Re: [Discuss-gnuradio] FFT of FFTs

[Eric Blossom]

On Sat, Jun 18, 2005 at 08:18:38PM -0400, James Cooley wrote:
> Hi all,
> 
> I'm trying to take an FFT of an FFT.... Basically, I want to tune to a 
> signal, and for a given RF frequency, try to spot periodic usage of that 
> frequency. Is this possible? What I have now is hopelessly slow, so I'm 
> not really sure if I've got it right.

Hi Jamie,

Here are a couple of suggestions.  If there are relatively few
frequencies that you want to observe for periodic occupancy, I would
suggest extracting the frequency bands of interest using a
gr.freq_xlating_fir_filter for each one.  If there are lots of them,
and they are evenly spaced, then the dft filterbank is what you want
to split them out (blksimpl/filterbank.py).

Once you've got your individual streams of signals, for each one I
would compute an estimate of whether it is occupied.  You could do
this by computing the magnitude of the stream (gr.complex_to_mag) and
then low pass filtering that with a gr.iir_filter, possibly followed
by a limiter (which would need to be written).  At this point, for
each of your input streams, you have an output stream that is
effectively a stream of 1's and 0's, where 1 means "is occupied".
Then run each of those streams into it's own FFT.   Point this whole
pipeline at some kind of TDMA input (GSM basestation?) and you
ought to see the slots (assuming the basestation isn't driving all
the slots all the time).

Eric