Ofdm modulation/demodulation problem

Hi
I m using USRP2 and USRPN210 DB RFX900.

Block diagram
MOD:random source–>ofdm–>usrp sink
DeMOD:usrp source–>ofdm demod–>>sink scope,fft

Problem is that:
Modulation is going fine,even at the receiver end the spectrum is just
like
it should be (block for OFDM )
but there is nothing at the output of OFDM demod block Sink Scope is
showing
nothing. http://gnuradio.4.n7.nabble.com/file/n38053/OFDM_DeMOD.png
http://gnuradio.4.n7.nabble.com/file/n38053/OFDM_Mod.png
http://gnuradio.4.n7.nabble.com/file/n38053/Spectrum.png
I have attached the mod,demod,received spectrum snaps.
Help is highly appreciated…:slight_smile:

Regards:
Bilal Abbasi
COMSATS IIT Islamabad
Pakistan.


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On 10/18/2012 04:47 AM, Bilal wrote:

but there is nothing at the output of OFDM demod block Sink Scope is showing
nothing. http://gnuradio.4.n7.nabble.com/file/n38053/OFDM_DeMOD.png
http://gnuradio.4.n7.nabble.com/file/n38053/OFDM_Mod.png
http://gnuradio.4.n7.nabble.com/file/n38053/Spectrum.png
I have attached the mod,demod,received spectrum snaps.
Help is highly appreciated…:slight_smile:

1 few things:

  1. Remove the throttles, the sample rate is already throttled by the
    usrp. There will be a fight.

  2. I see an exception printed in the GRC window for one of the
    screenshots. That might be important to share?

  3. The random source should probably be 0 to 1 << 32. I believe thats a
    32 bit integer stream. Things might not work as expected when the input
    bits are mostly 0.

-josh

1)removed still no output…:frowning:

2)thread[thread-per-block[0]: <gr_block gr uhd usrp source (12)>]:
caught
unrecognized exception

i think this is not an error as it appears when i close the window…

  1. i have connected scope with random source to see transmitted integers
    still no output at receiver.
    (tried even simple sin wave).

Regards
Bilal Abbasi


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On 10/19/2012 05:09 AM, Bilal wrote:

1)removed still no output…:frowning:

2)thread[thread-per-block[0]: <gr_block gr uhd usrp source (12)>]: caught
unrecognized exception

i think this is not an error as it appears when i close the window…

That might be ok. I think its a boost issue on ubuntu 10.04. So I am
guessing this is ubuntu 10.04, but i could be wrong :slight_smile:

  1. i have connected scope with random source to see transmitted integers
    still no output at receiver.
    (tried even simple sin wave).

The scope sink is working for you, right? No some weird OpenGL freezing
issue. If you make a simulation only flow graph like noise_source
->throttle->scope sink, you see the noise?

I suppose the first thing to do before transmitting OFDM would be to
verify the RX chain with some simple test tones. Is the antenna
selection correct, power levels, etc. Does the daughterboard report
locked when tuned?

-josh

PS: some of the uhd examples liek ascii_art_dft automatically check for
dboard lock; hope that helps

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