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Re: [Discuss-gnuradio] BPSK to QPSK

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] BPSK to QPSK
Date: Wed, 3 Mar 2010 11:30:31 -0500

On Wed, Mar 3, 2010 at 10:51 AM, Jason Uher <address@hidden> wrote:
>> Hum... I'm glad to hear that, although I thought that the pi- phase
>> ambiguity of Costas- loops made them unsuitable for QPSK-
>> constellations... But I guess there is something I'm missing in the code
>> then?
> You aren't missing anything, you just need to account for the possible
> rotation.
> The easy way to do this is with differential decoding, because the
> instantaneous phase is irrelevant.
> The harder-but-still-easy was is to transmit some know sequence at the
> beginning to determine the phase offset and adjust your decoding
> accordingly.
> Jason

Yes, what Jason said.

I think of synchronization as making sure the constellation is
aligned. The actual bit representations are the next step. Since we
put a 64-bit access code on the front of every packet, it should be
easy to add logic in the sink to determine what the rotation is, but
we haven't done anything about that.


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