Nonlinear equalizer for spectral efficient IM/DD FBMC/OQAM transmission system

Multi-carrier modulation is attractive for high-speed passive optical network. However, for a given system, its capacity is limited by the system bandwidth and the number of effective data subcarriers. With the increase of the baud rate and the number of data subcarriers, the signal distortion becomes serious. In this work, we propose a nonlinear equalizer (NLE), mainly composed of a 2-order Volterra filter and a complex-valued neural network channel estimator, to mitigate the severe signal distortion. We experimentally demonstrate and evaluate it in a spectral efficient 12.5/25-GBd intensity modulation direct detection filter bank multicarrier with offset quadrature amplitude modulation (FBMC/OQAM) transmission system with the 10-dB bandwidth of 9.6-GHz over 30-km standard single mode fiber. In the 12.5-GBd FBMC/OQAM transmission, when the number of the data subcarriers is increased to 448 with the total 512 subcarriers, the proposed NLE outperforms the linear equalizer (LE) and can achieve about 2-dB receiver sensitivity improvement at 3.8×10⁻³ bit error ratio (BER), the hard decision forward error correction (HD-FEC) threshold. When the symbol rate is increased to 25-GBd, the number of data subcarriers is limited to 256 because of the power fading of high-frequency components, the LE cannot recover the distorted signal well, while the proposed NLE can still achieve better BER performance.