被動式光學網路與集中式無線電接取網路的高傳輸效益之研究

The Study of PON and C-RAN with High Transmission Efficiency

顏志恆(Jhih-Heng Yan)

指導教授 ： 馮開明

國立清華大學/電機資訊學院/光電工程研究所/博士(2016年)

摘要

在今日，光接取網路是提供通訊服務不可或缺的一環。隨著近來服務內容的增長、例如高畫質影像串流和雲端運算，提升被動光學網路(PON)和無線電接取網路(RAN)的傳輸效益、包含頻譜效益和成本效益，已經成為銳不可擋的趨勢。近年，在無線通訊中十分成熟的正交分頻多工(OFDM)技術，成為這項工作的可以仰賴的解決方案之一。直接接收光學正交分頻多工(DDO-OFDM)技術已經確保可以增進整體傳輸效益。數以千計的學術和產業界研究計畫都致力於直接接收光學正交分頻多工在被動光學網路和無線電接取網路的研發工作。
在這個論文中，以建置成本與營運成本可負擔做為前提，為進一步地增長傳輸效益，我們研究並透過實驗證實三項解決方案。第一，我們提出多頻帶直接接收光學正交分頻多工系統，藉以大幅舒緩傳輸端與接收端裡電子元件技術的瓶頸。透過簡單的光學單頻帶(SSB)濾波器，在光學檢測器中產生的訊號間拍頻干擾(SSBI)可以大幅度避免。在50 GHz的傳輸頻寬中，整體的傳輸率達到150 Gb/s。第二，為進一步改善頻譜效益，我們在被動光學網路的接收端裡應用了遞迴式數位訊號處理技術，以削除訊號間拍頻干擾。透過重建多頻帶直接接收光學正交分頻多工訊號的訊號間拍頻干擾，這項數位訊號處理可以逐步地抹除訊號間拍頻干擾。結果顯示訊號表現在2次遞迴運算之後達到飽和，而傳統的空白保護頻帶減少了66%。第三，在多頻帶直接接收光學正交分頻多工系統中，加入偏振態多工技術。為了符合集中式無線電接取網路架構在下世代行動通訊的發展，傳統上以接收端設計為出發點的偏振態分工解多工技術、回授式偏振態追蹤技術或偏振態等化數位訊號處理皆非為我們系統所需。在這個研究中，我們在中央機房端產生具有兩個偏振態正交的光學載波的偏振態多工訊號。透過一個簡單的光學濾波器就可以完成偏振態解多工。基於這項無偏振態追蹤技術，我們提出兩項解決方案，分別是一個偏振態多工的多服務光載無線(RoF)行動網路前段和一個偏振態多工的光載中頻(IFoF)行動網路前段。一般系統使用的偏振態追蹤訓練訊號，在這兩項系統中將不再被需要，因此系統傳輸率將是一般單偏振態系統的兩倍。更甚者，兩個偏振態正交的光載波可以被重複利用於產生上傳訊號，而無須任何偏振態控制機制。因此，我們可以建立真正無須偏振控制的光學網路端點(ONU)和用戶端(UE)。於偏振態多工多服務光載無線行動網路前段的展示中，我們應用一個12 GHz的低頻訊號與一個62.5 GHz的毫米波訊號。而在偏振態多工光載中頻行動網路前段的展示中，前述的光學單頻帶濾波器將同時做為多頻帶解多工與偏振態多工解多工兩項技術使用。
總結前述，藉由應用多頻帶技術、遞迴式數位訊號處理，與偏振態多工三項技術，直接接收光學正交分頻多工系統的頻譜效益將在可負擔的成本之下獲得有效的增長。這些集中化的系統架構設計將依循下世代行動通訊對於系統複雜度集中化的大趨勢。因此，這些解決方案將能提供高度的頻譜效益與成本效益予被動光學網路與無線電接取網路，對未來光接取網路與其通訊技術的研究發展將有大程度的貢獻。

關鍵字

光纖通訊 ； 被動光學網路 ； 無線電接取網路 ； 行動通訊 ； 正交分頻多工 ； 直接接收 ； 數位訊號處理 ； 偏振態多工 ； 光載無線

並列摘要

Today, an optical access network always acts as one of the most important roles to provide communication services. With the progress of recent services, such as ultrahigh video streams and clouding computing, a laudable goal is to enhance the transmission efficiency of passive optical network (PON) and radio access network (RAN), mainly including the spectral efficiency and cost efficiency. In recent years, orthogonal frequency division multiplexing (OFDM) as a mature technique widely applied in wireless communications has become one of the promising solutions. A direct-detection optical OFDM (DDO-OFDM) signal format is guaranteed effectively enhancing the overall transmission efficiency. Tens and thousands of academic and industrial projects have devoted to the research and development of DDO-OFDM in PONs and RANs.
In this dissertation, to further enhance the transmission efficiency with affordable implementation and operation costs, we study and experimentally demonstrate three effective solutions. Firstly, a multiband DDO-OFDM system is proposed to largely relief the electrical bottlenecks in both the transmitter and receivers. With an applied simple optical single-sideband (SSB) filter, the deleterious photo-detection incurred signal-signal beating interference (SSBI) can be mostly avoided. The overall transmission data rate achieves 150 Gb/s with less than 50 GHz transmission bandwidth is used. Secondly, to further improve the spectral efficiency, an iterative DSP for SSBI reduction is employed in each receiver of the PON. By rebuilding the SSBI of the received multiband DDO-OFDM signals, such DSP can gradually wipe out the SSBI contaminations. The results show that the signal performance saturates within only two iteration loops, and the conventional blank bandgap can be narrowed by about 66%. Thirdly, a polarization division multiplexing (PDM) technique is then employed to the above multiband DDO-OFDM system. To follow the trend of centralized RAN architecture in next mobile communications, the receiver oriented complicated feedback polarization tracking mechanism or polarization equalizing DSP is not necessary in our proposed PDM systems. In this topic, a designed PDM signal with two polarization orthogonal optical carriers is generated in the central office, namely the transmitter. A simple optical filtering is enough for PDM demultiplexing. Two solutions based on this polarization-tracking-free mechanism are presented, a PDM multi-service radio-over-fiber (RoF) mobile fronthaul and a PDM intermediate-frequency-over-fiber (IFoF) mobile fronthaul. The conventional training signals for polarization tracking are thus eliminated in these two systems, where the data rates are exactly doubled with respect to those of single polarization systems. Furthermore, the polarization orthogonal optical carriers are reused for an upstream generation without any polarization controlling. Therefore, a true polarization controlling free optical network unit (ONU) and user equipment (UE) has been established. In a demonstration of the PDM multi-service RoF mobile fronthaul a 12-GHz low-frequency wave and a 62.5-GHz millimeter wave are applied. And in the PDM IFoF mobile fronthaul demonstration the noted optical SSB filter is applied for multiband and PDM demultiplexing simultaneously.
As a result, by employing the multiband technique, the iterative DSP, and the PDM technique, the spectral efficiencies of a DDO-OFDM system are effectively enhanced with affordable costs. The centralized system architectures also follow the big trend of system complexity centralization in the next generation mobile communications. Therefore, in both PONs and RANs, these presented solutions are proved and guaranteed with high spectral efficiency and cost efficiency, which are greatly benefit to the future researches and developments of optical access network communications.

並列關鍵字

optical fiber communication ； passive optical network ； radio access network ； mobile communication ； orthogonal frequency division multiplexing ； direct detection ； digital signal processing ； polarization division multiplexing ； radio over fiber

參考文獻

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