APCO 25 trunking receiver

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madengr
Posts: 34
Joined: Fri Mar 01, 2013 6:51 am

APCO 25 trunking receiver

Post by madengr » Fri Mar 01, 2013 6:32 pm

Looks as if with the bladeRF, all the hardware is there to do it for the 800 MHz band. I'd love to see a project started to implement this entirely in GNU radio. It can already be done in windows by chaining together various bits of software. Two instances of SDR#, each with it's own RTL-SDR dongle. One sits on the control channel and pipes the audio into Unitrunker, which then controls the other SDR# to tune the voice channel, whose audio is piped to DSD for decoding.

However with several 10's of MHz of realtime bandwidth, it can all be done with a single front end. With GNU radio I wrote a Python script to record multiple NBFM channels to disk with squelched audio, and mix the audio for real-time monitoring. Have tried it with 26 channels so far with no over-runs (VOLK is the key to doing this) from the USRP2. Thus I can simultaneously monitor every repeater in the 2m band and all the simplex channels, with no scanning.

So my dream app is to simultaneously monitor multiple trunking sites, each with their multiple voice channels, and streaming all the audio to disk, tagged with time and radio ID.

Ambitious? Yes, but it would be the ultimate scanner. What stinks is that is is presently beyond my ability. I'm an RF/Microwave hardware guy, whose GNU Radio dabbling is limited to GRC and Python. The C++ makes a whooshing noise as it goes over my head.

bpadalino
Posts: 303
Joined: Mon Mar 04, 2013 4:53 pm

Re: APCO 25 trunking receiver

Post by bpadalino » Mon Mar 04, 2013 5:36 pm

Do you have an example frequency plan you are running with?

I am playing around with writing a channelized filterbank for the FPGA which would allow you to stream the N streams, already filtered and channelized. The problem is that they require the channels to be at specific center frequencies away - so getting it down to a few streams of maybe 250kHz wide channels instead of one massive channel might be the possibility, with the 'fine tuning' done at the host PC?

Do you know which part of your GRC flowgraph is taking up the most CPU? That would be useful to determine which blocks would be better suited for FPGA processing instead of host processing.

madengr
Posts: 34
Joined: Fri Mar 01, 2013 6:51 am

Re: APCO 25 trunking receivers

Post by madengr » Mon Mar 04, 2013 9:23 pm

Code is here:

http://dl.dropbox.com/u/49570443/nbfm_rec.tar

The channels are scattered about, thus I'm using frequency translating FIR filters instead of the channelizer. About the only band plan hams can agree upon is 5 kHz steps. I have not done any profiling to see what blocks use the most cycles, however the translating filters and demod are always running. The rest is just audio resampling, some of which is squelched. I test it by entering several duplicate channels of 162.55 MHz, then a dummy channel several MHz away to open up the front end bandwidth, requiring large decimation in the translating filters. Audio gain has to be reduced since it is coherent.

Over the weekend I modified it by adding in the DSD block for P25 decoding. However I could only handle about 4 simultaneous channels over a few MHz. So the DSD block requires much, much more cycles than the simple NBFM demodulation.

I could see that FPGA channelizer being very usefull if you can do standard sound card rates (i.e. 192 kHz) since that seems to be what many windows based decoder programs use for ham applications.

madengr
Posts: 34
Joined: Fri Mar 01, 2013 6:51 am

Re: APCO 25 trunking receiver

Post by madengr » Thu Mar 07, 2013 5:57 pm

Looks like a GNU Radio trunking receiver for Motorola analog systems has already been done:

https://www.cgran.org/wiki/Smartnet

cmh716
Posts: 15
Joined: Fri Jul 26, 2013 3:16 am

Re: APCO 25 trunking receiver

Post by cmh716 » Thu Dec 05, 2013 2:34 pm

Hi madengr,

I don't suppose you have a gnuradio 3.7 and bladerf version of your multi receiver? Looks like gnuradio 3.7 broke it.

madengr
Posts: 34
Joined: Fri Mar 01, 2013 6:51 am

Re: APCO 25 trunking receiver

Post by madengr » Sat Mar 22, 2014 4:28 pm

Here is an updated version for 3.7. Still UHD though, not gr-osmosdr.

https://dl.dropboxusercontent.com/u/495 ... _rec37.tar

firefly
Posts: 1
Joined: Wed Aug 13, 2014 6:48 pm

Re: APCO 25 trunking receiver

Post by firefly » Thu Aug 21, 2014 10:02 am

Madengr,
It sounds like you have done a lot of good work. I am interested in a similar project and I was hoping to look through your code.
Would you mind updating your download link? It seems to be broken.

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