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I've read that QPSK works theoretically in order to encode two bits depending on the phase of the signal. However, practically a reference for the carrier signal would be needed due to some ambiguity of the phase.

DQPSK is supposed to solve this by using the previous phase as a reference to the next one.

I'm not understanding how the ambiguity of phase occurs in QPSK. Wikipedia alludes to this:

there is an ambiguity of phase if the constellation is rotated by some effect in the communications channel through which the signal passes.

Can anyone explain what occurs in the communications channel that changes this phase?

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