Combined Symbol-Pattern-Dependent Adaptive Equalization and Sequence Detection for Coherent Optical Fiber Communications

The key idea of this work is to treat the symbol- pattern-dependent inter-symbol-interference (ISI) that is avoided in conventional single-symbol modulation systems as useful symbol correlation, incorporate the feature of the symbol correlation into both adaptive equalization and sequence detection and thus, upgrade the system to a multi-symbol-oriented pattern-dependent equalization and detection regime. Specifically, we propose an adaptive equalization scheme sharing a pattern-dependent look- up-table with a maximum likelihood sequence estimation process to facilitate tolerance and mitigation on symbol-pattern-dependent ISI. The proposed scheme can be applied to various transmission systems with linear or nonlinear pattern-dependent ISI, such as bandwidth constrained systems with severe linear ISI, or high-power long-distance transmission systems with fiber nonlinearity induced self-phase modulation. In exemplary coherent transmission systems with digital narrow filtering before digital to analog conversion at transmitter, we experimentally demonstrate effective demodulation and detection of a polarization-division-multiplexing quaternary- phase-shift-keying signal with a constrained 3-dB transmitter bandwidth down to 1/8 of thesignal baud rate.