Hark: Breaking Receiver Power Limits of LoRa

Abstract

A long-standing challenge for LoRa devices has been to enable ultra-low-power reception that supports always-on listening while preserving the standard LoRa PHY and compatibility with existing COTS transmitters. Currently, COTS LoRa radios fail to support spontaneous actuation due to the prohibitive receive power (~43.5 mW) required to listen continuously, which effectively locks out applications that fall under LoRaWAN Class C. Recent works achieve substantially lower receive power, but either require modified physical layers and coordinated helper transmissions, or forgo dechirping and the range it provides. To this end, we present Hark, an ultra-low-power LoRa receiver that enables always-on listening at 1.90 mW while fully demodulating and decoding standard frames from unmodified COTS transmitters. We achieve this by pairing a novel digital wake-up pipeline with an analog tunnel-diode oscillator frontend that exhibits autodyning and is disciplined by a shared, unsynchronized, unmodulated carrier emitter. Specifically, Hark samples the channel at a fraction of the Nyquist rate to minimize compute and ADC power, yet successfully demodulates standard LoRa frames even below the noise floor. Hark demonstrates a communication range of nearly 500 m while still reducing receive power by over 22× compared to the COTS LoRa receiver. Hark therefore targets deployments where standard LoRa transmitters must remain unmodified and a receiver-side carrier emitter can be shared across multiple receivers.