Energy-scavenging devices with general-purpose microcontrollers can support arbitrarily complex sensing tasks in theory, but in practice, energy limitations impose severe constraints on the application space. Richer sensing such as image capture would enable many new applications to take advantage of energy scavenging. Richer sensing faces two key challenges: efficiently retaining the necessary amount of harvested energy, and storing and communicating large units of sensor data. This paper presents the WISPCam, a passive UHF RFID camera tag based on the Wireless Identification and Sensing Platform that overcomes these two challenges to support reliable image capture and transmission while powered by an RFID reader. The WISPCam uses a novel charge-storage scheme designed specifically to match the image sensor's needs. This scheme optimally balances capacitance and leakage to improve the sensitivity and efficiency of the power harvester. The WISPCam also uses a novel data storage and communication scheme to reliably support the transfer of complete images to an RFID reader application. The WISPCam makes battery-free image capture practical for applications such as mechanical gauge reading and surveillance, both demonstrated in this paper, and opens the door to richer sensing applications on battery-free devices.

-We present the first power over Wi-Fi system that delivers power and works with existing Wi-Fi chipsets. Specifically, we show that a ubiquitous piece of wireless communication infrastructure, the Wi-Fi router, can provide far field wireless power without compromising the network's communication performance. Building on our design we prototype, for the first time, battery-free temperature and camera sensors that are powered using Wi-Fi chipsets with ranges of 20 and 17 feet respectively. We also demonstrate the ability to wirelessly recharge nickel-metal hydride and lithium-ion coin-cell batteries at distances of up to 28 feet. Finally, we deploy our system in six homes in a metropolitan area and show that our design can successfully deliver power via Wi-Fi in real-world network conditions.

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