Implementation of the Unidirectional and Bidirectional Optical Wireless Communication System

戶外無線光通訊系統為最近興起的高速傳輸系統，其高頻寬以及相對低廉的設備費用為其最大之競爭優勢，故無線光通訊系統亦為最後一哩問題的方案之一。由於光的傳遞具有方向性，因此傳輸端與接收端之間必須完全淨空，才能順利的連結。另一方面，光在大氣中傳輸時，光的能量會漸漸的減弱，在能見度極低的情況下，其衰減程度甚至高達100dB/km以上。而減弱的因素主要為懸浮粒子所造成的吸收，散射以及大氣擾動。其中影響最大的為散射效應。以一般無線光通訊所使用的1.55μm波長來說，適合用米氏散射(Mie Scattering)來分析。 本論文主要分為三個部分，一是透過文獻分析米氏散射對於光傳輸時所造成的衰減；其二為實際測量一個傳輸距離為785公尺，傳輸8個通道，每個通道傳輸速率最高為10Gb/s的多波長多工戶外無線光通訊系統，對其做為期兩個禮拜的觀察與記錄。從實驗結果可以發現當出現下雨情形時，其系統的錯誤率雖然下降，但仍維持在可以接受的範圍中。最後我們則實際測試了一個傳輸距離為785公尺，雙向均傳輸四個通道，每個通道傳輸速率最高為10Gb/s的雙向戶外無線光通訊系統。經由實際的測量後發現，雙向傳輸的表現與單向傳輸之間的差距並不大，而比起單向傳輸的架構，雙向傳輸之架構更符合實際上網路的使用方式。 戶外無線光通訊系統雖然受限於大氣因素，無法取代光纖網路成為網路的骨幹系統，但其憑藉著高頻寬以及較低成本的優勢，可成為高速網路的替代方案，且由於其方便架設以及高機動性的優勢，更使其成為緊急狀況發生時的通訊方式。因此我們相信戶外無線光通訊系統是一個非常有發展潛力的無線通訊技術。The optical wireless communication system is a newly risen communication system. Both high transmission capacity and low equipment cost are main advantages of the optical wireless communication system, and result in an ideal solution for the last mile problem. Because the transmission of light is directional, we can only connect both transmitter and receiver when the line of sight is available. On the other hand, the light power will decrease when transmitting light in the atmosphere. The power attenuation can reach over 100dB/km if the visibility is extremely low. The main factors caused power attenuation are: absorption, scattering and turbulence. The scattering effect is the most significant effect of them. At the wavelength of 1.55μm, we can use Mie scattering theory to analyze the attenuation effect. This thesis composed by three major parts: The first one is to analyze power attenuation by Mie scattering theory. The second part introduces a 8-channel unidirectional optical wireless communication system with 785 meters and each channel rate up to 10Gb/s. From the experiment result we can found that in bad weather condition, such as raining, decrease the bit error rate performance , but the result is still acceptable. The last part shows a 4-channel bidirectional optical wireless communication system with 785 meters and each channel rate up to 10Gb/s and measure the bit error rate. The bit error rate performance of the bidirectional system is almost the same as the unidirectional system. Because of the high transmission rate and low hardware cost, the optical wireless communication system is a suitable solution for the last mile problem. Even more, this system is easy to setup, so it is a good method for emergency communication situation. We believe that optical wireless communication system is a high potential wireless communication technology