Wirelessly Controlled mm-Sized IoT Devices

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In the almost 120 years since Marconi’s first transatlantic wireless transmission, several generations of wireless devices have connected people with stations and with each other, resulting in the number of wirelessly connected devices recently exceeding the global human population. The next exponential growth in wireless devices will no longer be in access between people but in connecting objects and machines in the age of the Internet of Things (IoT). Projections show sensor demand reaching trillions of devices in the coming decades; this will largely be fueled by the emergence of smart sensors that combine computation, communication, and sensing. Ultra-low-power smart radios that can provide unique IP addresses and their locations are therefore a requirement for the emergence of the IoT.

Our IoT efforts focus on the design of ultra-small (mm-sized) ultra-low-power or wirelessly-powered sensor nodes and the wireless systems required to locate, power up, duty cycle, and communicate with these nodes. Addressing the challenges involved in the design of these tiny nodes and the surrounding wireless framework will open up new application areas for the IoT in commercial, medical, and industrial settings.

Ant-Sized Radio and RF Wireless Power Transfer

We have demonstrated a single-chip 24GHz/60GHz passive radio implemented in 65nm CMOS [1, 2]. This chip is fully self-sufficient, with neither pads nor any external components (e.g. power supply). It integrates RX and TX antennas and provides a communication range of up to 50 cm. A modified M-PPM 60GHz transmitter is used to communicate the data sequence as well as the local timing reference. Pulse signaling enables real-time localization through time-of-flight. The chip operates with a recovered power of less than 1.5 µW coming from the reader.

This power-harvesting pad-less millimeter-sized radio (“ant-sized radio”) uses two on-chip antennas for both power and communication. Radio placed atop U.S. penny for scale.

To cement the theory behind the design of the ant-sized radio, we have developed a modular analysis framework for far-field RF power transfer to millimeter-sized sensors [3]. We have considered the entire power transfer chain at the system level, taking into account the regulations governing the transmitter, the properties of the channel, and the characteristics of the receiver, which comprises an antenna, matching network, rectifier, and load. We have found that there exists an optimal frequency for power transfer to small nodes, and that for mm-sized sensors, the optimal frequency lies in the mm-wave regime (10s GHz). We have also established clear design guidelines for the power recovery circuitry of wirelessly powered sensors. These guidelines are meant to improve upon conventional approaches to the design of such systems, which are often designed to operate at a lower frequency than is optimal for power transfer.

Ultrasonic Wireless Power Transfer and Wake-Up

We have also explored the use of air-coupled ultrasound for wirelessly powering mm-sized nodes [4]. By taking into account ultrasonic transmission, propagative effects (including diffraction, absorption, and nonlinearity), safety/regulations, and transduction and rectification at the receiver, we have shown through simulation that ultrasonic powering can outperform RF-based systems in terms of the power delivered to small nodes at long range, using a precharged capacitive micromachined ultrasonic transducer (CMUT) as a proof-of-concept acoustic power recovery device.

In addition to power, ultrasound is also an attractive choice for implementing wireless wake-up signaling, which allows remote duty-cycling of IoT nodes. We have shown that a combined CMUT-CMOS system allows for competitive power consumption (i.e., lifetime) and sensitivity (i.e., range) compared to the state of the art in wake-up receiver technology while enabling miniaturization down to the mm-scale [5, 6].

Conceptual block diagram of ultrasonic signature transmission, propagation, and reception by an ultrasonic WuRx. The WuRx gates the operation of a duty-cycled IoT device.

Photograph of the constituent parts of the ultrasonic WuRx.

[6] A. S. Rekhi and A. Arbabian, “Ultrasonic Wake-Up with Precharged Transducers,” IEEE J. Solid-State Circuits (JSSC), vol. 54, no. 5, pp. 1475-1486, Feb. 2019.

[5] A. S. Rekhi and A. Arbabian, "A 14.5mm² 8nW -59.7dBm-Sensitivity Ultrasonic Wake-Up Receiver for Power-, Area-, and Interference-Constrained Applications," Proc. 2018 IEEE Int. Solid-State Circuits Conf., San Francisco, CA, 2018.

[4] A. S. Rekhi and A. Arbabian, "Wireless Power Transfer to Millimeter-Sized Nodes Using Airborne Ultrasound,” IEEE Trans. Ultrason. Ferroelectr. Freq. Control, vol. 64, pp. 1526-1541, Oct. 2017.

[3] J. Charthad, N. Dolatsha, A. Rekhi, and A. Arbabian, "System-Level Analysis of Far-Field Radio Frequency Power Delivery for mm-Sized Sensor Nodes," IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 63, pp. 300-311, Feb. 2016.

[2] M. Tabesh, N. Dolatsha, A. Arbabian, and A. Niknejad, "A Power-Harvesting Pad-Less Millimeter-Sized Radio," IEEE J. Solid-State Circuits, vol. 50, no. 4, pp. 962-977, Apr. 2015.

[1] M. Tabesh, M. Rangwala, A. M. Niknejad, and A. Arbabian, "A Power-Harvesting Pad-Less mm-Sized 24/60GHz Passive Radio with On-Chip Antennas," 2014 Symp. VLSI Circuits Dig. Tech. Pap., Honolulu, HI, 2014.