Performance Of PPM Wireless Optical Communication In Ocean Anisotropic Turbulence

In the past decade,applications such as marine environmental monitoring,underwater sensor networks,and military activities are quietly emerging,which have placed higher requirements on the data transmission rate of underwater media.Forcing people to use underwater wireless optical communication(UOWC)links instead of using low data rate voice communication links.Optical signals are attenuated due to the three mechanisms of scattering and absorption caused by various components present in the ocean and ocean turbulence caused by changes in refractive index.First,this paper studies the bit error rate(BER)of gamma-gamma M-ary pulse position modulated(PPM)optical wireless marine communication systems in anisotropic ocean turbulence.Therefore,according to the relationship between the scintillation index of the weak atmospheric turbulent spherical wave and the scintillation index of the weak ocean turbulent spherical wave,the marine equivalent "structure constant" represented by the ocean turbulence parameters and the anisotropy factor is derived.Based on the structural parameters and the generalized Huygens-Fresnel principle,the average received power of a collimated Gaussian beam with anisotropic ocean turbulence is established and used for the BER expression of a gamma-gamma PPM wireless optical marine communication system.Secondly,in order to further optimize the performance of the UOWC system,the UOWC system of partially coherent laser light sources is explored.Based on the cross-spectral density function and the gamma-gamma channel distribution,the performance index parameters of the partially coherent optical wireless marine communications system-BER and outage probability are studied.According to the formula,numerical simulations were performed for various turbulence parameters,source coherence parameters,PPM modulation order,data bit rate,the average current gain of avalanche photodiode(APD)and transmission distance for anisotropic ocean turbulence BER.Finally,the numerical simulation analyzes the relationship between the average signal to noise ratio(SNR)and the outage probability for different anisotropic ocean turbulence parameters,source coherence parameters,receiving aperture size,and transmission distance.The main conclusions are as follows.Anisotropic turbulence has a weaker intensity than corresponding isotropic turbulence when the ocean turbulence parameters such as the rate of dissipation of mean-squared temperature and the rate of dissipation of kinetic energy per unit mass of fluid remain the same.As the ocean becomes more anisotropic,the performance of the PPM wireless optical marine communication system is improved.Under a fixed anisotropy factor and a Gaussian beam with the same coherence,in the environment of small the rate of dissipation of mean-squared temperature and large molecular kinematic viscosity coefficient,there is a smaller BER and probabilityof interruption;temperature dominates ocean turbulence compared with salinity-dominated ocean turbulence,there is a smaller BER and outage probability.A large modulation order and bit rate,as well as an increase in link length will lead to system performance degradation.When the turbulent anisotropy of the ocean increases to a certain extent,the BER does not always decrease as the rate of dissipation of kinetic energy per unit mass of fluid increases.When a partially coherent beam is used as the system emission source,the system performance can be improved,but when the source coherence parameter is always increased,the BER is reduced first and then increased.In addition,no matter which beam source the transmitter uses,other parameters remain for a while.As the average APD gain increases,the BER decreases first and then increases.It shows that choosing the appropriate coherence light source at the transmitter and the appropriate average APD gain and large receiving aperture at the receiver are of great significance for optimizing the system.When the transmission distance is large and the turbulence intensity is large,a completely coherent optical system may be better than a partially coherent light,which indicates that the change of the coherence of the beam during ocean turbulence transmission needs to consider both the light source parameters and the turbulent environment(turbulence intensity,transmission distance).This article provides some reference value for the design and platform construction of UOWC system in anisotropic turbulence.