Domain-wise knowledge decoupling for personalized federated learning via Radon transform

Personalized federated learning (pFL) customizes local models to address heterogeneous data across clients. One prominent research direction in pFL is model decoupling, where the knowledge of a global model is selectively utilized to assist local model personalization. Prior studies primarily use decoupled global-model parameters to convey this selected knowledge. However, due to the task-related knowledge-mixing nature of deep learning models, using these parameters may introduce irrelevant knowledge to specific clients, impeding personalization. To address this, we propose a domain-wise knowledge decoupling approach (pFedDKD), which decouples global-model knowledge into diverse projection segments in the representation space, meeting the specific needs of clients on heterogeneous local domains. A Radon transform-based method is provided to facilitate this decoupling, enabling clients to extract relevant knowledge segments for personalization. Besides, we provide a distillation-based back-projection learning method to fuse local-model knowledge into the global model, ensuring the updated global-model knowledge remains decouplable by projection. A theoretical analysis confirms that our approach improves generalization. Extensive experiments on four datasets demonstrate that pFedDKD consistently outperforms eleven state-of-the-art baselines, achieving an average improvement of 1.21% in test accuracy over the best-performing baseline.