Frequency-Offset-Insensitive Equalizer for Coherent Transceivers Linear Impairment Compensation in Nyquist WDM System

As the per-channel data rate in Nyquist WDM systems increases, widely linear (WL) MIMO equalizers are required to compensate transceiver linear impairments. However, the crosstalk, filtering, and mixing effects arising from the interaction between frequency offset (FO) and transceiver impairments significantly degrade the FO tolerance of the WL-MIMO equalizer, limiting its applicability in Nyquist WDM systems. In this paper, we introduce an FO pre-compensation 8×2 WL-MIMO equalizer. By integrating a specially designed single-branch FO compensation structure with an 8×2 WL-MIMO equalizer based on the digital phase-locked loop least mean square algorithm, the proposed method achieves FO-insensitive transceiver impairment compensation with nearly unchanged computational complexity. Simulations of a Nyquist WDM system show that the proposed scheme achieves an FO tolerance exceeding 5 GHz. Moreover, a 32 GBaud dual-polarization 64 quadrature amplitude modulation signals transmission experiment over 75 km standard single-mode fiber further confirms that, while maintaining stable transceiver impairment compensation, the proposed scheme significantly extends the FO tolerance from 200 MHz to beyond 2 GHz compared with the conventional 8×2 WL-MIMO equalizer.