Single Sideband Modulated Coherent Optical Fast-OFDM with Frequency Oversampling

This paper investigates the inter-carrier interference (ICI) in single sideband (SSB) fast orthogonal frequency-division multiplexing (OFDM), whose subcarrier space is half of the conventional OFDM. The ICI due to the non-orthogonal transform in the fast-OFDM degrades system performance, and the single-tap equalizer is infeasible to compensate dispersion as the multiplication-convolution theorem cannot be applied to the transform pair. In this paper, we propose an efficient coherent optical fast-OFDM scheme for SSB modulation. In the proposed scheme, the subcarriers of halved space can be de-multiplexed by frequency-domain oversampling with appropriate operations. The ICI is completely avoided. Compared to other schemes, the single-tap equalizer can be easily applied at the receiver to compensate channel dispersion. Moreover, the closed-form expression is analytically derived for evaluating the performance degradation due to the ICI, and confirmed by the numerical analysis. The simulation results also show the advantages of the proposed scheme. Finally, experiments were carried out to realize a 36-Gb/s SSB fast-OFDM signal with a spectral efficiency up to 6 bit/s/Hz with 8-ASK for the first time.