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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: Sat, 29 Apr 2006 09:33:04 +0200
User-agent: Mutt/1.3.28i

Hi Prateek,

yes - I sampled another radio station (this time at 1,5 GHz -> broader
subcarriers) and the "jitter" was significantly reduced. I implemented
deinterleavingi, channel coding, and descrambling - but so far no success in
demodulating. Yesterday I wrote a sender to check my receiver but I
didn't do any testing yet. 

This is pretty much where I am now. It's all Matlab but I'd be happy to
share the code with you next week as I'm not home over the weekend.

I took a look at the Dream code too - a lot of this code could be reused
(syncronisation, frequency estimation). 

If we do an OFDM implementation we should spend some time designing it.
The resulting code should be as flexible as possible.

My deadline for this project is the beginning of October.


On Sat, Apr 29, 2006 at 01:12:23AM +0530, Prateek Dayal wrote:
> hi jens,
> did u make any progress in ur OFDM problem. I have some free time now and
> would like to work on this too in the gnuradio framework.
> Have u captured any more data that you can please share with me ?
> Regards
> Prateek
> On 4/6/06, Jens Elsner <address@hidden> wrote:
> >
> >Achilleas,
> >
> >thanks for your comments. I am beginning to like this problem, as it has
> >extended my knowlegde of OFDM tremendously - thanks everybody.
> >
> >Nevertheless I'd like to see it solved. We have ruled out pretty much
> >every effect - I'm beginning to suspect that the transmitter actually
> >sends
> >shifted symbols. The remaindre of this week I'll be pretty busy, but I'll
> >come back to
> >the problem and get data from other radio stations to verify.
> >
> >> >Frequency offset results in ICI.
> >> >     This is "white noise" interference.
> >>
> >> Correct: v0 will result in a time offset of N*v0
> >> which will manifest itself as ICI.
> >>
> >> There is MORE to it.
> >> It ALSO introduces an unknown phase offest (constant in each
> >subchannel):
> >> Consider the i-th component of consequtive OFDM symbols.
> >> They are effectively rotated by multiples of 2*pi*(N+G)*v0.
> >> This is the phase accumulated over an OFDM symbol duration.
> >> Although the linear phase increase will be compensated by D-QPSK, there
> >> will be a constant phase rotation equal to 2*pi*(N+G)*v0 in all
> >> demodulated QPSK symbols.
> >Yes, agreed. This effect can be seen nicely if you demodulate a symbol
> >and change the fractional frequency offset in small steps. The
> >constellation pulsates (white interference) and rotates (phase rotation).
> >
> >> BTW, If you make sure that you estimate v0 with an accuracy of 1e-6
> >> then the time offset is on the order of 1e-3 (negligible) and the
> >> frequency offset on the order of 1 degree (negligible).
> >I think I hit the right frequency by +- 1 Hz, subcarrier spacing being
> >1KHz. Should be alright.
> >
> >> BTW, because of the way you have coded your frequency correction (in
> >> your Matlab code) even perfect estimation of v0 will still result in the
> >> above phenomenon.
> >> If you correct for v0 INDIVIDUALLY in each OFDM symbol (as you do in
> >> your code), then--assuming perfect estimation of v0--there will be two
> >> effects:
> >> 1) within each OFDM symbol there will be no phase variation since you
> >> have corrected it already
> >> 2) each consequtive sample in the i-th subchannel will be advanced by
> >> 2*pi*(N+G)*v0 radians. This is the phase accumulated over an OFDM symbol
> >> duration.
> >> This is why it is important to derotate your entire frame at once.
> >Ok. I changed that in my code.
> >
> >> Having said that, it seems that even if this is done, the problem you
> >> observe is still there!
> >
> >> >I am clueless.
> >>
> >> I attach a piece of Matlab code that simulates both the Tx and Rx.
> >> Everything mentioned above can be demonstrated in this controled
> >experiment.
> >Great - nice way to learn about OFDM.
> >
> >> I still do not understand why the real-data application does not work.
> >Neither do I - I'll get other data next week.
> >
> >Jens
> >
> >
> >_______________________________________________
> >Discuss-gnuradio mailing list
> >address@hidden
> >http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> >
> --
> Prateek Dayal
> http://flickr.com/photos/prateekdayal

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