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Technical Perspective: The Power of Wi-Fi to Deliver Power

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In the late 1990s, the rapidly dropping costs of microprocessors, of wireless network interfaces, and of sensors led some researchers to propose a powerful vision: a vision that tiny, wirelessly connected, computerized sensors could be scattered about like grains of smart dust and that these ‘motes’ would self-organize into a network that would allow us to weave intelligence into the physical world.1,2 This would allow us to intelligently control a diverse array of physical systems, making them more efficient, less power hungry, and more responsive to human needs. For example, we could reduce the costs of heating and lighting a building, providing these services only to occupied rooms; could measure every tremor in an earthquake zone, predicting large quakes; or could let computers sense blood sugar levels and control insulin pumps, making life more pleasant for diabetics. Thousands of researchers were inspired by this vision to work on many aspects of these ‘wireless sensor networks,’ making this a rich field of scientific inquiry.

However, one aspect of wireless sensor networks has detracted us from realizing this powerful vision. This is the need to provide power to sensor motes. It has turned out that powering a sensor using batteries makes them large, expensive, and unwieldy. Instead of scattering them to the winds, they need to be very carefully sited, so that batteries can be replaced from time to time, and so that energy would not be wasted on expensive wireless packet transmissions. This has greatly reduced the scope of wireless sensor networks, making them more of a niche technology than one would otherwise expect.

Despite this setback, a small group of researchers have held true to the original vision. Their line of attack has been to use energy harvesting, that is, gathering energy from the environment. Approaches include using tiny photovoltaic panels to harvest light, piezoelectric crystals to harvest vibrations, and antennas to harvest microscopic amounts of energy from radio and TV signals. However, these approaches have had limited success because sensors that rely on energy harvesting alone cannot be guaranteed to receive energy when they need it: they may be in the dark, in vibration-free environments, or in remote areas with a quiescent electromagnetic spectrum.

In the following paper by Talla et al., the authors turn the problem on its head. Instead of focusing on energy harvesting, they focus on wireless energy transfer. In their approach, a sensor mote harvests energy wirelessly transmitted by a dedicated power supplier. By itself, this is not particularly novel, in that this has been used by Radio Frequency Identification (RFID) systems for many years. What is clever about the paper is the authors use ubiquitous Wi-Fi devices both to supply and to harvest radio frequency energy. More specifically, they modify standard Wi-Fi chipsets to transmit special power packets that can be used to deliver power to a mote. Moreover, to prevent the energy harvester on a mote (a capacitor coupled to an antenna) from losing energy due to self-discharge, they send power packets on multiple Wi-Fi channels. They also use a cleverly chosen power packet transmission schedule to ensure that power packets minimally affect data transmission to other nodes.

What is clever about the following paper is the authors use ubiquitous Wi-Fi devices both to supply and harvest radio frequency energy.

The net result is their system, whose transmitter uses a stock Wi-Fi chipset, and whose receiver uses custom hardware, can wireless power sensors, such as a camera and a temperature probe. They can also wireless trickle charge a standard battery over the air.

While this is a big step toward the original vision of wireless sensor networks, unfortunately, the ultimate vision is still not within grasp. Wi-Fi antennas are several centimeters long, which makes the sensors not quite dust-like. The range over which power can be transmitted using this approach is also fairly small, less than a few meters. Moreover, sensors powered in this way can only operate at tens of Hz, at best. No doubt, these limitations will be overcome in years to come.

Although this paper does not deliver on the original vision of wireless sensor networks, it is nevertheless well worth reading, if only as an exercise in lateral thinking. It challenges our customary view of Wi-Fi as a data transmission technology, and shows that Wi-Fi can be used to deliver power as well, a rather surprising observation. Maybe it will stimulate you too to use standard technologies in unconventional ways?

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    1. Estrin, D. et al. Instrumenting the world with wireless sensor networks. In Proceedings of the 2001 IEEE International Conference on Acoustics, Speech, and Signal Processing, Vol. 4.

    2. Kahn, J.M., Katz, R.H. and Pister, KSJ. Next century challenges: Mobile networking for 'Smart Dust.' In Proceedings of the 5th annual ACM/IEEE International Conference on Mobile Computing and Networking. ACM, 1999.

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