Equalization of Dispersion-Induced Crosstalk in Optical Offset-QAM OFDM Systems

In discrete-Fourier-transform-based offset quadrature amplitude modulation (QAM) orthogonal frequency division multiplexing (OFDM), the orthogonality between the inter-carrier interference (ICI) and the signal is broken as dispersion increases, which in turn limits the dispersion tolerance. The ICI on a subcarrier is only from its adjacent two subcarriers, but its influence depends on the frequency of the subcarrier. We find that it is the high-frequency subcarriers that limit the dispersion tolerance and thus propose to equalize these dominant subcarriers to improve the performance. Simulations of 112-Gb/s offset-4QAM and 224-Gb/s offset-16QAM OFDM are performed to verify the analysis. We also experimentally demonstrate the transmission of a 38-Gb/s offset-16QAM OFDM signal over 1200-km single-mode fiber without cyclic prefix (CP). The bit error rate (BER) is reduced from 10^-3 to 10^-4 at ∼ 16-dB optical signal to noise ratio by equalizing only the high frequency half of the subcarriers. This is in contrast to conventional OFDM which cannot realize a BER of 10^-3 at 400 km when CP is not applied.