In addition to the advantage of lighting, LED has short response time and high speed modulation characteristics. White LED light flashing is not easy to be caused by high speed modulation to the human eye to detect, can provide the function of data communication while lighting. In the period of 380 ~ 780 nm visible spectrum data communication technology referred to as "for the visible light communication (VLC) technology. VLC in the privacy communication, high precision, short safety in accurate location, transportation, communication and indoor navigation and other fields has great potential, especially can replace radio frequency (RF) solve the problem of "last 1 m".



Compared with radio waves, visible light communication has many advantages: 1) the amount of information on the Moore's law development, the radio spectrum has occupied a lot of band, VLC use is higher than 3 THZ spectrum of visible light and still belong to the white Spaces, without being limited by the use license; 2) visible light cannot penetrate the building wall, adjacent to each other in a closed cell VLC signals do not interfere with each other, high safety, good secrecy; 3) optical transceiver devices simple equipment, low cost; 4) the visible light wavelength belongs to the sub-micron, has obvious advantages in the direction of accurate positioning; 5) VLC can replace radio in some certain occasion is sensitive to electromagnetic interference, such as aircraft, hospitals, nuclear power plants or oil drilling, etc.) the application.



1, indoor positioning



Indoor positioning system using RF, bluetooth and ultrasonic methods such as system stability is not high, long response time, electromagnetic interference, low precision and accuracy. VLC is not affected by electromagnetic interference, through indoor fixed light can achieve rapid accurate positioning and navigation. The future of the place such as shopping malls, underground shopping center by intelligent pedestrian automatic support system based on VLC target positioning and guidance, reduce the customer can't find the goods location of damage. A similar application can be extended to the airport, museums and other digital radio positioning occasions. VLC indoor positioning system according to different requirements, the positioning accuracy is also different. For indoor pedestrian navigation positioning of a class or mentally disabled, moderate accuracy. But for a robot positioning accuracy is higher.



(1) indoor pedestrian navigation



Help visual disabilities to identify the location of the indoor navigation system, as shown in the figure below, the LED is installed on the ceiling, with a magnetic sensor smartphone users to hang in the neck, smart phones and headsets. Mobile phone through the visible light communication receive LED ID information, from the light source direction and positioning information; After the path optimization and transmitted to the headphone lead pedestrians will be results. Test results show that within 1 ~ 2 m testers can accurate positioning and navigation, and can in 59 ° ~ 66 ° direction within the scope of correction.



Combination of signs on the wall panel, LED light source development VLC navigation and positioning system: the system design of the filter to reduce the interference of background light. Through the VLC, user phone receiving sign panel LED from the navigation information. Navigation information including sign panel location information and the best visual disabilities achieve downloadable data information light counterparts. Navigation signal in the system of data transfer rate of 62.5 KB/s, and the data transfer rate of 1 MB/s. Tests, visual disabilities can receive about 5 m on headphones navigation signals coming in the distance the sign on the front panel LED light, when go to about 1 m far from the sign panel, the light intensity is enough, so the visual disabilities receiver can automatically download and store the text information.



(2) indoor positioning accurately



In robots and unmanned aerial vehicle (uav) (UVA) control system, many applications require accurate positioning. How to guarantee the system complexity is not high on the premise of high positioning accuracy is the research hot spot. Access to the target location methods are many, such as the Angle measurement signal reaches (AOA), time of arrival (TOA), to the time difference (TDoA) and received signal strength (RSS), etc. There is a need not synchronization based on RSS indoor positioning system. Test results show that with 95% accuracy within 5.9 cm realize accurate positioning. Still have a kind of based on received signal strength/time-division multiplexing mode (RSS/TDM) bit filling method of indoor positioning system. Bit filling method can realize all kinds of optical signal interference instantaneous signal strength measurement. 16 spacing between each other in the system for 40 cm leds composed of square array is fixed on the ceiling, VLC receiver installed on 170 cm from the ceiling of the robot. The error of the test results show that the robot in less than 3 cm, successfully realized the five target path planning of autonomous navigation.



Image sensor besides can realize image capturing and data reception, due to the interference light focus pixels and VLC ideal signal pixels, so than photoelectric diode as receiver has better anti-interference ability. The image sensor in VLC accurate location and used by people more and more attention. Wireless personal area network (WPAN) IEEE P802.15 standardization organization of the working group has been considering how to locate into camera communications standards.



The application research about the image sensor. The following figure (a) the display screen shows the receiver to image and position information obtained at the same time. (b) is the application of image sensor on robots. Two fixed image sensor on robots send information detection robot position by LED. One of the image sensor to capture incoming light direction, another image sensor to capture continuously high speed frame and parses the data from visible light. The test results show that the robot can achieve precise positioning in cm range. (c) is the use of image capture and visible light communication to achieve 50 m far outside the tank accurate positioning, positioning accuracy to achieve millimeter level.



