iPoint: A Platform-Independent Passive Information Kiosk for Cell Phones

We introduce iPoint, a passive device that can interact and deliver information to virtually any mobile phone equipped with a WiFi network interface and a camera. The iPoint does not need any battery but harvests energy from the phone WiFi transmissions. The iPoint delivers information to the mobile phone through a low power LCD display that can be captured and processed by a software application. We introduce a mechanism of Packet Width Modulation (PWM) to encode the phone requests in the length of WiFi packets. This allows the use of phones not equipped with RFID readers, and still allows the ultralow power microcontroller to decode the information. In this paper, we describe the architecture of iPoint, discuss the design choices of each component, and report on the experimental evaluation of our prototype. Various RadioFrequency energy harvesters are discussed and a WiFi tailored, modified Greinacher voltage multiplier, a highly efficient parallel full-wave rectifier, is designed, prototyped and fully characterized. It features an energy-harvesting efficiency of up to 72% and collecting up to 2.5 mW from a phone WiFi transmission. An ultralow-power micro-controller with LCD capability (TI MSP430F417) is optimized, and customized to interface with our RF Front End (RF-FE). A PWM demodulator is designed, as an integral part of the energy-harvester, and interfaced with the microcontroller. Finally, the mobile phone application for decoding the LCD output is presented. The evaluation is based on actual measurements carried on the third generation of prototypes we built.