Physical Activity Sensing via Stand-Alone WiFi Device

WiFi signals for physical activity sensing show great practical potentials for pervasive healthcare applications due to the widespread WiFi deployments and high levels of public acceptance of such systems. Traditionally, WiFi-based sensing uses the Channel State Information (CSI) from an off-the-shelf WiFi Access Point (AP) which transmits signals that have high pulse repetition frequencies. However, when there are no users on the local network only beacon signals are transmitted from the WiFi AP which significantly deteriorates the sensitivity and specificity of such systems. Surprisingly, WiFi based sensing under these conditions have received little attention given that WiFi APs are frequently in idle state. This paper presents a practical system based on passive radar techniques which does not require any special setup or firmware changes to be able to work with any commercial WiFi device. To cope with the low duty cycles associated with beacon signal transmissions, a modified Cross Ambiguity Function (CAF) has been proposed to reduce redundant samples. In addition, an external device has been developed to send WiFi probe request signals which stimulates an idle AP to transmit WiFi probe responses, thus generate usable transmission signals for sensing applications without the need to authenticate and join the network. Detection performance shows that the proposed concept can significantly improve activity detection and is a viable candidate in future healthcare applications.