Joint clock recovery and feed-forward equalization for PAM4 transmission.

Area of Science:

• Optical communications
• Signal processing

Background:

• Short-reach optical interconnects are crucial for cloud services, data centers, and IoT.
• 4-level pulse amplitude modulation (PAM4) offers high data rates at a low cost for these applications.
• Inter-symbol interference (ISI) and sampling clock offset (SCO) degrade PAM4 signal quality.

Purpose of the Study:

• To propose a joint clock recovery and feed-forward equalization (CR-FFE) algorithm for PAM4 transmission.
• To simultaneously address inter-symbol interference (ISI) and large sampling clock offset (SCO).
• To improve the robustness of PAM4 systems against clock inaccuracies.

Main Methods:

• Developed a CR-FFE algorithm that estimates timing error using fractionally spaced equalizer tap coefficients.
• Implemented a 50-Gbit/s PAM4 transmission experiment using a 10 GHz directly modulated laser (DML).
• Tested the algorithm's performance over 40 km of standard single-mode fiber (SSMF).

Main Results:

• The CR-FFE algorithm successfully eliminated ISI and tracked large SCO.
• The proposed algorithm demonstrated resilience to SCO up to 1000 ppm.
• Performance was evaluated against the 2x10^-2 Soft-Decision Forward Error Correction (SD-FEC) Bit Error Rate (BER) threshold.

Conclusions:

• The joint CR-FFE algorithm significantly enhances PAM4 transmission performance in short-reach optical interconnects.
• It offers a dramatic improvement in SCO tolerance compared to traditional cascaded CR and FFE methods.
• This algorithm provides a robust solution for reliable high-speed optical data transmission.