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

[Jens Elsner]

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