I’m always on the lookout for technologies that helps reduce the use of batteries for the IoT. With that in mind, I’m bullish on energy-harvesting sensors and radios, and excited to see more investment in wireless power at a distance. So this week, I dug into a paper from researchers at Universidad Carlos III de Madrid, Uppsala University in Sweden, and Spain’s IMDEA research nonprofit that discusses a way to use light and radio frequency backscatter to eliminate batteries in the IoT.
The paper outlines a concept the researchers call Passive LiFi, which uses light and radio frequency (RF) backscatter as energy sources and light as a means to most efficiently transmit data. It builds on LiFi, a technology that uses light to transmit and receive information. LiFi, which is short for “light fidelity,” has been around for a while, with big companies such as Disney and Signify investing in it, but it hasn’t yet hit the mainstream.
LiFi was introduced about a decade ago as a means of using light to transmit data. Currently, most of our wireless data transmissions, such as Wi-Fi and cellular, occur through the use of radio frequencies. But with LEDs, engineers can modulate the frequency of light in ways that transmit information. And of course you can also transmit data by flicking a light on and off.
LiFi is still fairly niche technology, in part because it requires a line of sight between the light and the device receiving the data. That means if you want to transmit information to, say, a phone, it needs to be out of someone’s pocket or purse and underneath the light source. And LiFi isn’t inherently energy efficient, largely because light sources generating the signal tend to be plugged into their own source of power.
But with PassiveLiFi, researchers are more interested in using LiFi to communicate small amounts of data over long ranges while simultaneously harvesting energy from the light source. In their paper, the PassiveLiFi researchers showed off a system with a light source and an IoT tag that was able to communicate at a distance of 305 meters (1,000 feet) with the tag consuming just 3.8 microwatts.
Researchers tweaked the traditional LiFi system in a few different ways. But the two most important things to know about Passive LiFi as compared to other wireless transmission schemes is that, one, it uses light to transmit data instead of RF; and two, that it uses both light and RF to generate energy.
The experimental system researchers set up consisted of an IoT tag, a set of solar cells, and a LoRa transmitter. They discovered they could create a wireless transmission scheme using light that would even work outdoors over long distances. The sun normally washes out the data transmission because it is so bright, and the same thing can happen indoors with certain other types of light, but the researchers are confident that those types of lights are being phased out.
They also discovered that they could increase the transmission of data from 1.5 meters to 3.5 meters indoors with a capacity of 280 kbps, which is a little more than double the previous capacity using LiFi. However, to build a truly self-sustaining tag that transmits data constantly, an application would only be able to send 250 bps, which is a minuscule amount of data. The capacity rises if you don’t constantly transmit or if you add an additional battery to the tag.
In short, the ideas here work, but they don’t transmit much data and would require a complete system consisting of a specialized transmitter inside light bulbs and specialized tags ready to harvest the light and send data. That’s not to say this isn’t worth investigating. There are plenty of environments where a well-lit sensor needs to transmit a few bits of information each day. Imagine sensors on a roadway with LiFi-enabled streetlights shining down on them. Or an office in which you might have occupancy sensors or temperature sensors powered by strategic Li-Fi transmitters in the campus lights.
The paper makes a case for these and other use cases. But if I were pressed to answer the question I asked in the headline (“Can PassiveLiFi make batteries for IoT obsolete?”), I’d say that the answer is a pretty big “no” unless we boost the harvesting efficiency of solar cells and the amount of information we can transmit using light.