應用於長距離被動光網路且具成本效益之光正交多工傳輸技術

Abstract

為了因應未來寬頻影音服務的需求，國際標準制訂組織FSAN正在訂定下世代被動光網路標準，並命名為NGPON2。NGPON2將會需要支援40-Gbps網路傳輸量，以提供每個ONU用戶1-Gbps的傳輸速率，並且將光纖涵蓋範圍由20公里提升至100公里。然而目前使用在2.5-Gbps GPON與10-Gbps XGPON的OOK光收發器無法直接升級使用在40-Gbps NGPON2。由於40-Gbps OOK光收發器所使用頻寬約將近40 GHz，因此會遭遇到嚴重的光纖色散干擾與接收端的雜訊，使得光纖傳輸距離被嚴重限制在5公里之內，無法滿足NGPON2的需求。 近來，基於直調偵測之光正交多工長距離被動光網路由於具有良好的頻寬使用效率，並可以充分地分配次載波頻率位置與調變格式，而受到大量的矚目，因此非常適合成為NGPON2的解決方案。然而，光直調偵測正交多工長距離被動光網路有兩個瓶頸需要克服：第一個為RF Power Fading限制住了100公里光纖傳輸系統的第一個Passband頻寬至僅僅3 GHz; 第二個為次載波交互混合干擾（SSII）會產生In-Band和Out-of-Band干擾，因此會導致接收訊號性能嚴重衰減。 在本論文中，我們同時從理論與實驗來分析討論RF Power Fading與SSII干擾的問題。同時，我們也提出了具偏壓控制之低叼啾並具Preemphasis與Post-Equalizer之光直調偵測正交多工傳輸技術，可以調整EAM叼啾以提升第一個Passband的頻寬。實驗結果顯示在3-GHz頻寬之內使用128 QAM與Preemphasis和Equalizer可以在100公里光纖傳輸距離達到21-Gbps傳輸速率。 我們也分析了20~100公里光纖傳輸系統的頻率響應，並發現在第一個Passband之後仍有其他Passbands可以被使用來提升頻寬。因此，我們發展了使用可調式光調變與位元負載配置之多頻段光正交多工傳輸技術，將頻寬使用達到最佳化。實驗結果顯示使用10-GHz電吸收光調變器與直接偵測，在20~100公里光纖傳輸距離下，皆可以達到40-Gbps傳輸速率。 最後，我們研發了SSII消除技術以估算SSII並進一步消除。在基於電吸收光調變器之直調偵測多頻段光正交多工傳輸系統中，我們領先完成了SSII消除技術開發與實驗展示。由於SSII嚴重地受到叼啾的影響，且我們發現僅考慮簡單的常數叼啾並無法正確消除SSII，因此我們發展了基於動態叼啾的SSII消除技術，可以大幅提升SSII估算精準度與SSII消除比例。實驗結果顯示相較於只考慮常數叼啾的SSII消除技術，使用具動態叼啾之SSII消除技術，SSII消除比率可由23.6%提升至74.4%，並可在32.25-Gbps與100公里之光正交多工傳輸系統中，提升2.8-dB光接收靈敏度。 因此，結合我們提出之可調式光調變與位元負載配置之多頻段光正交多工傳輸技術與新穎的SSII消除技術，可以成功地解決RF Power Fading與SSII的問題，使得光直調偵測正交多工長距離被動光網路成為一可行且具成本效益之NGPON2解決方案。

In order to meet the requirements of broadband multimedia services in the near future, FSAN (Full Service Area Networks) has been working on the new standard for next-generation passive optical network (NGPON) and named NGPON2, which can provide 40-Gbps capacity to support 1-Gbps data rate for each optical network unit (ONU), and extend the coverage range from 20-km to 100-km standard single-mode fiber (SSMF). However, current on-off-keying (OOK) based optical transceiver used in 2.5-Gbps GPON and 10-Gbps XGPON cannot be directly upgraded to support 40-Gbps NGPON2, since 40-Gbps OOK modulation will occupy near 40-GHz bandwidth and suffer from server limitation of chromatic dispersion and receiver noise, thus limit the SSMF transmission to be less than 5 km and cannot meet the NGPON2 requirement. Recently, optical OFDM LR-PON with intensity modulation and direct detection (IMDD), which boasts to offer high spectral efficiency and flexible bandwidth allocation, has been attracted a lot of attention, and is very suitable for NGPON2. However, optical IMDD OFDM LR-PON has two hurdles needed to be crossed. The first one is the RF power fading, which limits the available bandwidth of the 1st passband of 100-km SSMF to be less than 3 GHz, while the second one is the subcarrier-to-subcarrier-intermixing interference (SSII), which generates in-band and out-of-band interferences to degrade the performance of received OFDM signal seriously. In this thesis, we theoretically and experimentally analyze the RF power fading and SSII effects. Then, we proposed the bias-control low-chirp optical IMDD OFDM scheme, which can adjust the chirp of electro-absorption modulator (EAM) to increase the bandwidth of the first passband. The experimental results demonstrate a superior performance of 21-Gbps OFDM signal transmission over 100-km LR-PONs with spectrally-efficient 3 GHz by using a cost-effective and low-chirp EAM, and adopting the 128-QAM format and adaptive subcarrier preemphasis and post-equalizer. We also analyze the responses of 20~100-km SSMF and find that secondary passbands can be used to increase the bandwidth. Thus, we develop the dynamic multi-band OFDM with adaptive modulation and bit-loading algorithm to make the best of bandwidth utilization. The experimental results successfully demonstrate a superior performance of at least 40-Gbps OFDM-signal transmission over 20~100-km LR-PONs, which is achieved by using a 10-GHz EAM and direct detection, and adopting the bit-loading algorithm. Finally, we develop a novel SSII cancellation technique to estimate and eliminate SSII. For the first time, the SSII cancellation technique is experimentally demonstrated in an EAM-based IMDD multi-band OFDM transmission system. Since the characteristics of SSII are seriously affected by the chirp parameter, a simple constant chirp model, we found, cannot effectively remove the SSII. Therefore, a novel dynamic chirp model is developed to obtain better estimation and cancellation of SSII. Compared with 23.6% SSII cancellation by the constant chirp model, our experimental results show that incorporating the dynamic chirp model into the SSII cancellation technique can achieve up to 74.4% SSII cancellation and 2.8-dB sensitivity improvement in a 32.25-Gbps OFDM system over 100-km SSMF. Based on our proposed dynamic optical multi-band OFDM with adaptive modulation and novel SSII cancellation technique, the optical IMDD OFDM LR-PON will not be limited by the RF power fading and SSII, and become a feasible and economical solution for NGPON2.