RF Harvesting at 2.4 GHz for Scattering between Battery-less Transponder and Mobile Telephones

This work studies RF (radio frequency) harvesting of electromagnetic energy from intentionally generated wireless RF signals in the 2.4 GHz ISM band as power supply for a battery-less back-scattering transponder. According to our implementation, a first mobile telephone transmits a quasi-continuous Bluetooth advertizing signal and the transponder scatters and modulates this signal such that it is receivable by a Bluetooth receiver of a further mobile telephone. The scattering process includes 4 MHz frequency translation and frequency shift keying for information transmission. The modulation process is controlled by a low energy micro-controller, which obtains its operational energy from a RF harvesting circuit that accumulates energy from the incident RF signals. The low energy transfer of Bluetooth signals motivates the present feasibility study, which proposes a supplement of WLAN signals as additional source of energy for the battery-less transponder. The present study includes examining alternative WLAN modes: ”tethering”, ”WiFi direct” and ”ad hoc” and concludes that an Android specific form of WLAN tethering shows the most promising potential for our scattering application, which is based on regular, unmodified mobile telephones. Thus, by additionally using WLAN signals, we achieved a significant range improvement of the wireless energy transfer from the transmitting mobile telephone to the battery-less transponder, which encourages further endeavors in this direction.