Research On Underwater Wireless Optical Communication With Pointing Errors In Oceanic Turbulence

With the advance in the space-air-ground-sea integrated network(SAGSIN)technologies,underwater wireless communication(UWC),which plays an important role in S AGSIN,has drawn increasing attention.UWCs enable the realization of wireless data transmission through acoustic wave,radio frequency(RF)and optical waves.Considering the demand for low latency and high speed UWCs and the high latency of underwater acoustic communication(UAC)and the limited bandwidth of underwater RF communication,underwater wireless optical communication(UWOC),which can provide low latency and high speed data transmission,has been regarded as an attractive and viable alternative.However,UWOCs still meet some challenges need to be studied and solved,and the oceanic turbulence is one of the challenges.In the oceanic turbulent environment,light attenuation propagating in seawater,turbulence-induced negative effects,and pointing errors can degrade the performance of UWOCs.In order to accurately analyze and effectively improve the performance of an UWOC,it is necessary to synthetically investigate the effects of the oceanic turbulent environment on UWOCs.Particularly,it is important to note that pointing errors are common in UWOCs because of the sway of the platforms carrying the UWOC equipment caused by the disturbance of water flow and beam wander resulted by oceanic turbulence.Therefore,it is necessary to study UWOC in the oceanic turbulent environment by taking into the consideration the existence and influence of pointing errors.In conclusion,study of UWOCs with pointing errors in the oceanic turbulent environment carries major implications.The research related to the dissertation mainly focus on performance analysis and optimization of UWOC with pointing errors in the oceanic turbulent environment.The contents and contributions in the dissertation are summarized as follows.The accuracy of the existing UWOC pointing errors models are moderate and can be improved.To solve this problem,an analytical model of spatial relationships in an UWOC with pointing errors is developed in this dissertation.The analytical model involves both of the attitude adjustment related to the pointing adjustment of the optical transmitter and receiver,and the changes in the position and attitude(position and attitude deviations)of the platform carrying the UWOC equipment caused by the disturbance of water flow.The analytical model can improve the accuracy of misaligned displacements analysis in an UWOC link with pointing errors.In this study,firstly,a geometric model is presented to describe the position and attitude of a platform carrying UWOC equipment,and the coordinates transformation related to the changes in the position and attitude of a platform is analyzed.Then,the coordinates transformation related to the pointing adjustment of an optical transmitter and receiver is analyzed.Next,according to the position and posture of the transmitter platform and receiver platform,the pointing adjustment of the optical transmitter and receiver,which lead to the alignment between the optical transmitter and receiver,can be determined.In other words,the relative spatial relationships between the optical transmitter and the transmitter platform,and the relative spatial relationships between the optical receiver and the receiver platform are determined when the UWOC link between the optical transmitter and receiver is an aligned link.Finally,in a misaligned UWOC link,the relative spatial relationships between the optical transmitter and receiver can be analyzed and the misalignment displacements can be obtained by further taking into consideration of the changes in the position and attitude of the platforms carrying the UWOC equipment.The existing optical propagation properties analysis are with no consideration of misalignment.To solve this problem,an analytical model of the optical propagation properties in a misaligned UWOC link is developed in this dissertation.The optical propagation properties analytical model is developed according to the optical wave propagation theory in random medium,and involves the light propagation path in a misaligned UWOC link.The optical propagation properties analytical model is able to improve the accuracy of optical wave propagation properties analysis in UWOC with pointing errors.In this study,firstly,according to both of the optical wave propagation theory in random medium and the relative spatial relationships between the optical transmitter and receiver in a misaligned UWOC link,the beam wander variance and the scintillation index in the receiving plane of a beam propagating through oceanic turbulence are analyzed.Next,the average pointing errors in an UWOC in oceanic turbulent environment can be calculated by taking into consideration of the misalignment displacements caused by the position and attitude deviations of the platforms carrying the UWOC equipment and beam wander resulted by oceanic turbulence.Finally,according to the calculation results of misalignment displacements,root mean square beam wander,average pointing errors,and scintillation index,the effects of the changes in the position and posture of a platform,beam parameters,and oceanic turbulence parameters on the optical propagation properties in oceanic turbulence are studied.Pointing errors can result in degradation of UWOC performance in oceanic turbulent environment.To solve this problem,a new structure design of beam arrays is presented in this dissertation.In this new type beam arrays,the pointing direction of each single beam can be adjusted individually,and the adjustment scheme is mated to the statistical characteristics of pointing errors in a misaligned UWOC link.The new type beam arrays can enhance the transmission reliability of UWOC systems in oceanic turbulence.In this study,firstly,the concept and the spatial structure design of the new type beam arrays,which is named as pointing adjustable beam arrays(PABA),are proposed.A PABA consists of laser arrays and reflector arrays.In a PABA,the pointing direction of every single beam is different.In a PABA,the pointing directions can be divided into two categories.One part of the beams point at the center of the receiving plane,the other part of the beams point at the area around the center of the receiving plane.The target points of the beams that point at the area around the center of the receiving plane symmetrically located at a ring around the center of the receiving plane,and the radius of the ring is equal to the UWOC pointing errors in oceanic turbulent environment.Then,the spatial relationships associated with a PABA is analyzed,and the pointing adjustment principle of an arbitrary single beam in a PABA is introduced from the following three aspects,the relative spatial relationships between the incident beam and the reflected beam,determining the pointing direction of a reflected beam according to the attitude of a reflector,and determining the attitude of a reflector according to the pointing direction of a reflected beam.In a PABA,the pointing adjustment of each single beam is accomplished by adjusting the attitude of a reflector related to the single beam.Therefore,the key to making the pointing direction of a single beam in a PABA be same as the desired pointing direction is determining the adjusted angle of the related reflector in the attitude adjustment process.Finally,in consideration of the light attenuation propagating in seawater,oceanic turbulence,and pointing errors,beam propagation in the oceanic turbulent environment is simulated by wave optics simulation.According to the simulation results,the performance,such as path loss,scintillation index,mean signal-to-noise(SNR)and mean bit-error-rate(BER),of UWOC systems with beam arrays are evaluated.