台灣高鐵之光載無線通訊系統設計

Abstract

在本篇論文中，我們對於實作在WiMAX標準上的光載無線通信(RoF)技術有深入的討論，不管是同步優化技術，通道估計演算法還是後端解調效能。在同步方面，我們對於載波偏移估計，時間的粗同步與細同步都有充足的討論，另外對於頻率載波偏移追蹤(CFO tracking)也有完整的模擬分析。另一方面，在通道估計上，配合通道的頻域響應特性，我們發展出一套根據通道長度的適應性通道估計機制達到節省計算功率消耗的效果。我們進一步利用一正交分頻多工之通道量測系統在台灣高速鐵路的戶外與隧道環境下進行光載無線通信系統的量測，此量測系統並非昂貴且笨重的通道探測機，而是由一般的實驗儀器組成，因此在量測訊號上可以依據量測目的而設計。通道特性包含通道頻率響應、角擴散、功率延遲擴散、都卜勒頻譜與傳播損失都是我們量測的重點。由量測分析的結果，我們可以觀察出高鐵環境相當單純，其通道大部分都含有很強的直視波，且其傳播損失也很接近Friss formula所計算出來的值。我們亦根據量測的結果來探討台灣高鐵上WiMAX基地台配合光載無線通信(RoF)技術的佈建方案。此部分我們配合台灣所使用的WiMAX系統參數來模擬實際通訊傳輸時會產生的現象，並且提出三種狀況相互比較，一個是光纖佈建長度依造地理上的長度，另一個是在基地台端將所有光纖補成等長，最後一種狀況是利用開關控制演算法的機制來避免系統效能的降低。此模擬結果證明了使用開關控制演算法可以大幅提升系統效能。

In this thesis, we go into details on radio-over-fiber (RoF) wireless communication system based on WiMAX standard not only for synchronization but also channel estimation and detection performance. In view of synchronization, we discuss the synchronization procedure and algorithms such as carrier frequency offset (CFO) estimation, fine timing and CFO tracking. For another way, we describe the channel estimation methods for the channel measurement. According to the relationship between the variation of channel frequency response (CFR) and channel length, we propose an adaptive channel estimation mechanism which can save computing power. Furthermore, we apply an OFDM-based channel sounder to measure the channel characteristics of RoF system in Taiwan High Speed Rail (THSR) environment at the 2.5GHz band. This measurement system is made of general-purpose commercial equipments. Based on the requirement of channel measurement, the OFDM signal format is able to be flexibly designed. The channel characteristics we are going to measure are the frequency responses, delay spread, angular spread, Doppler spectrum and propagation loss. From the measurement result, a strong line of sight (LOS) component is usually observed and the measured path loss is close to the prediction by the Friss formula. Radio over Fiber (RoF) technique is attractive for both extending the wireless cell coverage and reducing the cost by using distributed remote antenna units (RAUs). In order to observe multipath effect of WiMAX-RoF system in THSR, we use the Elektrobit C2 channel emulator to simulate three cases which are non-equal fiber length, equal fiber length and RAUs switching. From the simulation, we prove that switching on only one RAU at a time can indeed promote the performance of the whole system.