From kimfmx at gmail.com  Sat Feb 15 08:33:49 2020
From: kimfmx at gmail.com (Kim Mortensen)
Date: Sat, 15 Feb 2020 09:33:49 +0100
Subject: DQPSK demodulation.
Message-ID: <CAB7W5DctefN662tshOyoeyszv_ZemvNLstfaRHE=F8NKU2ZMzQ@mail.gmail.com>

Hi,

Thanks for the add, I have been hinted this place, and im looking forward
to digging deeper into the realm of TETRA / DQPSK.

What im trying to do is write a C# program for demodulation of TETRA. and
have come so far as to be able to view the IQ vector run around on the
unity circle, but it appears a bit distorted. I think its due to a crude
low pass filter i used on the I/Q signal causing this. 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? and
do you have the open source code i can inspect, regardless if programming
language.

I know there is tons of programs out there that can decode and even produce
audio from TETRA DQPSK signals, and I have one installed, but i want to
have an edge to this, i need to understand and learn first hand. It may
take time, but thats how it is.

I have what is required regarding programming skills and radio knowledge -
I am radio Amateur, and 20 years ago i did have Digital communication
classes at Engineering college - But thats 20 years ago, and the equations
in the books are kind of far away in my head.. but the theory sticks.

Please help me get up to speed in decoding DQPSK, im not expecting to do
this quickly, but will be in steps when i have time, and will lead to
questions. But at some point i could be on your frontier digging into TETRA
as a whole.

Best Regards
Kim Mortensen
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From laforge at osmocom.org  Mon Feb 17 07:58:46 2020
From: laforge at osmocom.org (Harald Welte)
Date: Mon, 17 Feb 2020 08:58:46 +0100
Subject: DQPSK demodulation.
In-Reply-To: <CAB7W5DctefN662tshOyoeyszv_ZemvNLstfaRHE=F8NKU2ZMzQ@mail.gmail.com>
References: <CAB7W5DctefN662tshOyoeyszv_ZemvNLstfaRHE=F8NKU2ZMzQ@mail.gmail.com>
Message-ID: <20200217075846.GD1213860@nataraja>

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
implementation.

> 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.

Regards,
	Harald

-- 
- Harald Welte <laforge at osmocom.org>            http://laforge.gnumonks.org/
============================================================================
"Privacy in residential applications is a desirable marketing option."
                                                  (ETSI EN 300 175-7 Ch. A6)


From kimfmx at gmail.com  Mon Feb 17 20:46:56 2020
From: kimfmx at gmail.com (Kim Mortensen)
Date: Mon, 17 Feb 2020 21:46:56 +0100
Subject: DQPSK demodulation.
In-Reply-To: <CAB7W5DctefN662tshOyoeyszv_ZemvNLstfaRHE=F8NKU2ZMzQ@mail.gmail.com>
References: <CAB7W5DctefN662tshOyoeyszv_ZemvNLstfaRHE=F8NKU2ZMzQ@mail.gmail.com>
Message-ID: <CAB7W5Ddr9CymxMVCn4a5M+GwJeV1izpk-3Oew6obYhAta7-6cQ@mail.gmail.com>

Hi,

Would someone happened to have a sample file containing I/Q data from a
DQPSK signal. The content is not important, as long as its DQPSK -
Preferably in wav format.

Best Regards
Kim Mortensen

Den l?r. 15. feb. 2020 kl. 09.33 skrev Kim Mortensen <kimfmx at gmail.com>:

> Hi,
>
> Thanks for the add, I have been hinted this place, and im looking forward
> to digging deeper into the realm of TETRA / DQPSK.
>
> What im trying to do is write a C# program for demodulation of TETRA. and
> have come so far as to be able to view the IQ vector run around on the
> unity circle, but it appears a bit distorted. I think its due to a crude
> low pass filter i used on the I/Q signal causing this. 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? and
> do you have the open source code i can inspect, regardless if programming
> language.
>
> I know there is tons of programs out there that can decode and even
> produce audio from TETRA DQPSK signals, and I have one installed, but i
> want to have an edge to this, i need to understand and learn first hand. It
> may take time, but thats how it is.
>
> I have what is required regarding programming skills and radio knowledge -
> I am radio Amateur, and 20 years ago i did have Digital communication
> classes at Engineering college - But thats 20 years ago, and the equations
> in the books are kind of far away in my head.. but the theory sticks.
>
> Please help me get up to speed in decoding DQPSK, im not expecting to do
> this quickly, but will be in steps when i have time, and will lead to
> questions. But at some point i could be on your frontier digging into TETRA
> as a whole.
>
> Best Regards
> Kim Mortensen
>
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