Research And Implementation On Key Technology Of Outdoor Long-distance Visible Light Communication System

As a new kind of wireless optical communication technology, the Visible Light Communication(VLC) is causing a sensation around the world. Compared with other wireless technologies, VLC has high security, none electromagnetic interference, safety, etc. VLC expands the spectrum resource effectively, and can be compatible with the wireless communication network. Due to the complementary nature of VLC and WIFI technology, people also call it LIFI. Compared with the traditional lighting, LED light not only has low power consumption, small size, and long using time but also has high modulation bandwidth and short response time. As the lighting technology of the 21 st century, LED will greatly promote the development of VLC system in various fields. At present, there is a lot of research on the indoor VLC, and less on the outdoor VLC. The key problems to the outdoor VLC system are: a) the optical signal tracking; b) the synchronization of optical signal; c) the research of optical link; d) the research of optical modulation and demodulation technology. This thesis proposes an outdoor visible light communication system based on satellite time synchronization, and automatic tracking algorithm based on CCD-based camera. Finally, we test the communication performance. The main works of this thesis are as follows:1. Depending on the analysis of related theories, the thesis proposes a quasi-synchronization algorithm based on satellite time. After initialization, the controllers wait and accept synchronization timing signals from satellites, then control both transmitter and receiver to communicate.2. This thesis proposes an automatic tracking algorithm based on CCD-based camera. The algorithm calculates a determined direction and angle instruction which rely on the LED coordinate information on the CCD image, and then transmit the control commands to the platform for tracking.3. Depending on the detailed design, an experimental platform of outdoor VLC system is completed. The camera receives the correct signal when selecting the proper exposure time and threshold. The experiment which has GPS synchronization timing signals is significantly better than the early experiment without GPS. Due to the constraints of the CCD-based camera hardware, the current communication speed can be achieved between 100 bps and 200 bps. That means, better quality, and faster speed CCD-based camera can further improve the VLC speed. Using CCD image sensor to receive multiplexed optical signal can also improve communication rate. In the laboratory, when using eight LED lights which send one channel optical signal per LED light, CCD camera detects 8 signals in parallel and receives correctly, the communication rate increases to 8 times the original. For tracking experiments, the LED source has the ability to keep in the vicinity of the center position of the CCD image, which proves that it is feasible to use CCD camera for tracking the position of the light source.