FTN SSB 16-QAM Signal Transmission and Direct Detection Based on Tomlinson-Harashima Precoding With Computed Coefficients

We propose and experimentally demonstrate a Tomlinson-Harashima precoding (THP) scheme with the coefficients computed by simulations. It realizes spectrally efficient faster-than-Nyquist (FTN) signaling for quadrature amplitude modulation (QAM) format. In our scheme, the coefficients of the THP precoder are obtained by numerical simulations with an additive white Gaussian noise (AWGN) channel, in which the intersymbol interference (ISI) is induced by a digital FTN filter. Then, we experimentally apply these THP coefficients at the transmitter to mitigate the FTN-related ISI, while the ISI from the analog channel is suppressed by the receiver-side adaptive equalizer. By this means, the experimental estimation of the THP coefficients is not required, leading to a simplified system operation. We perform numerical simulations to study the impacts of FTN signaling and channel bandwidth limitation on the proposed THP scheme. A proof-of-concept experiment is conducted, in which a 28-GBaud FTN single-sideband (SSB) 16-QAM signal is transmitted over an 80-km single mode fiber (SMF). Compared to the conventional scheme with the experimentally estimated THP coefficients, a negligible sensitivity penalty is observed for the proposed scheme. We also achieve a record FTN rate of 21.73% and a high spectral efficiency of 4.30 b/s/Hz for the single-polarization 16-QAM format on a single wavelength.