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In recent years, more attention has been paid to FSO(free space optical) communication system for the advantages of high security, easy installation and deployment and high transmission data compare to conventional frequency wireless communication. FSO communication has a wider prospect in application of civilian and military. However, the FSO link is easily affected by the atmospheric effects such as cloud, rain, fog and so on, which leads to the decline of the performance of the communication system. Orthogonal frequency division multiplexing (OFDM) is a kind of multicarrier transmission in which high data rate streams are split into lower rate streams and then transmitted simultaneously over several narrow-band subcarriers. OFDM subcarrier can use many different modulation modes. One of the main modulation modes is multilevel quadrature amplitude modulation (MQAM). OFDM is known for its increased robustness against frequency selective fading, narrow-band interference, and high channel efficiency and it is widely used in broadband wireless communication systems. Meanwhile, most of the current studies about free space optical communication are based on atmospheric turbulence models with lognormal, Gamma-Gamma and M distribution. There are not appropriate for aperture averaging reception condition. In 2012, Barrios R and Dios F proposed a new Exponentiated Weibull atmospheric turbulence model for the first time. Exponentiated Weibull atmospheric turbulence model is suitable for the weak to strong turbulence and the average diameter of the aperture. Therefore, this paper aimed at the combined effects of the Exponentiated Weibull atmosphere turbulence, geometric spread and pointing errors on airborne FSO communication system, the bit error rate (BER) performance of the OFDM airborne FSO communication link is investigated. The OFDM optical communication link model based on M-QAM is built, and the electrical carrier to noise ratio for OFDM optical communication link is obtained. The closed form mathematical expression for the total average BER performance is theoretically derived. The relationship between the BER performance and the transmitted optical power under different parameters such as the atmosphere turbulence, the normalized jitter standard deviation and the normalized beam-width is analyzed by simulation. The simulation results show that with transmission optical power increased, the performance of optical link which only under the influence of atmospheric turbulence is better improved than which combined effect of atmospheric turbulence and pointing error. The pointing error has a obvious deterioration in the performance of the system. The performance of the bit error rate improved by increasing the transmission optical power when not considered pointing error is 3 orders of magnitude higher than when considered pointing error. The bit error rate increases with the increase of turbulence intensity, the normalized jitter standard deviation and the normalized beam-width. The airborne system performance is similarly improved in different modulation orders by increasing the transmitted optical power. The BER performance is obviously improved by increasing the transmitted optical power when the normalized jitter standard deviation is less than 0.7. In practical application, the derived average error rate closed expression can be used to estimate the performance of the system and provide reference for the design of the airborne FSO communication system.
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