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Digital coherent superposition (DCS) for optical orthogonal frequency-division multiplexing (OFDM) subcarrier pairs with Hermitian symmetry can simultaneously reduce the inter-carrier interference (ICI) from laser phase noise and cancel the fiber nonlinearity to the first order. In this paper, we study an interesting scenario, where we upgrade the conventional OFDM with 4-quadrature amplitude modulation (QAM) to DCS-OFDM with 16-QAM, which have equal spectrum efficiency and require no change on hardware. We conduct a simulation to compare the tolerance to the laser phase noise and fiber nonlinearity, which are the main concerns for such an upgrading. First, the 16-QAM DCS-OFDM relaxes the laser linewidth requirement and performs better with a smaller fast Fourier transform size. Second, the nonlinear tolerance of 16-QAM DCS-OFDM is better when the nonlinear effects appear. The nonlinear tolerance of DCS-OFDM depends on the dispersion compensation scheme. The maximum SNR improvement is 3.4 dB with full post-dispersion compensation, and it can increase to 6.6 dB with symmetrical dispersion compensation at the launch power of 4 dBm. Finally, we verify that the maximum SNR improvement of 16-QAM DCS-OFDM with full post-dispersion compensation is 3.7 dB at the launch power of 6.2 dBm in the experiment.

Comparison of ICI Reduction and Fiber Nonlinearity Tolerance for DCS-OFDM and Conventional OFDM With Equal Spectrum Efficiency