In addition to lighting LED there are five major segments of the market

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.

1, indoor positioning

There are many problems such as low stability, long response time, large electromagnetic interference, low precision and low accuracy in RF, Bluetooth and ultrasonic system. VLC is free from electromagnetic interference, and can be used for fast and accurate positioning and navigation. In the future, large shopping malls, underground shopping centers and other places can be targeted and guided by VLC based intelligent pedestrian support system to reduce the losses caused by the specific location of the goods. Similar applications can be extended to airports, museums, and other digital location broadcasts. VLC indoor positioning system according to different needs, the positioning accuracy is also different. For indoor pedestrian or mentally handicapped navigation class positioning, moderate accuracy can be. But the positioning accuracy of the robot will be higher.

(1) indoor pedestrian navigation

An indoor navigation system for visually impaired people, as shown in the following figure, LED is mounted on the ceiling, with a magnetic sensor mounted on the user's neck, a smartphone connected to the headset. The mobile phone receives the ID information from the LED through the visible light communication to obtain the direction of the light source and the positioning information. The test results show that the ~66 range can be accurately positioned and navigation in the 1~2m range, and can be achieved in the range of -59 degrees.

The VLC positioning navigation system is developed based on the LED light source of the wall panel. The system is designed to reduce the interference of background light. Through VLC, the user's mobile phone receives the navigation information from the panel LED. The navigation information includes the position information of the panel and the optimal light intensity for the visually impaired to reach the downloaded data information. In this system, the data transfer rate of navigation signal is 62.5kb/s, and the data information transmission rate is 1Mb/s. In the test, the visually impaired can receive about 5m far from the label plate LED light coming from the navigation signal through the headset, when walking distance to sign panel about 1m far, because the light intensity is enough, so the visually impaired receiver can automatically download and store text information.

(2) indoor accurate positioning

In many applications, such as robot and unmanned aerial vehicle (UVA) control system, precise positioning is needed. How to achieve high positioning accuracy under the premise of ensuring the complexity of the system is a hot research topic. There are many ways to get the target position, such as the measurement signal arrival angle (AOA), arrival time (TOA), arrival time difference (TDoA) and received signal strength (RSS), etc.. An indoor positioning system based on RRS without synchronization. The test results show that the accurate positioning is achieved with 95% accuracy in 5.9cm. There is also an indoor positioning system based on the received signal strength / time division multiplexing mode (RSS/TDM). The bit stuffing method can be used to measure the instantaneous signal intensity without interference of various optical signals. In this system, 16 LED lamps with a pitch of 40cm are fixed on the ceiling, and the VLC receiver is mounted on the ceiling 170cm robot. The test results show that the robot can realize the autonomous navigation of 5 target points in the error of less than 3cm.

In addition to the image sensor can realize image capture and data receiving, due to external interference light focusing pixels and VLC ideal signal focusing pixel difference, so the ratio of photoelectric diode as a receiver has a better anti-jamming capability. More and more attention has been paid to the application of image sensor in VLC. Wireless personal area network (WPAN) IEEE standardization organization P802.15 working group has been considering how to integrate the camera into the camera communication standards.

Research on several applications of image sensor. The following figure (a) shows the receiving end of the image and position information at the same time access. (b) is the application of image sensor in robot. Two image sensors fixed on the robot send information by LED to detect the position of the robot. An image sensor captures the direction of the incident light, and another image sensor captures the high speed frame image continuously and analyzes the data transmitted by the visible light. The test results show that the robot can achieve precise positioning in centimeter range. (c) is the use of images