visible light communication (VLC)
Visible Light Communication(VLC) is a wireless transmission technology in which visible light is modulated with the data signal. This technology is also referred to as Light Fidelity( LiFi). Generally, light modulation is used in optical transmission technology and also in infrared LANs. In both cases, it is non-visible light from light-emitting diodes(LEDs) or infrared LEDs( IR LEDs) that is modulated. It is different with the VLC method, which works with visible light with wavelengths between 375 nm and 780 nm: with light from LED lights.
Structure of a VLC data link
Conceptually, Visible Light Communication consists of the transmitter device, for example a light emitting diode, and a receiver device, for example a photodiode. The VLC transmitter is stationary and is also the LED lighting for the room or office. The receiving stations, the VLC receivers, can be stationary or mobile.
The VLC technology functionally realizes the physical layer. All protocols can be transmitted on this layer. For data transmission, the data from a server is converted into the corresponding protocol in the VLC transmitter and modulated into light signals. For this purpose, the signals can be resampled in pulse phase mod ulation( PPM), non return to zero ( NRZ) in combination with on-off keying( OOK) or another method. On the receiving side, the modulated light signals are converted into voltage signals in a photodiode or phototransistor and amplified. Since light pulses from white LEDs have different decay times for fluorescence and phosphorescence, the longer phosphorescence of the blue phosphor is filtered out.
Decisive for the reception range is the radiation range of the illumination module. It has a strong angular dependence, which is determined by the Field of View( FoV), the radiation area. Data can only be transmitted if there is a line of sight( LOS) between the VLC transmitter and receiver and if the VLC receiver is in the Field of View. As soon as the line of sight is broken, no transmission takes place. Data rates can reach up to 1 Gbps. With wavelength division multiplexing, data rates of up to 3 Gbit/s are also possible. In this process, three light-emitting diodes with the primary colors red, green and blue are modulated independently of each other, transmitted via the VLC channel and, on the receiving side, broken down into the individual colors again by an optical filter and demodulated.
In 2011, the Heinrich Hertz Institute (HHI) in Berlin achieved data rates of 800 Mbit/s over distances of a few meters via line-of-sight (LOS) in the laboratory. In the meantime, data rates of 3 Gbit/s are being realized. Over longer distances of more than one kilometer, data rates of a few megabits per second could be realized.
Areas of application for VLC technology
The VLC technology was dealt with by the ITU-T in a recommendation under G.vlc and standardized under the G.9991 standard. In addition, the IEEE working groups802.15.7, 802 .15.13 and 802.11bb are concerned with the standardization of Visible Light Communication (VLC) and LiFi. VLC transmission can be used wherever WLANs cannot be used, furthermore in airplanes, hospitals, in congress halls for simultaneous translation into different languages, in production, traffic engineering and in home networking for the transmission of multimedia content, music and television. The VLC concept can be well combined with Powerline because it allows other rooms to be included and bypasses the limitation of line-of-sight.