The Investigation On Certain Characteristics Of Underwater Wireless Optical Communiation Channels

Ordinary laser?hereafter referred as to laser?and orbital angular momentum?OAM?beam based underwater optical communications have advantages on the issuses of promoting the system capacity,transmission rate and transmission distance.Furthermore,considering the OAM beam with spatial helical phase structures provides a novel information modulation degree for underwater optical optical communication?UWOC?,which can tremendously promotes the sisytem capacity,transmission rate and spectral efficiency.Hence,by selecting the laser and OAM beam,the channel characteristics of UWOC are studied in this thesis.The related results and findings can not only provide theoretical fundations for channel estimation and equalizaton of UWOC,but also give theoretical references for the study and desgin of UWOC system.The highlights of this thesis are listed as follows:?1?Focusing on the requirements of the receiver aperture of UWOC system should cover the two-dimensional spatial distributions of optical signal spot as much as possible.By referring to the conception of 3d B bandwidth,the 3d B optical intensity spot radius?3d B OISR?is ultilized to characterize the dominating optical intensity distribution of laser under scattering underwater channel.The characteristics of 3d B OISR are investigated based on the Monte Carlo simulation method.The investigation shows that in the channel with a high density of scattering particles,the divergence angle of laser source plays a negligible role in 3d B OISR.This is important for optical communication as this clearly shows that the geometric loss is no longer important for the desgin of receiver aperture and optimal spacing of diversity scheme for UWOC system in the highly scattering channel.?2?Aiming at the scattering effects of underwater optical channel,the characteristics of proportion of scattering components of laser beam are analyzed in this thesis.The study shows that the proportion of scattering components of laser beam in the case I water cnannel is very small,the field of view is not a key parameter in the design of photoelectric device of UWOC system.The proportion of scattering componets increased singificantly in the case II water channel,which results in severe channel loss.Hence,under the constraints of optimal receiving signal to noise ratio,the value of feild of view sould be enlagred as big as possible.?3?Focusing on the continuously increased requirements of transmission rate and system capacity of UWOC system,and considering the structured light that represented by the OAM beam can satisfy the above mentiond requirements,based on the multiple phase screen method,the spatial distribution,spiral spectrum,and scintllation index of Laguerre-Gaussian?LG?beam,superposition states of LG beam and LG beam with higher-order radial indices are examined in this thesis.According to the investigation,the integrality of spatial distribution of the superposition states of LG beam and LG beam with higher-order radial indices are susperior to the LG beam.The LG beams with 1-order radial index has highest normalized relative received intensity,then followed by the superposition states of the LG beam,and the LG beam is lowest.When the equivalent tempreture structure parameterC_n²?28?1.0?10^{-1 5}K ²m^-2/3,transmission distance Z₀?50m and the ratio of tempreture gradient and salinity gradient contributions to the oceanic turbulence?=-2.5,the LG beams with higher-order radial indices have minimum average scintillation indices.