  The study on carrier phase estimation (CPE) approaches, involving a one-tap
normalized least-mean-square (NLMS) algorithm, a block-wise average algorithm,
and a Viterbi-Viterbi algorithm has been carried out in the long-haul
high-capacity dispersion-unmanaged coherent optical systems. The close-form
expressions and analytical predictions for bit-error-rate behaviors in these
CPE methods have been analyzed by considering both the laser phase noise and
the equalization enhanced phase noise. It is found that the Viterbi-Viterbi
algorithm outperforms the one-tap NLMS and the block-wise average algorithms
for a small phase noise variance (or effective phase noise variance), while the
three CPE methods converge to a similar performance for a large phase noise
variance (or effective phase noise variance). In addition, the differences
between the three CPE approaches become smaller for higher-level modulation
formats.
