Re: [Discuss-gnuradio] Regular FM radio fine, POCSAG horrible

[jean-michel . friedt]

If it can be of any help, I use POCSAG to illustrate one of the SDR labs I
teach: the flowcharts are pp.4 and 5 of http://jmfriedt.free.fr/tp_sdr.pdf 
(don't
mind the text in French, only look at the pictures) and the associated videos
are at http://jmfriedt.free.fr/ (first two ones from the top).

JM

> > I am sure GR can do that, but I can't ;-)
> I can't help but propose you change that ;) No, seriously,
> cross-compiling GNU Radio for an ARM sounds more complicated than doing
> non-coherent binary FSK demod, but then again, that might just be me :D.
> 
> In fact, you're absolutely right: getting a solid signal quality before
> attempting decoding might be a good idea. However, most probably pagers
> don't need awesome SNR, so "somewhat noisy" might still be ok.
> 
> so how do you get the samples into GNU Radio?
> I guess you use the gr-osmosdr source? which sampling rate? Where in
> your base band are your carriers?
> What does your flow graph look like?
> 
> Generally: If you have a RF recording, [1] might just profit from one
> more entry, and we'd have something more tangible to talk about :)
> 
> I'll outline the steps I'd do to try to achieve better signal:
> 
>  1. Record a signal and test with that -- doing everything live makes
>     things complicated and hard to reproduce.
>  2. Use a xlating FIR filter to move a single 12.5kHz channel to 0Hz, so
>     that either symbol is +- 4.5kHz
>      1. this will require that you design a filter. Don't worry, that's
>         relatively easy:
>          1. run gr_filter_design
>          2. select low pass, enter your source's sampling rate, set the
>             end of the pass band to let's say 5kHz and the start of the
>             stop band to 7.5kHz (If I understand wikipedia correctly,
>             channel spacing is 12.5kHz, and symbol deviation is
>             +-4.5kHz, so from the center of the lower channel to the
>             lower bit of the upper channel it's 12.5kHz - 4.5kHz = 8kHz).
>          3. You'll notice that if you start with a high sampling rate,
>             your filter gets ridiculously long. If that's the case, you
>             might want to reduce the sampling rate of your signal
>             source, or add a stage of half- or quarter bandwidth FIR
>             decimation (with a decimation factor of 2 or 4, respectively)
>      2. set the decimation of that xlating FIR to something reasonable,
>         so that rate_in/decimation > 12.5kHz/2, but not >>.
>          1. this way, you'll get "just enough" rate at the output. 
>      3. set the center frequency to the middle of your two symbol
>         frequencies in the input spectrum
>  3. add visualization sinks here and there, and verify :)
>  4. add a real high-pass filter
>      1. Your single-channel spectrum looks something like [1] with 0 Hz
>         in the middle.  Since we've filtered away stuff above 5kHz, we'd
>         now concern ourselves with filtering away everything below 4kHz.
>      2. Same procedure as for the xlating fir, but use the reduced
>         sampling rate and a 4 kHz high-pass with a 2kHz stop band or
>         something. The closer the stop band is to pass band, the longer
>         your filter gets.
>      3. In principle, a 4-5 kHz real-tapped bandpass xlating fir would
>         have done the same, but doing this step by step reduces error
>         probability.
>  5. repeat "add visualizations" :)
>  6. You should now have a clean signal with only two peaks in your
>     spectrum at +-4.5kHz; does your external decoder deal well with that?
> 
> In principle, you're extremely close to having your own decoder by now.
> Non-coherent BFSK decoding would simply do the same as step 2, but with
> two filters, each centered on either symbol frequency, baudrate-wide
> passband, decimating to the baudrate, followed by a
> complex-to-magnituded-squared conversion each, then something like
> division of the 1-filter magsquared by the 0-filter magsquared, followed
> by a threshold decision (threshold=1). You'd then be getting a raw
> POCSAG bitstream :D
> 
> Best regards,
> Marcus
> 
> 
> [1] from http://edge.rit.edu/edge/P09141/public/FSK.pdf ,
> Watkins-Johnson Company "Tech-notes Vol. 7 No. 5 September/October 1980:
> FSK: Signals and Demodulation", p. 8 FSK spectrum
> <http://edge.rit.edu/edge/P09141/public/FSK.pdf>
> 
> On 06/02/2015 12:04 AM, Stephan van Beerschoten wrote:
> >
> > I am sure GR can do that, but I can't ;-)
> > Also, I don't have a good waterfall at all of the pocsag broadcast,
> > which is probably part of why I can't make it out with my ears either.
> > Yes, I think I have too much noise.
> > I hope it can be overcome with the right settings and filters.
> >
> > I'll try to capture a screenshot of what I see. It's nothing like the
> > screenshots in Wikipedia.
> >
> > On Jun 1, 2015 5:47 PM, "Marcus Müller" <address@hidden
> > <mailto:address@hidden>> wrote:
> >
> >     Hi!
> >     I personally think the soundbite from wikipedia is broken, since
> >     it's 11kHz sampling rate violates Nyquist ;)
> >     Well, I must admit that my preferred way of analyzing this
> >     wouldn't be the audible reproduction; if you can see it clearly on
> >     the waterfall, and "optically" have enough dB between the carriers
> >     and noise, then you'll be fine decoding it.
> >
