  Sensor-based human activity recognition (HAR) has been an active research
area, owing to its applications in smart environments, assisted living,
fitness, healthcare, etc. Recently, deep learning based end-to-end training has
resulted in state-of-the-art performance in domains such as computer vision and
natural language, where large amounts of annotated data are available. However,
large quantities of annotated data are not available for sensor-based HAR.
Moreover, the real-world settings on which the HAR is performed differ in terms
of sensor modalities, classification tasks, and target users. To address this
problem, transfer learning has been employed extensively. In this survey, we
focus on these transfer learning methods in the application domains of smart
home and wearables-based HAR. In particular, we provide a problem-solution
perspective by categorizing and presenting the works in terms of their
contributions and the challenges they address. We also present an updated view
of the state-of-the-art for both application domains. Based on our analysis of
205 papers, we highlight the gaps in the literature and provide a roadmap for
addressing them. This survey provides a reference to the HAR community, by
summarizing the existing works and providing a promising research agenda.
