Atmospheric turbulence has a greater influence on the performance of the atmospheric laser communication system increasing the bit error rate (BER) and reducing the signal to noise ratio (SNR). If the bidirectional free space laser transmission link has channel reciprocity, the change of optical signal intensity at the two terminal is correlated, and channel state information (CSI) can be obtained at the transmitter, which saves channel space and improves real-time detection of instantaneous CSI and communication channel capacity. In this paper, under the condition of weak-fluctuation, according to the Rytov approximation theory, the relationship between the correlation of optical signal of receiving terminal and transmission path of bidirectional free space laser transmission link is deduced and analyzed, and then the coaxial atmospheric laser transmission link of bidirectional transmitting and receiving is built for field experiment. Experimental results show that the intensity of optical signal at the receiving terminalof bidirectional free space laser transmission link is correlated. The correlation coefficient is related to the location of transmission path. The probability density distribution of the optical signal collected by the experiment obeys the terminal and approximation theory, the relationship between the correlation of optical signal of receiving terminal and transmission path of bidirectional free space laser transmission link is deduced and analyzed, and then the coaxial is correlated. The correlation coefficient is related to the location of transmission path. The probability density distribution of the optical signal collected by the experiment obeys the lognormal distribution, and the real-time change trend of the intensity of the speckle signal at the both receivers is same. Therefore, the atmospheric channel of the bidirectional free space laser transmission link is reciprocal. The conclusion of this paper is of great significance for realizing high-rate and low bit error rate transmission in atmospheric channel.
Atmospheric turbulence has a great influence on the performance of the atmospheric laser communication system reducing the signal to noise ratio (SNR) and increasing the bit error rate (BER). However, there is rarely study on the effect of atmospheric turbulence on the power spectrum of the rectangular pulse. In this paper, a spectral analyzing method is used to analyze the influence of atmospheric turbulence on the signal. An experiment of laser beam propagation characteristic is carried out on a 6km horizontal atmospheric link, the wavelength is 808 nm. The signal is 100MHz rectangular pulse. The waveform of the rectangular pulse is collected by the oscilloscope, and the power spectral density of the signal is calculated and analyzed by the method of periodogram. Experimental results show that the response and noise characteristics of the laser and photoelectric detector have a great influence on the signal power spectrum distribution which can increase the noise component in the 10^6 Hz frequency range. After the atmospheric turbulence propagation, the signal power decreases in the whole frequency range. However, as the existence of atmospheric turbulence, the signal power increases in the atmospheric turbulence characteristic frequency (tens to hundreds of Hz). The noise power increases in the high frequency range (10^7~10^8 Hz).
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