Study On Key Techniques Of Short-Range Optical Wireless Communications

The optical wireless communications (OWC) is a cutting-edge technology that combines both wireless communication and optical communication. As OWC provides most of the inherent advantages of optical fiber communications while leveraging the flexibility of the RF wireless, it is rapidly becoming a potential solution to the global wireless spectrum shortage. Therefore, this technology has important implications for this emerging field and broad application prospects.Currently, OWC is still in its infancy stage with many technical constraints and challenges to be solved. The major drawback is that the substantial path losses and high shot noise from ambient light will bring uncertainty to the optical transmission, which requires the marriage of many techniques to ensure the reliability, flexibility and practicality of the OWC system.The study if this dissertation is supported by the National Natural Science Foundation of China (NSFC) and the National High-tech Research and Development Program (863Program), addressing key techniques regarding non-line of sight (NLOS) ultraviolet (UV) communication and indoor visible light communication (VLC). Special attention has been paid to the following three areas:further study on optical channel characteristics, performance improvement of the OWC system and the combination of lighting, communication and positioning, which are also the focus of this dissertation. The main innovative ideas are summarized as follows:Firstly, since few studies have been reported on atmospheric weather-dependent effects on UV channel modeling, a UV channel model which incorporates the impact of various weather conditions is built and validated according to extensive measurements.Secondly, few researches are conducted to investigate UV communication system using statistical simulation method. A performance study of short-range NLOS UV communication system based on a Monte Carlo system-level model is presented, in which the channel parameters, such as the path loss and the background noise, are experimentally measured using an outdoor UV communication test-bed. The system trade-offs among the distance, the link geometry, the noise level, the bit error rate (BER) and4modulation formats are simulated, and the results may provide guidelines to the system design.Thirdly, a moving average filter based method for UV communication is proposed and realized so as to improve the system performance. Experimental results show that the proposed method is able to improve the received signal-to-noise ratio (SNR) and correspondingly reduce BER by approximately two orders of magnitude.Fourthly, to suppress the damage from the interference, an OWC scheme based on direct sequence spread spectrum (DSSS) is proposed and realized. Experimental results show that the proposed scheme has a great capability to reduce interference. With the help of DSSS, the BER of the UV communication system can be reduced from2x10-2to less than2.5x10-7.Lastly, addressing the problem of high resolution indoor positioning in large buildings, a dual-tone multi-frequency (DTMF) based indoor positioning algorithm and a VLC based indoor positioning system are proposed, which can provide good positioning accuracy without interfering with the lighting and communicating function. Simulation results show that this method reduces positioning error to less than6mm (SNR=15dB), while in contrast, the state-of-the-art microwave indoor positioning can only reduce the error to tens of centimeters.