Asymmetric digital backpropagation for long-haul digital subcarrier multiplexing fiber optical transmission

Abstract

Digital Subcarrier Multiplexing (DSM) has emerged as a preferred solution over its Single-Carrier (SC) counterpart for high-speed long-haul Standard Single Mode Fiber (SSMF) transmission, owing to its flexible spectral allocation and adaptive transmission capabilities. However, fiber nonlinearity remains a fundamental limitation on the transmission bandwidth-distance product, while Digital Backpropagation (DBP) serves as a representative Nonlinearity Compensation (NLC) method. Traditional symmetric DBP assumes a uniform nonlinearity accumulation over the SSMF link, which deviates from the inherently asymmetric power profile in practice. Specifically, we propose an asymmetric DSM-DBP scheme based on a nonlinear accumulation asymmetry metric ΔE(ρ), which enables the efficient determination of the optimal dispersion splitting ratio ρ ∈ [0, 1] without exhaustive optimization. Consequently, we can efficiently determine the position of nonlinear compensation operators within each DBP step for high-speed long-haul DSM transmission systems. When the single-wavelength, four-subcarrier, 800 Gb/s Probabilistic Shaping Dual-Polarization 64-Quadrature-Amplitude-Modulation (PS-DP-64QAM) signals are transmitted, the asymmetric DSM-DBP increases the optimal launch power to 5dBm and achieves a Signal-To-Noise Ratio (SNR) improvement of 0.2 dB over symmetric DSM-DBP after the 1600 km SSMF transmission. Under a Normalized Generalized Mutual Information (NGMI) threshold of 0.89, the SSMF reach can be extended to 2400 km, representing a reach extension of approximately 160km compared to traditional symmetric DSM-DBP. Meanwhile, the robustness of the proposed asymmetric DSM-DBP is numerically verified for various modulation formats, variable baud rates, and different subcarrier configurations. These results highlight the scalability and practical relevance of the proposed asymmetric DSM-DBP for next-generation high-capacity optical networks.