Qpsk issues


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

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



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