Detect if certain channels are being used

GNU Radio
1

Hi folks,

I want to accomplish the following with my usrp:
Receive from 1400 Mhz till 1500 Mhz and detect if there is activity
(=signal, no matter what) on 1430, 1430 and 1490 Mhz
So i have to split the spectrum in 3 parts (= channels) and do a peak
detection on each.
But how do i split the spectrum of my source?

kind regards
Guy

2

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Hi Guy,

100MHz bandwidth is not easy to process. In fact, you can only capture
it at once if you have one of the brand new USRP X3*0.
But let’s assume you have that :slight_smile: and let’s assume you have the
processing power to handle that amount of data.

You have to realize that “signal, no matter what” at f1,f2,f3 is not
really a specification. A signal might be a spread-spectrum signal
that is far below interference from electronic devices etc., as well
as an extremly short impulse, caused by some electric flash or
whatever, it can be very little Energy in a very narrow channel or
very much energy spread over 10s of Mhz’s…

So, what you do is usually bandpass your signalled filter, and have a
model for your desired signal and look out for things that match that.
Depending on your model, this can be easy or impossible…

Greetings,
Marcus

On 05.02.2014 20:39, [email protected] wrote:

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3

Let me be more clear of what is my goal here:
We receive the 13cm ham band (2300Mhz - 2400 Mhz)
This is converted down (-900Mhz) to 1400 - 1500 Mhz) (L band)
This way normal satelite receivers can rx amateur television stations in
13cm band.

We have 3 input frequencies in use: 2330 Mhz, 2360 Mhz and 2390Mhz
For each input there are 2 receivers: 1 analoge end 1 digital. (this
makes 6 receivers)
these receivers are powered on 24/7. this consumes a lot of power
±120Watt

Are station has te become “green”

So thats why i was thinking of making a “detection” system. so that when
a signal comes in (being it analoge tv or digital tv)
the pair of receivers can be switched on for a while… and the video
signal can be recieved and relayed…

The detection may run at slow speeds… 2 or 3 times per second. to
reduce processing power.

I’m a newbee with gnuradio… but eager to learn!

thanks
Guy

----- Oorspronkelijk bericht -----
Van: “Marcus Müller” [email protected]
Aan: [email protected]
Verzonden: Woensdag 5 februari 2014 21:58:54
Onderwerp: Re: [Discuss-gnuradio] detect if certain channels are being
used

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Hi Guy,

100MHz bandwidth is not easy to process. In fact, you can only capture
it at once if you have one of the brand new USRP X3*0.
But let’s assume you have that :slight_smile: and let’s assume you have the
processing power to handle that amount of data.

You have to realize that “signal, no matter what” at f1,f2,f3 is not
really a specification. A signal might be a spread-spectrum signal
that is far below interference from electronic devices etc., as well
as an extremly short impulse, caused by some electric flash or
whatever, it can be very little Energy in a very narrow channel or
very much energy spread over 10s of Mhz’s…

So, what you do is usually bandpass your signalled filter, and have a
model for your desired signal and look out for things that match that.
Depending on your model, this can be easy or impossible…

Greetings,
Marcus

On 05.02.2014 20:39, [email protected] wrote:

_______________________________________________ Discuss-gnuradio
mailing list [email protected]
Discuss-gnuradio Info Page

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4

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As Martin said, I think with these specifications your problem becomes
manageable :slight_smile:
Get the detection running for a single channel, meaning that you tune
your USRP to the channel (you might even do that on the actual 13cm
band, daughterboards like SBX and WBX will support these frequencies,
IIRC, and save on noise figure), using a sampling rate that is not
much higher than your channel’s bandwidth. This will help you
eliminate noise.
analog TV transmissions are usually rather broad; digital TV often is
OFDM, but this of course depends on the tx. You should be able to
sense the former by filtering to an appropriate bandwidth and
detecting specifics of your analog tv signal (this might include
looking for line-duration-periodicity, or several carriers following
the typical B/W-audio-Colorchannel modulation scheme of PAL or NTSC),
the latter should be a little more complex to do correctly
(effectively involving trying to detect OFDM Frames), but will lead to
a very small false alarm rate.

As soon as it works for a single frequency, “just”* periodically
retune your USRP.

Greetings,
Marcus

*Ankit, if you’re reading this: at that speed it’s easy :wink: our next
prototype will be awesome.

On 05.02.2014 22:29, Martin B. wrote:

I’m a newbee with gnuradio… but eager to learn!
to channelize the signals (e.g. using FFTs) and define a threshold.

MB

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5

On 05.02.2014 13:13, [email protected] wrote:

So thats why i was thinking of making a “detection” system. so that when a
signal comes in (being it analoge tv or digital tv)
the pair of receivers can be switched on for a while… and the video signal can
be recieved and relayed…

The detection may run at slow speeds… 2 or 3 times per second. to reduce
processing power.

I’m a newbee with gnuradio… but eager to learn!

As Marcus said, this can become difficult fast, and a lot of people are
getting PhDs on topics like this. Since your subject seems pretty
clearly defined, you might be able to implement it without much effort.

Some thoughts:

  • Say you don’t want an X300, but a USRP with a lower bandwidth (e.g.
    B210). Since your update rate is small, you can simply tune to several
    segments and process them one at a time.
  • If your signals are generally narrow-band, it might suffice to
    channelize the signals (e.g. using FFTs) and define a threshold. Since
    USRPs are generally not calibrated, you need to do that – e.g. by
    estimating a mean noise floor.

In any case, you’ll accidentally miss signals and have false alarms
(e.g. you detect a signal that’s not there), each with a certain
probability, that will never be zero. It depends on many things, such as
averaging time etc.

If you can live with this, you can definitely implement this with GNU
Radio.

MB