Further more this means that the demodulated symbol (differences!) should always have phase 0, pi/4, 2pi/4, 3pi/4 (plus a *constant* offset of pi/8). But this offset is varying.
Am I right? And why should the transmitter introduce a varying offset? The problem is somewhere in the demodulation chain. Jens On Mon, Apr 03, 2006 at 11:23:49AM -0400, Robert McGwier wrote: > Jens: > > Then this means that the demodulated symbol should always be on points > abs(X) = abs(Y) = 1. HOWEVER, this must be built correctly into the > transmit symbols BEFORE they are transmitted for them to be received by > the simple division you have done below. I hope I get time to visit it > in greater detail today. > > Bob > > > > Jens Elsner wrote: > >Bob, thanks for the reply. > > > >This is exactly what I am doing: > > > >[...] > > dem = data ./ data_l; > >[...] > > > >A point-wise division by the last OFDM symbol. This also restores > >amplitude, but has the same effect on the phase. > > > >I do not control the receiver, my data is from a radio station. I am > >only trying to demodulate. I am synchronised in the > >frequency domain. Time domain synch is also appropriate. > > > >I guess I'll just have to get new data to verify this. It seems just too > >odd. > > > >Jens > > > >On Mon, Apr 03, 2006 at 10:48:32AM -0400, Robert McGwier wrote: > > > >>Jens: > >> > >>This pi/4-DQPSK. That means > >> > >>New-symbol * (complex conjugate (Old-symbol)) is pi/4 modulo pi/2. > >>Are you taking this into account on both the transmitter and the > >>receiver and it in all of the bins before your inverse fft provides the > >>time domain signal to transmit would be my best best. > >> > >>I will look at it more carefully today if I have time but this question > >>would be the first one I would ask the code to tell me. > >> > >>Bob > >> > >> > >> > >>Jens Elsner wrote: > >> > >>>Here is my little problem in animated frames. > >>> > >>>What you are seeing is already DQPSK. Displayed is the signal > >>>constellation for one OFDM frame, 75 symbols. The code was posted > >>>earlier. > >>> > >>>The problem: The (differential!) signal constellation has a constant > >>>phase offset for each OFDM symbol. Why? What to do about it? > >>> > >>>Jens > >>> > >>> > >>-- > >>AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, > >>NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman > >>Laziness is the number one inspiration for ingenuity. Guilty as charged! > >> > >> > > > > > > > -- > AMSAT VP Engineering. Member: ARRL, AMSAT-DL, TAPR, Packrats, > NJQRP/AMQRP, QRP ARCI, QCWA, FRC. ARRL SDR Wrk Grp Chairman > Laziness is the number one inspiration for ingenuity. Guilty as charged! > _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
