Re: [Discuss-gnuradio] Help on "easy" digital TX/RX

[Josh Blum]

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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