  Indoor human monitoring systems leverage a wide range of sensors, including
cameras, radio devices, and inertial measurement units, to collect extensive
data from users and the environment. These sensors contribute diverse data
modalities, such as video feeds from cameras, received signal strength
indicators and channel state information from WiFi devices, and three-axis
acceleration data from inertial measurement units. In this context, we present
a comprehensive survey of multimodal approaches for indoor human monitoring
systems, with a specific focus on their relevance in elderly care. Our survey
primarily highlights non-contact technologies, particularly cameras and radio
devices, as key components in the development of indoor human monitoring
systems. Throughout this article, we explore well-established techniques for
extracting features from multimodal data sources. Our exploration extends to
methodologies for fusing these features and harnessing multiple modalities to
improve the accuracy and robustness of machine learning models. Furthermore, we
conduct comparative analysis across different data modalities in diverse human
monitoring tasks and undertake a comprehensive examination of existing
multimodal datasets. This extensive survey not only highlights the significance
of indoor human monitoring systems but also affirms their versatile
applications. In particular, we emphasize their critical role in enhancing the
quality of elderly care, offering valuable insights into the development of
non-contact monitoring solutions applicable to the needs of aging populations.
