Research On Automatic Alignment Control Algorithm Of Underwater Wireless Optical Communication

With the development of wireless communication technology and the application of marine detection systems,underwater wireless communication technology has received great attention.At present,underwater wireless communication can be divided into three categories: underwater acoustic communication,radio frequency communication and optical communication.Underwater acoustic communication signals have low attenuation and long transmission distance,but the transmission rate is too low;RF communication transmission rate is high,but attenuation is large,and the transmission distance is too close;while underwater optical communication has a larger bandwidth and a higher transmission rate.In particular,underwater laser communication has the advantages of good security,high transmission rate,large communication capacity,and long transmission distance.In recent years,it has become an important research direction in the field of underwater wireless optical communication.Due to the small light spot and divergence angle of the light source in high-speed laser communication,after a certain distance of transmission,the light spot at the receiving end is still small.Moreover,the detection surface of the photodetector of the high-speed receiver is smaller than the received light spot,so it is difficult for the receiver and the center of the light spot to be in fine alignment,which makes it difficult to establish a stable underwater optical communication link.This paper studies the problem of alignment of underwater optical communication.First,the Monte Carlo simulation and statistical method is used to analyze the distribution of the received light intensity of laser photons transmitted in seawater,and the relationship between the received light intensity and the photon receiving position at different transmission distances is obtained.The Gaussian function was used to fit and analyze the simulation results.The distribution power of the receiver in the underwater wireless optical communication system with respect to the different spots of the light spot was studied,and a mathematical model of the received light intensity and the position of the receiver was established.Then,on the basis of the existing laser transmission model,considering the influence of the receiver position,a received power model including the distance parameter between the receiver and the center of the spot is obtained.Firstly,the traditional alignment control algorithm(hill-climbing method)is introduced,and the alignment performance is analyzed using matlab simulation.Based on this,an underwater wireless optical communication alignment algorithm based on EKF is proposed.Based on the received light intensity,the distance between the current position of the receiver and the maximum light intensity is estimated,and the active tracking alignment between the receiving end and the center of the light spot is achieved through a feedback algorithm.Simulations show that the alignment error at the receiving end was less than two millimeters,and the receivingefficiency exceeds 98% after stabilization.Finally,an underwater light spot acquisition experiment system was set up to collect and analyze the light spot images at the receiving end at different distances.The experimental results show that after the beam is transmitted 25 meters underwater,the received light intensity distribution is still approximately Gaussian,which is simulated with Monte Carlo The results are consistent,and the Gaussian distribution function of pixel coordinate position and received light intensity is obtained.We also set up the alignment control system at the receiving end.The motor control algorithm based on the mountain climbing method is adopted,and the upper computer program of the motor control module is designed according to the control theory of the stepper motor.Combined with the hardware platform,the experiment of automatic alignment control of the underwater wireless optical communication receiver was carried out.Based on the light intensity distribution relationship obtained from the experimental spot image,the offline method is used to verify the EKF-based alignment algorithm.The experimental results show that the underwater wireless optical communication alignment control system can accurately align the center position of the light spot,which has certain engineering significance.