Spectrum Sensing

Hello everyone,

I was going through some gnuradio documentation (on comsec.org wiki)
which says “…16-bit I and 16-bit Q data (complex), resulting in 8M
complex samples/sec across the USB (32 Mbps max. data rate). This
provides a maximum effective total spectral bandwidth of about
6MHz…”.

By Nyquist criteria, should not this be 4 Mhz? Also, I found this note
at some other place which said that we can sense spectrum upto +/-
15MHz from the carrier frequency. Which one of the above is correct?
And what is bottleneck for maximum amount of spectrum we can sense at
one time, is it limited by the usb bus speed?

Also, the in the program “usrp_spectrum_sense.py [options] min_freq
max_freq”, is there a limit on (max_freq - min_freq)?

Thanks,
Shravan.
PS: My DSP background is zilch.

Shravan -

On Thu, Nov 02, 2006 at 03:53:35PM -0600, Shravan Rayanchu wrote:

I was going through some gnuradio documentation (on comsec.org wiki)
which says “…16-bit I and 16-bit Q data (complex), resulting in 8M
complex samples/sec across the USB (32 Mbps max. data rate). This
provides a maximum effective total spectral bandwidth of about
6MHz…”.

By Nyquist criteria, should not this be 4 Mhz?

When you have both I and Q, we have sign information on the
frequency, so the full range of -4 MHz to +4 MHz is 8 MHz.
Of that, 75% is valid data, the rest includes aliases and
shouldn’t be trusted in most circumstances.

Also, I found this note
at some other place which said that we can sense spectrum upto +/-
15MHz from the carrier frequency. Which one of the above is correct?

Both. The choice of which 6 MHz band can be adjusted
digitally.

And what is bottleneck for maximum amount of spectrum we can sense at
one time, is it limited by the usb bus speed?

Yes.

- Larry

Hi Nikita,

In order to answer your question we need more information on what you
want
to do exactly. gr-fosphor takes all unprocessed samples and does
everything
on the GPU. Calculating spectrum, graphical output etc. As far as I know
there isn’t a way to get the samples back from the GPU.

Cheers
Johannes

Hi Nikita,

so basically what you want is a couple of calculated FFTs per second.
These
are then sent to the web service and delivered to who ever connects to
it.
Let’s assume every USRP has its own flowgraph. Then The thing to do is,
receive samples, convert them to a vector with the size of your FFT, use
a
‘Keep one in N’ block to decimate the number of FFTs per second and feed
the output to a FFT block. Then pass your results around as you like.

Happy hacking
Johannes

Hi Nikita,

uhd_fft uses a WX sink. It does its own FFT calculations. They are not
meant to be used elsewhere. I recommend using a FFT block and then
format
the output according to your needs.

Happy hacking
Johannes

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