Enhanced Phase Estimation for Long-Haul Multi-Carrier Systems Using a Dual-Reference Subcarrier Approach

Multi-subcarrier (MSC) modulation is recently standing out as a notable new feature in the latest generations of high-speed coherent optical transceivers, providing distinctive advantages over legacy single-carrier systems. However, the transition from single-carrier to MSC modulation still requires significant optimization of digital signal processing subsystems, in order to maximize the inherent potential of subcarrier multiplexing. In this article, the penalty associated with the usage of carrier phase estimation (CPE) methods inherited from single-carrier systems is quantitatively studied and the need for enhanced MSC-tailored solutions is evidenced. A novel pilot-based joint-subcarrier CPE method based on a dual-reference subcarrier (DRS) approach is described and numerically assessed. The proposed DRS-CPE exploits the frequency-dependent walk-off effect imposed by chromatic dispersion (CD) to enable the separation of phase noise processes incurred by the transmitter and local oscillator lasers, thereby allowing to effectively apply joint-subcarrier CPE independently of the symbol-rate per subcarrier. Our numerical results demonstrate that the proposed DRS-CPE enables to operate with near-optimum performance at symbol-rates per subcarrier as low as 1 Gbaud over ultra-long-haul distances.