Link Optimization And Transmission Characteristics Of Wireless Optical Vortex Communication In Weak Turbulence Absorbing Seawater

With the continuous depletion of terrestrial resources,and the ocean has rich fisheries and mineral resources,the exploitation of ocean resources becomes one of the important directions of socialism proposal in our country.Underwater wireless communication technology is the basis of the development and utilization of ocean resources.Underwater wireless optical communication(UWOC)has become a research hotspot in this field because to its high speed,high capacity and high reliability.However,the performance of the UWOC system is severely limited by the turbulence and absorption of seawater,the alignment error of the transceiver system in the optical transmission link,the jitter of the system platform,and the jitter and other pointing errors introduced by the turbulence.Therefore,the key to optimize the performance of UWOC system is to restrain the signal attenuation and loss caused by ocean turbulence,absorption and link pointing error.In view of this prominent problem,this paper has carried out the following research and made corresponding progress:(1)Under the paraxial approximation and Rytov approximation,the average bit error rate(ABER)of optical orbital angular momentum(OAM)bandwidth-limited carrier link is derived by defining the average signal-to-crosstalk noise ratio(ASCNR)including the system bandwidth.The closed-form expression of the average information capacity of perfect optical vortex(POV)carrier link with multi-band phase shift keying(M-PSK)modulation is proposed by using the established link bandwidth-limited ABER and the multi-band quadrature amplitude modulation(M-QAM)scheme.Based on the ABER of OAM mode and the average information capacity model of link in optical transmission system,the influence of modulation order and channel parameters on POV transmission is studied,and the optimal parameters of POV in specific communication links are obtained.With the increase of the virtual refractive index of seawater,the ABER increase and information capacity of the optical transmission system decrease.Using a large ring radius in the object plane of Fourier lens and high signal transmission power,the underwater optical transmission system with POV as signal carrier can realize information transmission with high information capacity and low ABER,and the ABER and information capacity are affected by both signal wavelength and seawater absorption coefficient.(2)The link pointing error was introduced to establish the information capacity model of PLG carrier underwater wireless optical communication link in weak turbulent absorption seawater.The following conclusions are drawn: pointing error significantly degrade the performance of optical transmission systems;The large optical OAM of topological charge and radial quantum number are not conducive to the transmission of high information capacity.The larger the spectral absorption coefficient of seawater is,the greater the information capacity loss of the link is.High signal transmission power can realize information transmission with high average information capacity.The information capacity is also affected by both the signal wavelength and the seawater absorption coefficient.(3)Considering the M-QAM and link pointing error,the ABER model of PLG vortex optical transmission system in weakly turbulent absorption seawater is established.Based on the established ABER model,the tolerance of the limited optical transceiver aperture modulation order,turbulent absorption of seawater and transceiver system to the link pointing error of the PLG vortex optical transmission system is analyzed.It is concluded that the communication link using high-order QAM and large-aperture optical transceiver has higher link pointing error tolerance than that using low-order QAM and small-aperture optical transceiver.In the case of larger link pointing error,the performance degradation of the link with larger receive aperture is less than that of the link with smaller receive aperture.The tolerance of the link with low radial order and OAM topological charge and high transciver signal nosie ratio(SNR)is greater than that of the link with high radial order and OAM topological charge and low SNR transciver system,and the PLG vortex has better tolerance to the pointing error than the Besser-Gaussian beam.(4)The spiral harmonic expansion used is derived from the Rytov approximation of the random coaxial-double position power Gaussian vortex(RCDPPGV)in weakly turbulent seawater,and the ABER of the RCDPPGV carrier underwater extinction communication link is derived.Furthermore,the channel capacity of the RCDPPGV carrier underwater extinction communication link is established by using the classical Shannon information theory of the stationary discrete memoryless system.The study found that the channel capacity and ABER is the nonlinear function of the dimensionless structural parameter α,when |α|=1,which reached the maximum and minimum values.Seawater absorption has a significant effect on the channel capacity,but has little effect on the ABER.The link with large receiving aperture,small transmitting beam diameter,long seawater window light source wavelength and large OAM channel number can achieve high channel capacity communication.