Forum: GNU Radio Help on "easy" digital TX/RX

Hello!
As to introduct me, im very new to GnuRadio and the general topic of 
SDR.
However, i have a project in which i should implement a very basic way 
of
sending packets. This given packet format has a constant preamble and a
constant length of  appended databits. For detecting those out of a 
stream
etc. ive already written a working matlab script, now im stuck at the
GnuRadio side.
As a simple Presentation it would be enough to save a captured bitstream
from GnuRadio to harddrive which i later load in Matlab and send it 
through
my script.

So my question is, how can setup the most easily a one way tx/rx 
connection
between 2 SDR's, without any packet format or, which simply gives me the
received bits? My basic idea is like that:
TX:
<http://gnuradio.4.n7.nabble.com/file/n38924/tx.png>
RX:
<http://gnuradio.4.n7.nabble.com/file/n38924/rx.png>
I simply cant get it working at all. It would be too kind if somebody 
could
give me some kind of introduction how to handle that.

Regards



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Hanz,

You have a few options.  You can build a digital tx/rx in GRC, run an
example like tunnel.py, etc.  I've also created a set of examples with a
libraby called pre-cog, which you can see here:

http://www.youtube.com/watch?v=f8emQ-TvD90

An updated url to the pre-cog github pages:

https://github.com/jmalsbury/pre-cog/wiki

If you do use pre-cog, feel free to send any questions.  It's a
work-in-progress and the documentation may not explain everything...

-John

Thank you for the advising! That looks like something what i could use.
I just came up with another question:
I simply tried to simulate a DPBSK Path with noise, loaded from a file 
and
sinked to a file. Between Demod and Sink i placed a "Pack K Bits" with 
K=8.
<http://gnuradio.4.n7.nabble.com/file/n38935/08.png>
But my received data just contains 0's! It has the right length, but 
just
zeros... Any ideas anyone?



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I now have 2 new problems :(
1. I have a random source hooked to a Modulator and then to USRP. From 
the
same device i listen at the same Frequency. But since the antennas are 
just
a few cm's away, the constellation graph is completely diffuse and the
points are anywhere, there arent any fixed points noticeable. Do I have 
to
change something at the gain or add something?
<http://gnuradio.4.n7.nabble.com/file/n38994/const.png>
2. How can i differentiate if there is a signal receiving or just some
background noise. I mean if i put the Demodulator directly after the 
USRP
and stream it to file, then of course ill mostly get just random 
amplified
noise...



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On 01/14/2013 02:56 PM, Hanz wrote:
> noise...
>

Few tips, the modulator tags packed bytes, so the random source should
be [0, 256)

The modulator produces amplitude 1.0 samples. You will want to scale
these down as fullscale can clip. Either multiply constant block or
fullscale stream arg in UHD block:
http://files.ettus.com/uhd_docs/doxygen/html/struc...

Put the output of the modulator on a the FFT plotter. You should see the
response of the matched filter.

-josh