C. Rajashekar Reddy

For the past decade, low-power communication has relied on a delegation architecture that offloads carrier signal generation to external emitter devices. Although this reduces transmitter and receiver power, the resulting three-device topology introduces deployment complexity that has hindered wider adoption. Returning to a true two-device architecture on a microwatt budget was considered impossible because stable microwave oscillators are inherently power-hungry. We introduce M2, which breaks the trade-off between stability and power. The key contribution is an oscillator that generates microwave signals below 105 μW while maintaining stability of a few ppm, achieved by coupling a tunnel diode with a high-Q SAW resonator. Over a multi-hour deployment, this stability is at least two orders of magnitude better than state-of-the-art tunnel diode oscillators and is maintained across controlled variations in temperature, humidity, and motion, as well as uncontrolled real-world conditions including a crowded university food court and outdoor environment with direct sun exposure. Furthermore, M2 exhibits self-oscillating mixing and autodyning, enabling standalone transmitters and receivers that achieve 135 m line-of-sight range and -75 dBm reception sensitivity, with ranges exceeding tens of meters in tag-to-tag topology. M2 takes a major step beyond the de facto delegation architecture, enabling a new class of stable, standalone microwatt microwave (M2) radio transceivers.

Wireless communication remains the most power-consuming operation in embedded systems. Low-power transmitters such as backscatter achieve microwatt-scale operation but produce weak signals susceptible to in-band interference and frequency-selective fading, particularly in non-line-of-sight settings. Multi-band transmission can mitigate these effects through spectral-domain redundancy rather than temporal-domain redundancy, improving reliability without sacrificing bitrate or latency. However, conventional multi-band radio transceivers require duplicated RF chains or power-hungry synthesizers. We introduce HILO, a tunnel diode-based frontend that enables dual-band operation from a single circuit. HILO inverts the conventional paradigm: rather than suppressing the harmonics inherent to tunnel diode nonlinearity, it harnesses them. An external device injection-locks the oscillator to a fundamental frequency, simultaneously stabilizing a higher harmonic and yielding two injection-locked carriers without a second oscillator or frequency multiplier. In transmit mode, HILO exploits self-oscillating mixing to modulate both carriers simultaneously; in receive mode, it uses autodyne downconversion at both frequencies. By turning an inherent nonlinearity into a feature, HILO achieves dual-band links while consuming under 210 μW.

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 10x 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.

Wireless communication remains the most power-intensive operation in embedded systems. Decades of research have enabled radio transmitters to operate at power levels as low as tens of μWs while maintaining practical communication ranges. However, achieving power-efficient reception over similarly useful distances has received significantly less attention. State-of-the-art low-power receivers typically rely on Schottky diode-based envelope detectors, which are inherently limited in sensitivity and unable to support complex modulation schemes. We introduce SoMix, the Single Oscillator Mixer receiver, a novel architecture that uses tunnel diode oscillators to overcome these limitations. Specifically, we demonstrate the autodyning property of tunnel diode oscillators, allowing a single circuit to generate both a carrier signal and perform signal downconversion, thus merging two traditionally power-hungry analog tasks into one energy-efficient step. The SoMix front-end consumes less than 100 μW while supporting high-sensitivity reception. Through injection-locking, SoMix stabilizes its tunnel diode oscillator using even a weak external carrier signal, allowing it to receive frequency-modulated transmissions from distances greater than 100 meters in line-of-sight environments. We also demonstrate that the SoMix exhibits robustness in complex real-world scenarios. SoMix outperforms state-of-the-art receivers in power, range, and functionality.

Continuous monitoring of vital signs has become increasingly important for digital healthcare and enhancing self-awareness. Wearable devices like smartwatches, earbuds, and rings are gaining widespread acceptance for health monitoring. However, two significant challenges remain: (i) the limited battery life of these devices makes them unsustainable for long-term use, and (ii) many older adults, who would benefit most from health monitoring, often face barriers due to limited digital literacy. To address these issues, we introduce BioPulse—a perpetual, patch form-factor device designed for continuous monitoring. BioPulse estimates key parameters for cardiac health such as heart rate, heart rate variability, and blood pressure. By utilising a sparse sampling algorithm alongside NFC-based energy transfer and communication, the system operates without a battery, achieving a 57.9% reduction in power consumption.

Sensing the motion of physical objects in an environment enables numerous applications, from tracking occupancy in buildings and monitoring vital signs to diagnosing faults in machines. Typically, these application scenarios involve attaching a sensor, such as an accelerometer, to the object of interest, like a wearable device that tracks our steps. However, many of these scenarios require tracking motion in a noncontact manner where the sensor is not in touch with the object. A sensor in such a scenario observes variations in radio, light, acoustic, and infrared fields disturbed by the object’s motion. Current noncontact sensing mechanisms often require substantial energy and involve complex processing on sophisticated hardware. We present TunnelSense, a novel mechanism that rethinks noncontact sensing using tunnel diode oscillators. They are highly sensitive to changes in their electromagnetic environments. The motion of an object near a tunnel diode oscillator induces corresponding changes in its resonant frequency and thus in the generated radio waves. Additionally, the low-power characteristics of the tunnel diode allow tags designed using them to operate on less than 100 μW of power consumption and with a biasing voltage starting at 70 mV. This enables prolonged tag operation on a small battery or energy harvested from the environment. Among numerous applications enabled by the TunnelSense system, this work demonstrates its ability to detect breathing at distances up to 30 cm between the subject and the TunnelSense tag.

Wireless communication remains a significant power-consuming task for embedded systems. When exacerbated by increased wireless contention, it results in frequent re-transmissions and, consequently, rapid battery depletion in wireless embedded systems. We introduce TunnelRadio, which is our ongoing effort to design low-power radio transmitters that achieve significant energy efficiency compared to commodity radio transceivers. TunnelRadio leverages tunnel diodes to design energy-efficient oscillators that generate a carrier signal and mix it with a baseband or audio signal, operating at microwatts of power consumption. Specifically, we design TunnelRadio to broadcast signals in the FM broadcasting band. In this work, we demonstrate that TunnelRadio can broadcast an audio signal up to a distance of 24 meters while consuming fewer than 150 microwatts. Additionally, we showcase the transmitter’s capability to transmit complex baseband signals, such as those modulated with chirps. Our preliminary work aims to pave the way towards more widespread use of the FM band, thereby enabling large-scale deployments of wireless embedded systems.