted there is a variable  "start_resamp" that indicates this.
> How do you know that the begining of frame is not BETWEEN two samples?
> in other words there is no fine timing synchronization. I wonder what 
> the effect of a small timing error is at the output of the FFT demodulator.

I didn't quite understand what you are saying until I though about it:
This should be equivalent to a small modulation in the frequency domain?

I just simulated that by:

% simulate fractional offset
d_resamp = d_resamp .* exp(j*linspace(0,2*pi*0.1,length(d_resamp)).');

No effect - after all I did resampling before.


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