| Free space optical communications (FSO) is also called atmospheric optical communications, it has become a hot research topic in the field of optical communications since1990s. Compared to the traditional optical fiber communications, the chief distinguishing feature of FSO is making optical signal get rid of fiber, it can be used in some special occasions. In general, the advantages of FSO is as followed:the frequency of optical signal is high enough that it is not easy to be interfered by electromagnetic waves around; the capacity is great enough that the transmission rate can achieve gigabit per second; it has independent communication band that doesn’t occupy the existing frequency band. Although FSO system has its particular advantages, its disadvantage is also obvious:Exposed in atmospheric channel, the optical signal can be disturbed easily. The activity of optical signal is always random and unpredictable. It caused great damage, and even interrupt the entire communication system. Adopting novel modulation and demodulation method suitable for FSO system can reduce the impact of atmospheric channel in the maximum extent, it has positive significance to promote the development of FSO technology.This dissertation is concerned about the modulation and demodulation technology in FSO system. The influence of atmospheric turbulence on the performance of communication systems is researched. Several new methods of modulation and demodulation are proposed, and the theoretical analysis and simulation is performed. The main contents and innovations of this dissertation are as follows:(1) A new PolSK modulation method is proposed to solve the problem that the random phase jitter impacts the polarization of optical signal. Two AM modulators replace single modulator in traditional PolSK modulation is adopted in the article. The method avoids the influence of light signal phase change of the polarization state. Theoretical analysis and experiment prove the feasibility of the system. (2) According to the average transmitted power of the traditional PolSK modulation is too high, while the anti-turbulence performance of PPM modulatbn with low average transmitted power is poor. A novel PPM-PolSK modulation combining the advantages of two modulations is proposed. The new modulation of PPM-PolSK can not only greatly reduce average transmitted power, but also improve the anti-turbulence performance. The article analyzes the performance of the PPM-PolSK modulation and simulated on simulation platform, verifies the result of theoretical analysis.(3) A novel structure of4-PolSK without using Stokes parameters receiver is proposed in this paper to avoid the complex structure in traditional M-PolSK system. The paper analyzes the system theoretically and simulates on simulation platform. Compared to2-PolSK, M-PolSK can raise transmission efficiency, but using Stokes parameters, receiver increases the complexity of entire system. The most remarkable advantage of the new communication system is simply using passive optical components would demodulate the optical signal. The result of the simulation platform proves that the system can work normally under weak atmospheric turbulence channel.(4) A new communication system of MIMO-CPolSK combined the MIMO system and CPolSK modulation is proposed in this article according to the MIMO technology would improve the performance of communication system. The paper discusses Mathematic model under weak turbulence channel and Gamma-Gamma model, then analyzes the performance of the system under both models and simulates on MATLAB platform The result of numerical simulation shows that BER performance is greatly promoted in MIMO system and the new type modulation is an achievable modulation method. |
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