Scattering Bluetooth Messages between Battery-Less Transponder and Mobile Telephones

This work concerns the operation of a battery-less transponder operated by intentionally generated wireless signals in the 2.4 GHz ISM band. The wireless signals consist of a power supplying data stream and specific Bluetooth messages. In response, the transponder back-scatters these messages to a conventional Bluetooth receiver. Our implementation uses two regular mobile telephones, one for transmitting the wireless signals, the other for receiving the scattered signals. The back-scattering applies frequency shifting of the incident Bluetooth messages for separating transmission and receiving processes. The special Bluetooth messages include specifically designed bit sequences that by themselves serve as short Bluetooth messages that are appropriately coded for detection by the Bluetooth receiver. In effect, generation of response messages is delegated from the battery-less transponder to the transmitting telephone. Experiments show that 2 MHz frequency shifting achieves better performance and/or efficiency than 4 or 6 MHz shifting, the latter based on third harmonics. At 2 MHz, the active power consumption of the transponder is <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\boldsymbol{347}\ \boldsymbol{\mu W}$</tex> and the wireless operational range reaches <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\approx \boldsymbol{6}$</tex> cm, which is a 50 % improvement over our previous work. Thus, the proposed design improves basic communication between a battery-less transponder and mobile telephones, which encourages further endeavors in this direction.