Synthetic Anchor-Assisted Prototype Alignment for Heterogeneous Federated Learning

Heterogeneous Federated Learning (HFL) aims to enable collaboration among clients with diverse model architectures and non-IID data distributions, where direct parameter aggregation is often infeasible and prototype-based aggregation may suffer from semantic drift. Existing methods usually construct global references from client-side representations, making the alignment process vulnerable to local statistical bias and noisy or corrupted prototype updates. In this paper, we propose SATPL, a synthetic anchor-assisted prototype alignment framework that introduces externally generated synthetic anchors as auxiliary semantic priors for HFL. Instead of treating Large Language Models (LLMs) as infallible semantic oracles, SATPL uses LLM-generated class descriptions and generative models to construct reproducible reference prototypes for supervised tasks with known class semantics. Each client maps its heterogeneous local representation into a shared anchor space through a lightweight projection head, while an alignment-weighted aggregation rule assigns lower weights to prototypes that are highly inconsistent with the corresponding anchors. The blockchain component is used as an auditable recording layer for anchor commitments and aggregation metadata, rather than as the source of the statistical robustness guarantee. Experiments under architecture heterogeneity, label skew, and random prototype corruption show that SATPL improves accuracy, convergence stability, and robustness over representative HFL baselines. Additional analyses evaluate anchor quality, temperature sensitivity, and system overhead, clarifying both the benefits and limitations of synthetic-anchor-based alignment.