Multisymbol QPSK Partitioning for Improved Frequency Offset Estimation of 16-QAM Signals

We propose and investigate a generalized quadrature phase-shift keying (QPSK) partitioning algorithm for carrier frequency offset (CFO) estimation in coherent optical 16-quadrature amplitude modulation (QAM) receivers. The algorithm utilizes the phase difference between the samples with different time-domain separations, so that the estimation accuracy can be significantly enhanced. We propose a computationally efficient procedure to implement the generalized CFO estimation algorithm. In a 28-GBd coherent 16-QAM system, simulation results and complexity analysis show that the time span of 16 can realize an optimal balance between performance and complexity. With the optimal time span, the complexity to achieve a normalized variance of 10^-8 for the CFO estimation error can be reduced by 150 times compared with the conventional QPSK partitioning algorithm.