|Subject:||Re: [Discuss-gnuradio] TETRA demodulator advices|
|Date:||Tue, 1 Dec 2015 10:33:14 -0500|
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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