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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: Wed, 5 Apr 2006 18:11:25 +0200
User-agent: Mutt/1.3.28i

Thanks for the reply! I'm really struggeling here... 

> Yes, I think this is a critical step. small letters are time domain and
> capital letters are frequency domain.
> x(n) -----------> X(k)
>          FFT
> x(n-n') ---------> e^(j*2*pi*n'*k/N) * X(k)
>             FFT
> for one OFDM symbol,  0 <= k <= N-1
> Therefore as you can see the phase shift will increase with k. For k=0,
> there will be no phase shift and for k=N-1, there will be a phase shift of
> almost 2*pi*n'. Therefore for a QPSK, when you map symbols to bits, there
> will be more errors for higher subcarrier (index).  You have to start
> sampling exactly after the cyclic prefix ends. In general if your timing
> error is towards cyclic prefix, you effectively cyclic shift your data. In
> case your shift is away from the end of the cyclic prefix, you take some
> samples from next symbol and introduce ISI. You need to see how this will
> effect you in DQPSK case. Please look at this link for relevant papers

The phase shift doesn't matter in differential demodulated systems.

To sum up: 

Frequency offset results in ICI. 
        This is "white noise" interference.

Time offset results in 

        a) ISI, if offset results in taking values from other symbols.
        This is also "white" interference.

        b) phase shift, if offset results in taking values from the
        cyclic prefix. This results in a "circle" structure, as the
        phase shift is higher for subcarriers of higher frequency.

The time offset effect b) is removed by differential demodulation, since
the phase shift will be the same.

The effect I am seeing is a constant phase shift for all subcarriers, 
different for each OFDM symbol. Nothing that could be explained with a
frequency offset or a time offset. 

I am clueless.

> In general if your relative frequency offset is .01 or less, I think you
> will not be affected much. But again I am not too sure for DQPSK. Again QAM
> is more sensitive to these errors than QPSK.
A small frequency offset results just in a SNR degradation - my
estimation is pretty accurate.


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