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

To meet the stringent cost and power constraints of high-speed long-haul optical transmission, this paper proposes an all-digital baud-rate joint clock recovery and adaptive equalization (JCA) scheme for 400G/800G-ZR coherent systems. The proposed scheme enables large chromatic dispersion (CD) distortion compensation, receiver IQ skew mitigation, and all-digital baud-rate clock recovery (CR) for the first time. It overcomes the high computational complexity and eliminates the 2× oversampling requirement of conventional all-digital CR schemes, while removing the need for additional hardware in existing baud-rate CR schemes. Moreover, it addresses key shortcomings of current baud-rate CR schemes, including sensitivity to laser frequency offset (LFO) and phase noise (LPN), as well as low tolerance to the CD. Simulation and experimental results demonstrate that, at a laser linewidth (LW) of 1.2 MHz, the proposed baud-rate timing phase error detector (TPED) reduces timing jitter by over 8 dB and 13 dB compared with existing MM-sign and ABS-sign TPEDs, respectively. The proposed scheme achieves a 75% reduction in computational complexity for 400G-ZR and an 88% reduction for 800G-ZR relative to conventional 2× oversampling schemes, while maintaining comparable performance. Compared with the existing baud-rate JCA scheme suitable for 400G/800G-ZR scenarios, the proposed approach enables receiver IQ skew compensation and significantly enhances CR performance, achieving a 3 dB sensitivity gain at a receiver IQ skew of 1 ps or a sampling clock offset (SCO) of 10 ppm, all with a marginal reduction in computational complexity. The proposed baud-rate JCA scheme thus presents a comprehensive, low-complexity, and high-performance solution for coherent 400G/800G-ZR transmission.