Feed-forward carrier phase recovery for offset-QAM Nyquist WDM transmission.

Due to the half symbol delay between in-phase and quadrature components for offset quadrature amplitude modulation (OQAM) signal, phase noise cannot only lead to constellation rotation but also introduce additional crosstalk. Therefore, OQAM signal has very poor tolerance to the laser linewidth. Here, we carry out a semi-analytical investigation of phase noise induced crosstalk during OQAM Nyquist WDM transmission, and find that the carrier phase recovery (CPR) has to be implemented prior to the inter-symbol-interference (ISI) equalization. Then, after a function separation of polarization de-multiplexing and ISI equalization, we propose a new DSP flow with a linewidth-tolerant blind feed-forward CPR scheme for OQAM signal. Its effectiveness is verified under the scenario of 5-channel 28-Gbaud polarization multiplexing (PM) OQAM Nyquist WDM systems. A tolerance of linewidth and symbol duration products of 6.5×10(-4) and 1.1×10(-4) is secured for 4-OQAM and 16-OQAM, respectively, given 1-dB required-OSNR penalty at BER = 10(-3).