In addition to lighting, LEDs have five powerful application directions

In addition to lighting advantages, LEDs also have characteristics such as short response time and high-speed modulation. The light flicker caused by high-speed modulation of white light LED is not easily detected by the human eye, and it can provide data communication function while lighting. This technology for data communication in the visible spectrum range of 380~780nm is referred to as visible light communication (VLC) technology. VLC has great potential in the fields of medium and short-distance secure and confidential communications, high-precision and accurate positioning, transportation communications, and indoor navigation. In particular, it can replace radio frequency (RF) to solve the "last 1m" problem.

Compared with radio waves, visible light communication has many advantages: 1) The amount of information is developing according to Moore's Law, and many frequency bands of the radio spectrum have been occupied. VLC uses the visible light spectrum that is higher than 3THz and is still a blank spectrum, and is not subject to license restrictions; 2) Visible light cannot penetrate building walls, and VLC signals in closed units are adjacent to each other. It will not interfere with each other, has high security and good confidentiality; 3) The visible light transceiver device is simple and low-priced; 4) The wavelength of visible light is in the sub-micron range, which has obvious advantages in accurate direction positioning; 5) VLC can replace the application of radio in some specific occasions that are sensitive to electromagnetic interference (such as aircraft, hospitals, nuclear power plants or oil drilling, etc.).

1. Indoor positioning

The RF, Bluetooth, and ultrasound methods used in indoor positioning systems have problems such as low system stability, long response time, large electromagnetic interference, and low precision and accuracy. VLC is not subject to electromagnetic interference and can achieve fast and accurate positioning and navigation through indoor fixed light sources. In the future, large shopping malls, underground shopping malls and other places can reduce losses caused by customers not being able to find the specific location of goods through the target positioning and guidance of intelligent pedestrian automatic support systems based on VLC. Similar applications can be extended to airports, museums and other digital positioning broadcast situations. The VLC indoor positioning system has different positioning accuracy according to different needs. For indoor navigation positioning of pedestrians or mentally retarded people, medium accuracy is sufficient. However, the positioning accuracy requirements for robots and other applications will be higher.

(1) Indoor pedestrian navigation

An indoor navigation system that helps visually impaired people identify their location, as shown in the picture below, LEDs are installed on the ceiling, a smartphone with a geomagnetic sensor is hung around the user's neck, and the smartphone is connected to the headset. The mobile phone receives the ID information from the LED through visible light communication and obtains the direction and positioning information of the light source; after optimizing the path, the result is sent to the headset to guide pedestrians. The test results show that testers can accurately position and navigate within a range of 1~2m, and can achieve direction correction within a range of -59°~66°.

VLC positioning and navigation system developed in conjunction with wall sign panel LED light source: This system is designed with filters to reduce background light interference. Through VLC, the user's mobile phone receives navigation information from the sign panel LED. Navigation information includes signage panel location information and optimal light intensity points for the visually impaired to reach downloadable data information. In this system, the data transmission rate of navigation signals is 62.5kb/s, and the data information transmission rate is 1Mb/s. During the test, the visually impaired can receive the navigation signal from the LED light on the sign panel about 5m away through headphones. When they walk about 1m away from the sign panel, because the light intensity is sufficient, the visually impaired receiver can automatically download and store the text information.

(2) Accurate indoor positioning

Precise positioning is required in many applications such as robots and unmanned aerial vehicle (UVA) control systems. How to obtain high positioning accuracy without high system complexity is a hot topic of research. There are many methods to obtain the target position, such as measuring signal angle of arrival (AOA), time of arrival (TOA), time difference of arrival (TDoA), and received signal strength (RSS). There is an indoor positioning system based on RRS that does not require synchronization. Test results show that precise positioning is achieved with an accuracy of 95% within 5.9cm. There is also an indoor positioning system based on the received signal strength/time division multiplexing mode (RSS/TDM) bit stuffing method. The bit stuffing method can measure instantaneous signal strength without interference from various optical signals. In this system, 16 LED lights spaced 40cm apart form a square array and are fixed on the ceiling. The VLC receiver is installed on the robot 170cm away from the ceiling. The test results showed that the robot successfully achieved autonomous navigation of the planned path of 5 target points within an error of less than 3cm.

In addition to being able to achieve image capture and data reception at the same time, the image sensor has better anti-interference ability than the photodiode as a receiver due to the difference between the focusing pixels of the external interference light source and the focusing pixels of the VLC ideal signal. The application of image sensors for accurate positioning in VLC has attracted more and more attention. The IEEE standardization organization P802.15 working group for Wireless Personal Area Networks (WPAN) is already considering how to integrate positioning into the camera communication standard.

Research on several applications of image sensors. The screen (a) in the figure below shows the simultaneous acquisition of image and position information by the receiving end. (b) is the application of image sensors on robots. Two image sensors fixed to the robot detect the robot's position through information sent by LEDs. One image sensor captures the direction of incident light, and the other image sensor continuously captures high-speed frame images and analyzes the data from visible light. Test results show that the robot can achieve precise positioning within a centimeter range. (c) uses image capture and visible light communication to achieve accurate positioning of an external water tank 50m away, with positioning accuracy reaching millimeter level.

