Gmsk_demod()

GNU Radio
1

Hello , I want to demodulate Gsm control channels , which i
have
captured using rfx900. i checked there specturm using fft,py file , Can
anyone kindly guide me , wht steps should i take from here , to
demodulate
them and also is there any way to analyze them in time-domain for a
given
carrier frequency.

Regards,

Muhammad Awais

2

On Tue, Jan 27, 2009 at 07:20:42PM +0500, M. Awais Arshad wrote:

Hello , I want to demodulate Gsm control channels , which i have
captured using rfx900. i checked there specturm using fft,py file , Can
anyone kindly guide me , wht steps should i take from here , to demodulate
them and also is there any way to analyze them in time-domain for a given
carrier frequency.

Regards,

Muhammad Awais

You may want to take a look at the OpenBTS code.

$ svn co http://gnuradio.org/openbts/trunk openbts

Eric

3

Hi,

I want to demodulate GSM control channels too. Particularly, I am
interested
in AGCH and PCH channels (GSM Down Link). Is OpenBTS what we are looking
for?
I am trying to use gssm but I am having some problems with the
installation.
Is better to use OpenBTS?

Thanks in advance.
Sincerely,

José Carlos

2009/1/27 Eric B. [email protected]

4

Hello, everyone,

We’ve just got two USRPs in our lab and we want to start using them to
do some demodulation.

I’d like to use one of them to transmit the sum of two (uncoded)
signals, useful and interference, with possible relative scaling to
simulate different channels. At the receiver side, I wanna then
implement different demodulators, like treating interference as noise
or successive demodulation (demodulate interference, cancel and
demodulate user).

Is there any reference I could take a look on how to implement this in
USRP? Many thanks!

–
Best wishes,

Yufeng

5

On Tue, Jan 27, 2009 at 05:06:39PM +0100, José Carlos R. wrote:

Hi,

I want to demodulate GSM control channels too. Particularly, I am interested
in AGCH and PCH channels (GSM Down Link). Is OpenBTS what we are looking
for?
I am trying to use gssm but I am having some problems with the installation.
Is better to use OpenBTS?

You’ll have to decide for yourself. OpenBTS is designed as a
basestation, not as a service monitor, but it may be possible to use
parts of it. You may also want to take a look at the AirProbe
project, which is building a service monitor.
https://svn.berlin.ccc.de/projects/airprobe/wiki

Eric

6

On Tue, Jan 27, 2009 at 12:23:41PM -0500, yufeng wang wrote:

demodulate user).

Is there any reference I could take a look on how to implement this in
USRP? Many thanks!

–
Best wishes,
Yufeng

Please look at yesterday’s message from me about suggestions for
newbies, then take a look at gnuradio-examples/python/digital

Eric

7

On Tue, 2009-01-27 at 12:23 -0500, yufeng wang wrote:

I’d like to use one of them to transmit the sum of two (uncoded)
signals, useful and interference, with possible relative scaling to
simulate different channels. At the receiver side, I wanna then
implement different demodulators, like treating interference as noise
or successive demodulation (demodulate interference, cancel and
demodulate user).

One place to start is with the digital-bert example application. It is
designed to implement a one-way transmission (using BPSK) between two
USRPs, i.e., it does not implement any packet-oriented overhead and has
the minimal flowgraph necessary to accomplish this.

The TX application sends a bit error rate testing waveform, basically an
LFSR-based pseudorandom bit stream modulated using BPSK and a
root-raised-cosine transmit filter. This is sent continuously.

The receiver application performs RRC receive filtering, carrier
frequency and phase recovery, bit timing recovery, BPSK demodulation,
and measures the bit error rate. It displays, once per second, the
current frequency offset, bit timing error, estimated signal to noise
ratio, and estimated bit error rate. It’s easy to see the effects of
multi-path, shadowing, and antenna efficiency this way.

You can modify these applications to implement different
modulation/demodulation (implementing QPSK vs. BPSK is rather
straightforward), and add whatever artificial channel impairments you
wish.

Once you have a feel for how things work at the PHY level, there is the
digital packet radio example application (“tunnel.py”), which not only
implements a highly configurable PHY layer, but has a (really) basic
CSMA MAC and an interface to the Linux IP networking layer, allowing
routing IP between two hosts using USRPs. That may be of interest to
you, but I recommend starting with the simpler continuous transmission
BERT example to get a start on your learning curve.

Johnathan