We report and experimentally investigate the performance of an adaptive decision-directed channel equalizer (ADDCE) in reduced-guard-interval dual-polarization coherent-optical orthogonal-frequency-division-multiplexing (RGI-DP-CO-OFDM) transport systems. ADDCE retrieves an estimation of the phase noise value after an initial decision making stage by extracting and averaging the phase drift of all OFDM sub-channels. Moreover, it updates the channel transfer matrix on a symbol-by-symbol basis. We experimentally compare the performance of the ADDCE and the conventional equalizer (CE) combined with maximum-likelihood (ML) phase noise compensation and inter-subcarrier-frequency-averaging (ISFA) algorithms. The study is conducted at 28 GBaud for RGI-DP-CO-OFDM systems with quadrature-phase-shift-keying (QPSK) and 16 quadrature amplitude modulation (16-QAM) formats. Using ADDCE, zero-overhead laser phase noise compensation is accomplished and the overhead due to training symbol (TSs) insertion is significantly reduced. In addition, ADDCE offers a superior performance over the CE in the presence of synchronization timing errors and residual chromatic dispersion (CD). We also achieve a longer transmission distance than when using the CE. At a forward-error-correction (FEC) threshold of 3.8 × 10−3, using a cumulative overhead of less than 2.6%, transmission distances of 5500 km and 400 km were achieved for the cases of QPSK and 16-QAM RGI-DP-CO-OFDM, respectively.
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