Tomlinson–Harashima Precoding For Dispersion Uncompensated PAM-4 Transmission With Direct-Detection

Combating the destructive effects of power-fading in fiber-optic systems using direct-detection is the key for increasing the transmission rates and/or reach. Some of the solutions used in the past include additional components, e.g., a dispersion-compensating fiber or an IQ-modulator, which increase the costs of the entire system. Another solution, the compensation of the channel distortions using decision-feedback equalization, is limited to its error-propagation effect. In this paper, we introduce Tomlinson-Harashima precoding for the compensation of power-fading, which allows us to use the efficiency of feedback equalization for channels with spectral zeros without the disadvantage of error propagation. In an experimental investigation with different scenarios, including transmission over 25 km up to 100 km of standard single-mode fiber with gross data rates ranging from 56 to 560 Gb/s, we show that Tomlinson-Harashima precoding significantly outperforms decision-feedback equalization in all cases. Using Tomlinson-Harashima precoding, power-fading can be efficiently compensated for using digital signal processing only, keeping the system flexible and at the same time cost-effective.