Analysis of Spectral Shaping on DFT-OFDM

Though widespread in wireless communication, orthogonal frequency-division multiplexing has not gained full recognition in coherent optical long-haul transmission systems. This is mainly because of its high peak-to-average power ratio (PAPR), which reduces the nonlinear tolerance and ultimately limits the maximum transmission distance. Recently, DFT-OFDM has been proposed and demonstrated as a promising algorithm for nonlinearity mitigation. In this letter, we further improve the nonlinear tolerance of OFDM by introducing spectral shaping in addition to DFT spreading. Spectral shaping spreads the signal power over some of the unmodulated subcarriers, therewith effectively reducing the PAPR. In this letter, it is shown that spectrally shaped DFT-OFDM outperforms DFT-OFDM for dispersion managed and non-managed links independent of fiber type. For fibers with lower dispersion, it is found that the gain in transmission reach due to spectral shaping is higher than for fibers with higher dispersion. Thus, spectral shaping is a powerful tool that allows us to tradeoff spectral width for transmission reach with low computational complexity.