應用同調迴旋極化調變改善無線光通訊系統在散射通道的影響

Coherent Circular Polarization Modulation for Optical Wireless Communications through Multiple-Scattering Channel

郭建志(Jian-Zhi Kuo)

指導教授 ： 吳靜雄

國立臺灣大學/電機資訊學院/電信工程學研究所/碩士(2006年)

https://doi.org/10.6342/NTU.2006.00617

摘要

由於無線光通訊（OWC）系統具有和光纖通訊網路匹配的高傳輸頻寬以及資料傳輸量，並且能夠解決在都會光纖通訊網路中無法將光纖延伸到各個用戶端的問題（the Last-mile problem），因而大大地降低了系統成本並具有很高的發展潛力，無線光通訊系統便成為了我們感興趣的研究目標。不同於在光纖通訊系統中資料是以光纖為傳輸媒介，在無線光通訊系統中，傳輸調變光波在大氣通道下所受到的衰減是我們要特別注意的。光波在大氣通道下的能量衰減主要可分為散射、吸收、以及大氣亂流三大類，其中尤以霧的散射最為嚴重。在本論文中，我們主要探討在無線光通訊通道中調變光波受到霧的散射所造成的嚴重影響。由於一般常用的雷射波長大約在近紅外光波段，而且霧的粒子半徑分布範圍約略稍大於傳輸光波波長，所以我們可根據米氏理論來討論散射對於傳輸效率所造成的影響。
引用了米氏理論得到散射強度頻譜函數，我們能夠計算光波受到單一粒子散射後，散射光波在各個方向強度的分布。我們更利用大氣輻射能量轉移方程式結合米氏散射來建立調變光波在大氣傳播的多重散射通道模型。經由模擬分析散射光波在通道中的頻率響應，我們能夠發現幾點重要特性：在不同天氣狀況下的通道響應，調變光波的散射強度分布以及調變光波的極化程度，都有著明顯優於未經調變光波的表現，特別是在濃霧中會有更強烈的效果。因此調變光波在散射通道傳播時極化程度所受到的改變以及接收端同調強度對於散射強度的比值，是我們提出利用極化偏移鍵移（Polarization Shift Keying）來完成同調檢測系統的重要依據。
考慮了迴旋極化雷射在散射通道傳輸的表現，我們提出了有別於過去傳統的極化偏移鍵移系統：一種簡化的迴旋極化偏移鍵移系統。此系統主要是利用簡單而快速的極化生成單元，經由改變調變雷射中兩線性極化分量的相位差來產生左迴旋以及右迴旋的調變光波，由二位元編碼來決定光波的極化旋轉方向。經由散射通道的模擬結果我們可以得到，調變光波受到散射後極化程度的衰減是可以容忍的。因此在接收器前端只要經由一個極化參數提取器（Stokes parameters extractor），接著計算傳輸光波中兩線性極化分量的相位差。此相位差是決定傳送資訊的重要參數，用來作為評估效能的主要依據。再經過二位元解碼就可以得到傳送的資訊。由於我們將傳送資訊載在調變光波中兩線性極化分量的相位差，所以此系統不需要可適性的極化控制單元。在此論文中，我們主要評估此同調檢測系統應用於無線光通訊在多重散射通道的效能。並且考慮通道中極化狀態的變動以及接收端散射功率的影響。在不同的天氣狀況下，對接收端的參數如接收場視角以及同調強度對散射強度比，訊號對雜訊比等特性加以分析。與直接強度檢測系統作比較後，我們有充分的理由利用迴旋極化偏移鍵移來達到同調檢測並大大地改進系統效能。

關鍵字

無線光通訊 ； 米氏散射 ； 輻射能量轉移方程式 ； 極化調變

並列摘要

Optical wireless communication (OWC) systems provide sufficiently high system bandwidth and data rate equivalent to fiber-optic network. This optical communication system makes connectivity possible between Internet provider and household subscribers to solve “The Last-mile” problem. Thus the OWC links can reduce the high installation cost of Fiber-optic networks. Such high developing potential has attracted our attention. The main difference between these two communication systems is the propagation channel, wired and wireless. The primary drawback for OWC is high dependence on adverse weather conditions. The atmospheric attenuation is mainly caused by scattering, absorption, and atmospheric turbulence, etc. The most detrimental environmental conditions concerned about are fog and haze. In this thesis, we start with the channel response of OWC systems through dense fog. Because the distributed particles diameter and the near infrared wavelength used are in the same order, we can apply Mie theory to calculate the scattering characteristics through fog particles.
The group of Dr. Ishimaru computed the scattering amplitude phase function by Mie theory and solved the radiative transfer equation in frequency domain to figure out the channel response of the multiple-scattering channel model for further application. From the simulation results we can see that frequency modulated wave shows superior properties over regular wave with narrower angular spectrum and lower order scattering effect. It also preserves higher degree of polarization (DOP), which is more evident for dense fog consideration. With the particular behavior of the polarization statistics and coherence through propagation, we exploit an effective circular polarization shift keying (PolSK) scheme to realize the coherent detection in OWC systems.
By taking the advantage of sufficiently high DOP and well behavior of circular polarized and frequency modulated wave, we employ a simple polarization modulation unit to generate left-handed-circularly and right-handed-circularly polarized wave. The direction of rotation is corresponding to the message bits and refers to the relative phase of two orthogonal polarized components of the optical wave. In the receiver front end an effective Stokes parameters extractor is used to extract the received Stokes parameters on the Poincare sphere. We can obtain the relative phase of the two orthogonal polarization components which is the important basis for transmitting information. Then we cover the information by binary decoding. Because we carry the information on the relative phase of the two orthogonal polarized components, adaptive polarization control unit is needless.
In this thesis, we evaluate the coherent circular polarization detection scheme applied to mitigate the attenuation caused by dense fog and take account of the interference of diffuse intensity and DOP attenuation. We analyze the characteristics of receiver field-of-view (FOV), coherence parameter, and signal-to-noise ratio (SNR) to idealize the OWC system performance.

並列關鍵字

Optical wireless communication ； Mie theory ； radiative transfer equation ； Stokes parameters ； degree of polarization ； polarization shift keying

參考文獻

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[3] I. I. Kim, B. McArthur, and E. Korevaar, “Comparison of laser beam propagation at 785 nm and 1550 nm in fog and haze for optical wireless communications”, in Optical Wireless Communications III, E. Korevaar, ed., Proc. SPIE 4214, pp.26-37, 2001.

[4] S. Karp, R. M. Gagliardi, S. E. Moran, and L. B. Stotts, Optical Channels: Fibers, Clouds, Water, and the Atmosphere, Plenum, 1988.

[5] A. Ishimaru, Wave Propagation and Scattering in Random Media, IEEE, Oxford U. Press, 1997.