2, intelligent transportation systems (ITS) information transmission



Intelligent transportation system (ITS) through timely acquire and transfer information about traffic conditions and so on to reduce the number of traffic accidents, traffic jams, and fuel consumption. Image processing can be used to help the driver to identify the traffic lights, obstacle detection, vehicle and vehicle and road facilities as well as between the capture of information, is one of ITS key technology. High speed data image sensor combined with VLC on ITS has a broad application prospect. Literature records the use of LED light and installed in the car with 1000 frame/s frame rate and pixel 1024 x 1024 pixel resolution camera device on the side of the road and the communication between the vehicle. Real vehicle tests show that this system can be in the vehicle at a speed of 30 km/h, implementation within the range of 35 m from 256 LEDs array of light transmission of data to receive.



At present, the use of high speed image sensor in the VLC needs to solve some technical problems, such as installing facilities are expensive, require complex real-time image processing technology, the camera frame rate is lower than the light-emitting diodes (leds) limiting the response speed of communication speed, etc. Literature reports the latest research and development of the optical image sensor (OCI) in the application of the wireless optical communication (owcs) system. Using complementary metal oxide semiconductor (CMOS) image sensor technology of the latest development of OCI has two characteristics: used for high-speed communications pixels (CPx) and reception for fast and accurate LED detection 1 bit logo image output function. The owcs system is known as the world's first and only 16.6 ms delay, 20 MB/s transmission rate of the pixel transmission communication system.



3, VLC high-speed data transmission



Large amount of data, such as high definition (HD) video streaming transmission, access to information such as high-speed download information flow and high speed data backup has become an integral and indispensable part of contemporary life. The beam of visible light small divergent Angle can be used for data transmission, and the path to the characteristics of low transmission loss make high-speed download data flow and the safety of the transmission bandwidth possible. As shown in the figure below, the OMEGA engineering developed a has a 100 MB/s 4 hd VLC data transmission of the data flow. Orthogonal frequency division multiplexing (OFDM) technique, with 16 leds to 10 m2 area within the scope of the photodiode detector for data transmission.



In VLC between receiver and transmitter add a focusing lens to achieve 5 m x 5 m x 3 m indoor demo system of high quality video and audio broadcast transmission: compared with the line-of-sight (LOS) link, Nd - LOS link has a good network node mobility, characteristics of traceability, but the link loss in transmission rate is not high. Studies in order to solve this problem, this paper implements the scope of 2 m, 400 MB/s transmission rate and ber RBE < 10-8 typical indoor bidirectional optical wireless communication system research (visible light leds as downward send light chain, IR is uplink data source). RGBLEDs using wavelength division multiplexing (WDM) can get higher data transmission rate. Document records the first based on WDM/discrete tone (WDM/DMT) modulation technique of VLC system, realized the technology of modulation and obtain the single white light LED RGB - 803 MB/s rate of data communication.



4, VLC application in aviation



VLC obvious advantages in the air. Visible light leds have been widely used on a new generation of commercial aircraft, through visible light replacement cable or cable communication can reduce the volume and weight, reduce the electromagnetic interference (EMI) and reduce the cost, etc., Boeing commercial airplanes platform also in carrying out a research into the future wireless optical network solutions. Passengers reading lamp based on VLC communication service unit (PSUs), entertainment system, and the application of cabin intercom: (a) this reading lamp to illuminate the passengers in the seating area, at the same time reading lamp as a source of optical communication notebook computer or other terminal receiver to passengers to transmit data. Passenger entertainment system is visible between the seat through the whole list to communicate with each other and make up the mesh network, realize the in-flight entertainment (iFE) transmission of information. Signals from the LED light can also be used as a low bandwidth voice communication, realize the internal intercom, as shown in (b).



5, underwater VLC



Underwater communication in the field of military, industrial, and science has important position. Underwater communication rate requirements within the scope of a few megabytes to dozens or even higher. Radio waves of the sea will be affected by high attenuation. Sound waves in the ocean transmission speed of 1500 m/s, delay time is long, limited bandwidth, high bit error rate, sound waves will be about the disturbance such as dolphins and whales and other sea animals. VLC is a good way to overcome the attenuation underwater and electromagnetic interference. About between the pilot and underwater remote truck VLC communication system development: the transmitter in the system is composed of 70 LED light source, support 120 ° Angle of data transmission, the transmission rate is as high as 20 MB/s. Document records the visible underwater optical communication system in nuclear power plants machine to detect the specific application. Underwater remote truck equipped with cameras and 20 m in the distance between the site control and video signal transmission. At present the main challenges facing underwater optical wireless communication system is under the influence of underwater properties especially the turbidity and difficult to realize long distance communication. In order to solve this problem, the national university of Singapore and the Massachusetts institute of technology carried out in 30 m of 1.2 Mb/s within the distance of underwater optical communication system research and demonstration.