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Re: [Discuss-gnuradio] usrp_spectrum_sense vs. usrp_fft

From: adib_sairi
Subject: Re: [Discuss-gnuradio] usrp_spectrum_sense vs. usrp_fft
Date: Thu, 3 Jun 2010 22:40:12 -0700 (PDT)

adib_sairi wrote:
> TMob wrote:
>> 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
> 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
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