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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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Simply propagating through space, or through a waveguide (like optical fiber), changes the phase of the carrier, according to

\$\Delta\phi = 2\pi d/\lambda\$

Of course if d is fixed, you can find a way to lock on to and recover the carrier.

But d is not strictly fixed. In an optical fiber, as the temperature varies, the fiber will expand and contract, changing the physical length of the path. At the same time the index of refraction of the glass making up the fiber will also change slightly, which might counteract or enhance the physical effects from expansion and contraction.

Over 1000's of km of fiber these path length change can add up to many many wavelengths even if there's just a few degrees (or even fractional degrees if you talk about underwater installations) of temperature variation.

Also, in long-distance fiber optic links, you have optical amplifiers spaced every so many km to maintain the signal amplitude. These optical amplifiers are also likely to have an effect on the carrier phase, and that effect is likely to have some thermal dependence.

I should also add that, due to the invention of the optical amplifiers I mentioned, coherent modulation is actually very rare in the real world. It was widely studied in the 1980's due to the factor of two improvement in channel capacity per watt for coherent modulation. But the development of the erbium-doped fiber amplifier (EDFA) made it more economical to simply increase power than to use the more complex coherent modulation and demodulation.

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