| Free space optical communication (FSO) is a kind of thermal wireless communication technology, it is the information carrier of the laser, through the free space to achieve the exchange of information and transmission, compared with the traditional wireless communication,optical communication with the characteristics of laser carrier, Make full use of the high frequency band of the laser, to achieve high-speed, free space high-speed transmission, so free space optical communication is considered a very potential communication technology. FSO in practical production applications, due to the atmospheric channel of the time-varying characteristics, as well as atmospheric turbulence,atmospheric attenuation effect, transmission carrier laser will have a very large impact, so that the reliability of communication systems worse. In order to deal with the influence of various interference factors on the atmospheric channel communication system, modulation and coding technology, adaptive technology and orbital angular momentum technology have been the focus of research in recent years. Adaptive transmission technology is the focus of this paper.In this paper, a new adaptive modulation system is proposed for FSO,and the related research is carried out. The main contents are as follows:Firstly, the channel characteristics of FSO are studied, and the mechanism of atmospheric attenuation and atmospheric turbulence and its influence on system performance are analyzed.Secondly, the FSO communication system model of subcarrier intensity modulation is studied. The performance of subcarrier intensity modulation under atmospheric turbulence is analyzed mathematically. On this basis, an adaptive modulation scheme is proposed to improve the system band utilization.Then, a typical algorithm of channel estimation for wireless channel is studied, and its performance is analyzed. The correlation algorithm is improved for the particularity of optical communication signal, and the channel estimation and SNR estimation strategy of FSO are proposed.Finally, an MPSK adaptive modulation system with constant power rate of change is studied in the atmospheric channel. A series of improved Bessel functions are used to give the mathematical expression of the spectral efficiency and bit error rate of the adaptive modulation system.Through the above work, the adaptive modulation scheme proposed in this paper can flexibly switch the modulation format and improve the quality of the atmospheric channel under the premise of ensuring the communication quality, so as to improve the spectrum utilization rate of the communication system and improve the transmission efficiency obviously. |
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