Re: [Discuss-gnuradio] Demodulating APCO P25 signals

[Bob McGwier]

Tom Rondeau wrote:
> stevie.glass wrote:


> 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.

And decrease the linearity demands on the amplifier as a result.
> 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