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
on 2006-04-03 13:05
on 2006-04-03 16:51
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!
on 2006-04-03 17:09
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