OFDM demodulation problem / example video


#1

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


#2

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 E. 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
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charged!


#3

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