Efficiently Assemble Normalization Layers and Regularization for Federated Domain Generalization
Authors: Khiem Le, Long Ho, Cuong Do, Danh Le-Phuoc, Kok-Seng Wong

Domain shift is a formidable issue in Machine Learning that causes a model to suffer from performance degradation when tested on unseen domains. Federated Domain Generalization (FedDG) attempts to train a global model using collaborative clients in a privacy-preserving manner that can generalize well to unseen clients possibly with domain shift. However most existing FedDG methods either cause additional privacy risks of data leakage or induce significant costs in client communication and computation which are major concerns in the Federated Learning paradigm. To circumvent these challenges here we introduce a novel architectural method for FedDG namely gPerXAN which relies on a normalization scheme working with a guiding regularizer. In particular we carefully design Personalized eXplicitly Assembled Normalization to enforce client models selectively filtering domain-specific features that are biased towards local data while retaining discrimination of those features. Then we incorporate a simple yet effective regularizer to guide these models in directly capturing domain-invariant representations that the global model's classifier can leverage. Extensive experimental results on two benchmark datasets i.e. PACS and Office-Home and a real-world medical dataset Camelyon17 indicate that our proposed method outperforms other existing methods in addressing this particular problem.