Research And Implementation Of Long Distance Visible Light Communication System Between Boats

The Visible Light Communication (VLC) is a kind of wireless optical communication technology. Visible light at wavelength from375nm to780nm is used as transmission carrcr, at the same time atmosphere is used as channel when photoelectric converter is used as receiver. Compared with other light sources, LEDs have advantages of low-power dissipation, long lifetime, small size, low heat and safety. With the development of lighting technology based on light-emitting diode (LED), visible light communication technology has been widely studied. Compared with other wireless communications, VLC has advantages of anti-electromagnetic interference, high security, no need to apply for radio spectrum, big channel bandwidth, high bit rate, low cost and easy to use. Currently, most researches are focused on indoor high-speed wireless access and indoor positioning. Long distance visible light communication studies are focused on intelligent transportation systems. For now, there are some problems in signal lamp communication on board, such as low bit rate, high error rate and the low degree of automation. This paper presents a signal lamp communication system between boats using VLC technology. The main problems of realizing the system are the signal loss and attenuation. According to the analysis of factors which leads to signal loss and attenuation, a new long distance visible light communication system is implemented with a set of devices and experiments. The main works of this paper are as follows.1. In this paper, factors leading to signal loss and attenuation are theoretically analyzed. The power budget and overall design of the system are presented based on theoretical analysis. The main factors leading to free space transmission loss are the semi-angle at half power of transmitter, transmission distance and antenna area of receiver. The factors leading to atmospheric attenuation are atmospheric scattering, atmospheric absorption and atmospheric turbulence. Based on the above analysis, the power budget are obtained under the conditions of7.4km transmission distance,3°semi-angle at half power of transmitter and20cm receiving lens. In this situation, power budget of the system are as follows:free space transmission loss is73.35dB, atmospheric attenuation is14.8dB, power of transmitter is bigger than40dBm and the sensitivity of receiver is lower than-60dBm.2. Depending on the analysis of related theories, this paper introduces the design of transmitter, which includes the selection of LED, secondary optical design of LED, the comparison between PPM and OOK modulation and design of drive circuit. Two methods to improve the optical power of transmitter are selection of commercial LED and drive circuit, in the same time the secondary optical design of LED is helpful to reduce the loss caused by half-power angle.3. Depending on the analysis of related theories, this paper introduces the design of receiver, which includes the selection of received optical lens, design of optical filter, analysis of optical noise and electrical noise and the design of low noise circuit. The method to enhance received gain is to choose received optical lens with large area. In this paper, design of optical filter and low noise circuit are used to reduce noise, enhancing the SNR of receipt signals.4. Based on the detailed design, a set of long distance visible light communication is finished, which is the basis of telecommunication experiment platform. In the indoor experiment, half-power angle of transmitter is3.2°, when in the outdoor experiment message can be sent and received in4km, with153.6kbit/s bit rate.