In this letter, we first present the theoretical basis for coherent optical OFDM systems in direct up/down conversion architecture. We then demonstrate the transmission performance through simulation for WDM systems with coherent optical OFDM (CO-OFDM) including the fiber nonlinearity effect. The results show that the system Q of the WDM channels at 10 Gb/s is over 13.0 dB for a transmission up to 4800 km of standard-single-mode-fiber (SSMF) without dispersion compensation. A novel technique of partial carrier filling (PCF) for improving the non-linearity performance of the transmission is also presented. The system Q of the WDM channels with a filling factor of 50 % at 10 Gb/s is improved from 15.1 dB to 16.8 dB for a transmission up to 3200 km of SSMF without dispersion compensation.
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