Re: [Discuss-gnuradio] Demodulating APCO P25 signals

[Tom Rondeau]

stevie.glass wrote:
> Hi,
>
> I'm trying to demodulate P25 signals using GNURadio.
>
> The specification says I should be able to use a single common DQPSK 
> receiver to recover either the 4 level FM  (C4FM) or the CQPSK signals 
> - the difference being channel bandwidth (12.5KHz vs 6.25KHz) and that 
> the DQPSK is also amplitude as well as phase modulated. For my first 
> attempt I've used the GNURadio DQPSK block - here's what I've done:
>
>     * Capture live data using usrp_rx_cfile.py (the sampling rate is
>       256KS/S so over 2MB for a short sample) [#1]. A screen capture
>       of the FFT (peak hold) can be found at [#2].
>     * Extract a 12.5KHz channel the capture and write the complex
>       baseband signal to file [#3]. So the spectrum now looks like
>       [#4] and an oscope snapshot is shown in [#5]. The final display
>       seems to show a phase-shifted signal so I am pretty confident
>       this far is ok.
>     * Wrote a program [#6] to take the complex baseband sample,
>       resample it (so the sampling rate is an integer multiple of the
>       symbol rate and at least 4x the highest possible component
>       frequency of the signal), and demodulate it using the DQPSK
>       block. The output is the dibit stream corresponding to the signals.
>
> I am not convinced the output its right - I'm not finding the frame 
> sync sequence that should be present in the symbol stream. I'm 
> thinking that perhaps the standard block isn't suitable for my use. 
> Can anyone advise?
>
> The other thing that causes me to think its not right is that 
> according the spec P25 uses a form of DQPSK where each successive 
> symbol is shifted in phase from its predecessor by 45 degrees (pi/4 
> radians). Yet the spec also includes the following table which seems 
> to shows a 90 shift between symbols. I am confused about the spec and 
> unsure as to what the DQPSK block expects.
>
> Dibits
>         Symbol
>         CQPSK Phase Change (degrees)
>         C4FM Deviation (KHz)
> 01
>         +3
>         +135
>         +1.8
> 00
>         +1
>         +45
>         +0.6
> 10
>         -1
>         -45
>         -0.6
> 11
>         -3
>         -135
>         -1.8
>
>
> So, is this really a pi/4 encoding or pi/2? Is the DQPSK block 
> suitable for demodulating P25? Am I using sensible values for the M&M 
> clock recovery/channel filter? Are there any other obvious mistakes?
>
> On a related note, there's a demodulator for a very similar system 
> (RD-LAP) described at http://radiorausch.googlepages.com. The author 
> has much more signal processing chops than I (the described code 
> doesn't use the DQPSK demod block but a custom one). Sadly, there's no 
> code or contact email there. If anyone knows the author perhaps 
> they'll point them to me so I can beg or borrow that code.
>
> Steve

Your oscope signal of the baseband looks like good, clean QPSK, though, 
so you're on the right track. The table you've posted is definitely pi/4 
DQPSK. What this does is shift the constellation by 45 degrees every 
symbol, which means that what used to be a 0 degree phase shift (00) 
becomes a +45 degree phase shift. The idea here is to minimize the phase 
transition between successive symbols so the maximum is +135 degrees 
instead of +180. It's a minor change in the modulator that helps lower 
the bandwidth of the signal.

The big problem, though, is that the DQPSK block in GNU Radio doesn't do 
the pi/4 coding scheme. You'll have to work on it to adjust the rotation 
by +45 degrees each symbol. This should be fairly easy to do.

Tom