Multistage carrier phase estimation algorithms for phase noise mitigation in 64-quadrature amplitude modulation optical systems

Two novel low-complexity multi-stage digital feedforward carrier phase estimation (CPE) algorithms for 64-ary quadrature amplitude modulation (QAM) are proposed and analyzed by numerical simulations. The first stage is composed of a Viterbi&Viterbi block, based on either the standard quadrature phase shift keying (QPSK) partitioning algorithm using only Class-1 symbols or a modified QPSK partitioning scheme utilizing both Class-1 and outer most triangle-edge (TE) symbols. The second stage applies the Viterbi&Viterbi algorithm after a 64-QAM-to-QPSK transformation, while the subsequent stages iterate a maximum likelihood estimation (MLE) algorithm for phase estimation. All proposed techniques are characterized by a high tolerance to laser phase noise: an OSNR penalty of 1 dB at bit error rate (BER) of 10−2, the proposed schemes can tolerate a linewidth times symbol duration product equal to 5.6e−5 and 7.1e−5, respectively. At 32 Gbaud, all of the above linewidth requirements can be met using commercial tunable lasers. The proposed schemes achieve a similar linewidth. tolerance with a reduced implementation complexity with respect to algorithms based on the blind phase search (BPS) method.