16 波長10Gb/s 光學無線傳輸實作

Abstract

在都會網路的點對點應用上，無線光通訊系統最大的競爭優勢在於高傳輸容量與低廉設備費用，為解決最後一哩問題替代方案。由於光對障礙物的穿透率極差且傳遞具有方向性，因此傳輸端與接收端之間需無任何阻擋物存在，也就是在兩端的視線(line of sight)之間淨空時，通訊品質才能達到最佳狀態。因此主要不利光在大氣傳輸的因素為：由懸浮粒子和分子所造成的吸收、散射與大氣擾動。尤其當濃霧發生時，每公里的衰減量高達100dB以上。散射效應共分三大類：瑞利散射(Rayleigh scattering)、米氏散射(Mie scattering)與幾何散射(Geometrical scattering)，各效應適用範圍與粒子半徑有關。以光源波長 為例，波長與霧氣粒子半徑相似，相較於瑞立散射或幾何散射，米氏理論是最適合用於分析散射的理論。米氏提供對於散射角度以及強度的數學分析，可以用來評估在不同狀況下光穿越大氣介質時造成的損耗以及計算光受到單一粒子散射後，各方向的散射強度。 本論文主要分為兩大部分，一是透過文獻分析米式散射對光傳輸造成的的衰減；另一則是實際測試一個16個通道，每個通道傳輸速率為每秒百億位元的多波長多工無線光通訊系統在大氣中傳輸，並量測實際錯誤率。距離在一至兩百公尺時，訊號受大氣衰減並不明顯，因此效能相當優越。增加傳輸距離至一公里以上後，由於光穿越大氣中的路徑增長，受到大氣干擾的現象十分明顯。在實驗中，我們以一個通訊距離為2.16公里的實驗作為代表。本文從理論以及實驗數據探討無線光通訊通道中調變光受到霧散射的影響，確實證明濃霧是造成光在大氣中傳輸最主要的障礙。無線光通訊雖然受限於氣候因素以至於不適合取代光纖網路作為網路骨幹，但它具有的存在較低成本下，達到射頻系統無法達到的高頻寬優勢卻可彌補光纖接取網路在都會中布建成本過高的問題。在緊急狀況，如地震或其它天災發生時，其快速架設與機動性高的優點，更可滿足快速傳遞訊息的通訊需求。因此我們相信無線光通訊是一個相當有發展潛力的替代通訊技術。

In the point-to-point application of urban networks, the advantage of optical wireless (OW) communication system is the high transmission capacity and the low equipment expense, which provides an ideal solution for the last mile problem. Because the propagation of light is directional, the high communication quality can be achieved if there are no obstacles between the transmitter and the receiver (line of sight). Therefore the main factor for transmitting light in the atmospheric is: absorption, scattering and turbulence caused by the suspended particles and molecules. Power attenuation per kilometer reaches as high as above 100dB especially when the thick fog occurs. The scattering effects can be assorted as three categories: Rayleigh scattering, Mie scattering and geometry scattering. Various effects can be applied in different range of wavelength. For example, at the wavelength of , if the radius of particles is similar to the wavelength, Mie theory is suitable for analyzing the scattering effect. Mie theory which provides mathematical analysis about the scattering angle and the intensity, it can be used to estimate attenuation caused by atmosphere and scattering intensity in different conditions. The thesis consists of two major parts. One is to analyze attenuation by Mie theory; the other part introduces a 16-channel OW system with channel rate of 10Gb/s and measures the bit error probability. When transmission distance is within a few hundred meters, the attenuation is small. Therefore the performance of the system is quite superior. Increasing transmitting distance above one kilometer, the attenuation is significant. In actual experiment, we demonstrate an OW transmission with 2.16 kilometers. The thesis discusses the influence caused by scattering from the theory and experimental data. We can understand the major challenge in the OW communication is fog in the thesis. But OW can provide high data rate and low hardware cost at the same time, we believe that OW communication is a high potential technology.