Dual Stage CPE for 64-QAM Optical Systems Based on a Modified QPSK-Partitioning Algorithm

In this letter, a novel two-stage digital feed forward carrier recovery algorithm for 64-ary quadrature amplitude modulation (QAM) is proposed and analyzed. Due to the absence of any feedback loop, the approach shows a high tolerance toward laser phase noise. Different steps involving partition, selection, and rotation of symbols are also discussed. For an OSNR penalty of 1 dB at bit error rate of 10^-3, the proposed scheme can tolerate linewidth times symbol duration product equal to 3.3×10^-5 and hence can be used with the commercially available state-of-the-art tunable lasers for 64-QAM transmission at 400 Gb/s.