LIFI hot VLC basic links, standards and development issues

Compared with incandescent lamp and fluorescent lamp, white light emitting diode (LED) has the characteristics of long life, high luminous efficiency, low power consumption, no radiation, good safety and high reliability. White LED is very competitive in the future market. Around about 14000000000 of the world's incandescent bulbs are converted into more energy-efficient LED. The Japanese government 10 years ago as LED lighting technology in twenty-first Century, the Chinese government has also issued a few years to gradually end the incandescent lamp sales policy.

In addition to lighting advantages, LED also has the characteristics of short response time and high speed modulation. The light flicker caused by high speed modulation of white LED is not easy to be detected by the human eye. The technology of data communication in the visible spectrum of 380~780nm is called visible light communication (VLC). VLC has great potential in the field of medium and short distance secure communication, high precision and accurate positioning, traffic and communication, and indoor navigation, especially in the field of radio frequency (RF), which can solve the problem of "last 1m". Compared with radio waves, visible light communication has many advantages: 1) the amount of information in the development of Moore's law, many radio spectrum band has been occupied by VLC is higher than that of 3THz and visible light spectrum spectrum are still blank, without use of licensing restrictions; 2) visible light can not penetrate the wall of the building, each other the adjacent closed VLC signal unit will not interfere with each other, high safety, good confidentiality; 3) visible light receiving device has the advantages of simple equipment, low price; 4) belongs to the sub micron wavelength of visible light, has obvious advantages in accurate orientation; 5) VLC can replace the radio in some specific occasions (sensitive to electromagnetic interference such as aircraft, hospitals, nuclear power plants or oil drilling, etc.) in the application.

VLC and RF compared to the most obvious problem is the visible light transmission rate is limited by the communication distance. VLC uses non coherent communication mode, VLC communication path loss is the 4 side of the distance, and relative to RF, the loss is the square of the distance. LED has the inherent nonlinear current intensity characteristics, the performance decreases sharply with the increase of temperature, and the output light color and the equipment life are reduced rapidly. In addition, the dimming of the lights will have a certain impact on the transmission power and transmission performance. These disadvantages limit the application of VLC, VLC can not completely replace the high-speed RF communication. How to make full use of the advantages of VLC and overcome the unfavorable factors to improve the performance of VLC communication is a hot research topic.

VLC technology was first proposed in 1999 by the University of Hong Kong Grantham Pang and carried out audio transmission for VLC research. Japan then launched an in-depth study of VLC. 2003, the visible light Communications Association (VLCC) was established in japan. At present, more and more organizations and organizations are committed to the research of the key technologies of VLC. Including the typical research institutions: the European project home Gigabit access network (OMEGA, HomeGigabitAccessNetwork), the optical communication center (UCL), Heinrich Herz, the German Fraunhofer Institute of communication, Holland PHILPS, France Telecom, University of Oxford, Australia and University of Science & Technology China, Fudan University, Monash University Leah. These institutions have made outstanding achievements in the theory, algorithm, simulation and experiment of VLC. However, the development of VLC is still in its infancy, and there is still a gap between mature industry, life and military applications. At present, with China's LED industry has been in rapid climb, "" Research on key technology of visible light communication system was a major breakthrough, real-time communication rate increased to 50Gbps (bits per second), the equivalent of 0.2 seconds to complete a high-definition movie downloads.

VLC basic link and communication standard

1.1 VLC basic link

Indoor VLC basic link

light source

White LED there are three main types: red green and blue mixed into white light RGB-LED, blue LED fluorescence powder and mixed into white light PC-LED and UV LED surface to produce white light by mixing red and green phosphors UV-LED. UV-LED in the formation of white light energy loss, low light efficiency, rare in practical applications. For PC-LED, the blue light LED modulation bandwidth is about 35MHz, affected by the yellow light component, its modulation bandwidth is only a few megabytes. In order to improve the modulation bandwidth, the blue light filter is usually added to filter the Yellow component. PC-LED low cost, simple drive, in the application of lighting in general. Three LED in RGB-LED can be independently modulated. The 15MHz modulation bandwidth can be obtained by using multi wavelength division technique for each RGB-LED. This type of LED is expensive, relatively complex, highly efficient and flexible lighting effect and high modulation bandwidth.

Receiver

PIN diode, avalanche diode (APD) and image sensor are used in VLC receiver. PIN photodiode low price, large receiving area, high sensitivity and less sensitive to temperature, widely used. APD photodiode has high sensitivity, small receiving area and high cost. The image sensor can obtain image and data information without interference. Because the image sensor detects only the intensity of the light transmitted by the LED, the image sensor can still receive the data it needs even if there are multiple light sources transmitting data at the same time. Image sensor is especially suitable for accurate positioning system.

1.2 communication standard

In 2007, Japan released the JEITACP-1221 "visible light communication system" and JEITACP-1222 "visible light ID system". In 2009, IrDA and VLCC jointly developed the "IrDA visible communication physical layer technical requirements". Europe