Wireless devices are so common today that if you were to try to count juts how many signals are passing through and reflecting off of your body, I suspect you will miss at least one. Phones, laptops, routers, tablets, some media players and more use Wi-Fi to communicate, flooding the areas around them with microwaves. Researchers at the University of Washington decided to see what functions the sea of waves could be used for and discovered that gesture recognition is one of them.
Even though Wi-Fi signals are able to pass through walls and people, the waves do not travel without impediment. As you move your body slightly alters the signal, and while the frequency shift is not enough to affect a data connection, it can be detected with a proper receiver. By analyzing the changes, the researchers discovered it is possible to detect a person's gestures using Wi-Fi in separate rooms from the source and receiver. You could control electronics anywhere within your home, with simple gestures, as could anyone else as the system has the potential to track multiple people.
This may sound like something too close to science fiction to be commercially available, but some off-the-shelf routers could be used after a software update, and the signal sources could be any of the myriad of Wi-Fi enabled devices already available.
Source: University of Washington