  The idea of exploiting the Wi-Fi bursts as the medium for sensing purposes,
particularly for the human targets in the indoor environment, was cultivated in
both radar and computer science communities and it has became a noticeable
research genre with cross-disciplinary impact in security, healthcare,
human-machine interaction etc.This article comparatively introduces passive
radar based and channel state information (CSI) based approaches. For each
means, the primary design principles, signal processing and representative
applications scenarios are shown. At last, some opportunities and challenges of
Wi-Fi sensing are pointed out for the sake of stepping closer to the
practitioners and end-users.
