Digital Signal Processing for Training-Aided Coherent Optical Single-Carrier Frequency-Domain Equalization Systems

In this paper, we present the design of digital signal processing (DSP) algorithms for training-aided coherent optical single-carrier frequency-domain equalization (SC-FDE) systems. Based on two training-aided channel estimation (TA-CE) schemes, the requirements for training sequence to achieve the minimum mean square error performance of channel estimation (CE) are outlined. Moreover, the practical implementation issues of constant-amplitude zero-autocorrelation sequence and Golay sequences are discussed. We propose to perform time-domain windowing to the CE taps for further noise suppression to improve the CE and achieve large overhead saving with the optimized CE. Furthermore, frame timing synchronization and frequency offset compensation algorithms based on Golay sequences are developed, which provide bandwidth-efficient solution with robust performance for long-haul transmission. Finally, a low-complexity fractionally spaced frequency-domain equalizer is reported to effectively reduce the computational complexity of the whole system. A total of 28-Gbaud coherent polarization division multiplexing (PDM) system simulations and 10-Gbaud coherent PDM system experiments are conducted to verify that the proposed DSP solutions provide robust performance and are suitable for implementation in high-speed long-haul digital coherent receivers.