Low complexity joint clock recovery and adaptive equalization based on a baud-rate timing phase error detector for short-reach digital coherent transmission

Abstract

Digital coherent receivers adopting joint clock recovery and adaptive equalization (JCA) can avoid failures of the adaptive equalizer (AEQ) or clock recovery algorithm (CRA) due to clock asynchrony and chromatic dispersion (CD). But in the previous JCA scheme, the AEQ has a heavy computational load as it has to generate two samples per symbol (SPS) for the subsequent timing phase error detector (TPED) which is the core of the CRA. Furthermore, the previous JCA scheme cannot compensate for receiver skew or accommodate Nyquist signals with small roll-off factors (ROFs). These shortcomings hinder its practical applications in ultrahigh-speed short-reach coherent transmission requiring low power consumption, high spectral efficiency, whilst being sensitive to receiver skew. To solve this problem, we propose a new JCA scheme by integrating a two-section real-valued (RV) AEQ with an all-digital feedback CRA based on a baud-rate TPED versatile for different ROFs. Experiments for 61-GBaud dual-polarization (DP) Nyquist 16QAM signals with an ROF of 0.01 show that, compared with the previous JCA scheme, the proposed scheme can reduce the AEQ computational load by about 70% for 10-km transmission, whilst improving the receiver sensitivity by more than 1.7 dB for a receiver skew of 1.5 ps. As far as we know, the proposed JCA scheme is the most comprehensive and efficient solution for ultrahigh-speed short-reach coherent transmission where CD, receiver skew, clock asynchrony, and Nyquist signals with small ROFs have to be dealt with.