【摘要】：近年來,自由空間光通信(FSO,Free Space Optical Communication)的研究開始復(fù)蘇。相較于射頻通信,FSO具有帶寬大,無需頻譜認(rèn)證,組網(wǎng)速度快,功耗低,通信設(shè)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(jī)變化,會產(chǎn)生光束擴(kuò)展,光束漂移和光強(qiáng)閃爍等影響,其中光強(qiáng)閃爍的影響最為顯著。光強(qiáng)閃爍會導(dǎo)致光束質(zhì)量嚴(yán)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(yán)重制約了FSO技術(shù)的發(fā)展。所以定量的對光強(qiáng)閃爍系數(shù)進(jìn)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(shù)。與傳統(tǒng)的Kolmogorov譜相比,采用三層高度譜進(jìn)行描述的湍流產(chǎn)生的閃爍效應(yīng)更加嚴(yán)重。在對數(shù)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(chǔ)上,進(jìn)一步推導(dǎo)了弱湍流中高斯波的閃爍系數(shù)表達(dá)式,利用LN信道衰落下誤碼率的表達(dá)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(shù)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(jié)論。在上述研究的基礎(chǔ)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(kuò)展Rytov理論推導(dǎo)了忽略內(nèi)尺度和外尺度效應(yīng)的上行鏈路和下行鏈路的光強(qiáng)閃爍系數(shù)模型,該模型適用于湍流由弱到強(qiáng)的所有區(qū)域。文中分析了三層高度譜的冪律指數(shù),近地湍流強(qiáng)度,波長,天頂角等因素對光強(qiáng)閃爍系數(shù)的影響。數(shù)值結(jié)果表明,閃爍系數(shù)隨冪律指數(shù)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(shù)達(dá)到最大值而后又開始下降。文中還推導(dǎo)了采用OOK和M-PPM調(diào)制方式下,通信系統(tǒng)誤碼率的閉合表達(dá)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(jù)分析結(jié)果可知,在實際系統(tǒng)中,考慮可能的湍流環(huán)境,設(shè)定可以通信的天頂角范圍,通過選用較長波長進(jìn)行通信,可以有效抑制光強(qiáng)波動。本文研究時在不同海拔高度用不同的冪指數(shù)值進(jìn)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) has begun to revive. Compared with radio frequency communication, FSO has the advantages of wide bandwidth, no need for spectrum authentication, fast networking speed, low power consumption, small size of communication equipment, high security and good security. At the same time, it will also face a big problem - atmospheric turbulence, atmospheric turbulence will cause random changes in atmospheric refractive index, beam expansion, beam drift and intensity scintillation, among which intensity scintillation is the most significant. As a result, the beam quality is seriously degraded and the BER at the receiver is increased, which greatly affects the stability and reliability of the FSO communication system and seriously restricts the development of the FSO technology. Therefore, it is necessary to study the scintillation coefficient quantitatively to optimize the transmission rate and capacity of the communication system. Then, the scintillation coefficients of plane wave and spherical wave in slant-path FSO link are analyzed based on three-layer altitude spectrum. Compared with the traditional Kolmogorov spectrum, the scintillation effect caused by turbulence described by three-layer altitude spectrum is more serious. The influence of zenith angle and wavelength of light wave on laser communication in weak turbulent channel is studied. Based on this study, the expression of scintillation coefficient of Gaussian wave in weak turbulent channel is deduced. The BER under different conditions is analyzed by using the expression of BER under LN channel fading. The finite BER in uplink and downlink is quantitatively analyzed. The influence of beam radius on scintillation coefficient and bit error rate is studied. It is concluded that the BER of downlink does not change with beam waist radius, and there exists an optimal beam waist radius in uplink which can minimize the BER. Rytov's theory deduces an intensity scintillation coefficient model for both uplink and downlink links, which ignores the effects of internal and external scales. The model is suitable for all regions of turbulence from weak to strong. The effects of power law exponent of three-layer height spectrum, near-earth turbulence intensity, wavelength, zenith angle on the intensity scintillation coefficient are analyzed. It is shown that the scintillation coefficient decreases with the increase of power law exponent and wavelength, and increases with the zenith angle. When the zenith angle is close to 70 degrees, the scintillation coefficient reaches its maximum value and then begins to decrease. The closed-form expression of BER for communication systems with OOK and M-PPM modulation is derived. The BER for different turbulent conditions is analyzed and obtained. The variation trend of BER with zenith angle, wavelength and other factors is analyzed. According to the analysis results, considering the possible turbulence environment, the range of zenith angle which can be communicated can be set and the fluctuation of light intensity can be effectively suppressed by choosing a longer wavelength to communicate. The continuous calculation is different from the previous analysis of different power rates and is closer to the actual slant communication system.
【學(xué)位授予單位】：西安電子科技大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2014
【分類號】：TN929.1

【參考文獻(xiàn)】

相關(guān)博士學(xué)位論文 前1條

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

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本文編號：2205253