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71-Mbit/s ultraviolet-B LED Li-Fi based on 8-QAM-OFDM modulation

71-Mbit/s ultraviolet-B LED Li-Fi based on 8-QAM-OFDM modulation

By BizLED Bureau

Sep 18, 2017: Researchers at King Abdullah University of Science & Technology (KAUST) have modulated the signal of an ultraviolet LED emitting at 294nm, and demonstrated a high-speed near-solar-blind communication link with a data rate of 71 Mbit/s.

They used non-visible light for communication systems in the UV-B band, which have several benefits. The researchers explain the process in a paper titled “71-Mbit/s ultraviolet-B LED communication link based on 8-QAM-OFDM modulation”, published in the Optics Express journal.

A demonstration of ultraviolet-B (UVB) communication link is implemented utilizing quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM). The demonstration is based on a 294-nm UVB-light-emitting-diode (UVB-LED) with a full-width at half-maximum (FWHM) of 9 nm and light output power of 190 μW, at 7 V, with a special silica gel lens on top of it. A −3-dB bandwidth of 29 MHz was measured and a high-speed near-solar-blind communication link with a data rate of 71 Mbit/s was achieved using 8-QAM-OFDM at perfect alignment. 23.6 Mbit/s using 2-QAM-OFDM when the angle subtended by the pointing directions of the UVB-LED and photodetector (PD) is 12 degrees, thus establishing a diffuse-line-of-sight (LOS) link. The measured bit-error rate (BER) of 2.8 ×× 10−410−4 and 2.4 ×× 10−410−4, respectively, are well below the forward error correction (FEC) criterion of 3.8 ×× 10−310−3. The demonstrated high data-rate OFDM-based UVB communication link paves the way for realizing high-speed non-line-of-sight free-space optical communications.

There are low background solar radiation and low device dark noise for the UV-band, especially for the UVC/B band (100-280nm/280-315nm), making the communication link near-solar-blind and impervious to noise. As UV radiations are scattered by many molecules and aerosols present in the atmosphere, the UV band is indicated to establish a non-line-of-sight (NLOS) communication link, with less restrictive requirements for pointing, acquisition, and tracking of the light beam.

Quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM) has been used to inject a modulated signal into a 294nm UVB LED with a special silica gel lens on top of it, the signal was then picked up by a silicon avalanche photodetector.

The UVB LED had a half-maximum (FWHM) of 9nm, it delivered 190μW of light output at 7V. The researchers reported a −3-dB bandwidth of 29MHz and a data rate of 71 Mbit/s when using 8-QAM-OFDM at perfect alignment. The link’s speed dropped down to 23.6 Mbit/s using 2-QAM-OFDM when the UVB-LED and photodetector where pointing at intersecting angles up to 12 degrees (for a diffuse-line-of-sight link), over an 8cm distance. This is to be compared with the previous record data rate of 2.4 kbps for links based on UV-LEDs.

The spectra of the UVB-LED is plotted, a ~294-nm peak emission with FWHM of ~9 nm was measured. The small signal frequency response of the UV channel is measured, which takes all the components into account in the system, like the transmitter, i.e. the UV LED, and the receiver, i.e. the APD. The UV LED bandwidth was measured using the Digital Phosphor Oscilloscope (DPO) for received signals with different amplitudes through varying their modulation frequency. The −3 dB bandwidth, i.e. the frequency at which the received signal voltage amplitude decreases to 12√12 is 29 MHz, was measured by keeping the distance between the LED and APD as close as possible (distance = 0).

Meanwhile, the measured bit-error rate (BER) for a direct line-of-sight link was 2.8×4 -10, and 2.4×4 -10 for a diffuse-LOS link, well below the forward error correction (FEC) criterion of 3.8×3 -10.

Although the communication links were only established over a few centimeters, the researchers expect that with the development of higher power UVB or UVC LEDs or laser diodes capable of emitting tens and hundreds of milliwatts, high-speed non-line-of-sight free-space optical communications could become a reality.

Source: KAUST

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