Coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems have inferior nonlinear performance due to their high peak to average power ratio (PAPR) characteristics. In order to mitigate fiber nonlinearities for the CO-OFDM systems, we propose a novel discrete Fourier transform (DFT)-precoded coherent optical OFDM combined with a Hermitian symmetry (DFT-precoded HS CO-OFDM) system, which is a combination of the DFT-spread OFDM and the Hermitian symmetry of the DFT. Furthermore, the proposed system can be easily implemented by using a fast Fourier transform (FFT) algorithm. Because the proposed system has a lower PAPR than the conventional CO-OFDM system, the DFT-precoded HS CO-OFDM system provides superior nonlinear tolerance. Moreover, the proposed system is superior to the conventional DFT-spread coherent optical OFDM (DFT-spread CO-OFDM) system in terms of maintaining low PAPR in optical fiber transmission. The low PAPR in the proposed scheme is suitable for long-haul optical transmission systems, compared with the conventional DFT-spread CO-OFDM. For the DFT-precoded HS OFDM system, we have developed a theoretical framework illustrating the principles of the proposed system. The numerical results show that the DFT-precoded HS CO-OFDM system outperforms the conventional CO-OFDM by 5.4% EVM performance after 1200-km transmission and the conventional DFT-spread CO-OFDM by 4.5% EVM performance after 2800-km transmission at a fiber launch power of -2 dBm.
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