Power/Performance Bits: Sept. 26

Long-range communication
Researchers at the University of Washington developed devices that run on almost zero power can transmit data across distances of up to 2.8 kilometers. The long-range backscatter system, which uses reflected radio signals to transmit data at extremely low power, achieved reliable coverage throughout 4800-square-foot house, an office area covering 41 rooms and a one-acre vegetable farm.

It consumes 1000 times less power than existing technologies capable of transmitting data over similar distances, according to the researchers.

The system has three components: a source that emits a radio signal, sensors that encode information in reflections of that signal and an inexpensive off-the-shelf receiver that decodes the information. When the sensor is placed between the source and receiver, the system can transmit data at distances up to 475 meters. When the sensor is placed next to the signal source, the receiver can decode information from as far as 2.8 kilometers away.

The sensors are also cheap, with an expected bulk cost of 10 to 20 cents each.

A flexible epidermal patch prototype, which could be used to collect and wirelessly transmit medical data, that successfully transmitted information across a 3,300 square-foot atrium. (Source: Dennis Wise/University of Washington)

The advantage to using reflected, or backscattered, radio signals to convey information is a sensor can run on extremely low power that can be provided by thin cheap flexible printed batteries or can be harvested from ambient sources. The disadvantage is that it’s difficult for a receiver to distinguish these extremely weak reflections from the original signal and other noise.

To overcome the problem, the team introduced a new type of modulation, called chirp spread spectrum, into its backscatter design. Spreading the reflected signals across multiple frequencies allowed the team to achieve much greater sensitivities and decode backscattered signals across greater distances even when it’s below the noise.

In one demonstration, the research team built a contact lens prototype and a flexible epidermal patch that attaches to human skin, which successfully used long-range backscatter to transmit information across a 3300-square-foot atrium. Prior smart contact lens designs typically had a 3-foot range.

The long-range backscatter system will be commercialized by Jeeva Wireless, a spin-out company founded by the UW team of computer scientists and electrical engineers, which expects to begin selling it within six months.

Wearable solar cells
Scientists from RIKEN and the University of Tokyo developed an ultra-thin photovoltaic device that could be integrated into textiles. Coated on both sides with stretchable and waterproof films, the solar cell continued to provide electricity from sunlight after being soaked in water and being stretched and compressed.

The flexible organic photovoltaic cells are based on a polymer blend material called PNTz4T. They deposited the device in an inverse architecture, which they had previously developed, onto a 1-µm-thick parylene film.

The device was then placed onto acrylic-based elastomer and the top side of the device was coated with an identical elastomer, giving it a coating on both sides to prevent water infiltration. The elastomer, while allowing light to enter, prevented water and air from leaking into the cells, making them more long-lasting than previous experiments.

a, Photograph of the washing process for the devices conforming to a dress shirt. Scale bar, 1 cm. b, Structure of the free-standing OPVs. c, Chemical structure of PNTz4T. d, Schematic of the double-side-coated OPVs. (Source: Hiroaki Jinno, et al.)

The device showed an efficiency of 7.9%, producing a current of 7.86 milliwatts per square centimeter. To test its resistance to water, the cell was soaked it in water for two hours, and found that the efficiency decreased only by 5.4%. To test the durability, they subjected it to compression, and found that after compressing by 52% for twenty cycles while placing drops of water on it, it still had 80% of the original efficiency.

Wireless power and data from the same antenna
Researchers from North Carolina State University developed a system that can simultaneously deliver watts of power and transmit data at rates high enough to stream video over the same wireless connection.

Wireless power transfer technologies use antennas with a narrow bandwidth to minimize the power lost in generating magnetic fields, particularly for transmission over inches or feet. Because using a narrow bandwidth antenna limits data transfer, devices incorporating wireless power transfer have normally also incorporated separate radios for data transmission, increasing the cost, weight and complexity of the device.

The team realized that while high-efficiency power transfer, especially at longer distances, does require very narrow band antennas, the system bandwidth can actually be much wider.

“People thought that efficient wireless power transfer requires the use of narrow bandwidth transmitters and receivers, and that this therefore limited data transfer,” said David Ricketts, an associate professor of electrical and computer engineering at NC State. “We’ve shown that you can configure a wide-bandwidth system with narrow-bandwidth components, giving you the best of both worlds.”

With this wider bandwidth, the team then envisioned the wireless power transfer link as a communication link, adapting data-rate enhancement techniques, such as channel equalization, to further improve data rate and data signal quality.

The researchers tested the system with and without data transfer. They found that when transferring almost 3 watts of power, enough to power a tablet during video playback, the system was only 2.3% less efficient when also transmitting 3.39 megabytes of data per second. At 2 watts of power, the difference in efficiency was 1.3%. The tests were conducted with the transmitter and receiver 16 centimeters, or 6.3 inches, apart, demonstrating the ability of the system to operate in longer-distance wireless power links.