When light becomes data

By Grace Segran

SMU Office of Research & Tech Transfer – According to a survey carried out for Direct Line by Opinium Research online, WiFi is the number one thing that their respondents could not live without. However, as we all know firsthand, WiFi is not without its problems. No matter where we are in the world, we’ve experienced internet connectivity problems at one point or another.

Now imagine a world where you can connect to high-speed internet without interference by just flicking on your light switch. LiFi – Light Fidelity – a wireless connection that could be 100 times faster than WiFi is a complementary technology to WiFi, and can be used to provide relief to the already congested radio spectrum in providing internet access to the general public. LiFi is a Visible Light Communications (VLC) system, a new paradigm that could revolutionise the future of wireless communication.

Other potential VLC applications include intelligent transport system, smart cities, localisation in warehouses/robotics, human sensing, safe and hazard-free data access in hospitals, toys and theme parks, indoor point-to-point communication, and vehicular communication.

VLC

VLC is a wireless method that enables high-speed transmission of data with visible light by modulating the intensity of light given off by a light source. The signal is received by a photodiode device (light sensors that converts light energy into electrical energy) that transforms the data into forms that are readable and readily consumed by end users.

“VLC technology is intended to overcome today’s crowded radio spectrum,” says Ila Gokarn, a PhD candidate in Computer Science, advised by Professor Archan Misra, at SMU’s School of Computing and Information Systems. “VLC uses the modulation of visible light – changing the intensity or colour of the LED (light emitting diode) light source to denote individual bits at high frequencies which are imperceptible to the human eye. Light (similar to 5G or WiFi) is an electromagnetic wave, and such waves can be used for free space communication via encoding bits (information) into the pattern of such waves.”

In January, Gokarn won the Best Research Demonstration Award at COMSNETS 2021, the International Conference on COMmunication Systens & NETworkS. The full paper of the demo “VibranSee: Enabling Simultaneous Visible Light Communication and Sensing” was presented at the 18th International Conference on Sensing, Communication, and Networking SECON 2021 where she was also awarded the N2Women Young Researcher Fellowship. Currently, she is working in the field of pervasive computing with a focus on cognitive edge computing paradigms and platforms.

The research

Gokarn’s work also involves Visible Light Sensing (VLS), a paradigm where strobing visible light from an LED, with the support of observer cameras and photodiodes, is used to detect high frequency mechanical vibrations and low frequency human gestures.

“The aim in VLS is to adjust the frequency of the strobing signal adaptively until it matches the unknown vibration frequency, at which point the vibrating object will appear to be visually stationary to an observer camera or photodiode (the same principle underlying the wagon-wheel effect in motion pictures),” she told the Office of Research and Tech Transfer.   

VLC and VLS use the same medium i.e. visible light from an LED, but Gokarn says that past research and practical applications have explored the two paradigms in isolation from each other. “Our objective is to close this gap as a unified framework using both VLC and VLS to close the control-feedback loop using visible light and yield numerous pervasive, industrial, and IoT applications.”

For example, a factory floor robot equipped with strobing LED lights can perform sensing of mechanical vibrations of factory floor machines with VLS, while also delivering control and communications via VLC.

“By sensing the mechanical vibrations, the robot can understand the current machine conditions and whether it is operating within acceptable bounds,” Gokarn explains. “If it is out of the acceptable operating bound, the robot can send control commands to the machine that can alter its operation in a way that neutralises any operating error in real time. This also acts as preventive maintenance by ensuring the machine is in optimal condition over time.”

“Our work is the first to explore the interplay and define the mathematical relationship between VLC and VLS,” says Gokarn. “Two other works have preceded us in simultaneous VLC and VLS – Amjad and Dressler showed that low frame-rate cameras supporting VLS are unaffected by VLC signals, while Varshney et al detected human gestures using light incident on solar cells but use RF-backscatter to communicate sensor readings using microwatts of power. Neither of these works, however, managed to successfully solve the challenge of simultaneously supporting high data rate VLC and highly accurate VLS.”

Outcomes

The researchers envision a new class of IoT and industrial applications that take advantage of simultaneous VLC and VLS to enable better perception of our physical world. They demonstrated an inherent trade-off between the outcomes of VLC and VLS.

VLC is about goodput,a portmanteau of good and throughput, with the latter describing the rate at which data is traversing a link while the former refers to useful data. For example, retransmissions because of congestion would mean the same data being part of throughput more than once; the amount of data the link carries may be impressive but it is not useful. For VLS, it is about sensing coverage in the context of mechanical vibration detection.

“This trade-off is determined by the duty cycle of the stroboscopic light that is key to VLS and the main carrier/component to VLC,” says Gokarn. “Our single and multi-LED architectures supported by our adaptive design and prototype, VibranSee, moderates this trade-off and provides 100% coverage for VLS while maintaining reasonable (>75%) VLC communication goodput.  We believe that our results will allow other industry researchers to adopt the use of VLC and VLS in a broader set of future industrial applications.”

Back to Research@SMU Aug 2021 Issue