AudioCast: Enabling Ubiquitous Connectivity for Embedded Systems through Audio-broadcasting Low-power Tags

Wireless connectivity challenges hinder the deployment of embedded systems. We introduce AudioCast to address two critical issues: spectrum scarcity-induced contention and high power consumption in transmitters. The widespread availability of broadcast radio receivers (for example, FM radios using the 88-108 MHz spectrum) and access to underutilized lower-frequency spectrum motivate the design of AudioCast. The lower-frequency spectrum offers superior radio-wave propagation characteristics, exhibiting at least 10 × lower path loss than the 2.4 GHz and 5 GHz Industrial, Scientific, and Medical (ISM) bands while avoiding congestion and interference. These properties enable reliable and long-distance communication, even for weakly radiated signals. AudioCast builds on these properties and the unique negative resistance of a tunnel diode. AudioCast rethinks the architecture of radio transmitters using a tunnel diode oscillator to generate carrier signals and self-modulate them with baseband signals. This results in frequency-modulated transmissions at an overall power consumption below 200 μW. Unlike related systems based on the backscatter mechanism, AudioCast does not require an externally generated carrier or rely on ambient signals. We argue that AudioCast represents an example of a new class of transmitters which we conceptualize as Beyond-Backscatter transmitters. Through experiments, we demonstrate that AudioCast achieves a transmission range of up to 130 m in line-of-sight and tens of meters in non-line-of-sight conditions respectively. These transmissions are decodable by ubiquitous commodity FM receivers in cars, homes, and phones. We evaluate AudioCast through theoretical analysis, benchtop experiments, and urban/indoor field deployments. Additionally, we prototype and demonstrate novel applications, including low-power voice transmissions and hand gesture communication, enabled by AudioCast's range and power efficiency.