Energy harvesting visible light communications power wearables

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Interest in visible light communications has remained strong for more than a decade, with highlights such as the rise of Li-Fi – an optical version of Wi-Fi. And new research showing the capacity of the human body to harvest energy from flashing LEDs adds another reason for developers to pursue visible light communications. In the study [PDF], scientists based in the US showed that energy harvesting visible light communications could repurpose waste electromagnetic emissions as a power source for wearables sensors.

Human antenna

Copper coils fashioned into rings and bracelets, which could be made for less than a dollar, were used to collect radio frequency noise produced by solid-state lighting. And the researchers showed that wearing the metallic collectors amplified the amount of energy gathered by a factor of ten compared with using a bare coil.

Visible light communications is appealing as the approach makes use of electric lighting that’s typically already around us in homes, offices, factories, and other buildings. Information can be sent wirelessly by turning LEDs on and off very quickly (faster than can be detected by the human eye to avoid distracting anyone nearby).

“Anything with a camera, like our smartphones, tablets or laptops, could be the receiver,” said Jie Xiong, an expert in information and computer sciences at the University of Massachusetts Amherst, who led the research. During a previous study, Xiong and colleagues highlighted that LEDs leak RF energy – a property that could potentially be used as a ‘side-channel’ to snoop on transmissions – which prompted the team to consider energy harvesting.

Bracelet+

The group has named its energy harvesting visible light communications system, Bracelet+. And during test scenarios, units were found to be capable of gathering micro-watts of power from an off-the-shelf LED lamp configured as a prototype visible light communications transmitter.

According to the experts, micro-watts of harvested power could be sufficient to drive wearable devices such as temperature or glucose sensors. An energy harvesting design worn around the waist was found to capture slightly more power. But the developers opted for a wrist-mounted bracelet given the gains in terms of user convenience. Based on Federal Communications Commission (FCC) and Food and Drug Administration (FDA) safety thresholds, the researchers believe that a design such as Bracelet+ would be safe for human health.

“[The] FCC requires the power density in the low radio frequency band (3 MHz – 30 MHz) to be below 0.2 𝑚𝑊/𝑐𝑚² and [the] FDA requires the power density to be below 10 𝑚𝑊 /𝑐𝑚²,” notes the team in its write up. “The maximum instantaneous RF power in our experiment is around 0.01 𝑚𝑊 /𝑐𝑚², which is well below the thresholds.”

In tests, the setup was capable of harvesting useful amounts of energy from a single LED. And there could be scope for designs to power larger electronics. To boost coverage, commercial visible light communications systems operate with multiple transmitters, which would expose the copper bracelet to more waste RF emissions.

Transmission gains

Using visible light communications can overcome transmission issues when frequencies within the licensed spectrum are all being used by other providers and applications. And optical communication is very much part of the conversation on what 6G (the follow-up to 5G telecoms infrastructure) will look like. There are other reasons for using a light-based system, which also have an energy harvesting theme.

Writing in the Nature journal, Communications Engineering, telecommunications specialists in China describe how optoelectronics can be used to monitor fish. Their setup features InGaN/GaN LEDs, which – as the group notes – are suitable for underwater wireless light communication thanks to the relatively small water attenuation in the blue-green spectrum region.

The waterproof design features sensors powered by the incident communications blue-green light source through an energy harvesting arrangement. This removes the need for battery power and enables the fish-monitoring elements to be as lightweight as possible.

Shining a flashlight onto the sensors activates them and allows the circuits to respond – for example, by broadcasting identifiers corresponding to the host fish. The packaging is sufficiently robust to withstand temperatures as low as -20 degC. But the design still permits fish to move freely.

In-vehicle communications

Energy harvesting visible light communications are being considered for a range of applications, including onboard vehicles. Engineers in Romania and France have proposed a system that utilizes a car’s internal lighting to send messages wirelessly within the vehicle’s cabin.

Vehicle occupants had connectivity regardless of where they were located in the cabin, thanks to an array of optical fibers mounted across the ceiling. The proof-of-concept design features an ARM Cortex M7 1008 MHz microcontroller, which is used to process data in real time. The chip can also perform data analysis and provide the bit error rate (BER) to enable communications link evaluation. In testing, the unit had a BER lower than 10^-7, according to the team.