Study On Propagation Characteristics Of Wireless Optical Wave In Anisotropic Oceanic Turbulence

As an important base for human sustainable development,the ocean is rich in biological,mineral resources and energy.Its position in the economic,cultural and military fields is gradually emerging.Scientific research,environmental monitoring,resource extraction and other increasingly frequent ocean activities,as well as the extensive application of technologies such as autonomous underwater vehicles and underwater sensor networks,require more demanding underwater wireless optical communication(UWOC)technologies.Compared with underwater acoustic communication and radio frequency communication,UWOC has attracted extensive attention due to its higher transmission rate,higher security and lower cost.However,UWOC faces many challenges due to path loss,pointing error,and turbulence effects.The absorption and scattering of optical waves by underwater media such as chlorophyll mainly cause signal attenuation and delay spread.Pointing errors cause degradation of the received optical signal quality.Underwater turbulence can cause beam shift,beam expansion,spot jitter,and intensity scintillation,which can greatly reduce communication system performance.Therefore,it is of great significance to study the signal fading characteristics caused by many factors and their suppression methods to improve the effectiveness and reliability of UWOC.This paper mainly studies the scintillation index model of spherical waves under anisotropic oceanic turbulent channels and the use of M-ary pulse position modulation(PPM)technology to alleviate the signal fading caused by various underwater channel influencing factors.Based on the fading model,the key performance of the optical communication system is studied and analyzed,and the specific work includes:1.Aiming at the influence of anisotropic oceanic turbulence on the scintillation index,the anisotropic ocean turbulence power spectrum model is derived,and the expression of aperture average scintillation index under the influence of non-zero inner scale and finite outer scale is given.The effect of relevant channel parameters and system parameters on the scintillation index are analyzed in detail.The conclusion can be drawn that the scintillation index increases with the outer scale of the turbulence,while the inner scale induces the opposite trend.Under weak ocean turbulence conditions,the influence of the inner scale on UWOC is greater than that of the outer scale.In addition,aperture averaging can well mitigate scintillation caused by turbulent effects.2.Aiming at the influence of anisotropic tilt angle on the scintillation index in anisotropic oceanic turbulence,the links of the UWOC system in horizontal and vertical directions are studied respectively.An anisotropic oceanic turbulence spectrum model considering the anisotropic tilt angle is deduced,the scintillation index expression of the horizontal and vertical links is given.The results show that the effect of the anisotropic tilt angle on the scintillation index of the horizontal link is not significant,while in the vertical link,the scintillation index strongly depends on the ocean depth and anisotropic tilt angle.3.For the performance evaluation of the UWOC system in the ocean channel,Mary PPM technique is used to establish a composite channel model considering the influence of path loss,pointing errors and ocean turbulence,and the key performance of the UWOC system is analyzed.On the one hand,based on the established composite channel model,the closed-form expression of the average symbol error rate(SER)of the UWOC system is deduced;On the other hand,considering the possibility of actual implementation and safety factors,the closed-form expression of the average channel capacity of the system under the limitation of average power and peak power is proposed.In addition,the influences of noise and water conditions on the performance of the UWOC system are further analyzed.The results show that the higher order modulation can improve the average SER performance of the system.Compared with having both average power and peak power constraints,the UWOC system has higher channel capacity when only average power constraint.Furthermore,in the ocean environment,thermal noise and quantum noise have more severe effects on the average SER and channel capacity than background noise.In this paper,the results can provide a theoretical basis for the design and performance evaluation of high-speed communication systems under underwater channels,and have broad application prospects and practical significance for carrying out oceanic activities.