Every smart phone is a backscatter reader: Modulated backscatter compatibility with Bluetooth 4.0 Low Energy (BLE) devices

TLDR

The study demonstrates that modulated backscatter signals can be engineered to produce channelized band‑pass signals similar to those of conventional wireless devices. The authors implement a proof‑of‑concept BLE backscatter system that generates three channel‑bandpass signals by modulating a single continuous‑wave carrier with a novel combination of fundamental‑mode and harmonic‑mode subcarrier modulation, leveraging the widely available Bluetooth 4.0 Low Energy standard. The prototype shows that unmodified BLE devices, such as an Apple iPad, can receive 1‑Mbps FSK packets indistinguishable from standard advertising, achieving over 100‑fold lower energy per bit (28.4 pJ/bit versus >10 nJ/bit) while remaining fully compatible without hardware or software changes.

Abstract

In this work, we show how modulated backscatter signals can be crafted to yield channelized band-pass signals akin to those transmitted by many conventional wireless devices. As a result, conventional wireless devices can receive these backscattered signals without any modification (neither hardware nor software) to the conventional wireless device. We present a proof of concept using the Bluetooth 4.0 Low Energy, or BLE, standard widely available on smart phones and mobile devices. Our prototype backscatter tag produces three-channel bandpass frequency shift keying (FSK) packets at 1 Mbps that are indistinguishable from conventional BLE advertising packets. An unmodified Apple iPad is shown to correctly receive and display these packets at a range of over 9.4 m using its existing iOS Bluetooth stack with no changes whatsoever. We create all three BLE channels by backscattering a single incident CW carrier using a novel combination of fundamentalmode and harmonic-mode backscatter subcarrier modulation, with two of the band-pass channels generated by the fundamental mode and one of the band-pass channels generated by the second harmonic mode. The backscatter modulator consumes only 28.4 pJ/bit, compared with over 10 nJ/bit for conventional BLE transmitters. The backscatter approach yields over 100X lower energy per bit than a conventional BLE transmitter, while retaining compatibility with billions of existing Bluetooth enabled smartphones and mobile devices.