400G Probabilistic Shaped PDM-64QAM Synchronization in the Frequency Domain

A low complexity digital signal processing architecture for dual-polarization coherent detection is presented. It synchronizes impairments in the frequency domain and uses synergies between the filtering processing stages to minimize overall complexity. We show how chromatic dispersion, skew, carrier frequency offset, timing error, and the state of polarization can be mitigated with a single filtering stage in the frequency domain. The error estimation in the frequency domain is based on the modified Godard algorithm, which calculates the clock tone with less than two samples per symbol and small timing jitter. The synchronization in the frequency domain is independent of the modulation format and therefore a good candidate for higher order modulation formats and so the synchronization of probabilistic shaped (PS) signals based on a high cardinality rectangular QAM format. We show the synchronization of PS polarization-division multiplexed (PDM)-64QAM signals with a symbol rate of 42.7 GBd in the frequency domain. The simulative results for the influence of the FFT size and the number of FFT blocks used for the error estimation are shown. Finally, we show experimental results for four different probabilistic shapings in back-to-back and transmission over 300 km, which are synchronized with the introduced digital signal processing (DSP).