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[Discuss-gnuradio] qpsk issues

From: Jason Uher
Subject: [Discuss-gnuradio] qpsk issues
Date: Tue, 7 Apr 2009 14:24:05 -0500


 I am transmitting a sequence of MIMO frames using gnuradio and using
attempting to correct the received signal for phase/CFO.

One problem that I am having trouble dealing with is an apparently
random 180degree phase shift in the received signal caused by
something (i.e., not my modulation scheme).  I notice this by visual
inspection because each frame starts with all antennas sending a saw
tooth ([(1+j), (0,0), (-1-j),(0,0,)]) for easy synchronization, and as
you can see in the attached diagrams, there is a clear shift during
that sequence.  Additionally, I have attached the output of the
modified costas loop (from a different frame) with the bit times
marked to clearly demonstrate the phenomena, you can see the change
around the 6th or 7th marker.

In both plots the X axis is the sample number and the Y axis is the amplitude.

My question is: what might be causing this shift and how could I solve it?



Attachment: pre_costas_phase_change.png
Description: PNG image

Attachment: post_costas_phase_change.png
Description: PNG image

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