π/4 DQPSK

[nasimnian]

Hello,

Considering ETSI standards (ETSI EN 300 392-2 V3.4.1 (2010-08)) for TETRA signals, the modulation used is typically π/4 DQPSK. However, in your work, "Tetra Toolkit," the output in the constellation appears to be QPSK. Is there a block in GNU Radio to convert π/4 DQPSK to QPSK? Could you explain the .grc and how the Tetra Demod manage this conversion?

Thanks in advance!

[Tim---]

Hi !

Yes, the modulation is indeed π/4 DQPSK. If I remember correctly, that's the role of the "Differential Phasor" block before the constellation decoder in the tetra_demod block.
We are only interested in the phase difference between our constellation points. Since this is π/4 DQPSK, our difference will be one of [π/4, 3π/4, -π/4, -3π/4].

So basically, the Differential Phasor will convert our annoying 8 points constellation to a proper QPSK constellation. We can then use the constellation decoder to get our bits.

(Note that it is different to DQPSK, where we first decode a QPSK constellation, then do a difference on the output bits, as seen in generic_demod)

I hope my explanation is kind of clear !