Abstract
A pilot-based adaptive equalizer is investigated for high cardinality polarization-division-multiplexing quadrature amplitude modulation transmission systems. Pilot symbols are periodically inserted for joint estimation of the dynamic state of polarization (SOP) and carrier phase, in a least mean square (LMS) sense. Compared to decision-directed least mean square (DDLMS) equalization and radially-directed equalization, the proposed equalizer can achieve robust equalization and phase estimation, especially in low optical signal-to-noise ratio (OSNR) scenarios. In an experiment on 56 GBaud PDM-64 QAM transmission over 400 km standard single-mode fiber, we obtained at least 0.35 bit per symbol generalized mutual information (GMI) improvement compared with other training symbol-based equalization when tracking 600 krad/s dynamic SOP. With the joint estimation scheme, the equalization performance will not be compromised even if the SOP speed reaches 600 krad/s or the laser linewidth approaches 2 MHz. For the first time, it is demonstrated that the pilot-based equalizer can track dynamic SOP rotation and compensate for fiber linear impairments without any cycle slips under extreme conditions.
Highlights
In metro and long-haul fiber links, the requirement to reliably transmit high speed data has stimulated the evolution of wide band optical transmission systems
We propose a pilot-based multiple-input multiple-output (MIMO) equalizer where pilots are used for joint adaptive equalization and carrier phase estimation (CPE), which can mitigate the polarization mode dispersion (PMD) effects and achieve a reliable tracking of the fast dynamic state of polarization (SOP) change that may occur in modern transmission systems
An external cavity laser emitting at 1550 nm was modulated by a dual polarization (DP) IQ modulator which was driven by 91 GSa/s 8-bit digital-to-analog converters (DACs)
Summary
In metro and long-haul fiber links, the requirement to reliably transmit high speed data has stimulated the evolution of wide band optical transmission systems. (DDLMS) [13,14] or other proposed data-aided equalization algorithms [22,23], we introduce the pilot symbols for two purposes: by comparing the pilot and the recovered signal, the carrier phase is estimated directly in the equalization process without requiring the decision-directed digital phase locked loop (DPLL). The aim of this step is to prevent the adaptive equalizer (AEQ) from being affected by the fast phase fluctuation and to reduce latency during the adaptive operation compared with decision-directed algorithms or frequency domain equalization [24]. The simulation and experiment results show that the proposed AEQ outperforms blind [12] or other pilot-based algorithms [23] and can work without significant loss of generalized mutual information (GMI) at SOP speed of 600 krad/s and laser linewidth of 2 MHz in 56 GBaud 64 QAM systems
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