> >     Now, I trust you're actually seeing excessive noise -- this might
> >     point to problems with your receiver (unsuitable antenna, too much
> >     noise in the amplifier, too little gain, intermodulation). The
> >     first step in limiting noise is always adding appropriate
> >     filtering. Can you add a FIR that selects your POCSAG channel out
> >     of your sampling bandwidth?
> >
> >     Best regards,
> >     Marcus
> >
> >     On 06/01/2015 11:28 PM, Stephan van Beerschoten wrote:
> >>
> >>     You're right in that I need more than GR. The audio of a pocsag
> >>     broadcast is very distinct. It's also clearly visible on a
> >>     waterfall.
> >>     The problem is that I have too much static in there. Way too much
> >>     noise. I can't get the gqrx module (where I tune and see the
> >>     waterfall) set right so the reception is fine.
> >>     I think the Wikipedia article had a soundbite of a pocsag
> >>     encoding. If you listen to it you'll notice it's very distinct.i
> >>     just have 90% noise and I can hear the broadcast in the very
> >>     background.
> >>
> >>     On Jun 1, 2015 5:25 PM, "Marcus Müller" <address@hidden
> >>     <mailto:address@hidden>> wrote:
> >>
> >>         Hi again,
> >>
> >>         Ok, I'm not familiar with the standard POCSAG, but if you got
> >>         a signal that you still need to decode with something else,
> >>         how do you know you don't get clear reception? What is your
> >>         measure for "good reception"?
> >>
> >>         As far as I read the English wikipedia, POCSAC uses a 4.5kHz
> >>         binary FSK, so can you see the two alternating frequency e.g.
> >>         in a waterfall plot of your RX signal?
> >>         Ideally, you'd directly be able to see the 512, 1200 or 2400
> >>         baud.
> >>
> >>         To explain a bit more:
> >>         GNU Radio is not a decoder for any specific standard; think
> >>         of it as the LEGO of SDR. You can build amazing things with
> >>         it, in fact, there's a lot of examples that come with GNU
> >>         Radio, and useful and complex standard implementations (FM
> >>         receiver, DTV transmission!), but if you need to have
> >>         something that's not there, you might need to a) use someone
> >>         else's Out-Of-Tree module or b) implement that functionality
> >>         yourself. So I must admit that I don't have the slightest
> >>         idea which settings you're referring to :) Maybe you're
> >>         interested in a quick&dirty introduction to GNU Radio [1].
> >>
> >>         In the case of POCSAG, I remember gr-pocsag being a thing
> >>         (search for pocsag on cgran.org <http://cgran.org>); I can't
> >>         remember the original author, and I presume it's pretty much
> >>         dead -- but I'd love to be proven wrong.
> >>         Also, pyboms has pocsag-mrt package, but that seems to rely
> >>         on GNU Radio 3.6.2, if the Readme is correct, so that's
> >>         pretty dead, too.
> >>
> >>
> >>         Best regards,
> >>         Marcus
> >>
> >>         [1] https://github.com/iZsh/pocsag-mrt
> >>         On 06/01/2015 10:18 PM, Stephan van Beerschoten wrote:
> >>>
> >>>         It is. I plan on running the output through a utility that
> >>>         can decode it. However, before that can happen I need to
> >>>         find out how I can get a clear reception of the broadcast.
> >>>
> >>>         On Jun 1, 2015 4:15 PM, "Marcus Müller"
> >>>         <address@hidden <mailto:address@hidden>>
> >>>         wrote:
> >>>
> >>>             I'm a bit confused, I though POCSAG was a text pager system?
> >>>
> >>>             On 06/01/2015 10:04 PM, Stephan van Beerschoten wrote:
> >>>             > Hi Guys,
> >>>             >
> >>>             > I compiled gnuradio for my ODROID ARM platform, and I
> >>>             can listen to
> >>>             > regular wideband radio just fine.  I am using a
> >>>             Generic RTL2832U with
> >>>             > Rafael Micro R820T tuner.
> >>>             >
> >>>             > The radio quality is fine, and even when using the
> >>>             rtl_fm tool
> >>>             > directly (off topic for this list), it works.
> >>>             >
> >>>             > However, when I switch channels to 155.520 to capture
> >>>             POCSAG
> >>>             > broadcasts I cannot get a clear reception. I can't
> >>>             find any decent
> >>>             > documentation on GR to tell me what each setting is,
> >>>             and I am not a
> >>>             > HAM radio operator so some of the basics evade me.
> >>>             >
> >>>             > I can't get decent POCSAG reception with the rtl_fm
> >>>             tool either, so
> >>>             > this is probably a setting thing somewhere.
> >>>             >
> >>>             > Why can't I get clear reception? Any pointers?
> >>>             >
> >>>             > Stephan
> >>>             >
> >>>             > _______________________________________________
> >>>             > Discuss-gnuradio mailing list
> >>>             > address@hidden <mailto:address@hidden>
> >>>             > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> >>>
> >>>
> >>>             _______________________________________________
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> >>>
> >>
> >
> 


-- 
JM Friedt, FEMTO-ST Time & Frequency/SENSeOR, 32 av. observatoire, 25044 
Besancon, France