French research institute CEA-Leti has achieved a data transmission rate of 7.7 Gbps in visible light communications (VLC) using a single GaN blue micro- light-emitting diode (LED), marking another step towards commercialisation and widespread use of the technology.
VLC, commonly called li-fi (short for 'light fidelity'), is an emerging wireless communication system that offers an alternative or a complementary technology to radio frequency (RF) systems such as wi-fi and 5G. It is considered to be a promising technology for security-related applications because light propagation can be confined to a room with no information leakage, as opposed to wi-fi communication, which penetrates walls. Li-fi also holds promise for ultra-highspeed data transmission in environments where RF emissions are controlled, like hospitals, schools, and airplanes.
Single microLED communications offer an ultra-high data-transmission rate for a variety of opportunities for new applications. These include industrial wireless high-speed links in demanding environments such as assembly lines and data centers, and contact-less connectors, or chip-to-chip communication. But their weak optical power limits their applications to short-range communications. In contrast, matrices of thousands of microLEDs contain higher optical powers than open mid- and long-range applications. However, preserving the bandwidth of each microLED within a matrix requires that each signal has to be brought as close as possible to the micro-optical source.
The University of Edinburgh in the UK, led by Harold Haas, recognised for coining the term 'li-fi' has also had success with micro-LEDs. In work reported in Photonics Research, segmented violet micro-LEDs were used with electrical-to-optical bandwidths up to 655 MHz. An orthogonal frequency division multiplexing (OFDM) based VLC system with adaptive bit and energy loading was demonstrated and a data transmission rate of 11.95 Gbps was achieved with a violet micro-LED, when the nonlinear distortion of the micro-LED is the dominant noise source of the VLC system. A 7.91 Gbpss data transmission rate was reported below the forward error correction threshold using a single pixel of the segmented array when all the noise sources of the VLC system are present.
CEA-Leti’s expertise in the microLED epitaxial process produces microLEDs as small as 10 microns. The smaller the emissive area of the LED, the higher the communication bandwidth – 1.8 GHz in the institute’s single-blue microLED project. The team also produced an advanced multi-carrier modulation combined with digital signal processing. This high-spectrum-efficiency waveform was transmitted by the single LED and was received on a high-speed photodetector and demodulated using a direct sampling oscilloscope.
CEA-Leti testbed for single GaN blue microLED that achieved throughput of 7.7 Gbps
'This technology has exciting potential for mass-market applications,' said Benoit Miscopein, CEA-Leti research scientist. 'Multi-LED systems could replace wi-fi, but wide-scale adoption will require a standardisation process to ensure the systems’ interoperability between different manufacturers. The Light Communications Alliance was created in 2019 to encourage the industry to implement this standardisation.'
In addition to a stand-alone wi-fi-like standard, the possibility to include this new technology as a component carrier in the downlink of 5G-NR, a radio-access technology for 5G mobile considerations, is also under investigation to bring a large additional license-free bandwidth.
'This may be feasible because CEA-Leti’s li-fi physical layer relies on the same concepts as wi-fi and 5G technologies,' said Miscopein. 'Matrices of thousands of microLEDs could also open the way to mid- to long-range applications, such as indoor wireless multiple access.'
Preserving the bandwidth of each microLED within a matrix requires that each signal is generated as close as possible to the micro-optical source.
'To meet this challenge, we expect to hybridise the microLED matrix onto another matrix of CMOS drivers: one simple CMOS driver will pilot one microLED,' Miscopein said. 'This will also enable the additional feature of piloting each microLED pixel independently, and that allows new types of digital-to-optical waveforms that could eliminate the need for digital-to-analog converters commonly used in the conventional ‘analogue’ implementations of li-fi.'
While the Light Communications Alliance will promote interoperability between different manufacturers’ li-fi systems, CEA-Leti will continue its research in two areas:
- A better understanding of the electrical behavior of single LEDs in high frequency regimes and the link between bandwidth and electromigration patterns, and
- Techniques to improve the range and/or increase the data rate using multi-LED emissive devices. This requires adapting the waveform generation as well as a CMOS interposer to drive the matrix on a pixel basis.
