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 _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
