Help on "easy" digital TX/RX

[Hanz]

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:

RX:

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

[John Malsbury]

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

On Thu, Jan 10, 2013 at 12:33 PM, Hanz <[hidden email]> wrote:

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]

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.

But my received data just contains 0's! It has the right length, but just zeros... Any ideas anyone?

[Hanz]

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?

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...

[Josh Blum-3]

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

> 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...
>

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/structuhd_1_1stream__args__t.html

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

-josh

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