Source polar coding is a potential solution for short blocklength-based low-latency key generation with limited sources, which is a critical aspect of six generation (6G) Internet of things. However, existing source coding schemes still suffer from significant degradation in key generation rate and reconciliation reliability in short blocklength regime. To address this issue, we introduce a multilevel source polarization-adjusted convolutional (PAC) coding framework. Furthermore, we propose a novel code construction algorithm that jointly leverages polarization effects and the maximum likelihood (ML) decoding error coefficient. Simulations demonstrate that the multilevel source PAC scheme with the proposed code construction achieves superior key generation rate under key disagreement constraints compared to conventional and multilevel source polar coding methods even in short blocklength regimes.

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