BER Performance Analysis Of Atmospheric Laser Communication System In Slant Path

Recently, the study of Free Space Optical(FSO) communication begins to revive. Comparing with RF systems, FSO has more advantages in power, volume, security, unlicensed frequency and bandwidth. However, due to the properties of atmospheric transmission channels, the wave beam suffers severe attenuation caused by the absorption and scattering. Meanwhile, the performance of FSO system degrades seriously due to the irradiance fluctuations(scintillations) as well as the beam spread and beam wander caused by atmospheric turbulence. As optical beams propagate in the atmosphere, the irradiance scintillation leads to the deterioration of beam quality and increment of the Bit Error Rate(BER), which severely affects the reliability and stability of the FSO system and limits the advancement of the FSO technique. Therefore, it is significantly necessary to analyze the irradiance fluctuations quantitatively, in order to optimize the transmission rate and the capacity of FSO system.In the dissertation, when the turbulence is considered weak, the scintillation index expressions of the plane wave and sphere wave in the slanted atmospheric path are derived based on the three-layer altitude spectrum. The results show that the turbulence-induced irradiance fluctuations based on the three-layer altitude spectrum is more severe than that based on the traditional Kolmogorov spectrum. Furthermore, we study the BER performance of the FSO communication system in the Log Normal(LN) channel as well as numerically analyze the effects of the zenith angle and the wavelength on the laser communications in weak turbulence channels. On the basis of the aforementioned research, the scintillation index expression of Gaussian wave is derived in weak turbulence, and the BER performances in different situations are analyzed according to their expressions under LN channel fading. Also, we give the expressions of the scintillation index and BER as functions of the beam wave radius in the uplink and downlink. It can be concluded that the BER does not change with the waist radius in the downlink, while an optimal waist radius exists to minimize BER in the uplink.Then, the scintillation index models in the downlink and uplink with the inner and outer scale effects neglected are derived according to the three-layer altitude spectrum and the extended Rytov theory. The models can be applied to weak-to-strong turbulence regime.In the dissertation, we discuss the effects of the power law exponent, wavelength and the zenith angle on the scintillation index. Numeric results show that increasing spectrum power exponent and wavelength lead to the BER’s decrement. However, the BER grows firstly if the zenith angle goes up and reaches the maximum value when the zenith angle is 70°. Afterwards, the BER drops as the zenith angle. Additionally, close expressions of BER with the OOK modulation and M-PPM scheme are deduced. The analysis of BER in different turbulence situations demonstrates the BER’s changes as the zenith angle and wavelength. According to the results, the turbulence effects can be mitigated effectively if we choose the suitable zenith angle range and the long light wavelengths to communicate with the turbulence condition taken into account in actual communication systems. It should be noted that our research adopt the spectral exponent function for the continuous calculation in different atmosphere layers, which is different from previous studies but more close to the actual slanted communication system.