The first Wi-Fi demonstration of crowd counting through walls has been given by researchers at the University of California, Santa Barbara (UCSB). The technique, which requires only a wireless transmitter and receiver, could have a variety of applications, including smart energy management, retail business planning and security.
"Our proposed approach makes it possible to estimate the number of people inside a room from outside," said Yasamin Mostofi, a UCSB professor of electrical and computer engineering, whose lab is responsible for the work. "This approach utilizes only Wi-Fi RSSI measurements, and does not rely on people to carry a device."
The transmitter and receiver are behind opposite walls in the team’s experiments; within those walls is a room with as many as 20 people zigzagging one another. The transmitter’s received signal strength indication (RSSI) is measured by the receiver, which is then able to estimate the number of people in the room.
Previous work in the Mostofi Lab has pioneered sensing with everyday radio frequency signals such as Wi-Fi. A similar crowd-counting demonstration was given in 2015, but with the transmitter and receiver in the same area as the people. “Enabling through-wall crowd counting is considerably more challenging, due to the high level of attenuation by the walls," Mostofi explained.
That challenge was solved by mathematically characterizing the content of the signal between two consecutive events — times when the signal strength drops due to human presence and movement. The approach comes from renewal process literature, a theoretical field that has found applications in areas such as reliability and risk analysis. The researchers refer to the periods between consecutive events as inter-event times.
"We have observed that while the signal magnitude can be severely attenuated through walls, the inter-event times corresponding to the events of significant signal drops are more robust to wall attenuations," said Saandeep Depatla, the lead Ph.D. student on this project.
The Mostofi Lab has tested its new technology extensively, in different locations, with different wall properties and with several different numbers of people — and with a counting accuracy of two people or less, 100% of the time, using only one Wi-Fi link. Their setup, furthermore, consists solely of off-the-shelf Wi-Fi transceivers.