Comprehensive noise model impact for classical signals and CV-QKD coexistence in multi-core fiber

Mitigating inter-core crosstalk is critical for the coexistence of classical optical signals and continuous-variable quantum key distribution (CV-QKD) in multi-core fibers (MCFs). In this work, a comprehensive excess-noise model is developed for MCF-based classical/CV-QKD coexistence, including direct inter-core crosstalk (ICXT), SpRS-ICXT, FWM-ICXT, and XPM-induced phase noise. Based on this model, we analyze the effects of classical launch power, channel spacing, inter-core coupling coefficient, and modulation variance on the secure key rate and total excess noise. A constrained optimization of classical launch power, channel spacing, and modulation variance is further performed under classical-link feasibility requirements, including receiver sensitivity, received-power margin, and implementation loss. The results show that ICXT is the dominant impairment under the considered MCF conditions, and that the optimized configuration achieves higher SKR and longer secure reach than fixed baseline settings while maintaining classical-channel availability. This work provides a theoretical noise-modeling and constrained-optimization framework for practical classical/CV-QKD coexistence in MCFs.