US Patent 9520041 Monitoring intrusion in an area using WIFI-enabled devices

For monitoring an area, signal strength samples are obtained (S1) by sampling a data signal transmitted from at least one first device and received by at least one second device, and measuring a signal strength of the sampled data signal. The signal strength samples are processed (S2) by forming consecutive first groups of N consecutive signal strength samples, calculating first standard deviations for each of the consecutive first groups, forming consecutive second groups of N consecutive first standard deviations, and calculating second standard deviations for each of the consecutive second groups. Based on the second standard deviations, it is detected (S3) if a fluctuation in the data signal received by the second device has occurred that represents an intrusion in the area.