Multi-stage feed-forward optical carrier phase estimation based on QPSK partitioning for 64-QAM signals

We numerically investigate two multi-stage feed-forward optical carrier phase estimation (CPE) methods for coherent optical 64-QAM signals, which are QPSK partitioning with maximum likelihood (ML) method and QPSK partitioning with blind phase search (BPS) method. Both methods are proved to be possible solutions for CPE of 64-QAM signals, with good laser phase noise tolerance and simple complexity. For QPSK partitioning with ML method, we creatively introduce cascaded multi-stage ML following QPSK partitioning, which leads to further improved laser phase noise tolerance compared to 1-stage ML condition. For QPSK partitioning with BPS method, BPS with 8 or 12 test phases and test phase range of π/4 is found to be suitable for practical application, because it can achieve good laser phase noise tolerance while keeping low complexity, and the use of BPS with more test phases in this method has limited advantage over single stage BPS with the same test phases.