Federated Learning allows training machine learning models by using the computation and private data resources of a large number of distributed clients such as smartphones and IoT devices. Most existing works on Federated Learning (FL) assume the clients have ground-truth labels. However, in many practical scenarios, clients may be unable to label task-specific data, e.g., due to lack of expertise. In this work, we consider a server that hosts a labeled dataset, and wishes to leverage clients with unlabeled data for supervised learning. We propose a new Federated Learning framework referred to as SemiFL in order to address the problem of Semi-Supervised Federated Learning (SSFL). In SemiFL, clients have completely unlabeled data, while the server has a small amount of labeled data. SemiFL is communication efficient since it separates the training of server-side supervised data and client-side unsupervised data. We demonstrate various efficient strategies of SemiFL that enhance learning performance. Extensive empirical evaluations demonstrate that our communication efficient method can significantly improve the performance of a labeled server with unlabeled clients. Moreover, we demonstrate that SemiFL can outperform many existing FL results trained with fully supervised data, and perform competitively with the state-of-the-art centralized Semi-Supervised Learning (SSL) methods. For instance, in standard communication efficient scenarios, our method can perform 93% accuracy on the CIFAR10 dataset with only 4000 labeled samples at the server. Such accuracy is only 2% away from the result trained from 50000 fully labeled data, and it improves about 30% upon existing SSFL methods in the communication efficient setting.
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