AudioCast: Enabling Ubiquitous Connectivity for Embedded Systems
through Audio-broadcasting Low-power Tags
Proceedings of Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies (IMWUT), June 2025, Demonstrated at ACM MobiSys 2025 and to be presented in the conference ACM UbiComp 2025
PDF copy of the scientific and peer-reviewed publication
Authors
Rajashekar Reddy Chinthalapani, National University of Singapore
Dhairya Jigar Shah, National University of Singapore
Nobel Ang, National University of Singapore
Ambuj Varshney, National University of Singapore
PI: Prof. Ambuj Varshney, National University of Singapore
Reach out: ambujv@nus.edu.sg
Abstract
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 X lower path loss than the 2.4 GHz and 5 GHz Industrial, Scientific, and Medical bands while avoiding congestion and interference. These properties enable reliable and long-distance communication, even for weakly radiated signals. AudioCast builds on these properties.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 microwatts. 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.
Visual Narrative
Overview
FM broadcast receivers are ubiquitous, but their usage is declining and being phased out in various parts of the world. This shift presents an opportunity to repurpose the spectrum for Internet of Things
The FM-broadcast band offers superior propagation characteristics, enabling long-range communication even in the presence of obstacles such as vegetation and vehicles
AudioCast enables low-power transmissions that can be received by FM broadcast receivers, which are ubiquitously present in our environment
AudioCast achieves a long communication range of up to 130 meters in line-of-sight environments, enabling its usage to support applications such as farm-scale sensing and support for city-wide sensor deployments
Design
Imagine a ball climbing a hill yet in one strange section, it speeds up instead of slowing down. This visual paradox reflects the negative resistance region of a tunnel diode, where increasing voltage results in decreasing current. AudioCast leverages this unique property.
AudioCast transforms sensor data bits into audio broadcasts by leveraging the negative resistance characteristics of a tunnel diode. AudioCasts demonstrates and uses the self-modulation property to self-modulate the tunnel diode oscillator.
AudioCast simultaneously broadcasts to multiple devices. Multiple AudioCast tags can coexist by leveraging frequency diversity to enable concurrent transmissions.
The power consumption of the AudioCast front-end is under 200 microwatts, enabling prolonged operation on small batteries or even battery-free operation using harvested energy.
Applications enabled by AudioCast
Temperature sensor that communicates information wirelessly to a receiver located tens of meter away in a home.
Controlling appliances, lights through battery-free gesture tracking and wireless communication devices.
AudioCast enables FM-broadcast receivers in cars and mobile phones to opportunistically backhaul sensor readings, even in the absence of conventional networking infrastructure such as cellular towers, for offloading sensor data from scenarios like sensors deployed in a remote farm.
AudioCast enables the realization of low-power trackers, which can be designed in compact form factors such as pendants, to capture vocal interactions and transmit them to smartphones or other devices for transcription.
Reference
ACM Reference Format
C. Rajashekar Reddy, Dhairya Shah, Nobel Ang, and Ambuj Varshney. 2025. AudioCast: Enabling Ubiquitous Connectivity for Embedded Systems through Audio-Broadcasting Low-power Tags. Proc. ACM Interact. Mob. Wearable Ubiquitous Technol. 9, 2, Article 27 (June 2025), 32 pages. https://doi.org/10.1145/3729471
@article{reddy2025audiocast,
author = {C. Rajashekar Reddy and Dhairya Shah and Nobel Ang and Ambuj Varshney},
title = {AudioCast: Enabling Ubiquitous Connectivity for Embedded Systems through Audio-Broadcasting Low-power Tags},
journal = {Proceedings of the ACM on Interactive, Mobile, Wearable and Ubiquitous Technologies (IMWUT)},
volume = {9},
number = {2},
article = {27},
year = {2025},
month = jun,
pages = {1--32},
doi = {10.1145/3729471}
}
Funding
This work is fully supported by the Advanced Research and Technology Innovation Centre (ARTIC) under Grant (Project Number: WDSS-RP1, WBS: A-8000976-00-00). In addition, some parts of this work are supported by a Tier 1 grant from Ministry of Education (A-8001661-00-00). It is also supported by a Startup Grant from ODPRT (A-8000277-00-00) and an unrestricted gift from Google through their Research Scholar Program (A-8002307-00-00-00), all of which are hosted at the National University of Singapore.