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[Discuss-gnuradio] low receive signal power

From: Rahul Dhar
Subject: [Discuss-gnuradio] low receive signal power
Date: Sat, 23 Apr 2005 03:53:20 -0400
User-agent: Mutt/1.4i

I'm building off of the fsk_tx/rx examples and implementing a
CSMA/CA-like protocol.  I'm trying to implement carrier sensing, and I'm
using gr_simple_squelch, as Eric suggest (off-list conversation).
However, I'm seeing a very low receive signal (about 2-3 dB).  The
fsk_rx shows receive signal at 20dB when there's nothing being
transmitted (20dB of just noise?) and around 30dB when I fire up fsk_rx.
The fsk examples do some extra stuff for displaying, like additional
filtering and FFTs, but would that affect the Rx power?  Presumably, I
should be able to take the gr_complex items given by the USRP, compute
power, and pass them along to the demodulator.

Carrier sense just calculates the power using gr_single_pole_filter
(d_iir) with alpha of 0.01 (tried 0.1, 0.001, and a few other values).
I set the threshold to 20dB, but maybe that's too high.

for (int i = 0; i < noutput_items; i++) {
   mag_sqrd = in[i].real()*in[i].real() + in[i].imag()*in[i].imag();
   f = d_iir.filter(mag_sqrd);
   if (f > threshold) {
      // handle carrier busy
   else {
      // handle carrier idle

Does this make sense at all?


Rahul Dhar
Actually, my goal is to have a sandwich named after me.

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