The Key Technology Research On Underwater Wireless Optical Communication Systems

Over the past decade, with the rapid development of the optical or acoustic underwater imaging technologies,as well as the increasingly of the autonomous underwater vehicle (AUV), and wireless sensor networks technologies, it provides a new method for the continuous, systematic, large spatial and temporal scale underwater exploration. However, these observation techniques require the support of communications with high data rate to achieve data transmission and control information exchanges. For example, the transmission of high-definition real-time video signals needs about 13 Mbps transmission capacity. In the military field, especially during the war at sea, high date rate of sound and image information transmission between underwater submarines, surface ships, and monitoring sensor becomes more important. Although the traditional acoustic communication technology having advantage at the transmission distance, reliability, etc., the disadvantage of a low transmission rate, narrow bandwidth, long delay, high power consumption and large size, made the acoustic communication technology difficultly to achieve Mbps transfer rate even in a short distance. Therefore, it is required to develop a highly efficient transmission technology for underwater applications.Underwater optical wireless communication can overcome the disadvantage of underwater acoustic communications of narrow bandwidth, subject to environmental, the low applicable carrier frequency, and high transmission time delay. First of all, as the frequency of optical wave is very high, the information carrying capacity is higher than acoustic wave and the transmission rate of visible spectrum for underwater communication can achieve Gig bit per second(Gbps); Secondly, the characteristics of anti - interference ability, and less susceptible by the changes in temperature and salinity of sea water of optical communication, have good performance in underwater electronic countermeasures; Third, the transmission is directional and if it is intercepted, this will result in disruption of communication links, so that users can easily detect the failure of communication links. Therefore, it has a high degree of security and confidentiality; Fourth, the optical wave length is short, and it makes the antenna of the sender and receiver quite small. It can significantly reduce the size and weight of the system design. Recently, with the rapid development of compact photovoltaic devices in lower power consumption, it is possible to design small size and low power consumption optical communication system for underwater equipments.This paper presents the research works of the key technical issues in high transfer rate underwater wireless optical communication system design. It is summarized as following:(1)The characteristics of the underwater channel of optical transmission are discussed. Based on the analysis of the impact of the water scattering and absorption, an analysis model of underwater optical communications system based on empirical formula of bio-optical fitting is presented. It is shown that the model can assess the role of the attenuation for underwater optical wireless communication systems in different regions or different depth, and provide analytical methods for the design of the holistic underwater optical communication systems.(2)An experimental system for underwater optical communications evaluation based on the light-emitting diode (LED) and photomultiplier tube is developed. The experiments of underwater optical communication in the laboratory are carried out and the impact of different particles concentration for the quality of communications is discussed.(3) Several current commonly used modulation techniques are evaluation in terms of the bit error rate, bandwidth and the optical power requirements, and the results show that the PPM modulation with lower power requirement, simple equipment, is a better choice for the underwater optical communications. In order to improve performance of PPM in bandwidth utilization, a new SPPM modulation method has been developed, it make use of the advantages of PPM modulation on the high-power utilization with low the transmission bandwidth to improve the data rate at a certain extent, however the BER performance of SPPM modulation is worse than DPPM and DH-PIM modulation technology. So it is only as a try method, and for the specific applications, it requires further improvement.(4) The method to improve the performance of wireless communication with error control coding method is discussed and characteristics of various linear block codes are presented. The combination of BCH and RS coding scheme with 2DPSK and OOK modulation for underwater optical wireless communication system are evaluated and the performance of the results are presented.