Hardware-Efficient Multi-Format Frequency Offset Estimation for M-QAM Coherent Optical Receivers

In future elastic optical networks, hardware-efficient and multi-format frequency offset estimation (FOE) is critical for digital coherent receivers (DCRs) to alleviate the requirement of wavelength matching while reducing the power consumption as well as the footprint of digital signal processing units. Here, we propose a blind feedforward and low-complexity FOE algorithm based on the techniques of extended QPSK partitioning and quasi-linear approximation. With processing in polar coordinates, a simple novel error function is first defined to measure the phase deviation induced by frequency offset so that robust FOE can be directly determined by the quasi-linear relationship between the phase deviation and the corresponding offset distance. Besides, a multi-timespan estimation method is also introduced to further mitigate the phase noise impact on FOE. Not only the simulation results of 4/16/32/64-quadrature amplitude modulation (QAM) demonstrate its feasibility and versatility, but also the complexity analyses show that hardware complexities are significantly reduced. The bit error rate versus OSNR performance is experimentally investigated in double-polarization 4/16/32-QAM measurements. The superiority of the proposed FOE makes it highly desirable for application in practical multi-format DCRs.