Problem with packet decoder for gmsk

Hello all,
I’m working on gmsk transmitter and receiver past 6 months , the
problem
that I’m facing is in the packet decoder.

I tried with wav file and signal source it worked good.

my block is of this type

Random_source=>Packet_encoder=>gmsk_modulator=>rational_resamlper_1=>uhd_sink

uhd_source=>rational_resampler_2=>low_pass_filter=>gmsk_demodulator=>packet_decoder=>scope_sink/file_sink

In rational_resampler_1 with interpolation=2, decimation=1
rational_resampler_2 with interpolation=1, decimation=2

I’m able to detect fft and constellation using scope_sink after
gmsk_demodulator,
but after packet_decoder I’m not able to see any signal in the
scope_sink
or there will be no data in the file if I write using file sink, it
will
be of 0 size, no data will be written.

I used packet decoder with default values ,no output in scope_sink or
file
_sink, threshold =-1, access code empty.
even I tried with changing the values of threshold from 50 to -50 , no
output, kept access code empty.

How to configure the packet decoder or any alternative idea.
I’m working on the transmitting data/file that has to be seen on
scope_sink
or to be written in the file using file sink.
In this case the input data is random source how to receive it, If I
want
to transmit .txt/.dat file how to construct block
should follow same step or something else.

Thanks
in advance

Ank


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On 03/26/2014 12:30 PM, Ank wrote:

uhd_source=>rational_resampler_2=>low_pass_filter=>gmsk_demodulator=>packet_decoder=>scope_sink/file_sink

There are a lot of things that can go wrong in this setup. Maybe the
sync is not locking fast enough, and destroying the preamble. It’s hard
for us to figure this out from a distance.

Try replacing the UHDs with a channel model, and go crazy with that. See
what happens.

M

Thanks Martin, i tried with replacing uhd with channel model, I had
good
signal at the receiver after gmsk_demodulator but I’m facing the same
problem i.e with packet decoder no out put after here.

Even I tried with removing uhd and directly connecting them even though
I
have the same problem like below shown

Random_source=>Packet_encoder=>gmsk_modulator=>rational_resamlper_1=>–!

!

!---------------------------------------------------------------------------------------------------------------------!

!---------=>rational_resampler_2=>low_pass_filter=>gmsk_demodulator=>packet_decoder=>scope_sink/file_sink

no output/signal after packet decoder,what would be the problem which
side I
have to concentrate to remove the error.

Thank you

Ank


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To expand on Martin’s comments, here are some of the issues that could
be
present:

  • Add a mult_const block or otherwise reduce the amplitude of the i/q
    signal going to the usrp_sink from +/- 1.0 to 0.5 or 0.7. This
    ensures you
    are not non-linear in the transmitter.
  • tx and rx_gain -> if you have these two devices a few feet apart
    (3-5
    dB for both tx and rx should work)
  • Frequency errors between the two USRP devices (or is this on a
    single
    device?) Ultimately, you could include some automatic, course
    frequency
    correction
  • In my experience, the GMSK demod will lock within 100-200 symbols.
    Since this is a continuous streaming flow graph driven by the random
    source, transient behaviour shouldn’t be an issue. You should have a
    continuous stream of samples to lock to. So I don’t think lock/sync
    time
    for the clock recovery is an issue.
  • Have you changed any parameters on the packet encoder/decoder
    blocks
    from the defaults? What is the bit/symbol and samps/symb set to?
    What’s
    the samps/symbol set to on the gmsk mod and demod?
  • Instead of a random source, test with a vector source and a known
    pattern.
  • To give a quick sanity check on your encoder/decoder config,
    connect
    the two without any channel. If that doesn’t work, you definitely
    have
    some config issues.

Also, once you’re through with this experiment, you might consider using
gr-mac, which provides similar functionality. Balint and I have been
hacking on it late tonight, you can find the latest here:

https://github.com/balint256/gr-mac

Also, a known working version that is a bit older and less bleeding
edge:

https://github.com/jmalsbury/gr-mac

http://conferences.sigcomm.org/sigcomm/2013/papers/srif/p31.pdf

-John

Lot of people around here at IIT Bombay also complain that there is no
data
at the output of packet decoder.

Try replacing the random source (binary output) with a signal source(set
it
to float). And try to plot the output of packet decoder(float output) in
a
gui sink. That should work, because there exists a working example among
GNU Radio example which uses this flow graph.

I can’t say what is going wrong in your flow graph. But I kind of feel
that
we need to dig into the implementation of the block to know what is
going
wrong.

Manu.

And here’s a flowgraph with some of the recommendations above
(attached).

-John M.

This worked good, but with usrp its still no output, I’m working on
that.

As i’m student, my project is on gsm/gsm-bts .

Can I implement this modulation scheme for for physical/L1 layer of
gsm.

Are there any modules available for gsm frame and burst, for L2,L3
layers.

Thank you all

Ank


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