All-digital baud-rate joint clock recovery and adaptive equalization for 400G/800G-ZR transmission

Area of Science:

• Optical Communications
• Digital Signal Processing
• High-Speed Transmission Systems

Background:

• High-speed long-haul optical transmission systems (400G/800G-ZR) face stringent cost and power constraints.
• Existing clock recovery (CR) and adaptive equalization schemes have limitations in computational complexity, oversampling requirements, and tolerance to impairments like chromatic dispersion (CD), laser frequency offset (LFO), and phase noise (LPN).
• There is a need for efficient and robust solutions for baud-rate clock recovery and equalization in coherent systems.

Purpose of the Study:

• To propose an all-digital baud-rate joint clock recovery and adaptive equalization (JCA) scheme for 400G/800G-ZR coherent systems.
• To enable large chromatic dispersion (CD) distortion compensation, receiver IQ skew mitigation, and all-digital baud-rate clock recovery (CR) for the first time.
• To overcome the high computational complexity and eliminate the 2x oversampling requirement of conventional all-digital CR schemes.

Main Methods:

• Development of a novel all-digital baud-rate joint clock recovery and adaptive equalization (JCA) scheme.
• Implementation of a new baud-rate timing phase error detector (TPED) to reduce timing jitter.
• Simulation and experimental validation of the proposed scheme for 400G/800G-ZR coherent systems.

Main Results:

• The proposed baud-rate TPED reduces timing jitter by over 8 dB and 13 dB compared to existing MM-sign and ABS-sign TPEDs, respectively.
• Achieved a 75% reduction in computational complexity for 400G-ZR and an 88% reduction for 800G-ZR compared to conventional 2x oversampling schemes.
• Demonstrated receiver IQ skew compensation and a 3 dB sensitivity gain at 1 ps IQ skew or 10 ppm sampling clock offset (SCO).

Conclusions:

• The proposed all-digital baud-rate JCA scheme offers a comprehensive, low-complexity, and high-performance solution for 400G/800G-ZR coherent transmission.
• The scheme effectively compensates for large CD, mitigates receiver IQ skew, and provides robust baud-rate CR.
• It overcomes limitations of existing schemes, including high computational complexity and sensitivity to impairments, while requiring no additional hardware.