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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 12:39:15 +0200 |
User-agent: |
Mutt/1.3.28i |
Further more this means that the demodulated symbol (differences!) should
always have phase 0, pi/4, 2pi/4, 3pi/4 (plus a *constant* offset of pi/8).
But this offset is varying.
Am I right?
And why should the transmitter introduce a varying offset? The problem is
somewhere in the demodulation chain.
Jens
On Mon, Apr 03, 2006 at 11:23:49AM -0400, Robert McGwier wrote:
> Jens:
>
> Then this means that the demodulated symbol should always be on points
> abs(X) = abs(Y) = 1. HOWEVER, this must be built correctly into the
> transmit symbols BEFORE they are transmitted for them to be received by
> the simple division you have done below. I hope I get time to visit it
> in greater detail today.
>
> Bob
>
>
>
> Jens Elsner wrote:
> >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
> >odd.
> >
> >Jens
> >
> >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,
> >>NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
> >>Laziness is the number one inspiration for ingenuity. Guilty as charged!
> >>
> >>
> >
> >
>
>
> --
> AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats,
> NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman
> Laziness is the number one inspiration for ingenuity. Guilty as charged!
>
- [Discuss-gnuradio] OFDM demodulation problem / example video, Jens Elsner, 2006/04/03
- Re: [Discuss-gnuradio] OFDM demodulation problem / example video, Achilleas Anastasopoulos, 2006/04/04
- Re: [Discuss-gnuradio] OFDM demodulation problem / example video, Jens Elsner, 2006/04/04
- Re: [Discuss-gnuradio] OFDM demodulation problem / example video, Robert McGwier, 2006/04/04
- Re: [Discuss-gnuradio] OFDM demodulation problem / example video, Prateek Dayal, 2006/04/05
- Re: [Discuss-gnuradio] OFDM demodulation problem / example video, Robert McGwier, 2006/04/05
- Re: [Discuss-gnuradio] OFDM demodulation problem / example video, Jens Elsner, 2006/04/05
- Re: [Discuss-gnuradio] OFDM demodulation problem / example video, Robert McGwier, 2006/04/05
- Re: [Discuss-gnuradio] OFDM demodulation problem / example video, Jens Elsner, 2006/04/06
- Re: [Discuss-gnuradio] OFDM demodulation problem / example video, Jens Elsner, 2006/04/05
- Re: [Discuss-gnuradio] OFDM demodulation problem / example video, Jens Elsner, 2006/04/04