DQPSK demodulation.

Harald Welte laforge at osmocom.org
Mon Feb 17 07:58:46 UTC 2020

Hi Kim,

On Sat, Feb 15, 2020 at 09:33:49AM +0100, Kim Mortensen wrote:
> From reading books and sites i find that I should probably used a
> raised cosine filter to keep the signal form beeing distorted (ISI).
> am i correct in my assumption? 

I'm not the expert on this subject, but RRC filters are used in a lot
of receivers for (at least) GMSK or DQPSK, including the osmo-tetra

> and do you have the open source code i can inspect, regardless if programming language.

osmoocm-tetra includes a Pi/4 DQPSK demodulator, you can find it in
osmo-tetra.git at src/demod/cqpsk.py.  It uses the root-raised-cosine
filter provided by gnuradio.

In general, we would of course appreciate if you could contribute to
the osmo-tetra implementation, rather than creating another implementation
of something that already works in osmo-tetra. But it's of course your
time and your call how you want to spend it :)

However, if you have your C# demodulator in a working stage, you could
integrate it with the phy/mac implementation of osmo-tetra by generating
the same soft-bit input file format that 'tetra-rx' consumes.


- Harald Welte <laforge at osmocom.org>            http://laforge.gnumonks.org/
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