【摘要】：近年來,自由空間光通信(FSO,Free Space Optical Communication)的研究開始復蘇。相較于射頻通信,FSO具有帶寬大,無需頻譜認證,組網速度快,功耗低,通信設備尺寸小,安全性高,以及保密性好的優(yōu)點。但是,FSO通信鏈路處于大氣層中,傳輸光束會不可避免地受到大氣分子及大氣中粒子的吸收和散射衰減。同時還會面臨一個很大的問題——大氣湍流,大氣湍流會引起大氣折射率的隨機變化,會產生光束擴展,光束漂移和光強閃爍等影響,其中光強閃爍的影響最為顯著。光強閃爍會導致光束質量嚴重下降,接收端誤碼率增大,極大的影響了FSO通信系統(tǒng)的穩(wěn)定性和可靠性,嚴重制約了FSO技術的發(fā)展。所以定量的對光強閃爍系數進行研究,對于優(yōu)化通信系統(tǒng)傳輸速率和傳輸容量是十分必要的。本文首先在弱湍流情形下,基于三層高度譜,分析了平面波和球面波在斜程FSO鏈路中的閃爍系數。與傳統(tǒng)的Kolmogorov譜相比,采用三層高度譜進行描述的湍流產生的閃爍效應更加嚴重。在對數正態(tài)(LogNormal,LN)信道衰落中分析了通信系統(tǒng)的誤碼率性能,量化分析了不同天頂角和光波波長對弱湍流信道激光通信的影響。在該研究的基礎上,進一步推導了弱湍流中高斯波的閃爍系數表達式,利用LN信道衰落下誤碼率的表達式分析了不同情形下的誤碼率。量化分析了上行鏈路及下行鏈路中有限的光束半徑對閃爍系數及誤碼率的影響,得到了下行鏈路誤碼率不隨束腰半徑變化,上行鏈路存在一個最優(yōu)束腰半徑可以使得誤碼率最小的結論。在上述研究的基礎上又分析了在斜程大氣激光通信系統(tǒng)中,基于三層高度湍流譜,采用擴展Rytov理論推導了忽略內尺度和外尺度效應的上行鏈路和下行鏈路的光強閃爍系數模型,該模型適用于湍流由弱到強的所有區(qū)域。文中分析了三層高度譜的冪律指數,近地湍流強度,波長,天頂角等因素對光強閃爍系數的影響。數值結果表明,閃爍系數隨冪律指數和波長增大而減小,隨天頂角增大而增大,在天頂角接近70°時,閃爍系數達到最大值而后又開始下降。文中還推導了采用OOK和M-PPM調制方式下,通信系統(tǒng)誤碼率的閉合表達式。分析了對不同湍流情形誤碼率,并得到了誤碼率隨天頂角,波長等因素的變化趨勢。根據分析結果可知,在實際系統(tǒng)中,考慮可能的湍流環(huán)境,設定可以通信的天頂角范圍,通過選用較長波長進行通信,可以有效抑制光強波動。本文研究時在不同海拔高度用不同的冪指數值進行連續(xù)計算,與以往分析不同冪率取值情況不同,更加貼近實際斜程通信系統(tǒng)。
[Abstract]:In recent years, the research of free space optical communication (FSO,Free Space Optical Communication) has begun to recover. Compared with RF communication, FSO has the advantages of large bandwidth, no spectrum authentication, fast networking speed, low power consumption, small size of communication equipment, high security, and good confidentiality. However, because the FSO communication link is in the atmosphere, the propagation beam will inevitably be absorbed and attenuated by the atmospheric molecules and particles in the atmosphere. At the same time, there is also a big problem of atmospheric turbulence. Atmospheric turbulence will cause random changes of atmospheric refractive index, which will produce the effects of beam expansion, beam drift and intensity scintillation, among which the effect of intensity scintillation is the most significant. The light intensity flicker will lead to the serious deterioration of beam quality and the increase of bit error rate (BER) at the receiver, which greatly affects the stability and reliability of FSO communication system, and seriously restricts the development of FSO technology. So it is necessary to study the light intensity scintillation coefficient in order to optimize the transmission rate and capacity of communication system. In this paper, the scintillation coefficients of plane wave and spherical wave in oblique FSO link are analyzed based on the three-layer height spectrum in the case of weak turbulence. Compared with the traditional Kolmogorov spectra, the scintillation effect of the turbulence described by the three-layer height spectrum is more serious. The bit error rate (BER) performance of the communication system is analyzed in logarithmic normal (LogNormal,LN) channel fading, and the effects of different zenith angles and wavelength of light wave on the weak turbulence channel laser communication are quantitatively analyzed. On the basis of this study, the expression of the scintillation coefficient of Gaussian waves in weak turbulence is further derived, and the BER in different cases is analyzed by using the expression of BER in LN channel fading. The influence of limited beam radius in uplink and downlink on scintillation coefficient and bit error rate (BER) is analyzed quantitatively. It is concluded that the BER does not vary with the beam waist radius in downlink. There is an optimal beam waist radius for uplink to minimize the bit error rate (BER). On the basis of the above research, the model of intensity scintillation coefficient of uplink and downlink is derived by using the extended Rytov theory based on the three layer high turbulence spectrum in the oblique atmospheric laser communication system. The model is applicable to all regions where turbulence is weak to strong. In this paper, the influence of the power law exponent of three-layer height spectrum, the intensity of near-Earth turbulence, the wavelength and the zenith angle on the scintillation coefficient of light intensity is analyzed. The numerical results show that the scintillation coefficient decreases with the increase of power law exponent and wavelength, and increases with the increase of zenith angle. When the zenith angle is close to 70 擄, the scintillation coefficient reaches the maximum value and then begins to decrease. The closed expression of bit error rate (BER) of communication system using OOK and M-PPM modulation is also derived. The error rate of different turbulence is analyzed, and the change trend of error rate with zenith angle, wavelength and other factors is obtained. According to the analysis results, in the actual system, considering the possible turbulence environment, setting the range of zenith angle that can communicate, and selecting the longer wavelength to communicate, the fluctuation of light intensity can be effectively suppressed. In this paper, different power exponents are used for continuous calculation at different altitudes, which is different from the previous analysis of different power rates, which is closer to the actual slant range communication system.
【學位授予單位】：西安電子科技大學
【學位級別】：碩士
【學位授予年份】：2014
【分類號】：TN929.1

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相關博士學位論文 前1條

1 易湘;大氣激光通信中光強閃爍及其抑制技術的研究[D];西安電子科技大學;2013年

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本文編號：2205254