Research On Modulated-Carrier Underwater Wireless Laser Communications Based On Analytic Monte Carlo Methods

Underwater wireless optical communication has very important economic value and strategic significance in marine communications, environmental monitoring, mineral search and underwater facilities monitoring applications. Although in the presence of transmission "window" of optical signals in seawater, the transmission distance of the existing laser communication systems has been greatly limited, mainly due to the strongly forward scattering of seawater, resulting in a broadening effect in the time/spatial and the polarization domain. The problem of how to suppress the scattered light to improve communication distance becomes the bottleneck of the underwater laser communication studies currently.It is not practical to increase the distance of the underwater optical communication through simply increasing the transmitter power. The reason is that the useful information is drowned by scattered optical signal. The strength of the optical signal carrying the communication information Communication is very small at the receiving end and did not or occurs only minimal scattering in the water. On the other hand, the scattering signal intensity is relavively large and causes the distortion of receiving waveform. If there is a method suppressing the forward scattering signal intensity to improve the contrast of the collimation signal that carries the information, it is possible to achieve the increase of the distance’of the underwater optical communication.Through the analysis of seawater Mie scattering theory and its function, the seawater forward scattering model is established in this paper. The results of model analysis show that the amplitude-frequency response of seawater prior to the scattering channel interference of scattered light showing a low-pass characteristics, and the amplitude-frequency response of collimated light carring useful communication information is uniform function. By ultilizing a high-frequency carrier to modulate the transmitting information signal, the useful ballistic optical signal in the frequency domain characteristics can be detected out of the scattering noise after the transmission in sea channel by a bandpass filter with its center frequency equals the modulation frequency at the receiving end. Thus, the article proposes a carrier modulated carrier method to suppress forward scattering in underwater optical communication. Based on this method, the scattering channel transmission characteristics and the transmission distance are studied.The establishment of the the carrier modulation underwater laser communication link depends on the time/frequency domain transmission characteristics analysis of the carrier modulation seawater optical communication channel. Traditional analytic method exists larger error. On the other hand, the experimental measurement method is limited by the sink’s geometric size. The parameter adjustment is not flexible. Moreover, the water quality parameters differ from a real seawater. Thus, theses methods are not practical. Based on the traditional statistical Monte Carlo simulation, the paper proposed a analytical Monte Carlo method by analysis of the photon parsing component that directly reaching the receiver after each microscopic collision. The HG function is used as the scattering fitting phase function. After that the single light pulse and the carrier modulation light pulse model are build based on the single photon Monte Carlo simulation model. This model is an effective solution to the problem of time/frequency domain analysis of the actual seawater scattering channel. With respect to the traditional algorithm, the computing time effectively increased164.57times. The model become a powerful tool of the underwater laser communication channel characteristics studies at this stage.By comparing the receiving the light intensity and the received signal bandwidth of tank experiments and the analytical Monte Carlo model simulation results with a variaty of water quality parameters, the effectiveness of the model is verified. On this basis, the model is applied to the simulation of real seawater quality parameters. The widely used Petzold’s three types water quality parameters of seawater forward scattering channel is applied. In addition to the traditional pattern of collimation peak+scattering tailing in previous studies, the simulation results show that there are two other patterns exist. The result renews our understanding of the characteristics of the channel impulse response of wireless laser communication in seawater. After that, the channel capacity under different attenuation length of seawater is analyzed through the FFT transform. It is found that its channel capacity approximately obeys the law of exponential decaying. It provides a simple channel estimation method for real seawater laser wireless communication link.Finally, the article carrier modulation signal input underwater wireless communication to resolve the Monte Carlo model closer to the actual application of laser modulation technique using Gaussian function modulated carrier modulated communication signal output waveform characteristics. Conditions download wave modulation maximum communication distance of the three types of marine water quality parameters analyzed in the received signal spectrum can be recognizable by the carrier modulation frequency. Simulation results show that the maximum communication distance of the three types of seawater was:ocean seawater140m; coastal waters75m Gulf seawater19m. The visible carrier modulation communications technology in communication distance ocean seawater increased more than2times. Traditional communication performance analysis does not contain the offshore waters and the Gulf seawater analysis, these two the seawater carrier modulation communication performance can not be compared.Finally, the analysis of the composition carrier modulation laser communication system in all aspects of its system parameters indicators. The results showed that the tune Q-switched Nd:YAG frequency doubling laser peak power of up to114MW, to effectively meet the demand for long-distance communications design; the laser modulation technically, two improved modulation of the Fabry-Perot modulator and within the modulation scheme alternative; optical filter member, the filter bandwidth of the best performance of the molecular iodine, wave number of0.1or less, the first choice of the optical filter member; two programs and the performance parameters of the photodetector member also analyzed.