In-Band Estimation of Optical Signal-to-Noise Ratio From Equalized Signals in Digital Coherent Receivers

We propose a novel method of in-band estimation of optical signal-to-noise ratio (OSNR) using a digital coherent receiver, where OSNR is determined from second- and fourth-order statistical moments of equalized signals in any modulation format. Our proposed method is especially important in recently-developed Nyquist wavelength-division multiplexed (WDM) systems and/or reconfigurable optical-add/drop-multiplexed (ROADM) networks, because in these systems and networks, we cannot apply the conventional OSNR estimation method based on optical-spectrum measurements of the in-band signal and the out-of-band noise. Effectiveness of the proposed method is validated with computer simulations of Nyquist-WDM systems and ROADM networks using 25-Gbaud quadrature phase-shift keying (QPSK) and 16 quadrature-amplitude modulation (16-QAM) formats.