Re: [Discuss-gnuradio] Theory - GSM traffic detection with a USRP board

[Martin Dvh]

Carlo E. Prelz wrote:
> Good day everyone. There must be a wide variety of persons reading
> this list. I hope there is some good soul there who happens to
> concretely understand the wherefores of radio waves, and has the
> ability to break the bread of this knowledge with those (like me) who
> are more prone to logics than to electronics.
> 
> I write code for a living, and life has been so generous as to allow
> me to deal with interesting stuff and apply them in the world of arts
> (have a look at my simple site if you are interested - www.fluido.as).
> 
> I decided to engage in a project whose target is to detect the
> existing cell-phone traffic in a specific surrounding and represent it
> in some visually or acoustically interesting way. This is a personal
> project, so I won't care too much if it gets to no concrete
> result. But I invested in an expensive USRP board with appropriate
> daughterboards (dbs-rx, range 500 MHz to 2.6GHz), so I would like to
> at least give it a concrete try. The boards are connected to
> log-periodic antennas provided by Ettus. 
> 
> Sadly, I did not notice in due time that the Gnuradio code base is
> developed in C++/python. Had I seen this I would probably have not
> started the project at all. I work in C/Ruby, a combination that is
> decidedly at odds with the chosen languages of Gnuradio. 
> 
> I have had a very hard time extricating the logic for configuring the
> USRP and setting the various parameters - frequency, gain, bandwidth,
> decimation. But I am now reasonably certain that this part works OK
> now in my code.
> 
> I am receiving a stream of data samples from the USRP. The
> daughterboards return I and Q channels. I feed these to fftw, and
> obtain the power spectrum of the required frequency band. So, for
> example, with decimation=16 I receive a power spectrum covering 4
> MHz. I confirm that this part is ok because I receive the same peaks
> in the same areas with my software that I receive with the Gnuradio
> tools. E.g: I have a peak at 832MHz (should be UHF 66 - here in
> Holland most of TV broadcast is on cable, so there is little activity
> on UHF) both with gnuradio-examples/python/usrp/usrp_fft.py and with
> my application.
You can look at these sites to find all digital and analog radio and TV 
frequenties and transmitters in The Netherlands:

Nozema:
http://www.nozema.nl/Content.asp?ID=6

Agentschap Telecom:
http://www.at-ez.nl/dav/

FM and TV info site:
http://www.radiowereld.nl/fmtv/
> 
> Now it is time to focus on the GSM bands. GSM900 should cover,
> according to what I found on the net, 880MHz to 915MHz for the uplink,
> in channels spaced 200kHz. What I found on this band was constant and
> relatively strong signals spaced 4MHz - some stronger than others, the
> strongest of them all at 896MHz and 900MHz. I could confirm that there
> were carriers on these frequencies by listening on my hand-scanner.
> But I then found out that those carriers would disappear when the USRP
> was turned off. It has to be said that, to the scanner, the whole band
> appears rather deserted (when the USRP is off). There is a carrier at
> 912MHz, but not much more. This last signal is indeed seen by the
> USRP.
> 
> So, this is my question: I expected to find a lot of carriers active
> at short bursts in the GSM900 band. This seems not to be the case,
> even if there are lots of active cellphones around. Am I looking at
> the wrong range? Or does the activity of cellphones manifest itself
> somewhat differently, so that no traditional carriers can be detected?
> And in this case, could you offer some pointers? I also had a look at
> the 1.8GHz range (1710-1785 MHz), with similar results. These, I
> believe, are the two bands that are used in this country for mobile
> phone traffic.
You are looking at the right bands.
Although the downlink is higher then you looked.
GSM900  uplink  880 -  895 Mhz downlink 925 - 960 Mhz
GSM1800 uplink 1770 - 1780 Mhz downlink 1805 - 1880 Mhz

For the GSM channels in use in Holland see:
http://nl.wikipedia.org/wiki/GSM_(communicatie)#Frequentieverdeling

Probably your handscanner sees the GSM channels only as noise since the 
channels are much wider then a scanner can cope with (probably 25 kHz max)
The other possibility is that all traffic is going on in the high band 
(1700/1800 Mhz)
Or something is wrong in your setup.
What antenna are you using?

The handscanner will see something if the usrp is turned on.
The scanner is then picking up the local oscillator of the DBSRX which is a 
constant carrier close to the frequency you are receiving.
The scanner is very close to the dbsrx and a constant carrier is very 
narrowband, so the scanner will pick this up.


> 
> I apologise in advance for my ignorance... I studied (long time ago)
> as an architect.
> 
> Carlo

I hope this helped,
Martin

>