Polybinary Shaping for Highly-Spectral-Efficient Super-Nyquist WDM QAM Signals

In Super-Nyquist wavelength-division multiplexed (WDM) systems, in which WDM channel spacing is smaller than a signal baudrate, there is a tradeoff relationship between the WDM crosstalk and inter-symbol interference due to the tight spectral shaping. For optimizing the tradeoff relationship, polybinary shaping with maximum likelihood sequence estimation is introduced. In this paper, we numerically and experimentally investigate the bit-error rate characteristics of the Super-Nyquist WDM quadrature amplitude modulated signals with polybinary shaping in order to clarify the relationship between the spectral efficiency and required signal-to-noise ratio (SNR). These results indicate that the Super-Nyquist WDM technique based on polybinary shaping is effective to optimize the spectral efficiency for the target required SNR equivalent to the target transmission distance.