An apparatus for detecting events includes a processor and a storage device storing instructions that when executed by the processor cause the processor to train a classifier for classifying channel state information (CSI) and detecting an event based on the classification of the CSI obtained during a monitoring phase. The training of the classifier includes: for each of the known events to be detected, during a time period in which the known event occurs in a venue, obtain training CSI of a wireless multipath channel between a wireless transmitter and a wireless receiver in the venue, in which the training CSI is derived from one or more probing signals sent from the transmitter through the wireless multipath channel to the receiver, and train the classifier based on the known events and the training CSI associated with each of the events.