In this work, we innovatively equalize optical intensity-modulated and directly detected (IMDD) four-level pulse amplitude modulation (PAM-4) signals using a complex-valued decision feedback equalizer (CDFE). Through mapping adjacent symbols of PAM-4 signals onto the complex domain, the influence of strongest inter-symbol interference (ISI) can be alleviated during the decision process in a decision feedback equalizer (DFE), effectively combating burst-error propagation when signals are noisy. Moreover, signal-adaptive manipulations of DFE parameters in both the time and the amplitude domain are performed by using an ultra-stable timing recovery and level-adaptive decision. Performance evaluations are made on vertical cavity surface emitting laser (VCSEL) modulated and multimode fiber (MMF) transmitted 100-Gbit/s optical PAM-4 signals. Based on experimental results of the short-reach optical communication, the proposed DFE outperforms the traditional DFE with a 0.5-dB system power budget gain at the 7% overhead (OH) forward error correction (FEC) bit error rate (BER) threshold.
In this work, for the first time to the best of our knowledge, we introduce the iterative pruning technique into the transfer learning (TL) of neural network equalizers (NNE) deployed in optical links with different length. For the purpose of time saving during the training period of NNE, TL migrates the NNE parameters which have been already trained on the source link to the newly-routed link (the target link), which has been proved to outperform the training initialized with the random state. Based on simulations, we proved that iterative pruning technique could further enhance the convergence speed during TL between the source and target links. Moreover, we quantitatively investigate the marginal effects of pruned threshold and pruned span on the convergence performance in various transmission distance scenarios. In addition, we observed a trade-off between performance stability and complexity of NNE, which requires to be optimized compromisingly by choosing an appropriate equalizer scale.
The demand for data rate of client-side transceivers is increasing due to the development of data center optical interconnect. To meet the high-speed requirement, the recent intensity modulation and direct detection transmission technologies are discussed.
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