Relation-Guided Versatile Regularization for Federated Semi-Supervised Learning

Federated semi-supervised learning (FSSL) target to address the increasing privacy concerns for the practical scenarios, where data holders are limited in labeling capability. Latest FSSL approaches leverage the prediction consistency between the local model and global model to exploit knowledge from partially labeled or completely unlabeled clients. However, they merely utilize data-level augmentation for prediction consistency and simply aggregate model parameters through the weighted average at the server, which leads to biased classifiers and suffers from skewed unlabeled clients. To remedy these issues, we present a novel FSSL framework, Relation-guided Versatile Regularization (FedRVR), consisting of versatile regularization at clients and relation-guided directional aggregation strategy at the server. In versatile regularization, we propose the model-guided regularization together with the data-guided one, and encourage the prediction of the local model invariant to two extreme global models with different abilities, which provides richer consistency supervision for local training. Moreover, we devise a relation-guided directional aggregation at the server, in which a parametric relation predictor is introduced to yield pairwise model relation and obtain a model ranking. In this manner, the server can provide a superior global model by aggregating relative dependable client models, and further produce an inferior global model via reverse aggregation to promote the versatile regularization at clients. Extensive experiments on three FSSL benchmarks verify the superiority of FedRVR over state-of-the-art counterparts across various federated learning settings.