2. Information transmission of intelligent transportation systems (ITS)

Intelligent transportation systems (ITS) reduce traffic accidents, traffic jams, and fuel consumption by promptly acquiring and transmitting information about traffic conditions and other aspects. Image processing can be used to help drivers identify traffic lights, detect obstacles, and capture information between vehicles and vehicles and road facilities. It is a key technology of ITS. High-speed data image sensors combined with VLC have broad application prospects in ITS. The literature documents the use of LED light and a camera mounted on a car with a frame rate of 1000 frame/s and a resolution of 1024pixel×1024pixel for communication between roadside devices and vehicles. The actual vehicle test verified that the system can accurately receive data transmitted from a light source composed of an array of 256 LEDs within a range of 35m when the vehicle is traveling at a speed of 30km/h.

Currently, there are some technical problems that need to be solved when using high-speed image sensors in VLC: installation facilities are expensive, complex real-time image processing technology is required, and the camera frame rate is lower than the response speed of light-emitting diodes, which limits the communication speed. The literature reports the application of the newly developed optical communication image sensor (OCI) in wireless optical communication (OWC) systems. The newly developed OCI using complementary metal oxide semiconductor (CMOS) image sensing technology has two features: communication pixels (CPx) for high-speed signal reception and 1-bit logo image output function for fast and accurate LED detection. The OWC system is known as the world's first pixel transmission communication system with only 16.6ms delay and 20Mb/s transmission rate.

3. VLC high-speed data transmission

The acquisition of large amounts of data, such as high-definition (HD) video streaming transmission, high-speed information stream downloading and high-speed data backup, has become an indispensable part of contemporary life. Visible light can use light beams with small divergence angles for data transmission, and its low path transmission loss makes high-speed bandwidth

Secure data stream downloading and transmission is possible. As shown in the figure below, the European OMEGA project has developed a VLC data transmission with 4 HD data streams at 100Mb/s. Orthogonal frequency division multiplexing (OFDM) technology is used to transmit data to photodiode detectors within an area of ​​10m2 through 16 LED light sources.

Demonstration system for achieving high-quality broadcast transmission of 5m×5m×3m indoor video and audio by adding a focusing lens between the VLC receiver and transmitter: Compared with the line-of-sight (LOS) link, the Nd-LOS link has the characteristics of good network node mobility and strong tracking, but its link loss results in a low transmission rate. Research was carried out to address this issue, and a typical indoor two-way optical wireless communication system with a range of 2m, a transmission rate of 400Mb/s and a bit error rate of RBE<10-8 was achieved (visible light LED is used as the downlink transmission light source, and IR is the uplink data transmission light source). RGBLEDs use wavelength division multiplexing (WDM) technology to achieve higher data transmission rates. The literature records the first VLC system based on wavelength division multiplexing/discrete multi-tone (WDM/DMT) modulation technology, which realizes the modulation of a single white light RGB-LED and obtains data communication at a rate of 803Mb/s.

4. VLC application in aviation

VLC has obvious advantages in aviation. Visible light LEDs have been promoted and applied in the new generation of commercial aircraft. Communication through visible light instead of optical cables or cables can reduce size and weight, reduce electromagnetic interference (EMI), and reduce costs. Boeing Commercial Aircraft Platform is also conducting research on future wireless optical network solutions. Applications of passenger reading light service units (PSUs), entertainment systems and cabin intercoms based on VLC communication: In (a) below, the reading light illuminates the passenger seating area, and at the same time, the reading light serves as an optical communication source to transmit data to the passenger laptop or other terminal receiver. The passenger entertainment system forms a high-speed mesh network through visible light communication between entire rows of seats to realize the transmission of in-flight entertainment (iFE) information. The light signal from the LED light can also be used as low-bandwidth voice communication to achieve in-machine intercom, as shown in (b).

5. Underwater VLC

Underwater communications play an important role in military, industrial and scientific fields. Underwater communication speed requirements range from several megabytes to tens of megabytes or even higher. Seawater is highly attenuated by radio waves. The transmission speed of sound waves in the ocean is 1500m/s, with long delay time, limited bandwidth, and high bit error rate. Sound waves can also cause interference to marine animals such as dolphins and whales. VLC can well overcome problems such as attenuation and electromagnetic interference underwater. Regarding the development of the VLC communication system between the operator and the underwater remote transport vehicle: the transmitter in this system is composed of 70 LED light sources, supporting data transmission within a 120° viewing angle range, and the transmission rate is as high as 20Mb/s. The literature documents specific applications of underwater visible light communication systems in machine inspection at nuclear power plants. The underwater remote transport vehicle is equipped with a camera to transmit video and control signals to a site 20m away. The main challenge currently facing underwater optical wireless communication systems is the difficulty in achieving long-distance communications due to the influence of underwater properties, especially turbidity. In response to this problem, the National University of Singapore and the Massachusetts Institute of Technology developed an underwater optical communication system that achieves 1.2Mb/s within a distance of 30m. Research and Demonstration.

For more relevant information, please click on the Sosoled website (wwwsosoledcom) or follow the WeChat public account