Channel Spacing Monitor Based on Periodic Training Sequence in DWDM System

To pack dense-wavelength-division-multiplexing channels with even higher spectral efficiency, precise monitoring and controlling the channel spacing between adjacent channels is beneficial. In this paper, we propose an accurate and robust channel spacing monitor scheme based on a designed periodic training sequence. The digital signal processing is implemented in a coherent receiver to estimate the carrier frequencies of two adjacent channels and the channel spacing is calculated thereafter. The periodic structure of the training sequence allows for the processing of multiple period autocorrelation to alleviate the influence of the noise. The proposed scheme is demonstrated in a 4×32 GBaud Nyquist dual-polarization 16 quadratic-amplitude modulation system. It is verified that the channel spacing estimation accuracy can be improved by performing the multiple period autocorrelation. The results show that the estimation error is less than 50 MHz under different conditions. Robustness of the monitor against various transmission effects, such as chromatic dispersion, polarization effects, and fiber transmission, is also demonstrated.