Optical coherent burst-mode receivers with delayed channel equalisation feedback from parallel and pipelined design

A digital optical coherent burst-mode receiver (BMR) with 112-Gb/s polarisation-division multiplexing-quaternary phase-shift keying (QPSK) modulation is proposed with parallel digital signal processing and pipelined design. The convergence rate property of the digital channel equaliser with non-negligible computational delay is studied extensively using simulations. Owing to the interaction between the channel equalisation and frequency offset estimation (FOE), a novel equaliser adaptation method using the latest available equaliser output and the latest available FOE result is proposed for a faster convergence than the conventional method. Simulation results reveal that the computational delay have a great impact on the convergence rate. A smaller step size for tap coefficient updating with slower convergence rate should be used to avoid divergence. Compared with the FOE module, the computational delay from the channel equaliser has the dominant effect on the convergence. The design of the equaliser with short computational delay is the key for a fast convergence rate. The parallel and pipelined digital optical coherent BMR after successful initialisation has negligible error rate performance loss when compared with the ideal serial signal processing within a wide frequency offset range.