DCT-precoding technique in optical fast OFDM for mitigating fiber nonlinearity

We construct a discrete Fourier transform (DFT)-precoded optical fast orthogonal frequency division multiplexing (FOFDM) system. However, the DFT-precoded FOFDM system requires a higher modulation order (M²) compared with the conventional FOFDM system (M) owing to the complex conjugate data. The higher modulation order results in an increase in the peak-to-average power ratio (PAPR), which degrades the nonlinear tolerance. To overcome this problem, we propose a discrete cosine transform (DCT)-precoded optical FOFDM system. The proposed system does not require a higher modulation order. Therefore, the proposed DCT-precoded optical FOFDM system has a lower PAPR characteristic than the DFT-precoded FOFDM system. The lowest PAPR characteristic of the proposed system enables superior nonlinear tolerance compared with other FOFDM systems. The numerical results show that the proposed system outperforms the conventional FOFDM system by an optical signal-to-noise ratio of 2 dB at 3.8× 10^-3~BER for an 800-km transmission.