Abstract
We investigate the effect of nonlinear self-phase modulation (SPM) on
clock recovery in long-haul coherent optical communication systems with low
to moderate baud rates. We consider dual-polarization, multilevel quadrature
amplitude modulation, and evaluate conventional timing estimators as well
as a modified Cramér–Rao bound. As the latter is hard to compute
in the presence of nonlinear channel impairments, we derive an approximate
closed-form expression. Contrary to linear channel impairments, the nonlinear
impairments can degrade the performance for higher input powers. In addition,
the bounds exhibit oscillatory behavior due to the oscillations in the signal
spectrum in the presence of SPM.
© 2012 IEEE
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