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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: Mon, 3 Apr 2006 17:09:17 +0200
User-agent: Mutt/1.3.28i

Bob, thanks for the reply.

This is exactly what I am doing:

    dem = data ./ data_l;

A point-wise division by the last OFDM symbol. This also restores
amplitude, but has the same effect on the phase.

I do not control the receiver, my data is from a radio station. I am
only trying to demodulate. I am synchronised in the 
frequency domain. Time domain synch is also appropriate.

I guess I'll just have to get new data to verify this. It seems just too


On Mon, Apr 03, 2006 at 10:48:32AM -0400, Robert McGwier wrote:
> Jens:
> This  pi/4-DQPSK.   That means
> New-symbol  *  (complex conjugate (Old-symbol)) is   pi/4 modulo pi/2.  
> Are you taking this into account on both the transmitter and the 
> receiver and it in all of the bins before your inverse fft provides the 
> time domain signal to transmit would be my best best.
> I will look at it more carefully today if I have time but this question 
> would be the first one I would ask the code to tell me.
> Bob
> Jens Elsner wrote:
> >Here is my little problem in animated frames.
> >
> >What you are seeing is already DQPSK. Displayed is the signal
> >constellation for one OFDM frame, 75 symbols. The code was posted earlier.
> >
> >The problem: The (differential!) signal constellation has a constant phase 
> >offset for each OFDM symbol. Why? What to do about it?
> >
> >Jens
> >  
> -- 
> AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
> Laziness is the number one inspiration for ingenuity.  Guilty as charged!

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