Using non-visible light for communication systems in the UV-B band has several benefits, the researchers explain in a paper titled "71-Mbit/s ultraviolet-B LED communication link based on 8-QAM-OFDM modulation", published in the Optics Express journal.
Among other things, 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.
What's more, because UV radiations are scattered by many molecules and aerosols present in the atmosphere, the UV band is particularly indicated to establish a non-line-of-sight (NLOS) communication link, with less restrictive requirements for pointing, acquisition, and tracking of the light beam.
The researchers used quadrature amplitude modulation (QAM) orthogonal frequency-division multiplexing (OFDM) 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 authors 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.
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.