Wi-Fi RF energy harvesting for battery-free wearable radio platforms

Wearable devices have huge market potential, but their usage has been limited because means to power such devices is still a challenge. Batteries, the most widespread solution, add size, weight and need periodic recharging which is a huge deterrent. In this work, we propose to eliminate batteries by leveraging Wi-Fi transmissions from nearby devices such as access points and smart phones to deliver power wirelessly to wearable devices. We develop a wearable temperature sensor, which harvests energy from Wi-Fi transmissions and transmits data back to an access point. We enable this application by designing an efficient 2.4 GHz Wi-Fi harvesting front-end to power an ANT radio platform with a smartphone as the Wi-Fi source. We study and analyze the effect of OFDM modulation, wide bandwidth (72 MHz) and the bursty nature of 802.11 Wi-Fi signals on the sensitivity, efficiency and the output power of the RF harvester. Our prototype achieves a sensitivity of -16.5 dBm with 100 % duty cycle Wi-Fi transmissions for a target output voltage of 2.05 V and 2.5 μW leakage at the storage capacitor node. This translates to an operating range of about 11.5 cm from a 2 dBm Wi-Fi transmitter on a smartphone and 92 cm from a 20 dBm Wi-Fi access point with a 3 dBi antenna on the wearable device.