Re: [Discuss-gnuradio] detect if certain channels are being used

[Marcus Müller]

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As Martin said, I think with these specifications your problem becomes
manageable :)
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* ;) our next
prototype will be awesome.


On 05.02.2014 22:29, Martin Braun wrote:
> On 05.02.2014 13:13, address@hidden wrote:
>> 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!
> 
> 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
> 
> 
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