In recent years, with the development of white light-emitting diode ( LED ) technology, Visible Light Communication (VLC) has become one of the research hotspots of next-generation wireless communication technology. VLC is also called LiFi (Light Fidelity). In 2011, German physicist Hardal Hass from the University of Edinburgh gave a speech on LiFi technology at the TED conference. For the first time, "VLC" was called "LiFi".
LiFi is an emerging wireless communication technology based on light (rather than radio waves) that combines the lighting and data communication functions of light, as shown in Figure 1. LiFi modulates the signal on the LED light source without affecting the LED illumination, and transmits the data by quickly switching a high-frequency flicker signal that the human eye cannot perceive.
Advantages of LiFi
Compared with the current mainstream WiFi communication technology, LiFi has the following advantages:
(1) In terms of capacity, the spectrum of radio waves is very crowded, and the spectrum width of visible light (about 400 THz) is 10,000 times more than that of radio waves;
(2) In terms of efficiency, the efficiency of the radio wave base station is only 5%, most of the energy is only consumed by the cooling of the base station, and the data of the LiFi can be transmitted in parallel while improving the efficiency;
(3) In terms of practicality, radio waves are only acquired in the base station, and WiFi cannot be used on the aircraft, in the operating room or at the gas station, and each lamp in the world can be easily connected to the LiFi hotspot;
(4) In terms of safety, radio waves are easily invaded, and visible light cannot pass through walls or even curtains, providing privacy and security for the network.
As a new technology that combines lighting and communication, LiFi does not affect the quality and requirements of lighting, especially in the development of light sources, while pursuing high transmission rates. The light source of LiFi must have the advantages of good modulation performance, high transmission power and high response sensitivity of the communication light source, and it also needs to meet the characteristics of high brightness, low power consumption and wide radiation range of the illumination source.
LiFi light source selection
1, LED
At present, most of the light sources used in LiFi technology are white LEDs, and a large part of the reason is due to the rapid development of LED technology . The realization of white LEDs mainly includes: blue LED chip excitation yellow-green phosphor converted into white light (PC-LED), ultraviolet light or ultraviolet LED excitation three primary color phosphors to produce white light and red, green and blue three kinds of LED chips are packaged in Mix together to produce white light (RGB-LED). The commercial white LED products at this stage are mainly divided into two categories according to the different spectral components: PC-LED and RGB-LED. The spectrum of the two types of white LEDs is shown in Figure 2.
The modulation bandwidth of the LED determines the channel capacity and transmission rate of the communication system . Studying the modulation characteristics of the LED device is one of the key issues to improve the performance of the new LiFi system. The LED modulation bandwidth is defined as the frequency at which the AC light output from the LED drops to 50% of a reference frequency value (-3 dB). Since the photoelectric response of the spectral portion of the yellow phosphor of the PC-LED is relatively lagging, as shown in FIG. 3, the modulation bandwidth of the LiFi light source is limited to within a few megahertz, thereby limiting the communication rate of the entire system, even at the receiving end. The blue filter also failed to significantly improve the defects of the light source.
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