Application analysis of clipping and digital resolution enhancer in high-speed direct-detection PAM4 transmission

In this paper, the application of a clipping and digital resolution enhancer (DRE) is numerically investigated in 56-GBaud direct-detection four-ary pulse amplitude modulation (PAM4) transmission systems. The influence of the tap length and the digital-to-analog converter (DAC) samples per symbol on DRE gain is first studied. After optimizing clipping probability, sampling jitter is introduced. The combination of clipping and DRE can increase jitter tolerance by ∼1.7% sampling time for 4-bit DAC. Then, the required optical signal-to-noise ratio (OSNR) at the hard-decision forward error correction (HD-FEC) threshold of 3.8×10^-3 is investigated in back-to-back case and 80-km fiber transmission. It is shown that the combination of clipping and DRE can reduce the required OSNR significantly for 3-bit and 4-bit DAC. Finally, the performance gain introduced by clipping and DRE is analyzed from a perspective of the transmitted average signal power and the received mean-squared quantization error.