When the optical signal is transmitted in atmospheric channel in the wireless optical communication (WOC) systems, the inter-symbol interference (ISI) will be caused by the atmospheric motion and the fading characteristics of the atmospheric channel. In addition, device noises such as photodetectors noises can also affect the signal in the real communication systems. It is difficult to eliminate both types of interference by the single equalization algorithm at the same time, therefore, a cascaded equalization algorithm is proposed. First of all, we use the MATLAB to simulate the cascaded equalization algorithm, the convergence performance of the algorithm is analyzed, the bit error rate (BER) under different signal-to-noise ratios (SNR) are also compared. Then, the algorithm is verified under laboratory conditions, we calculate the BER of the receiving signal, which is processed by the cascade equalization algorithm. The simulation analysis and experiments results show that, the convergence performance of the cascaded equalization algorithm is improved effectively, the ISI is decrease, the BER can reduce to about 10<sup>-8</sup> , decreased by 4 orders of magnitude than without equalization. It can improve the communication performance of the WOC systems, the validation of the cascaded equalization algorithm is verified.
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).