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Wi-fi used as a test and measurement tool

According to, researchers in University of California, Santa Barbara, led by professor Yasamin Mostofi have shown that wireless signals can do more than provide Internet access. They have demonstrated that a Wi-Fi signal can be used to count the number of people in a given space, leading to diverse applications, from energy efficiency to search-and-rescue.

“Our approach can estimate the number of people walking in an area, based on only the received power measurements of a Wi-Fi link,” said Mostofi, who added that this test did not require people to carry Wi-Fi-enabled telecommunications devices for them to be counted.

To accomplish the people-counting, the researchers put two Wi-Fi cards at opposite ends of a target area, a roughly 70-square-meter space. Using only the received power measurements of the link between the two cards, their approach can estimate the number of people walking in that area.

So far, they have successfully tested with up to and including nine people in both indoor and outdoor settings.

The findings of Mostofi's research group are scheduled for publication in the Institute of Electrical and Electronics Engineers Journal on selected areas in Communications' special issue on location-awareness for radios and networks.

“This is about counting walking people, which is very challenging,” said Mostofi. 
“Counting this many people in such a small area with only Wi-Fi power measurements of one link is a hard problem, and the main motivation for this work.”

This people-counting method relies in large part on the changes of the received wireless signal, according to the researchers. The presence of people attenuates the signal in the direct line of sight between the Wi-Fi cards if a person crosses the line of sight, and human bodies also scatter the signal—resulting in a phenomenon called multi-path fading—when they are not in the direct line of sight path.

By developing a probabilistic mathematical framework based on these two key phenomena, the researchers have then proposed a way of estimating the number of people walking in the space. 
With the near-ubiquity of Wi-Fi in many settings, the researchers' findings have the potential for many diverse applications.

For instance, the ability to estimate the number of people in a given space could be used in smart homes and buildings, so air conditioning and heating could be adjusted according to the level of occupancy.

 Previous work in the research lab involved imaging stationary objects/humans through walls with Wi-Fi signals, and Mostofi plans to eventually bring the two projects together in the future.


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