Improvement in the nonlinear tolerance for a DFT-spread CO-OFDM system using M-ASK modulation and Hermitian symmetry of a DFT

We propose a novel discrete Fourier transform (DFT) spread M-ary amplitude shift keying (M-ASK) coherent optical orthogonal frequency division multiplexing (CO-OFDM) with a Hermitian symmetry system to mitigate nonlinear effects by reducing the peak-to-average power ratio (PAPR) of the OFDM signals. The proposed DFT-spread M-ASK CO-OFDM with the Hermitian symmetry system has a lower PAPR than the conventional M²-quadrature amplitude modulation (QAM) CO-OFDM system using the DFT-spread scheme. Furthermore, to reduce the PAPR further as compared to the DFT-spread M²-QAM CO-OFDM system, the proposed system uses the Hermitian symmetry of the DFT, which enables the proposed system to have a lower modulation order of M instead of M². The significantly reduced PAPR of the proposed system provides superior nonlinear tolerance. We analytically investigate the PAPR distributions of the OFDM systems. We also perform numerical simulations to investigate the PAPR characteristics and nonlinear tolerance of the OFDM systems. Our numerical results show that the proposed system outperforms the conventional CO-OFDM and DFT-spread CO-OFDM systems in terms of the bit error rate and PAPR.