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Re: [Discuss-gnuradio] Demodulating APCO P25 signals

From: Bob McGwier
Subject: Re: [Discuss-gnuradio] Demodulating APCO P25 signals
Date: Tue, 19 Feb 2008 14:40:50 -0500
User-agent: Thunderbird (Windows/20071031)

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.


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