1 Introduction
Machine learning has been widely adopted in many applications in people's daily life [1], [2], [3]. Specifically for healthcare, researchers can build models to predict health status by leveraging health-related data, such as activity sensors [4], images [5], and other health information [6], [7], [8]. To achieve satisfying performance, machine learning healthcare applications often require sufficient client data for model training. However, with the increasing awareness of privacy and security, more governments and organizations enforce the protection of personal data via different regulations [9], [10]. In this situation, federated learning (FL) [11] emerges to build powerful machine learning models with data privacy well-protected.