Deterministic-ISI-mitigation enabled blind adaptive equalization in coherent optical systems with narrowband...

Digital narrowband pre-filtering is an effective approach to enabling high-spectral-efficiency transmission in coherent optical systems, but the resulting severe inter-symbol interference (ISI) can significantly impair conventional blind adaptive equalization (AEQ). Under strong bandwidth limitation, e.g., the 3-dB electrical transmitter bandwidth is 1/8 of the baud rate, the dominant ISI is mainly determined by the fixed transmitter-side narrowband filter and thus exhibits a deterministic correlation structure. Conventional AEQ treats this structure entirely as interference to be compensated, which degrades its reliability, whereas we propose a training-overhead-free DIM-assisted demodulation scheme that treats it as useful symbol correlation and explicitly exploits it. The DIM is a front-end symbol-window weighting module placed before AEQ, with coefficients optimized to maximize the desired-symbol energy. It partially suppresses the dominant deterministic ISI and facilitates the convergence of the subsequent AEQ. A matched post-filter is further introduced to suppress the high-frequency noise enhancement caused by near-full-response equalization and to restore the symbol correlation required for subsequent sequence detection. A linear baseband model is also established to analyze the evolution of ISI and noise through the major DSP stages. Experimental results for a 60-GBd PDM-QPSK system with a transmitter 3-dB bandwidth of 7.5 GHz show that the proposed DIM-CMA scheme achieves a 2.6-dB Q-factor improvement over conventional CMA after 1800-km transmission.