  Recent years have seen major innovations in developing energy-efficient
wireless technologies such as Bluetooth Low Energy (BLE) for Internet of Things
(IoT). Despite demonstrating significant benefits in providing low power
transmission and massive connectivity, hardly any of these technologies have
made it to directly connect to the Internet. Recent advances demonstrate the
viability of direct communication among heterogeneous IoT devices with
incompatible physical (PHY) layers. These techniques, however, require
modifications in transmission power or time, which may affect the media access
control (MAC) layer behaviors in legacy networks. In this paper, we argue that
the frequency domain can serve as a free side channel with minimal
interruptions to legacy networks. To this end, we propose DopplerFi, a
communication framework that enables a two-way communication channel between
BLE and Wi-Fi by injecting artificial Doppler shifts, which can be decoded by
sensing the patterns in the Gaussian frequency shift keying (GFSK) demodulator
and Channel State Information (CSI). The artificial Doppler shifts can be
compensated by the inherent frequency synchronization module and thus have a
negligible impact on legacy communications. Our evaluation using commercial
off-the-shelf (COTS) BLE chips and 802.11-compliant testbeds have demonstrated
that DopplerFi can achieve throughput up to 6.5~Kbps at the cost of merely less
than 0.8% throughput loss.
