This article appeared in Electronic Design and has been published here with permission.
As an optically based complement to RF-based Wi-Fi, Li-Fi (light fidelity) offers distinct attributes including potentially extremely high throughout over limited distances and immunity from (and non-sourcing of) EMI/RFI. One other characteristic of an optical link can be considered either a benefit or a drawback: Its line-of-sight path provides outstanding immunity to eavesdropping and hacking, but also limits user mobility.
Adoption of Li-Fi in the marketplace has been very limited thus far. However, there’s an industry association that provides standards and support, and there’s the potential for using a single LED bulb/photoreceptor unit as both light source and Li-Fi node (see Resources below).
Researchers, of course, see pushing the envelope of optical-based data links as an area of great interest. A team at Leti (a research institute of CEA Tech, Grenoble, France) has achieved a visible light communication (VLC) test-bed transmission at 7.7 Gb/s (significantly exceeding the previous 5.1-Gb/s record) using a single, 10-µm diameter, gallium-nitride (GaN) blue micro LED (Fig. 1). (In general, a smaller emissive area of the LED yields a higher bandwidth—here, 1.8 GHz in the institute’s single-blue micro LED project.)
In addition to the micro LED, the team also developed an advanced multi-carrier modulation scheme combined with digital signal processing to achieve their results. This high spectrum-efficiency waveform was transmitted by the single LED, received on a high-speed photodetector, and demodulated using a direct sampling oscilloscope (Fig. 2).
This class of experimental test bed requires many electro-optical components as well as test-and-measurement equipment for support, including these:
- Keithley (Tektronix) 2601 Current/voltage Source /DC Supply
- Mini-Circuits ZHL-1000-3W 3W, 500 to 1000 MHz Amplifier
- Tektronix AWG5204 Arbitrary Waveform Generator
- Microworld MWRF-ECP18 18-GHz RF probe
- Femto HSPR-X2 GHz Ultra-Fast Photoreceiver
- Thor Labs BBE1-E02 - 1" Broadband Dielectric Elliptical Mirror
- Tektronix 7254 Digital Oscilloscope
- Picosecond Pulse Labs (Tektronix) 5575A Bias Tee
While the Light Communications Alliance (created in 2019) is intended to encourage the industry to implement standardization and promote interoperability between Li-Fi systems from different manufacturers, CEA-Leti is planning to continue its research in two areas:
- Developing a better understanding of the electrical behavior of single LEDs in high-frequency regimes and the link between bandwidth and electromigration patterns.
- Investigating techniques to improve the range and/or increase the data rate using a matrix of multi-LED emissive devices. This requires adapting the waveform generation as well as a CMOS interposer to drive the matrix on a pixel basis.