Performance Comparison of Offset-16QAM and 16QAM for Nyquist WDM Superchannel With Digital Spectral Shaping

We investigate five-channel polarization division multiplexed Nyquist WDM superchannel using offset-16QAM and 16QAM modulation format, under the condition of transmitter-side digital spectral shaping technique, respectively. The offset-16QAM scheme can greatly relax the implementation complexity, because the tap number of 35 is sufficient for the required finite impulse response (FIR) filter, compared with 81-tap FIR filter of the 16QAM scheme. About 1.3-dB back-to-back sensitivity improvement is obtained for the offset-16QAM scheme. In particular, the offset-16QAM scheme has better tolerance of DACs' constraints, including the untracked jitter, sampling rate, and resolution. Finally, the practical implementation of offset-16QAM-based Nyquist WDM superchannel with TS-DSS is evaluated, by taking the phase difference deviation among wavelength channels and phase noise into account. No more than 1.3-dB required-OSNR penalty can be obtained, in case the phase difference is varied from 0° to 360°. When the laser with a linewidth of 100 kHz is used, there still exists about 1-dB overall performance improvement, compared with the 16QAM scheme.