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Wirelessly Controlled mm-Sized IoT Devices

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.

In this context, wirelessly powered, battery-free radios are the ultimate frontier in scaling the size and cost of a communication node. These radios face unique challenges in addressing the need for sufficient data rate without using a power supply, and indeed, there are several key challenges that still need to be addressed in this area. Cost, node density, communication latency, data rate capacity, localization, and miniaturization are the issues faced in the design of these devices. Addressing these challenges will open up new application areas for the IoT in commercial, medical, and industrial settings.

In the first phase of this project, 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 up to 50 cm. A modified M-PPM 60GHz transmitter (6 bits per slot) 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.

Taking this work forward, 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 derived the dependence of the optimal frequency and maximum powering range upon the important parameters of the power transfer chain, establishing 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.

Our latest efforts in this area have explored the use of airborne 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. As proof of concept, we have demonstrated the use of a precharged capacitive micromachined ultrasonic transducer (CMUT) to recover 5 μW at a range of 1.05 m, which is ~4x and ~10x better than previous ultrasonic and RF-based systems, respectively.

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.

(a) Diagram of possible IoT applications for wirelessly powered mm-sized receivers. (b) Block diagram of the wireless power transfer chain analyzed in our system-level work on far-field RF power transfer to mm-sized nodes.

Our analysis of far-field RF power transfer to mm-sized nodes shows that the optimal operation frequency lies in the mm-wave regime, but that the received power is relatively robust to small variations in frequency.

[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.

[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.

[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.

[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.