| The integration of air,space,ground and sea is an important strategy for future communications development.As an important part of it,underwater communication has aroused great interest in various countries.Underwater wireless communication technologies are mainly divided into three types: microwave communication,underwater acoustic communication and underwater optical communication.Underwater wireless optical communication has the characteristics of large bandwidth,strong anti-interference ability,good confidentiality,and small size.It has great application potential in the fields of military and broadband access networks.In the actual underwater wireless optical communication link,seawater has attenuation characteristics such as absorption and scattering of light,and the influence on light changes with wavelength.For example,there is a blue-green spectral transmission window(450?550nm)in seawater,therefore,the preferred spectrum range for underwater optical communication is relatively limited.The current spectrum resources are still sufficient,but with the intensification of prospecting aircraft and submarines in the ocean in the future,as well as the development and application of marine optical networks,the demand for underwater optical communications is increasing,and the problem of spectrum resources will gradually emerge.In order to alleviate this problem,this article conducts research on the simultaneous co-frequency full-duplex underwater wireless optical communication system to improve the spectrum utilization rate and ensure the communication efficiency.When performing simultaneous co-frequency full-duplex communication,due to the absorption and scattering of light by seawater,multipath fading occurs when light propagates in the forward direction,causing inter-symbol interference,therefore,the bit error rate performance is poor at a link distance of several hundred meters in a turbid sea environment;in addition,during the forward propagation of light,part of the light will be backscattered back to the transmitter plane of its own receiver,and serious self-interference will be formed in the turbid sea,which will affect the performance of full-duplex communication.In view of the above-mentioned problems that occur in the simultaneous co-frequency and full-duplex wireless communication of light in the ocean,in this paper,the forward and backward propagation characteristics of light in seawater channels are systematically studied respectively;equalization technology is used to solve the problem of inter-symbol interference caused by multipath;precoding technology is used to solve the problem of selfinterference at the transmitter caused by backscattering.Finally,on the basis of the aforementioned research,this article applies precoding technology to full-duplex underwater wireless optical communication,designs and builds a hardware system based on FPGA,and completes related tests.The main work of this paper is as follows:(1)The basic architecture of the underwater wireless optical communication system is introduced,the attenuation characteristics of the seawater optical channel are described in detail,and the coherent optical modulation technology and incoherent optical modulation technology used in the underwater wireless optical communication system are explained and analyzed,and the superiority of the intensity modulation method is clarified(2)The model of underwater optical communication system based on equalization technology is introduced.The Monte Carlo method is used to model and analyze the forward propagation process of light in seawater,and the time delay characteristics of the seawater optical channel are explored.The traditional equalization techniques such as LMS,LS,and MMSE are derived and explained,and a time-domain pre-equalization algorithm based on a priori model is proposed.The underwater wireless optical communication system based on DCO-OFDM modulation under real seawater channel parameters is simulated,and by comparing the bit error rate of the system when using different equalization algorithms,the performance of each algorithm is analyzed.Compared with other traditional algorithms,the proposed algorithm based on a priori model has a at least 1.6d B improvement in bit error rate performance.(3)The model of underwater wireless optical communication system based on precoding technology is introduced.The Monte Carlo method is used to analyze the backscattering process of light in seawater,and the effects of parameters such as the launching field of view,the receiving field of view,the aperture of the transmitter,and the water quality on the backscattering power are explored and summarized.Based on the idea of the spatial optical link equation,a modeling method of the underwater wireless light backscattered power distribution model is proposed,and the backscattered power distribution of light in the turbid harbor water quality under different condition parameters is modeled and verified.The principles of ZF precoding and MMSE precoding algorithms are introduced.The ocean optical backscatter channel obtained from Monte Carlo research is used as the communication channel.The full-duplex underwater wireless optical communication system equipped with precoding algorithms is simulated to verify the precoding algorithm.Coding technology can effectively suppress system self-interference and compare the suppression effects of different algorithms.(4)Use FPGA as the core hardware platform,an underwater full-duplex wireless optical communication system based on the combination of ZF precoding algorithm and analog circuit adder for self-interference suppression processing was designed and built,and the transmission performance is tested.The test results show that the system with selfinterference suppression processing has a good effect of simultaneous co-frequency fullduplex communication,and the bit error rate performance is significantly better than the system without self-interference suppression processing.It is proved that precoding has a certain self-interference suppression effect in underwater wireless optical communication.The use of the proposed self-interference suppression processing method can effectively suppress the self-interference in full-duplex underwater wireless optical communication and reduce the bit error rate.The bit error rate test results show that the system meets the expected requirements. |
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