Research On Several Key Technologies Of Coherent Underwater Optical Wireless Communication Systems

In comparison to the conventional acoustic counterparts,the underwater optical wire-less communications can provide a much higher data rate,higher security,lower interfer-ence and lower latency.Because of the above mentioned advantages,underwater optical wireless communication has become an attractive alternative to meet the requirements of high speed underwater communication systems in applications such as underwater wire-less sensor networks.However,the researches on underwater optical wireless commu-nications are still insufficient in both academia and industry.Therefore,it is necessary to carry out further researches on such systems to improve system performance,so as to meet the ever-increasing demand for underwater resource depletion and high-speed un-derwater wireless communications in recent years.Considering the excellent background noise rejection of coherent optical systems,this dissertation mainly focuses on the coher-ent underwater optical wireless communication systems.And several key issues such as channel modeling,equalization,phase noise estimation and space-time coding techniques have been discussed in such systems.By applying the newly proposed method in this dissertation,the key performance such as channel estimation accuracy,bit error rate and transmission data rate of the coherent underwater optical wireless communication system can be significantly improved.Specifically,the main research contents and innovations of this dissertation are summarized as follows:Firstly,the conventional impulse response model of the underwater optical wireless channel has a relatively low estimation accuracy,estimation efficiency,and a relatvely weak physical meaning.Therefore,this dissertation analyzes the impulse response of un-derwater optical channels by Monte-Carlo simulations.By analyzing the convexity of the simulation results,this dissertation proposes a new model for the impulse response in underwater channels.Compared to the conventional models,this new model has sig-nificantly improved on the estimation accuracy and estimation efficiency with enhanced physical meaning interpretation.Secondly,according to the above mentioned model,the performance of the high speed underwater optical wireless communication system will be degraded by the inter-symbol interference.Therefore,by applying Monte-Carlo method,this dissertation com-prehensively analyzes and compares the performance of different equalization techniques on suppressing the inter-symbol interference.According to the conclusions in this disser-tation,different equalization techniques should be adapted to different channel conditions.In addition,by adjusting the field of view of the receiving antenna,the equalization perfor-mance of the receiver in turbid water channel will achieve an optimal trade-off between the received power and the inter-symbol interference,and thus significantly reduce the signal power requirements of the transmit antenna.Thirdly,turbulence is another major feature of underwater channels,and the phase noise is an important non-ideal effect of coherent systems.Considering the phase estima-tion problem in turbulent channels,this dissertation discusses a method to further improve the accuracy of phase noise estimation in turbulent underwater channels by mathematical modeling and derivation.The results indicate that the newly proposed phase estimation method can significantly improve the accuracy of phase noise estimation as well as de-crease the system BER.Fourthly,in order to further improve the system performance,this dissertation dis-cusses the possibility of applying V-BLAST MIMO scheme to simultaneously increase the transmission bit rate and reduce the bit error rate of the system by analyzing the pairwise error probability of the system.Compared with the conventional space-time coding tech-nology in the optical wireless communication systems,the V-BLAST code will further increase the transmission bit rate in the high SNR area.In the end,this dissertation simulates a system that simultaneously considers all the above mentioned non-ideal effects.The simulation results show that the equalization tech-nique can effectively suppress the influence of inter-symbol interference on the bit error rate while significant SNR penalty still exists in the low SNR region.In the meanwhile,the new phase estimation technique can significantly reduce the bit error rate of the system in the high SNR region.In addition,the V-BLAST code improves the diversity gain and the transmission bit rate of the system.In summary,the research results in this dissertation can provide effective theoret-ical guidance for designing an underwater optical wireless communication system with lower bit error rate and higher data rate,which are the constantly discussed topics in the communication systems.