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

[Jens Elsner]

Achilleas,

thanks for taking the time. My goal is to implement COFDM in gnuradio, 
DAB is a nice start.

To your points:

> I got interested in this discussion and looked at the
> standard briefly and at your matlab code.
> I have a couple of initial points; I will spend more time sometime this 
> week on your code:
> 
> 1) where in the standard it says that this is pi/4 DQPSK ?
> I only saw that the modulation used is DQPSK.
Page 161, 14.7. "pi/4 shift DQPSK" (standard ETSI EN 300 401 V1.4.1)

> 2) You have done frequency correction manually: did you do this by 
> looking at the DC subcarrier (no power transmitted here) in the
> received signal prior to OFDM demod?
Yes. As can be seen in the code, I used frequency domain correlation
with the pilot symbol. Works perfectly and can also be automated.
There are a lot of papers on frequency offset estimation in OFDM.

> Frequency synchronization in OFDM is very crucial, because the OFDM 
> symbol rate is small! To see why, try to understand the effect of a 
> small frequency error at the output of the FFT demodulator.
Of course - I need to hit the subcarriers within a few Hz. But that
works, as can be seen from the demodulated results.

> 3) You have found the beginning of the frame manually. In your matlab 
> code you posted there is a variable  "start_resamp" that indicates this.
> How do you know that the begining of frame is not BETWEEN two samples?
> in other words there is no fine timing synchronization. I wonder what 
> the effect of a small timing error is at the output of the FFT demodulator.
Time sync is not a problem in OFDM - the guard interval takes care of
that. Nice property. Try playing along with the "start_resamp" value -
it will work for an offset up to about 504 (this is the length of the guard
interval).

Jens