Hi, I am currently trying to use the USRP to sense the 802.11 channels for activity. So far, I am using the usrp_spectrum_sense to do this. Each time I get the callback from gr.bin_statistics_f, I calculate the signal power in the returned data vector using the following formula: for bin in m.data: signalPower += 20*math.log10(math.sqrt(bin)/tb.fft_size)-20*math.log10(tb.fft_size)-10*math.log(tb.power/tb.fft_size/tb.fft_size) signalPower /= tb.fft_size According to previous posts, this should give me the signal power at the given center frequency in dBm. Unfortunately, it turned out that the values that I get using this code, vary very much, e.g. with the FFT size and the gain. When I leave gain and FFT size per default I get values from -28 through +5 (dBm) which definitely does not correspond to dBm. Is there any mistake in the formula? Is this really dBm that I get? Because the usrp_fft.py example shows more realistic values (around -50 - -60dBm) than the usrp_spectrum_sense.py, I was wondering if somebody could explain how usrp_fft gets to these values. All I can see in the source code there is that a USRP source is defined and connected to the scope. But where is the conversion into dBm done? Can this be applied to usrp_spectrum_sense somehow? Thanks, TMob -- View this message in context: http://www.nabble.com/usrp_spectrum_sense-vs.-usrp... Sent from the GnuRadio mailing list archive at Nabble.com.

on 2009-01-28 17:35

on 2010-06-04 00:01

TMob wrote: > 20*math.log10(math.sqrt(bin)/tb.fft_size)-20*math.log10(tb.fft_size)-10*math.log(tb.power/tb.fft_size/tb.fft_size) > Because the usrp_fft.py example shows more realistic values (around -50 - > -60dBm) than the usrp_spectrum_sense.py, I was wondering if somebody could > explain how usrp_fft gets to these values. All I can see in the source > code there is that a USRP source is defined and connected to the scope. > But where is the conversion into dBm done? Can this be applied to > usrp_spectrum_sense somehow? > > Thanks, > > TMob > did anyone have the answer to this question? is it because of the usrp_fft.py have the windowing block? regarding the windowing block, do any body know why the blackmanharis is chosen? Adib ----- Mohd Adib Sarijari Universiti Teknologi Malaysia www.fke.utm.my www.utm.my -- View this message in context: http://old.nabble.com/usrp_spectrum_sense-vs.-usrp... Sent from the GnuRadio mailing list archive at Nabble.com.

on 2010-06-04 07:43

adib_sairi wrote: >> power in the returned data vector using the following formula: >> and FFT size per default I get values from -28 through +5 (dBm) which >> Thanks, > ----- Mohd Adib Sarijari Universiti Teknologi Malaysia www.fke.utm.my www.utm.my -- View this message in context: http://old.nabble.com/usrp_spectrum_sense-vs.-usrp... Sent from the GnuRadio mailing list archive at Nabble.com.