提高下一代長距離光纖接取網路頻譜效率之開發和研究

Abstract

長距離 (大約100 公里) 波分多工 (WDM) 光接取網路被視為未來極具潛力的架 構。它可降低網路中的耗能並通過簡化網路結構而減少成本。在以往國科會 (NSC) 的計劃中，我們已經成功地驗證了100 公里、高於512 分流比 (split-ratio) 的長距 離光接取網路。我們還提出並驗證了若干種減緩瑞利背向散射 (Rayleigh backscattering) 的方法，並成功應用於載波分佈 (carrier-distribution) 長距離光接取 網路中。另外，我們更創建了由長距離被動光網路 (PON) 和長距離光纖微波 (ROF) 的整合系統，為固定和移動用戶提供一個整合的寬帶平台。 為了服務更多的用戶，並滿足每位用戶數據量呈指數增加的需求，僅僅依靠增 多波分多工 (WDM) 通道的數量是遠遠不夠的。原因在於光放大器的帶寬有限，只 能在30 nm 的帶寬內提供光放大。所以這就迫使光網路提升頻譜效率 (spectral efficiency)，即在固定的波分多工通道中使用更高的位元速率 (bit-rate)。雖然頻譜效 率可能不是目前接取網絡中最重要的問題，但在長距離光接取網路是極為重要的(因 為它集成了目前的都會網路和接取網路部分)。 本計劃主要就 4 個領域進行研究：(1)分析並確定提升長距離光接取網路頻譜效 率時可能會碰到的問題及挑戰。由於目前常用的不歸零 (NRZ) 格式的頻譜效率僅 為0.4 bit/s/Hz，我們將會採用正交波分多工 (orthogonal-WDM) 技術來提升頻譜效 率。(2) 我們會研究使用高頻譜效率的正交振幅調變 (QAM) - 正交頻分複用 (OFDM) 訊號在長距離光網路。(3)我們將驗證一個 4 波段 (bands) 高頻譜效率的 OFDM 長距離被動光網路，其數據傳輸率依中央控制中心 (CO) 與用戶之間的距離 而作出動態變化。(4) 最後，我們將驗證一個訊號再調變 (signal remodulation) 的高 頻譜效率光網路。下行訊號是OFDM，而上行再調變訊號是OOK。也會分析利用 反射式半導體光放大器 (RSOA) 作為抹掉下行OFDM 訊號的效率。

The wavelength-division-multiplexed (WDM) long-reach (LR) (~ 100 km) optical access network is considered as a promising candidate for the future. It can reduce the network power consumption and the cost by simplifying the network architecture. In the past 4 years NSC projects, we have successfully demonstrated a 100-km-reach and high split-ratio (> 512) LR access networks. We have also proposed and demonstrated several Rayleigh backscattering (RB) mitigation schemes for the successful implementation of the carrier-distributed LR optical access networks. In addition, we have also proposed and demonstrated schemes for the convergence of LR passive optical network (PON) with the LR radio-over-fiber (ROF) system, providing an integrated broadband platform for both fixed and mobile subscribers. In order to support more subscribers and to accommodate the exponentially growth in traffic demands by each subscriber, just increasing the number of WDM channels in the WDM LR optical network is not enough, since the optical amplifier used in the LR optical network has a fixed and limited bandwidth of about 30 nm. This forces the networks to increase the spectral efficiency (i.e., to use higher per-channel bit rates at fixed WDM channel spacing). Although spectral efficiency may not be the top priority issue of present access networks, it is not true for the case of LR access (since it integrates the present metro and access sections). The goals of this project focus on 4 areas: (1) We will analyze and identify the challenges to increase the spectral efficiency in LR optical networks. As the commonly used non-return-to-zero (NRZ) format has a spectral efficiency of only 0.4 bit/s/Hz, we will propose and demonstrate by using “orthogonal-WDM” to increase the spectral efficiency. (2) We will study and demonstrate using the high spectral-efficient quadrature amplitude modulation (QAM) orthogonal frequency division multiplexed (OFDM) signal for the LR optical network. (3) A four-band high spectral efficient OFDM LR-PON with be demonstrated with dynamic data-rate adjustment depending on the distances between the central office (CO) and subscribers. (4) Finally, a signal remodulated high spectral-efficient access using downstream OFDM and upstream on-off-keying (OOK) will be demonstrated. The analysis of using reflective semiconductor optical amplifier (RSOA) to erase the downstream OFDM for the upstream signal remodulation will be given.