Frequency Offset Estimation for 32-QAM Based on Constellation Rotation

Traditional frequency offset estimation (FOE) algorithms, such as fast Fourier transform-based FOE (FFT-FOE) and differential FOE (Diff-FOE), are no longer suitable for 32-QAM optical coherent receiver. Here, we propose a feed-forward and non-data-aided FOE algorithm for 32-QAM based on constellation rotation. After equalization, the constellation points of the second and the forth ring are selected and rotated with an angle of π/4. Thus, those points gather along diagonal lines, which enable us to use traditional Diff-FOE for 32-QAM signal. The performance of our proposed algorithm is numerically and experimentally verified under the scenario of back-to-back transmission with dual-polarization 32-QAM signals. Our proposed FOE algorithm is more robust to amplified spontaneous emission noise in comparison with recently proposed QPSK-selection assisted FFT-FOE, while the complexities are reduced by factors of 1.7 and 3.7 in the forms of multipliers and adders, respectively.