Bowers will concentrate on maximizing the efficiency of the systems used to transmit and receive data in wireless sensor networks.
Recharging a cellphone every few days is a manageable task - just find an outlet. Recharging the trillions of devices expected to be deployed on the Internet of Things over the next 20 years is not.
Adding to the complexity: Humans increasingly are linking devices together into wireless sensor networks, sensing information from their environments and communicating that information back and forth. To harness the full power of wireless sensor networks for tasks such as monitoring patients’ health remotely, saving lives during an emergency, or sensing environmental conditions, the wireless devices will have to be self-charging. This means that they must not only be capable of scavenging their energy from their surroundings but also be small and efficient enough that their energy requirements won’t outstrip their meager supply. And given the number of devices required, they better be inexpensive.
Over the last decade, a group of University of Virginia School of Engineering researchers has moved to the forefront of those systematically knocking down the barriers to creating self-sufficient sensors, also called nodes. One of these researchers is Assistant Professor Steven Bowers of UVA’s Charles L. Brown Department of Electrical and Computer Engineering. This year, the National Science Foundation presented Bowers one of its prestigious CAREER Awards, enabling him to concentrate on maximizing the efficiency of the systems used to transmit and receive data from one node to another while moving communications from the crowded sub-5 GHz part of the communications spectrum to the 24 GHz range.
Compounding the challenge, Bowers must achieve this level of efficiency across an extremely broad range of conditions. For instance, in a sensor network powered by sunlight, some nodes will be in the sun and others will be in shade.
“The power available from energy harvesting can vary by several orders of magnitude depending on the environment,” Bowers said. “We must be able to manage power across the system to take advantage of areas of high-energy productivity and compensate for those that are low.”
Read about the CAREER Award for Song Hu, assistant professor in UVA's Department of Biomedical Engineering.
Highly Efficient and Adaptable Communications
To ensure such large-scale ubiquity, it is critical to also keep costs down. Bowers is using standard complementary metal oxide semiconductor (CMOS) technology that powers the vast majority of today’s microprocessors. Moore’s law, the exponential trend over time that transistor sizes get smaller and enable higher and higher performance, can also be flipped, so that if performance is held constant, the cost per chip becomes very inexpensive if deployed at scale.
“Providing you can scale it, there is nothing close to CMOS for delivering the low cost per device we require to make these networks practical,” he said.
Within the constraints of a CMOS-based system, Bowers is tackling the challenges that arise from moving to 24 GHz communication. In addition to moving out of the sub-5 GHz range, now dominated by WiFi and Bluetooth, to a portion of the wireless spectrum that is not as heavily used, the switch to 24 GHz will allow Bowers to shrink the antenna. The size of an antenna is inversely proportional to the frequency.
“This is one of the last significant components that we have yet to miniaturize,” Bowers said. This is important because the goal of researchers redesigning sensor nodes is to reduce them to half a centimeter on a side.
In making this change, however, Bowers is significantly upping the ante. Operating devices at higher frequency also typically requires more energy. One of the goals of Bower’s CAREER Award research is to find ways to minimize this increase while maintaining performance and reliability.
Ensuring communication in these nodes can be conducted with a high level of efficiency across a broad power range is fundamental to the success of these networks — and this range is not only large but dynamic. For instance, as the sun crosses the sky, areas of sun and shade, high energy and low energy, will vary. In these circumstances, network designers would like to be able to offload network tasks to nodes with sufficient energy to conduct them and adjust the internode communications so that high-energy nodes would take on more of the burden of communication. For instance, they could transmit signals at higher power, which lowers the energy that other nodes would need to receive them. Another advantage of this strategy is that all devices on the network could be identical, switching roles as needed and sparing system designers the cost and complexity of having a central base station.
Creating communications hardware that can sustain this flexibility is extremely difficult.
“There are no free lunches. Any time you add actuation to a circuit — design it so that it can change and adapt — there is a functional cost associated with it,” Bowers said. “The actuation could require additional power or lead to a decrease in performance.”
As part of his CAREER Award, Bowers will explore these tradeoffs.
A Collaborative Approach
Creating a dynamic, low-energy communications system for sensor networks will require collaboration across a variety of electrical and computer engineering disciplines, including radio frequency, analog, digital and electromagnetic hardware design. In essence, Bowers will be drawing on elements from all these areas as he works toward his solution. But the results will be worth the effort.
“The promise of realizing such sensor systems could be transformative,” Bowers said. “Applied to business, they could increase efficiency and reduce energy consumption. Applied to emergency services like firefighting, they could save lives.”