Simultaneous quantum-classical communication (SQCC) protocols offer a practical approach to continuous-variable quantum key distribution (CV-QKD) by encoding quantum and classical signals onto the same optical pulse. However, like most QKD protocols, their performance is limited when experimental parameters, such as modulation variance, are optimized based on stationary channel assumptions. In fluctuating environments, such as free-space links, this can result in suboptimal key rates and reduced transmission distances. In this work, we introduce Gaussian postselection into the SQCC framework, enabling a software-based optimization of the modulation variance after channel estimation. This passive approach enhances key rates in both asymptotic and finite-size regimes without requiring hardware modifications and remains effective even when receiver imperfections are taken into account. We demonstrate that our protocol improves the transmission distance and robustness of SQCC relative to the standard fixed-variance SQCC protocol, and approaches the performance of a fully preoptimized system across both fiber and free-space channels. In particular, we show that the protocol enables full communication windows under ideal weather conditions and maintains higher duty cycles during adverse weather in satellite-to-ground scenarios. These results highlight the practicality of postselection-based SQCC for real-world quantum communication over both terrestrial fiber networks and satellite-based free-space links.

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• Quantum channels
• Quantum communication
• Quantum communication, protocols & technology
• Quantum cryptography