Wi-Fi signals are typically information carriers between a transmitter
and a receiver. In this paper, we show that Wi-Fi can also
extend our senses, enabling us to see moving objects through walls
and behind closed doors. In particular, we can use such signals to
identify the number of people in a closed room and their relative
locations. We can also identify simple gestures made behind a wall,
and combine a sequence of gestures to communicate messages to
a wireless receiver without carrying any transmitting device. The
paper introduces two main innovations. First, it shows how one can
use MIMO interference nulling to eliminate reflections off static
objects and focus the receiver on a moving target. Second, it shows
how one can track a human by treating the motion of a human body
as an antenna array and tracking the resulting RF beam. We demonstrate
the validity of our design by building.
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