Hi,
I am writing a TETRA demodulator with GNU Radio Companion without any
OOT block.
My flowgraph works but as I don't have a signal processing background,
I'm not really sure that my choice of the blocks and their parameters
is the best for my purpose.
The TETRA modulation uses PI/4 DQPSK with a bitrate of 36 kHz (symbol
rate of 18 kHz), and a RRC filter.
For now, my decoder consists of :
* Freq Xlating FIR Filter : to center and low-pass the signal, and
lower the sample rate to have about 2 SPS.
* Fractional resampler : to have exactly 2 SPS
* Feed Forward AGC : to normalize the amplitude to 1
* FLL Band Edge : to fine-tune the frequency
* Polyphase Clock Sync : to sample the symbols at the right timing
* Differential Phasor : to get the difference between two symbols
* Constellation decoder : to discretize the symbols
* Unpack K bits : to convert the symbols to bits.
The flowgraph and screenshots can be found at
https://github.com/Tim---/tetra-toolkit/tree/master/grc .
Do you have any advice on how to improve this flowchart ?
Bonus questions :
* using the FLL band edge after the low-pass seems odd because, even if
it can recover frequency offset, some of the bandwidth will be lost.
Should I use the error ouput of the FLL to change the freq used by the
Freq XLating FIR FIlter ? If yes, is there an example of this somewhere
?
* at first I wanted to use the Constellation receiver, but from what I
understand the constellations in gnuradio work by first discretizing
the signal, then doing the diff.
This does not work with PI/4 DQPSK because you need to first diff the
complex samples, then discretize.
Is there any way I could cheat to create a PI/4 DQPSK constellation in
gnuradio ?
Thanks in advance,
Timothée.
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