Research On The Optical Properties Of The Ground-to-train Wireless Communication System

Wireless optical communication(WOC) is a new promised kind of forefront communication technology. Since it has several advantages, such as easy-to-setup, wide frequency band, large information capacity, small scattering angle, large optical gain, strong anti-interference ability, low power etc., it has broad application prospect in the field of wireless communications. Then it has a broad application prospect in the field of wireless communications.And the ground-to-train WOC is a new subject in the field of wireless communication. However, the atmosphere turbulence leads to the attenuation of scintillation, beam wander and spread, the misalignment between transceivers result in path loss, both of them can affect the performance of ground-to-train WOC system seriously. Therefore, an accurate understanding of how the atmospheric turbulence influences the signal light transmission is of great significance.To reduce turbulence effect on the optical performance of the ground-to-train WOC system, we studies the system structure and ink performance under the influence of atmospheric turbulence. Firstly, we reconstruct the structure of the ground-to-train WOC link, establish the model for the received power and average channel capacity, study the performance of the link (such as link margin, signal-to-noise radio (SNR), bit error rate (BER) and channel capacity).Through the numerical simulation analysis, we study the effects from the turbulence parameters (such as structure parameters, the scintillation fluctuation, turbulence index, etc.) and link parameters (operating distance, normalized beamwidth, normalized jitter, average signal-to-noise ratio, etc.) on the performance of optical link.The main results are as follows:(1)Considering the effects of atmosphere turbulence for the system, the best position of the transmitter, the link margin with different weather conditions, and the SNR are calculated through the simulation computation. In the medium to strong scintillation fluctuation area, the main factors which affect the average link channel capacity comes from two aspects: atmospheric turbulence and the pointing error. When other parameters are fixed, increasing turbulence structure parameters, operating distance, normalized beamwidth, normalized jitter and average SNR simply or decreasing turbulence channel visibility, turbulence index, can reduce the average equivalent channel capacity. But when the operation is more than 100m, the impact of these factors could be weakened.(2) Signal beam can be information coding by fractional orbital angular momentum. For the ground-to-train WOC link, we set fractional Bessel-Gaussian beams as a signal beam, the signal source. The signal mode probability and crosstalk mode probability are oscillation curve at the receiving plane, but when the fractional index is half-integer, both of curves are smooth. With constant parameters, larger beam waist can result in enlarging both of the signal mode probability and crosstalk mode probability. Increasing turbulence fluctuations will reduce the average probability densities of signal OAM modes slightly, and increase the average probability densities of crosstalk OAM modes obviously. But in the medium to strong scintillation fluctuation area, the scintillation rolling parameters could make the peak of crosstalk OAM modes probability densities move to the beam center at the same time. In the weak to moderate fluctuation area, the smaller the beam waist is or the larger the spatial coherent radius is, the greater the signal normalized powers are, which is opposite to the crosstalk normalized powers. While it is more complicated for the moderate to strong scintillation area.