Digital PLL based frequency offset compensation and carrier phase estimation for 16-QAM coherent optical communication systems

We modify a 2
^nd
-order digital phase-locked loop (PLL) with additional frequency-offset compensation and pilot-symbol aided cycle slip recovery modules, and show that the modified 2
^nd
-order PLL can successfully recover the carrier phase for a 16-QAM coherent detection system with a frequency offset up to 10% of the symbol rate.